Optimized, versatile
diamond-based sensors:materials, fabrication andnovel applications

Dissertation
zur Erlangung des Grades
des Doktors der Naturwissenschaften
der Naturwissenschaftlich-Technischen Fakultät
der Universität des SaarlandesvonRichard Nelz
Saarbrücken
2019

Tag des Kolloquiums:
Dekan:
Berichterstatter:
Vorsitz:
Akad. Mitarbeiter:

07.05.2020
Prof. Dr. Guido Kickelbick
Dr. Elke Neu-Rung
Prof. Dr. Uwe Hartmann
Prof. Dr. Rolf Pelster
Dr. Stephan Kucera

iii

Abstract
Quantum sensing as one of the backbones of the second quantum revolution is about toenable a variety of novel applications requiring good spatial resolution and sensitivity. The
atomic-sized, negatively charged nitrogen vacancy (NV) color center in single crystal diamond was found to enable magnetic eld sensing at the nanoscale. Magnetic sensing using
NV centers is enabled by bright photostable emission and optically addressable spin states.

√

Due to its extraordinary coherence time, sensitivities of few nT/

Hz can be achieved underambient conditions. To enhance the spatial resolution of NV-based sensing, it is necessary
to approach the NV center to a sample to investigate. Here, a challenging nanofabrication
procedure is needed to sculpt the diamond into a photonic nanostructure usable as a scanning probe tip.
In this thesis, we report on the progress towards optimizing the applicability of NV centers as quantum sensors. We investigate novel material systems promising for upscaling
nanofabrication. By introducing a novel approach to enhance the adhesion of etch masks
and novel plasma treatments, we optimize the reliability of the nanofabrication procedure.
In addition, we study a novel near-eld interaction-based sensing resource. By investigating the interaction of shallow NV centers with a monolayer of WSe2 , we were able to show
simultaneous near-eld and magnetic eld sensing using the NV center.

Kurzzusammenfassung
Als eine der Säulen der zweiten Quanten-Revolution ermöglicht die Quantensensorik vieleneue Anwendungen, die eine gute Ortsauösung und Sensitivität benötigen. Das atomar
kleine, negativ geladene Sticksto-Fehlstellen (NV) Farbzentrum in einkristallinem Diamant ermöglicht das Detektieren von Magnetfeldern auf Nanomaÿstäben. Magnetfelddetektion mittels NV Zentren wird durch helle, photostabile Emission und optisch adressierbare Spin-Zustände ermöglicht. Aufgrund seiner auÿergewöhnlichen Kohärenzzeit erreicht√

es Sensitivitäten von einigen nT/

Hz unter Umgebungsbedingungen. Zur Verbesserung

der Ortsauösung NV-basierter Sensorik, muss das NV-Zentrum an die zu untersuchende
Probe angenähert werden. Dies erfordert einen herausfordernden Nanfabrikationsprozess,um den Diamanten in eine photonische Struktur zu formen, die als Rastersonde nutzbar
ist.
Diese Arbeit beschreibt Fortschritte zur Optimierung der Anwendbarkeit von NV-Zentren
als Quantensensoren. Wir untersuchen neuartige Materialien, die vielversprechend für die
Skalierbarkeit des Prozesses sind. Durch neue Ansätze zur Verbesserung der Adhäsion vonÄtzmasken und neue Plasmabehandlungen optimieren wir die Zuverlässigkeit der Nanofabrikation. Zudem analysieren wir einen neuen, auf Nahfeldwechselwirkung beruhenden Sensorikansatz. Bei der Untersuchung der Wechselwirkung von oberächennahen NV-Zentren
mit monolagigem WSe2 konnten wir das gleichzeitige Erfassen von Nah- und magnetischen
Feldern mittels NV-Zentren zeigen.

iv

v

Articles included in this thesis
The present thesis is a cumulative work including the six publications listed below. The
sorting of the list represents the chapters of this thesis in which the respective publications
occur.

The contribution of the author of this thesis is stated on the rst page of eachchapter.

Chap. 2

Color center uorescence and spin manipulation in single crystal, pyramidal diamond
tips
Richard Nelz, Philipp Fuchs, Oliver Opaluch, Selda Sonusen, Natalia Savenko, Vitali
Podgursky, and Elke Neu
Applied Physics Letters 109, 193105 (2016); doi: 10.1063/1.4967189

Chap. 3

Toward wafer-scale diamond nano- and quantum technologies 1
Richard Nelz, Johannes Görlitz, Dennis Herrmann, Abdallah Slablab, Michel Challier, Mariusz Radtke, Martin Fischer, Stefan Gsell, Matthias Schreck, Christoph
Becher, and Elke Neu
APL Materials 7, 011108 (2019); doi: 10.1063/1.5067267

Chap. 4

Reliable Nanofabrication of single-crystal diamond photonic nanostructures for nanoscalesensingMariusz Radtke, Richard Nelz, Abdallah Slablab, and Elke Neu
Micromachines 10, 718 (2019); doi: 10.3390/mi10110718

Chap. 5

Plasma treatments and photonic nanostructures for shallow nitrogen vacancy centersin diamond
Mariusz Radtke, Lara Render, Richard Nelz, and Elke Neu
Opt. Mater. Express 9, 4716-4733 (2019); doi: 10.1364/OME.9.004716

Chap. 6

Nanoscale sensing based on nitrogen vacancy centers in single crystal diamond andnanodiamonds: achievements and challenges 2
Mariusz Radtke, Ettore Bernardi, Abdallah Slablab, Richard Nelz, and Elke Neu
Nano Futures, in press (2019); doi: 10.1088/2399-1984/ab5f9b

1 This paper was selected as an Editor's Pick
2 This paper is a topical review

viChap. 7

Near-eld energy transfer between a luminescent 2D material and color centers indiamondRichard Nelz, Mariusz Radtke, Abdallah Slablab, Zai-Quan Xu, Mehran Kianinia,
Chi Li, Carlo Bradac, Igor Aharonovich, and Elke Neu
Adv. Quantum Technol. 1900088 (2019); doi: 10.1002/qute.201900088

Contents
1. Introduction

1

1.1.

NV centers in diamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5

1.2.

Nanofabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

1.3.

Experimental setup and methods. . . . . . . . . . . . . . . . . . . . . . .

12

2. Color center uorescence and spin manipulation in single crystal, pyramidal
diamond tips

17

3. Toward wafer-scale diamond nano- and quantum technologies

23

4. Reliable Nanofabrication of single-crystal diamond photonic nanostructures
for nanoscale sensing

29

5. Plasma treatments and photonic nanostructures for shallow nitrogen vacancy centers in diamond41

6. Nanoscale sensing based on nitrogen vacancy centers in single crystal diamond and nanodiamonds: achievements and challenges

61

7. Near-eld energy transfer between a luminescent 2D material and color
centers in diamond93

8. Conclusion and Outlook

103

8.1.

Materials for sensing applications. . . . . . . . . . . . . . . . . . . . . . .

8.2.

Optimized nanofabrication processes. . . . . . . . . . . . . . . . . . . . .

106

8.3.

Novel applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

108

8.4.

Outlook109. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. Supplementary informations

104

113

A.1. Chap. 2: Color center uorescence and spin manipulation in single crystal,pyramidal diamond tips

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

A.2. Chap. 3: Toward wafer-scale diamond nano- and quantum technologies

. .

113
118

1. Introduction
The breakthrough discoveries and theoretical descriptions of the photoelectric eect andthe black-body radiation as well as the introduction of quanta started the rst quantumthrevolution in the 20

century.

This established new laws in form of quantum mechan-

ics to describe the physical properties of atoms, solids and electro-magnetic elds.

The

understanding of the basic concepts of these new laws enabled the development of manytechnologies relevant for our modern society: For instance, the fundamental understanding of light matter interaction lead to the establishment of solar cells as well as to thedevelopment of lasers. But the rst quantum revolution only enabled the understandingof quantum mechanics [1] while new developments in the 2000s allowed the active employment of quantum mechanics as a resource for novel applications. These ndings initiatedthe second quantum revolution, where scientists found ways to actively create coherent
superpositions or entanglement of quantum states.
The progress made in the last decades is pioneering the development of novel technologies
like quantum computers [26], quantum cryptography [79] or quantum optics [1013].
The later, for instance, lead to the disproof of Einstein's view on quantum mechanics from1935 [14, 15]. For most of these quantum-based applications, reliable sources of single photons are mandatory. In particular, the deterministic on-demand creation of photons is ofmajor interest. Quantum photonics was used to optimize the properties of single photons
and their collection eciency [1618].

To eventually design a quantum network compa-

rable to the established internet, extensive quantum engineering is necessary [19]. Whiledierent quantum systems can act as e.g. memories like atomic ensembles [20, 21] or as
repeaters [22], single photons are optimal to be used as qubits to entangle dierent nodes
in the network [23, 24].
In general, single photons and quantum coherence represent the base onto which the backbones of the second quantum revolution are built.

In addition to the above mentioned

elds, quantum sensing and metrology is of major interest for a variety of novel applications leading to a better understanding of the microscopic properties of material systems,
atoms and spins as well as processes in living cells.
extraordinary spatial resolutions and sensitivities.

Most of these applications require

Especially color centers in diamondare promising quantum systems to fulll these requirements while they are able to senseelectric [25], magnetic [26] and optical near elds [27] as well as strain [28] and temperature [29]. In this thesis, we focus on the nitrogen vacancy (NV) center in diamond.
The NV center in diamond, which will be further introduced in Sec. 1.1, oers a remarkableapplicability as a quantum sensor [26]. Here, we distinguish between ensemble and single
center applications:First, we start with the NV ensemble which is extraordinary use-

ful in wide-eld applications like e.g. monitoring the activity of electrical and integrated

2

1. Introduction

circuits [30, 31], imaging magnetic nanoparticles inside living cells [32] or characterizing
fundamental material properties as crystal stress in diamond [33]. In addition, ensembles
were used to study the fundamental properties of the NV centers' spin resonances.

For

instance, the inuence of an external electric eld onto the coherence properties of thespin resonances was used to conrm the lack of inversion symmetry of NV centers in diamond [34]. Due to the large number of involved NV centers with dierent orientations in
the ensemble, it is possible to reach even sub-pT sensitivity [35], to extract the vectorial
components of magnetic elds [36] and to use the NV ensemble as a radio frequency spectrum analyzer [37]. Here, often nanometer-thin layers of NV centers close to the surface of
diamond are used to investigate the properties of a sample placed on top of the diamondwith a good spatial resolution. However, this resolution is mainly limited by diraction(i.e. the point spread function) of the used setup.
To overcome this limitation, it is necessary to use single NV centers in diamond. Due tothe atomic size of a single center, the spatial resolution is only limited by the distance
between the NV center and the sample under investigation. Using single NV centers inbulk single crystal diamond or nanocrystals opens up new possible applications like e.g. theusage of the NV center as a magnetic eld probe to image and detect magnetic resonances(MRI and NMR) [3840] and to measure magnetic elds on the nanoscale [41]. Further, it
is possible to image the magnetic eld of a single electron spin in ambient conditions [42].
Additionally, by bringing samples or particles close to a NV center embedded in bulk material, it is possible to image the magnetic resonances of the particles [4346] and to study
ballistic transport in metals [47] as well as spin waves [48]. In addition, using single NV
centers as quantum sensors also draws interest of other communities like life sciences [49].
Here, especially the chemical inertness and non-toxicity of diamond is favorable for theinsertion of NV centers into a living cell for e.g. bio-labeling [50], to remotely monitor
chemical processes in the cells [51], for particle tracking [52] and in vivo temperature measurements [53].
In recent years, near-eld interactions in form of an energy transfer between two close
dipoles have become a commonly used tool to study processes in biological environments inlife sciences [5456]. This interaction is called Förster Resonance Energy Transfer (FRET).
By bringing two dipoles in close proximity of a few nanometer, it is possible that a donor
dipole transfers its energy non-radiatively to an acceptor dipole. The main requirement
for an ecient transfer is a signicant spectral overlap between the donor's emission and6
the acceptor's absorption bands. By exploiting the strong 1/distance -dependency of theinteraction, it is possible to use FRET for superresolution imaging of dipoles.

Despitethe common usage of FRET in life science applications, the eect remains almost unexploited in the quantum sensing community. However, due to the broad emission spectrumof the NV center in diamond it is possible to nd a variety of potential FRET partners
rendering FRET a viable resource for sensing applications in general.

At the beginningof this decade, there were some rst experiments using FRET to establish an interaction
between NV centers and molecules or graphene [27, 57]. These attempts used NV centers innanocrystals attached to a scanning probe tip to study the interaction. While NV centersin nanocrystals might suer from a bad surface of the nanocrystals and a limited quantum

3eciency, using NV centers in single crystal diamond should lead to stable NV centers witha potentially unity quantum eciency and a reliable control of the NV-sample-distance.
However, the usability of NV centers in single crystal diamond for FRET was not demonstrated yet.
One promising approach to harness the full potential of the NV center for sensing applications uses high-purity single crystal diamond while the NV center's surrounding is
structured to form a photonic nanostructure in a complex top-down approach.

These

structures are usable as scanning probe tips [58] in an atomic-force microscope as proposedby Chernobrod and Berman in 2005 [59]: Here, especially the photonic enhancement of
the photoluminescence as well as the good coherence properties due to the low densityof impurities in the material aids to increase the achievable sensitivity [17, 60, 61]. Here,the tip-like geometry of the structure and the ability to approach a shallow NV center
embedded in the structure to a sample surface enables high spatial resolution sensing inthe sub-100 nm-regime [58].

This initiates completely novel applications for NV center-based sensing like, for example, imaging magnetic structures like vortices or domains anddomain walls [6268], characterizing magnetic properties of thin lms [6971] or by usinga NV center at cryogenic temperatures to image superconductors and superconducting
vortices [7275]. Further, NV centers used as scanning probe tips were used to image currents [76] and magnetic resonances on the nanoscale [77].
While single crystal diamond scanning probes are highly promising for high-resolution
and -sensitivity sensing, a persistent challenge in this approach is the complex, multistep
process to fabricate such nanostructures (see Sec. 1.2). Here, especially the yield of thefabrication has to be optimized by using reliable processes and suitable diamond material.
In this thesis, we will show routes to overcome these challenges. The progress we achieved
is summarized in Fig. 1.1.

First, we worked on the scalability of NV-based sensing byinvestigating novel material systems. We started by investigating commercially available
single crystal diamond scanning probe tips with a pyramidal shape. Here, the tips wereformed in a bottom-up approach by growing the diamond pyramids in a chemical vapor
deposition process. This will be the topic of Chap. 2 published as Ref. [78]. Afterwards,
we studied the properties of a wafer-sized single crystal diamond based on the successful
scaling of heteroepitaxial nucleation during chemical vapor deposition.

Here, we foundsingle native NV centers which we exploited to characterize the material's suitability as ahost for NV-based scanning probe devices (see Chap. 3 published as Ref. [79]).
As we found promising new material systems usable for sensing applications, we workedon the optimization of the nanofabrication procedure. Here, we mainly followed two approaches: First, we optimized the reliability of the fabrication. We used silicon as a thinadhesion layer for the etch mask and sophisticated plasma etching of the structures. This
will be introduced in Chap. 4 published as Ref. [80]. Second, we studied the inuence ofthe plasma treatment on shallow NV centers in the diamond.

Here, we focused on thecomparison of dierent plasma treatments to remove mechanical stress due to polishingthe diamond surface. By implementing novel unbiased plasmas in our process, we foundstable shallow NV centers in nanostructures promising for sensing applications (see Chap.
5 published as Ref. [81]).

4

1. Introduction

Building a diamond sensor
Novel materials
NV
100mm

Wafer-scale SCD:
Chapter 3

Pyramidal scanning probes:
Chapter 2

Optimized nanofabrication
ICP-RIE plasma

HSQ mask
Si layer

Reliable nanofabrication:
Chapter 4

Plasma treatment:
Chapter 5

Novel sensing applications
Se
W
Se

C

Interaction with graphene:
Chapter 6
Figure 1.1.:

FRET with WSe2:
Chapter 7

Schematic representation of the topics with their respective occurrence in this thesis.
We worked on all steps needed to improve the scalability and reliability of thenanofabrication process needed to build a NV-based diamond sensor. Further, we
introduced a novel sensing resource for NV centers in single crystal diamond in formof near-eld interaction, namely FRET, promising for a variety of applications.

Exploiting near-eld interaction as a resource for sensing would enable a new functionality
for NV centers in single crystal diamond.

To investigate their usability for applications

based on near-eld interaction, we rst performed a proof-of-principle experiment: By applying graphene on a diamond containing a shallow NV ensemble, we were able to study thequenching of the NV center's photoluminescence induced by the interaction. This will bepart of the topical review in Chap. 6 published as Ref. [82]. After successfully demonstrat-

5

1.1. NV centers in diamonding the suitability of NV centers as a donor for FRET processes via the interaction withgraphene, we wanted to further evaluate the interaction. Here, we transferred a monolayerof a luminescent two-dimensional material, namely WSe2 , on the surface of a diamondcontaining shallowly implanted NV centers.

By investigating the properties of the NV

ensemble undergoing FRET with akes of WSe2 , we found an outstanding novel feature of
the NV center: as the NV center was able to retain its capability to sense magnetic elds
while interacting with WSe2 , NV centers can be used as a multifunctional sensor.

Thiswill be described in Chap. 7 published as Ref. [83].
In Chap. 8, we shortly discuss and summarize the results of the publications included inthis thesis. In addition, we will give a brief outlook on potential future applications of theNV center in single crystal diamond as a quantum sensor.

As some basic concepts as well as details of the experimental setup have been omitted forbrevity in the manuscripts that form the main body of this thesis, we briey summarize
these in the following. We will give a short introduction to the NV center in diamond in
Sec. 1.1. Afterwards, we will summarize the process needed to fabricate scanning probe
devices in Sec. 1.2. Sec. 1.3 will introduce the experimental setups and methods in detail.

1.1. NV centers in diamond
The NV center is an optically active defect center consisting of a nitrogen atom and aneighboring vacancy within the diamond lattice. The NV center is oriented along one of
the equivalent h111i crystal axes dening the quantization axis, and belongs to the C3ν

symmetry group [84]. There are mainly two approaches to incorporate NV centers into thediamond. First, it is possible to incorporate nitrogen vacancies directly during the growth
process of the diamond in a chemical vapor deposition (CVD) reactor or in a High Pressure
High Temperature (HPHT) synthesis by creating substitutional nitrogen which can thenbe used to create NV centers. The main drawback hereby is to precisely control the lateralplacement of the NV centers in the diamond itself. While most of these techniques sueradditionally from an insucient control of the depth distribution of the NV centers, there
are some promising approaches using complex δ -doping processes during the growth of the
diamond [85]. However, implanting nitrogen into an ultra-pure diamond lead to a better
positioning of the centers potentially down to nanometer resolution by using pierced AFM
tips [86], nanomasks [87] or focused electron irradiation [88]. By subsequently annealing◦
the diamond above 600 C, the vacancies generated during the implantation can diuse in

the lattice towards substitutional nitrogen and then form the NV color center.
The nitrogen vacancy center in diamond can occur in dierent charge states: While the+positively charged NV is non-uorescent and is only accessible under enormous experimental eort [89], the neutral and negatively charged NV center oers radiative transitions
between discrete energy levels in the large bandgap of diamond. It is possible to spectrally
distinguish between both optical active charge states because of their dierent spectral po-

6

1. Introduction

sitions. While the purely electronic transition from the excited to the ground state, termed0−zero-phonon line (ZPL), of the NV lies at 575 nm, the ZPL of the NV
lies at 637 nm.
Both charge states have in common that they have a strong interaction with lattice vibrations leading to a broad phononic sideband with a width of more than 100 nm [see Fig. 1.2
a)].
Due to its extraordinary capabilities for sensing applications, we focus in the following
on the negatively charged NV center which will be termed NV center in the following forsimplicity. We will start by further discussing the electronic structure of the color center.
To describe the NV center in diamond, it is necessary to consider six electrons: two fromthe nitrogen atom and three from each of the dangling bonds of the carbon atoms surrounding the vacancy.

The sixth electron can be acquired from the vicinity of the NV

center, most likely from nearby substitutional nitrogen. This results in an overall spin of

1. The electronic conguration, as depicted in Fig. 1.2 b), gives rise to two triplet states3
3
1( A2 for the ground state and E for the excited states) with two singlet states ( E for1energetically lower state and A1 for the higher state) in between. While the transition
between the ground and excited state results in the ZPL at 637 nm, the radiative transition
between the two singlet states give rise to the infrared ZPL at 1042 nm [90] [solid lines inFig. 1.2 b)] which only oers a quite weak intensity compared to the optical transition due
11
to strong competitive non-radiative decay channels [84] [dashed line between A and E in
Fig. 1.2 b)].
The ground and excited state splits into two spin sub-levels ms = 0 and ms = ±1 sep-arated by a zero eld splitting of Dgs= 2.87 GHz and Des = 1.42 GHz mainly due tospin-spin-interaction. By applying an external magnetic eld, the degeneracy of the spinstates ms = ±1 is lifted by the Zeeman eect.

The splitting of these two transitions isgiven by 2γNV BNV where BNV is the magnetic eld projection onto the NV quantization

axis and γNV = 2.8 MHz/G is the gyromagnetic ratio of the NV center.
The import role for sensing applications is given by the special dynamics of the NV center's
excited state: While the transition from ground to excited state is highly spin preserving,
the decay of the excited state is spin dependent: As represented by the dashed lines fromthe excited state to the singlet state system in Fig. 1.2 b), the ms = 0 excited state hasa high probability to decay back into the ms = 0 ground state with a lifetime of 12 ns,
whereas the ms = ±1 excited state has a higher probability to decay non-radiatively into

the single state system. This additional non-radiative decay channel results in a shorter

lifetime of 7.8 ns [91]. The population of the singlet states decays (potentially emitting an
infrared photon) to the lower singlet state with a lifetime of several hundreds of nanoseconds [92].

Here, the non-radiative transition from the lower singlet state to the groundstate manifold is slightly preferential into the ms = 0 spin state [84]. Because of the opical
cycling of the ms = 0 state and the dynamics of the ms = ±1 excited state as well as the
singlet state system, it is possible to polarize the NV center into its ms = 0 ground state
after a few optical cycles with a delity of up to 95 % [93].

In addition, these dierent

probabilities for the transition to the long lived singlet system in the excited state leadto a spin-dependent optical emission rate and thereby gives an opportunity to eciently
distinguish between ms = 0 and ms = ±1 by comparing the integrated uorescence signal

71.1. NV centers in diamonda)b)

Conduction Band

3

E

7.8ns

Des

12.0nsms = ±1
ms = 011042nm

637nm

1

300ns

A

E

ms = ±1
3

A2

Dgs

ms = 0

Valence Band

Figure 1.2.:a) Emission spectrum of the negatively charged NV center in diamond. A clear
ZPL at 637 nm as well as the broad phononic sideband are visible. b) Electronic
structure of the NV− center in diamond. The zero eld splitting of the spin-levels
is Dgs = 2.87 GHz for the ground state and Des = 1.42 GHz for the excited state,
respectively. The solid lines indicate radiative transitions, while the dotted lines
indicate non-radiative transitions. Here, the transition from the ms = ±1-excitedstate to the singlet states is stronger then from the ms = 0 state leading to aspin-dependent uorescence as described in the text.

upon optical excitation.
In addition, this readout can be used to measure the above mentioned splitting of theground state spin states.

By applying a magnetic eld at a microwave frequency andsweeping its frequency, it is possible to induce spin ips when the microwave frequency
matches the transition frequency between the ms = 0 and the ms = ±1 states. These spin
ips lead to a reduction of the overall uorescence rate when the NV is continuously ex-

cited. This is called optically detected magnetic resonance [ODMR, see Fig. 1.3 a)]. Whena magnetic eld is applied on axis and thereby the degeneracy between the ms = ±1 states
is lifted, two resonances can be detected when sweeping the microwave frequency.

Thiscan be used to evaluate an unknown magnetic eld quantitatively.
The main measure for the performance of a sensor is its sensitivity η which describes theminimal magnetic eld that can be detected in a time T . Thus η describes the smallest
magnetic eld strength which can be sensed in 1 s with a signal-to-noise ratio (SNR) of 1.
In general, the intensity I of a NV center during an ODMR measurement can be describedas

I(f ) = I0 · 1 − CF

f − fNV
∆f



(1.1)

8

1. Introduction

with the average intensity I0 , the ODMR contrast C , the driving microwave frequency f ,the resonance frequency fNV and the full width at half maximum ∆f of the NV center's
spin resonance as well as the line shape prole F of the resonance. As any uctuation ofthe eld strength leads to a shift of the resonance frequency the sensitivity depends on theminimum intensity dierence depending on the resonance frequency |δI/δf | which can bedetected. This leads to a sensitivity

√
h ∆f
h
I
 0  ≈ PF√η=gµB max δI
gµB C I0

(1.2)

δf

with the numerical parameter PF related to the specic lineshape F of the resonance.

√

I0 as well as the contrast
C and the linewidth ∆f of the resonance. Hereby, the linewidth is fundamentally limited?
?
by the inhomogeneous dephasing rate Γ2 = 1/T2 of the NV center's electron spin which isThus, the sensitivity mainly depends on the photon shot noisedened by the surrounding spin bath.
Under continuous excitation from the exciting laser and the microwave frequency, thisfundamental limit can not be reached due to power broadening. In addition, the contrast

C also depends on the laser as well as the microwave power impinging the NV center [94].
To describe the functional connection, we rst need to introduce the saturation parameters = Pexc /Psat given by the ratio between the excitation power and the saturation power
which will be further introduced in Sec. 1.3 and the Rabi frequency ΩR = gµB BMW /~depending on the amplitude of the incoming microwave eld BMW . Therewith it is possible
to calculate the ODMR contrast

C=Θ

Ω2R
∞
Ω2R + Γ∞
p Γc

2
s
1+s

(1.3)

∞
with a normalization factor Θ, the polarization rate Γp dened as the inverse combined∞
lifetime of the singlet system and the optical cycling rate Γc dened by the inverse excited∞∞state lifetime [94]. As both rates Γp and Γc are constant for a given NV center, it isvisible that for a xed BMW the contrast increases with decreasing optical power and fora xed optical power the contrast increases with increasing microwave power. For strong
microwave power the contrast saturates towards Θ. Under power broadening, the resonancehas a Lorentzian prole with the linewidth ∆f depending on the optical and microwave
power [94]Γ∞∆f = c
2π

s

s
1+s

2

+

Ω2R
∞
Γ∞
p Γc

(1.4)

With these expressions for the contrast and the linewidth, it is possible to nd a Rabi
frequency which optimizes the sensitivity η for continuously measured ODMR at a xedexcitation power [94]

ΩR,opt =

q
s
∞
2Γ∞
p Γc
1+s

(1.5)

9

1.1. NV centers in diamond

To harness the full potential of the NV center in diamond in terms of sensitivity and therewith sensing capability, one approach can be to improve the photoluminescence (PL) rate
and therewith the photon shot noise of the NV centers by embedding them into optimized
photonic nanostructures (see Sec. 1.2) [64]. Additionally, it is possible to use more complex
measurement schemes developed in nuclear magnetic resonance (NMR) to further optimize
the coherence of the NV center. Hereby, a two level system is coherently manipulated while
its states interact with a magnetic eld. Here, dierent pulsed manipulation protocols haveto be applied to increase the sensitivity.
The basic coherent spin manipulation is to drive Rabi oscillations, which is shown in Fig.
1.3 b).

By applying a magnetic eld, we are able to select one of the spin transitions.

a)

c)

b)
Laser
Microwave
Laser
Microwave

Figure 1.3.:a) Optically detected magnetic resonance for dierent applied magnetic elds. A
clear splitting of the resonances is visible. b) Rabi oscillations on the transition|0i → |1i. The oscillation is damped due to decoherence eects. The pulse sequence is shown as the inset. c) Spin-Echo measurement (see inset for the full pulse
sequence) with spin revivals induced by the Larmor precession of near 13 C nuclear
spin. The envelope of spin revivals yields the coherence time T2 of the NV center.

Here, we use a magnetic eld oscillating at the transition frequency between the ms = 0
and ms= 1 ground states with dierent pulse durations.

This pulse changes then theamplitude of the spin states by coherently rotating the spin from ms = 0 ≡ |0i towards

ms = 0 ≡ |1i. By extending the pulse duration, the population oscillates following a co-sine between both spin states. This oscillation is damped due to a complex dependency ofdierent parameters like the coherence time of the NV center and the driving conditions of
the oscillation. However, the important parameter of the Rabi oscillation is its frequency
and especially the so-called π -pulse duration describing the pulse duration for which a full10

1. Introduction

spin ip occurs. This is important to further characterize the coherence time T2 of the NV
center via a Spin-Echo measurement [see Fig. 1.3 c)]. Here, we rst bring the NV center's
spin state in an equal superposition1√ (|0i + |1i) .
2

(1.6)

We then allow the NV center to relax for a given time τ /2 acquiring a phase caused bymagnetic elds in the surrounding. Subsequently, applying a π -pulse lead to a swap of thephases of the eigenstates. Afterwards, a second relaxation time τ /2 followed by a read-out
pulse cancels the accumulated phase caused by slowly varying magnetic elds in the rstrelaxation time. This leads to a nal superposition which is insensitive to static magnetic
elds. The full pulse sequence is shown as an inset in Fig. 1.3 c). The read-out signal I(τ )can then be derived as [58, 95]:

" 
2 #
  n Xj
τ − j · τRτ
exp −I(τ ) ∝ exp −
T2
TDec
0

(1.7)

13
Here, we use τR as the revival period of the Larmor precession of the
C nuclear spin
13
near the NV centers and TDec as the correlation time of the
C nuclear spin bath. n isdepending on the specic decoherence process. On the one hand, Spin-Echo measurements
yield the T2 coherence time of the NV centers but on the other hand they sense alternating
magnetic elds. The sensitivity is hereby further depending on the contrast C , the mean
intensity I0 and the spin read-out time tL [63]:

1
1
~
√
√
gµB C I0 tL T2

ηAC =

(1.8)

For shallow, single NV centers in nanofabricated structures as described in the following√

section, the sensitivity reaches the nT/

Hz-regime [58].

While the above described sensing approaches are based on the spin properties of the NV
center, we now want to describe attempts using the dipole of the NV center in other words
the optical transitions for sensing applications. Here, we employ the near-eld interactionof NV centers with other dipoles, namely Förster Resonance Energy Transfer (FRET). In
general, FRET describes a process where a donor dipole transfers its energy non-radiatively
to an acceptor dipole. The transfer eciency EF RET scales with the distance d between
the dipoles as [27, 57, 83]

 nR
EF RET =d(1.9)

with the FRET radius R describing the distance where the FRET eciency is 50 % andthe power n depending on the nature of the participating dipoles. For instance, two pointlike dipoles like e.g. a NV-dye molecule pair lead to n = 6 while a point-like dipole and a
two-dimensional dipole like e.g. a NV-WSe2 or NV-graphene pair lead to n = 4 [27]. The
main requirement for FRET is a spectral overlap between the donor's emission and the111.2. Nanofabrication

acceptor's absorption band. In addition, the FRET radius depends on other parameterslike the quantum eciency of the donor and both dipole moments.

FRET creates anadditional non-radiative decay channel for the donor leading to a shorter eective excitedstate lifetimeτef f = 1/γef f of the donor.

Typically, the lifetime changes induced via

ecient FRET are readily observable with our homebuilt confocal setup which will be
described in Sec. 1.3: nRγef f = γR + γN R = γR + γR ·d(1.10)

with the decay rates γR and γN R for the radiative and non-radiative decay, respectively.
Due to the strong distance-dependency, FRET is a viable novel approach for nanoscale
imaging of e.g. molecules [57] or for the characterization of novel materials [83].

1.2. Nanofabrication
As introduced in Sec. 1.1, enhanced PL rates of NV centers optimize the sensitivity andtherewith their usability in sensing applications. This can be achieved by sculpting the diamond into photonic structures. A promising approach is to structure cylindrical nanopillars
as they potentially enhance the PL rate by a factor of > 5 [17, 61, 64]. Here, the nanopillar
acts as a waveguide leading to a better collection of the emitted photons. To fabricate such
structures, we start with a cleaned diamond substrate onto which a silicon adhesion layer isdeposited (see Chap. 4). Afterwards, we spin-coat hydrogen silsesquioxane (HSQ) electron
beam resist onto the diamond surface. By using electron beam lithography we form etchmasks.

Afterwards, the pillars are etched in an inductively coupled plasma reactive ion

etch (ICP-RIE) process (see Chap. 4 and 5 for a detailed description of the process). Dueto the anisotropic nature of the plasma, it is possible to etch close to cylindrical pillars◦
with very steep sidewalls (≤ 5 ) [61].
To use the nanopillars for applications where a good spatial resolution is needed, it is necessary to approach the NV center in the pillar to a distance of several nanometer to theinvestigated sample. Here, using the pillar-structures as a scanning probe tip analogous to
an atomic-force microscope (AFM) is of major interest [64]. The main challenge hereby isto integrate the pillar into the AFM. We address this challenge by rst etching cantileverlike platforms into the diamond and then pillars onto the platforms.

Afterwards, it isnecessary to thin down the diamond until the platforms are free standing. This is achievedby etching the diamond from the backside. The freestanding devices are schematically depicted in Fig. 1.4. We glue the free-standing platform to quartz micro-capillaries [see Fig.
1.4 and 1.5 c)] which are then attached to the AFM head [64]. Using such approaches, it ispossible to control the geometry of the diamond scanning probes. However, typical (highbias) ICP-RIE plasmas potentially damage the surface of the diamond [96].

Motivatedby the potential damage, we study the inuence of bias-free plasmas on the NV centers'
stability and coherence (see Chap. 5).

12

1. Introduction

quartz capillarygluing point

cylindrical nanopillar
cantilever-like platform
Figure 1.4.:

NV center

Schematic image of the nanofabricated structure. Here, the shallow NV center isembedded in a cylindrical nanopillar usable as a scanning probe tip. To use thesedevices in an AFM, it is necessary to glue them to a quartz micro-capillary which
is attached to an AFM head.

1.3. Experimental setup and methods
To harness the full potential of NV center sensing with good sensitivity and nanoscale spatial resolution, it is necessary to combine confocal microscopy and AFM [62]. While force
feedback keeps the NV center close to the sample surface during scanning, we evaluate theNV center's spin dependent uorescence to sense magnetic or electrical elds. To further
study the near-eld interaction of the NV centers with nearby dipoles, we implement pulsed
excitation and time-resolved PL detection. To realize the above described techniques, twodedicated confocal PL microscopy setups, one combined with AFM functionality, have been
designed and built during this thesis.
As the functionality of the two confocal microscopes employed to perform the measurementsin the publications is mainly identical, we here summarize the features of the combined
confocal microscope/AFM setup. The AFM setup, depicted in Fig. 1.5 a), contains two sixaxes movement stages: one for positioning the diamond tip in the focus of the confocal microscope (NV-scanner) and one for positioning and scanning the sample (sample-scanner).
The stages have three piezoelectric steppers (attocube, 2× ANPx101, 1× ANPz101, controlled by ANC350) for coarse positioning and a three-axes piezoelectric scanner (attocube,
ANSxyz100, amplied by ANC250) for ne positioning in the nanometer-regime. While
the NV-scanner and the x- and y-axis of the sample-scanner are controlled with an analog voltage of a data acquisition system (National Instruments, PXIe-1073 with modules:
PXIe-6612 for timing, PXI-6733 for analog output channels, PXI-6220 for multifunctional
acquisition), we use a scanning probe microscopy (SPM) controller (attocube, ASC500) tocontrol the z-axis of the sample-scanner and therefore the distance between the diamond13

1.3. Experimental setup and methodsa)b)quartz capillarytuning forkc)d)quartz capillarygluing point

nanopillar
diamond scanning probe

Figure 1.5.:

AFM setup a) as described in the main text with zooms onto the tuning fork b) anda scanning electron image c) of the nanostructure attached to the quartz capillary.
d) Exemplary measurement of an AFM test-sample in grid-geometry showing a nicedepiction of the test structure. The white scale corresponds to 3 µm.

tip and the sample. Here, we establish force feedback by attaching the diamond pillar to atuning fork [see Fig. 1.5b) and c)] where we use a dither-piezo at a dened frequency which
excites oscillations of the tuning fork at its resonance. The tuning fork itself is made ofquartz. To read out the response of the tuning fork, the piezoelectric eect is used. Whenthe tuning fork oscillates, free charges are created which can be amplied and converted
to a voltage signal (Femto, HQA-15M-10T, Amplication: 10 V/pC) measurable with the
SPM controller.

To control the distance between the diamond tip and the sample, we

excite the tuning fork with a xed amplitude and at a constant frequency.

We chose afrequency in the slope of the resonance to optimize the gradient of the response of thetuning fork.

While approaching the sample to the scanning probe tip, the resonance ofthe tuning fork shifts and therewith the voltage we read-out changes.

While scanning,

the AFM controller maintains a constant voltage signal (set point) from the tuning fork
and thus keeps the distance between the diamond tip and the sample constant. Fig. 1.5d)shows an exemplary scan of an AFM test structure obtained with this setup. As describedin Sec. 1.2, the diamond tips are attached to a quartz capillary [see Fig. 1.5 c)]. To perform

14

1. Introduction

the measurements mentioned here, we glue the quartz capillary with the diamond tips tothe tuning fork. Afterwards, we cut the capillary close to the gluing point to reduce theweight attached to the tuning fork. This is depicted in Fig. 1.5 b).
To detect the PL of NV centers, we bring them into the focus of our home-built confo-

telescopef = 100

f = 75cal scanning microscope (see Fig. 1.6). Here, we continuously excite the NV center with

From AOM Setup

Pol.-control
QWP

QWP

Flipmount

Antenna

D

HWP
CC

CCD

APD 2

APD 1

FilteringPinhole 50µm

Clean-Up

Dichroic Mirror

Breadboard with HBT
MagnetFigure 1.6.:

Homebuilt confocal microscope to detect the photoluminescence of single NV centersin diamond. For details, see text.

a 532 nm diode-pumped solid-state (DPSS) laser (Laser Quantum, gem 532) or with a
594 nm DPSS laser (Cobolt, Mambo). The latter enables charge state detection of theNV center. To further realize pulsed excitation, we use an acousto-optic modulator (AOM;
Crystal Technologies, 3200-146) with a rise-time of ≈ 13 ns in double-pass conguration foreach laser system controlled by a TTL pulse generator (SpinCore, PulseBlasterESR-PRO).
The excitation light is then focused onto the NV centers through a 100× microscope objective (Olympus, LMPLFLN100X, numerical aperture: 0.8). We collect the PL of the NV
centers focused on a 50 µm pinhole to optimize the transmission of the setup. To separate
the PL from the excitation light, we use a dichroic mirror (cut-in wavelength:
and a 650 nm dielectric longpass lter.

650 nm)The collected PL is sent to either two highly-ecient photon counters (Excelitas SPCM-AQRH-14, quantum eciency:≈ 70 %) in a

Hanbury-Brown Twiss (HBT) interferometer setup or to a grating spectrometer (Princeton
Instruments, Acton Spectra Pro 2500, Pixis 256OE CCD). To perform time-resolved PL
analysis and lifetime measurements, we use a pulsed laser (NKT Photonics, EXW-12, pulse
length: ≈ 50 ps) equipped with a dedicated lter system (NKT Photonics, SuperK Varia)

tuneable from 450 nm to 850 nm as an excitation source and a single-photon counting timecorrelator (PicoQuant, PicoHarp 300). We model the instrument response function of the
setup by a Gaussian function with a width of 326 ps. To perform ODMR measurements,

15

1.3. Experimental setup and methods

the setup is equipped with a microwave source (Stanford Research System, SG396) and a
microwave amplier (Mini Circuits, ZHL-42W+). The microwave is delivered through a
movable near-eld antenna [62] made out of a 25 µm gold wire. The microwave circuit isalso equipped with TTL-controlled switches (Mini Circuits, ZASWA-2-50DR+) enabling
precise control of the microwave to coherently manipulate the NV center's ground state
spin.
Next to standard confocal scanning, lifetime imaging and spin-manipulation of the NV
centers, the setup is able to perform a variety of other measurements. First, an importantcharacteristic of a NV center in a photonic structure is its PL saturation [see Fig. 1.7a)].
To investigate PL excitation, we measure the excitation power dependent PL rate of theNV center, which we model using:I(P ) = I∞P
+ cBG P
P + Psat(1.11)

with the intensity I , the excitation power P , the saturation power Psat and the maximum
countrate of the NV center I∞ .

I∞ indicates the performance of the photonic structure

and B = cBG P estimates the background PL level of the NV center. Using equation 1.11,
we optimize the signal S -to-background B ratio s = S/(S + B), which is important formost measurements.
Another important characteristic of the NV centers is their photon statistics. This cana)Figure 1.7.:b)a) Calculated saturation measurement for a single NV center in a photonic nanostructure. Here, Psat = 300 µW and I∞ = 650 kcps was used. b) Calculated g (2) function for a single NV center with a strong inuence of the third state but without
any background uorescence. Here, we used a = 0.5, τ1 = 20 ns and τ2 = 250 ns.

be measured by time-correlating the collected photons in the HBT interferometer and(2)(2)
calculating the second-order autocorrelation g-function [see Fig. 1.7b)]. g(τ ) is hereby

16

1. Introduction

dened as

hI(t)I(t + τ )i.
(1.12)
hI(t)i2
Here, h·i indicates the time average of the intensity I(t) at time t. By replacing intensities(2)
with number of photons n(t), it is possible to rewrite g(τ ):g (2) (τ ) =g (2) (τ ) =hn(t)n(t + τ )i
hn(t)i2(1.13)

1<1 .
m(1.14)

By introducing m as the number of single photon emitters, one can showg (2) (0) = 1 −This reduction of correlation events for τ = 0 is referred to antibunching. For a three levelsystem with a shelving state in addition to the ground and excited state, which is the case(2)
for NV centers, g(τ ) can be written as:

g (2) (τ ) = 1 − (1 + a) exp(− |τ | /τ1 ) + a exp(− |τ | /τ2 )Hereby, the parameter(1.15)

τ1 describes the anti-bunching timescale while τ2 describes thea is given by the inuence of the third state onto

bunching on intermediate time scales.

the population dynamics. It was shown experimentally that these parameters signicantly
depend on the excitation power compared to the saturation power [9799]. By allowing a(2)(2)
non-perfect signal-to-background ratio s, the measured g-function gmeas (τ ) is given by:

In particular, measuring g

(2)


(2)
gmeas
(τ ) = 1 + s2 g (2) (τ ) − 1(1.16)(τ ) was important in Chap. 3 and 5. Here, we focus especiallyon measuring the properties of single NV centers rendering it necessary to characterize
their photon statistics.

Furthermore, we measured the PL saturation of NV centers in

Chap. 2 and 5 to investigate the PL enhancement of the photonic structures used in therespective chapter.
Another important technique especially to study the near-eld interaction of NV centerswith a nearby dipole or to investigate the photonic environment is Fluorescent Lifetime
Imaging (FLIM). Here, the lifetime of the NV center is measured on each position while
scanning the sample.

Here, we implemented advanced data evaluation techniques using

dedicated Matlab codes to extract the lifetime.

In the case of a spectral overlap of theemission of the NV center and the emission of the interacting dipole but distinguishable
lifetimes, it is further possible to use pulsed excitation to increase the NV/dipole PL ratio
by appropriate time-gating of the detection. This leads to a time-based ltering of the PL
signal.

Here, we typically use an integration time in the order of seconds to be able toclearly distinguish between both present lifetimes. However, it is possible to reduce theintegration time to a few tens of millisecond in the case of pure NV center PL employing
advanced approximation methods [100]. FLIM was used e.g. to evaluate changes in NV
centers' lifetime caused by the photonic environment (see Chap. 2) and to characterize theinteraction between a NV ensemble with other dipoles (see Chap. 6 and 7).

2. Color center uorescence and spin
manipulation in single crystal,pyramidal diamond tips
Richard Nelz, Philipp Fuchs, Oliver Opaluch, Selda Sonusen, Natalia Savenko, Vitali
Podgursky, and Elke Neu
Applied Physics Letters 109, 193105 (2016)
doi: 10.1063/1.4967189

The above stated manuscript discusses for the rst time the usability of commercially available diamond scanning probe tips with a pyramidal shape for quantum sensing applications
based on color centers in diamond.
Richard Nelz (R.N.) performed all measurements characterizing the nitrogen vacancy centers present in the material and evaluated all presented data. These measurements andtheir evaluation constitute the main body of the manuscript. Further, R.N. designed andrealized the homebuilt confocal microscope that was used to characterize color centers in
the material. The investigated diamond material was supplied by V. Podgursky, N. Savenko
and the company Artech Carbon OÜ (Tallinn, Estonia). In addition, R.N. actively contributed to modeling the diamond tips via simulations to describe the photonic propertiesof the structures as well as verifying these simulations using appropriate measurements. To
further study the interaction of the color centers in the material, R.N. developed a model
estimating the density of color centers in dierent regions of the diamond tip. This model
indicates the distance between centers explaining lifetime changes of nitrogen vacancy centers in the material. R.N. actively contributed to all discussions within the collaboration
concerning conceiving and conducting experiments and evaluating data. Lastly, R.N. outlined and wrote the rst draft of the manuscript as well as the supplementary material,
designed all gures and actively contributed to all major revisions of the article.

APPLIED PHYSICS LETTERS 109, 193105 (2016)

Color center fluorescence and spin manipulation in single crystal, pyramidal
diamond tips
Richard Nelz,1 Philipp Fuchs,1 Oliver Opaluch,1 Selda Sonusen,1 Natalia Savenko,2Vitali Podgursky,3 and Elke Neu1,a)1

Universit€
at des Saarlandes, FR 7.2 Experimentalphysik, 66123 Saarbr€
ucken, Germany€ J~
Artech Carbon OU,
oe 5, 10151 Tallinn, Estonia
3
Tallinn University of Technology, Department of Materials Engineering, Ehitajate tee 5, 19086 Tallinn,
Estonia2(Received 26 August 2016; accepted 24 October 2016; published online 7 November 2016)
We investigate bright fluorescence of nitrogen (NV)- and silicon-vacancy color centers in pyramidal, single crystal diamond tips, which are commercially available as atomic force microscopeprobes. We coherently manipulate NV electronic spin ensembles with T2 ¼ 7.7(3) ls. Color center
lifetimes in different tip heights indicate effective refractive index effects and quenching. Using
numerical simulations, we verify enhanced photon rates from emitters close to the pyramid apex
C 2016 Author(s). All article content,
rendering them promising as scanning probe sensors. V
except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4967189]

Sensitively measuring magnetic fields with high spatial
resolution is of paramount importance for many applicationsin science and industry. Generally, sensing with nanometer
resolution requires atomic sized sensors. Point defects incrystals (color centers) localize bound electrons on an atomic
scale (<1 nm). Here, color centers in diamond, especiallynegatively charged nitrogen-vacancy centers (here termed
NV centers), are very promising as they are photostable,
show bright luminescence and highly coherent, controllable
spins together with optically detected magnetic resonance(ODMR).1 ODMR enables to read out spin states via luminescence, and thus even single centers can be used as low
back action, quantum-enhanced sensors. Imaging magnetic
fields using NV centers, e.g., contributed insights into superconductivity2 or magnetic materials for spintronics.3 In life
sciences, highly sensitive measurements of magnetic fields
enable nuclear magnetic resonance detection even of single
proteins.4
Employing scanning probe methods enables sensing withnanometer resolution (atomic force microscopy AFM).1–3,5 In
essence, a nanostructure that reliably enables scanning embedded color centers in close proximity to a sample surface (tiplike geometry) is mandatory to fully harness the method’s
potential. Optimally, the structure also allows for efficientcollection of the color center’s fluorescence, consequently
enhancing sensitivity. Top-down approaches sculpting scanning probe nanostructures from single crystal diamond5 produce stable probes with coherent spins and are thus preferredover approaches where nanodiamonds are attached to tips.3
However, such approaches require extensive efforts in nanofabrication.5 In contrast, bottom-up approaches form nanostructures during diamond growth that are often attached tothe bulk diamond material and are not straightforwardly usableas scanning probes.6 An approach avoiding this problem is thegrowth of diamond pyramids detachable from the growtha)Electronic mail: elkeneu@physik.uni-saarland.de0003-6951/2016/109(19)/193105/4

substrate and transferable one-by-one to AFM cantilevers.7
AFM scanning probes using such pyramids are commerciallyavailable8 and might significantly broaden the application
range of scanning probe imaging with color centers.
We here demonstrate the basic applicability of commercial single crystal, pyramidal AFM tips for magnetic sensing
by showing coherent manipulation of in-situ created NV center spins in the devices. We investigate nature and origin ofcolor centers in the tip and simulate how the pyramidal shape
influences luminescence collection. Furthermore, we apply
reactive ion etching to the tips, illustrating routes towards
optimizing ready-to use devices for sensing via removal of
diamond.
The single crystal diamond AFM tips from Artech Carbon
are synthesized in a microwave plasma enhanced chemical
vapor deposition (CVD) process based on a CH4/H2 gas mixture. Note that the same CVD technology is used for all tips
from Artech Carbon. CVD is performed on silicon (Si) substrates pretreated with micron sized diamond powder toenhance the diamond nucleation density. Refs. 7 and 9 summarize details on fabrication of geometrically similar diamondtips using an alternative CVD technique. The parameters of
our CVD process ensure that (100) facets grow slowest andthus with minimized defect density.7 In contrast, on lateral surfaces of crystals growing with (100) facets, highly defective,
nano-crystalline diamond forms. As a consequence, pyramidal
diamond micro-crystals with square, (100)-oriented basal
planes evolve, which are embedded into the nano-crystalline
material. Heat treatment in air entirely removes the latter andtransferring the pyramids to AFM cantilevers becomes feasible.7 We note that the pyramids are mounted to the cantilevers
using epoxy. The apex of the pyramid often has a radius ofcurvature of 2–20 nm, thus rendering these diamonds ideallysuitable as ultra-hard AFM tips for high resolution topography
imaging or nano-indentation measurements.
To investigate the topography of the pyramids, werecord high resolution scanning electron microscopy (SEM)

109, 193105-1

C Author(s) 2016.
V

193105-2

Nelz et al.

images, as displayed in Fig. 1(a). The rough, step-like surfaceof the pyramid does not straightforwardly reveal its single crystal nature. We prove single crystal nature using NV centers in
the tip (see below), consistent with Raman microscopy characterization provided by the manufacturer.8 The pyramid in the
SEM image has a basal plane of 3.5  3.5 lm2 and a height of
10 lm. The cone angle of the tip is as low as 10 close to theapex (last  0.5 lm) and 20–25 closer to the base.
For sensing and quantum information, negatively charged
nitrogen- and silicon-vacancy centers (here termed NV andSiV centers) are of major interest. We investigate the photoluminescence (PL) of both types of color centers in a home built
confocal microscope (Numerical aperture NA 0.8, 532 nm
laser excitation). Confocal filtering of PL is ensured using asingle mode optical fiber. We record PL spectra using a grating spectrometer (Acton Spectra Pro 2500, Pixis 256OE
CCD), whereas highly efficient photon counters (Quantum
efficiency  70%, Excelitas SPCM-AQRH-14) enable quantifying PL intensity. To measure PL lifetimes of color centers,
we employ pulsed laser excitation (NKT EXW-12, pulse
length  50 ps, wavelength 527–537 nm) and time correlated
photon counting (PicoQuant, PicoHarp 300).
In a first step, we investigate the diamond pyramids’ PL
under laser excitation in general. All 10 investigated pyramidsshow bright PL with detected photon rates easily exceeding
106 cps (Mcps) (wavelength range 680–720 nm) for moderate
excitation power (<500 lW) all over the tip. To further quantify the intensity of this PL, which is mainly due to NV centers, we investigate the PL saturation behavior focusing thelaser onto the pyramid apex (see supplementary material).
Estimated NV PL count rates into the first lens of our confocal
microscope exceed 500 Mcps (Psat ¼ 380 lW) corresponding
to an emitted power of 100 pW, which is accessible with
common commercial photodiodes.
We now consider how the pyramids’ PL varies with thespatial position of the laser focus. To this end, we record
the PL in planes parallel to its basal plane (xy-planes) via
scanning the pyramid through the laser focus. To enable3-dimensional imaging, we change the position of the laser
focus with respect to the height of the pyramid (z-position)and repeat the measurement. Focusing the laser close to thebasal plane clearly reveals the square footprint of the pyramid as deduced from SEM images, while focusing the laser
close to the pyramid’s apex reveals PL stemming from a single point (data see supplementary material).

FIG. 1. (a) SEM image of a diamond pyramidal tip on a Si cantilever. Inset:
View from the apex of the pyramid revealing the step-like surface (b) PL
spectra recorded at apex (A), in the middle (B), and at the base of the pyramid (C). The SiV PL is rapidly decreasing compared to the NV PL. Notethat all spectra are normalized to maximum of the NV PL. We note that we
observe only minor broadband PL due to the epoxy at the pyramid base.

Appl. Phys. Lett. 109, 193105 (2016)

We now evaluate PL spectra recorded in different heightsof the pyramid [see Fig. 1(b)]. For all heights, we clearlyobserve the characteristic spectrum of NV centers with azero-phonon line (ZPL) at 638 nm [linewidth 4.1(2) nm]
and broad phonon sidebands between 650 and 800 nm.
When exciting PL closer to the pyramid apex, we
furthermore observe the ZPL of SiV centers at 738 nm [linewidth 4.9(1) nm]. SiV PL is significantly more pronounced thecloser to the pyramid apex a spectrum is recorded. For spectra
recorded at the apex, the SiV ZPL strongly dominates over theNV phonon sideband. PL spectra with the described features
are consistently observed from all 10 investigated tips.
To interpret the PL spectra, it is useful to recall that thepyramid’s apex is formed in the initial phase of CVD growth
on a Si substrate: A CVD plasma etches Si substrates, consequently diamond films grown on Si contain up to 1019 cm3
Si atoms.10 However, Si incorporation decreases significantly by more than one order of magnitude as soon as thegrowing diamond fully covers the substrate.10 Thus, we
expect a higher concentration of Si impurities close to theapex formed in the starting phase of the growth. In contrast,
nitrogen has been added to the gas mixture during the CVD
process (0.1% volume fraction) and forms NV centers in allheights in the diamond pyramid. We investigate the brightness of NV and SiV PL in dependence of the z-position of
the laser focus in more detail as shown in Fig. 2(a). Both PL
signals decrease in brightness from apex to base; however,the SiV PL decreases faster, indicating a stronger confinement of the area with high SiV density close to the apex.
Despite a significantly reduced diamond volume
exposed to the laser, NV and SiV PL are enhanced close to
the apex, which we also attribute to photonic effects: we simulate the emission patterns of dipoles with horizontal andvertical alignment with respect to the pyramid base in the

FIG. 2. (a) NV and SiV PL along the z-axis of the tip. (b) Simulated collection factor along the z-axis of the tip for horizontal and vertical dipoles. (c)Simulated collection factors for different NA and far field patterns as inset.
For these simulations, we place the dipole at the location that maximizes thecollection factor [see (b)]. (d) Measured excited state lifetimes along thez-axis of the pyramid; note the increase of the lifetime towards the apex as
well as a decrease very close to the apex. The z-axis origin (z ¼ 0) represents
the tip apex in (a) and (d) determined as the z-position for which maximallyspatially confined PL from the tip occurs (note the depth of focus of our confocal microscope of  0.9 lm).

193105-3

Nelz et al.

middle of the xy-plane for different z-positions. We use theFinite Difference Time Domain (FDTD) method implemented with a commercial software (Lumerical FDTD solutions). As sensing applications often demand to maximize
the PL rate in the detection optics, we investigate the collection factor defined as the ratio of the power emitted into the
NA to the emitted power in homogeneous diamond as a figure of merit, exemplarily for k ¼ 700 nm. The results of thesesimulations are displayed in Figs. 2(b) and 2(c) (details see
supplementary material). The simulated data, which qualitatively agree with our measurements [Fig. 2(a)], show that the
tip’s shape leads to an enhanced photon rate in our NA for
PL from color centers close to the apex. This situation isadvantageous for scanning probe applications, as these centers are simultaneously in close proximity to the sample surface. Moreover, our simulations indicate that (for horizontal
dipoles) even a moderate NA of 0.6 only reduces the collection factor by 25%.
To determine if the diamond tips are suitable for magnetometry applications, we perform ground state opticallydetected magnetic resonance (ODMR) of the NV centers.
~ reveals four pairs of
Applying a static magnetic field B
ODMR resonances, corresponding to the four equivalenth111i orientations of NV centers in the diamond lattice (datasee supplementary material). The occurrence of four pairs ofresonances witnesses the single crystal nature of the tip as
any misoriented, polycrystalline grains would lead to additional resonances or blurring of the ODMR-spectrum. For~ onto one of thethe following measurements, we align B~ ish111i axes and use the NV sub-ensemble to which Baligned. We verify coherent manipulation of NV electronic
spins via Rabi oscillations [see Fig. 3(a)] and measure theircoherence times T2 and T2.
Using a Spin echo-sequence, we measure T2 [see Fig.
3(b)] at the base of 4 pyramids and find on average
T2 ¼ 7.7(3) ls. We check T2 via a Ramsey-type experimentand find T2 ¼ 0:24ð7Þ ls (data not shown). We verify
T2 ¼ 7.2(1) ls for NVs close to one pyramid’s apex [Fig.
3(b)]. Thus, T2 is preserved despite SiV incorporation andpotentially lower crystal quality. T2 and T2 are almost oneorder of magnitude higher than in high pressure high temperature diamond (typ. 100 ppm substitutional nitrogen [N]s,T2  1 ls1), however, much lower than in high purity CVD
diamond (1 ppb [N]s, T2  300 ls1). Using T2 and T2 as wellas the PL count rate I0 ¼ 162.7 Mcps, the readout duration

FIG. 3. (a) Measurement of a Rabi oscillations between the ms ¼ 0 and the
ms ¼ 1 states of the Zeeman shifted sub-ensemble with a Rabi frequencyof 3.76 MHz. (b) Spin echo measurement at apex and base, fitted employing
the formalism derived in Ref. 11.

Appl. Phys. Lett. 109, 193105 (2016)

tL ¼ 350 ns and the contrast of the ODMR C ¼ 7.4%, we calculate the DC and AC magnetic field sensitivities1.pﬃﬃﬃﬃﬃﬃ
1
1
pﬃﬃﬃﬃﬃﬃﬃ  pﬃﬃﬃﬃﬃ  130:6 nTHz;
cNV C I0 tL
T2
rﬃﬃﬃﬃﬃ
.pﬃﬃﬃﬃﬃﬃ
T2
gAC ¼ gDC  22:9 nT
Hz:
T2

gDC 

1

(1)

(2)

Previous works report that lifetime changes of color centersevidence interaction with various defects12–14 as well as thephotonic environment.15 To investigate lifetime changes indetail, we measure the excited state lifetime sNV (sSiV) of
NV (SiV) centers along the z-axis of the pyramid [see Figs.
2(d) and supplementary material]. At the base (z  10 lm),
where we identified a reduced density of SiV centers, wefind sNV ¼ 11.79(3) ns, which well agrees with NV lifetimes
in single crystal bulk diamonds [sNV, bulk ¼ 12.9(1) ns]16
whereas sSiV ¼ 0.89(1) ns, as previously observed inpolycrystalline diamond.17 Approaching the pyramid
apex (z  2–4 lm), sNV [sSiV] increases to 13.07(3) ns[1.92(9) ns], potentially due to a decrease in the effective
refractive index close to the nanoscale apex.15 In the regionof increased SiV density very close to the apex(z  0–1 lm), sNV drops to 11.44(8) ns. In the presence ofgraphitic or disordered carbon, sNV shortens.12,13 In thiscontext, a high level of Si doping, here possibly induced bysubstrate etching,10 introduces structural defects.18 Thus,
this shortening of sNV might be attributed to incorporated Si
potentially lowering crystalline quality and inducing nondiamond phases. Moreover, [N]s can significantly quench
NV PL via dipole-dipole interaction.14 Such energy transfer(F€
orster resonance energy transfer, FRET) requires an overlap of the quenched color center’s (donor) emission bandand the quenching defect’s (acceptor) absorption band.
Thus, also a high density of SiV centers potentiallyquenches NV PL: NV emission (650 to 750 nm) welloverlaps with SiV absorption (540 to 740 nm19), especiallywith its dominant ZPL. FRET between NV and SiV centers
has so far not been demonstrated; however, NV centersshow efficient FRET transfer to dye molecules with absorption in a similar spectral range (50% efficiency at  6 nm
distance).20 We estimate the average distance between centers in the tip to be <10 nm (see supplementary material);
thus, FRET might be contributing to the quenching.
As a first step towards optimization of the tip for magnetometry, we etch it in an inductively coupled reactive ion
etching plasma (Ar/O2, 50 sccm each, 18.9 mTorr, 500 W
ICP, 200 W RF power see also supplementary material)
enabling highly anisotropic etching of diamond. We remove
up to 1 lm of diamond, as measured from the pyramidheight reduction, in several etch steps. We note that the pyramid still reveals a comparably defined apex of <200 nm
size and NV centers close to the apex retain coherenceðbase;unetchedÞ
ðapex;etchedÞ
(T2¼ 9:4ð2Þ ls; T2¼ 8:9ð2Þ ls). Surface
roughness remains mainly unchanged, while SiV PL significantly decreases, as we remove highly Si-doped diamond(details see supplementary material). We thus establish aprocedure to tune the NV/SiV ratio even for readily mounted
devices to tailor them for magnetometry.

193105-4

Nelz et al.

In conclusion, we show that single crystal diamond
AFM tips from Artech Carbon show bright PL from in-situ
created NV and SiV centers, easily measurable with standard
photodiodes. In addition, we demonstrate their suitability
pﬃﬃﬃﬃﬃﬃ
for magnetometry
pﬃﬃﬃﬃﬃﬃ (T2 ¼ 7.7(3) ls, gDC  130:6 nT= Hz;gAC  22:9 nT= Hz). FDTD simulations indicate high PL
rates from centers close to the pyramid apex.
The outstanding advantage of the pyramidal tips is theircommercial availability as well as a comparably low production cost per tip and high resolution topography imaging, compared to top-down approaches.5 The main drawback is a highcolor center density precluding to use single centers for imaging. Consequently, color centers in 1 lm tip height, limitedby the confocal microscope, contribute to sensing and significantly limit spatial resolution. Nevertheless, the tips might,
e.g., be utilized to monitor the activity of integrated circuits
via magnetic measurements;21 here, nanoscale resolution is
not mandatory and scanning devices avoid the necessity toplace a diamond chip on the circuit under investigation. For
single color center sensors, diamond purity has to be enhanced
by reducing nitrogen in the process and the use of more etch
resistant growth substrates (e.g., iridium). Creation of color
centers might be feasible via recently developed nanoimplantation techniques,22 whereas mounting of the devices to transparent cantilevers, e.g., made from silicon nitride would allow
optical access through the scanning device.
See supplementary material for additional information
about our experimental setup, ODMR-spectra, technicaldetails on the plasma treatment, the 3D fluorescence imaging, fluorescence saturation, and details on the numericalsimulations.
This research has been funded via the NanoMatFutur
program of the German Ministry of Education and Research(BMBF) under Grant No. FKZ13N13547 and Enterprise
Estonia (EAS) Project No. Lep16012. We thank J€
org
Schmauch for recording SEM images as well as Dr. Rene
Hensel, Dr. Tobias Kraus, Dr. Daniel Brodoceanu, andSusanne Selzer (INM, Saarbr€
ucken) for providing plasma
etching tools and assistance.

Appl. Phys. Lett. 109, 193105 (2016)
1

L. Rondin, J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky, and V.
Jacques, Rep. Prog. Phys. 77, 056503 (2014).
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L. Thiel, D. Rohner, M. Ganzhorn, P. Appel, E. Neu, B. M€
uller, R.
Kleiner, D. Koelle, and P. Maletinsky, Nat. Nanotechnol. 11, 677 (2016).
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J. P. Tetienne, T. Hingant, L. J. Martınez, S. Rohart, A. Thiaville, L. H.
Diez, K. Garcia, J. P. Adam, J. V. Kim, J. F. Roch, I. M. Miron, G.
Gaudin, L. Vila, B. Ocker, D. Ravelosona, and V. Jacques, Nat. Commun.
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I. Lovchinsky, A. O. Sushkov, E. Urbach, N. P. de Leon, S. Choi, K. De
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uller, L. McGuinness, F.
Jelezko, R. L. Walsworth, H. Park, and M. D. Lukin, Science 351, 836(2016).
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P. Appel, E. Neu, M. Ganzhorn, A. Barfuss, M. Batzer, M. Gratz, A.
Tsch€ope, and P. Maletinsky, Rev. Sci. Instrum. 87, 063703 (2016).
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I. Aharonovich, J. C. Lee, A. P. Magyar, D. O. Bracher, and E. L. Hu,
Laser Photonics Rev. 7, L61 (2013).
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A. N. Obraztsov, P. G. Kopylov, B. A. Loginov, M. A. Dolganov, R. R.
Ismagilov, and N. V. Savenko, Rev. Sci. Instrum. 81, 013703 (2010).
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See www.scdprobes.com for more information.
9
A. Obraztsov and P. Kopylov, “Diamond nano-tip and method for production thereof,” WO Patent Application No. PCT/EP2009/056,100 (2009).
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J. Barjon, E. Rzepka, F. Jomard, J.-M. Laroche, D. Ballutaud, T.
Kociniewski, and J. Chevallier, Phys. Status Solidi A 202, 2177 (2005).
11
L. Childress, M. V. Gurudev Dutt, J. M. Taylor, A. S. Zibrov, F. Jelezko,
J. Wrachtrup, P. R. Hemmer, and M. D. Lukin, Science 314, 281 (2006).
12
B. R. Smith, D. Gruber, and T. Plakhotnik, Diamond Relat. Mater. 19, 314(2010).
13
L.-H. Chen, T.-S. Lim, and H.-C. Chang, J. Opt. Soc. Am. B 29, 2309
(2012).
14
D. Gatto Monticone, F. Quercioli, R. Mercatelli, S. Soria, S. Borini, T.
Poli, M. Vannoni, E. Vittone, and P. Olivero, Phys. Rev. B 88, 155201(2013).
15
A. Beveratos, R. Brouri, T. Gacoin, J.-P. Poizat, and P. Grangier, Phys.
Rev. A 64, 061802 (2001).
16
A. T. Collins, M. F. Thomaz, and M. I. B. Jorge, J. Phys. C: Solid State
Phys. 16, 2177 (1983).
17
A. V. Turukhin, C.-H. Liu, A. A. Gorokhovsky, R. R. Alfano, and W.
Phillips, Phys. Rev. B 54, 16448 (1996).
18
A. Bolshakov, V. Ralchenko, V. Sedov, A. Khomich, I. Vlasov, A.
Khomich, N. Trofimov, V. Krivobok, S. Nikolaev, R. Khmelnitskii, and V.
Saraykin, Phys. Status Solidi A 212, 2525 (2015).
19
L. J. Rogers, K. D. Jahnke, M. W. Doherty, A. Dietrich, L. P.
McGuinness, C. M€
uller, T. Teraji, H. Sumiya, J. Isoya, N. B. Manson, andF. Jelezko, Phys. Rev. B 89, 235101 (2014).
20
N. Mohan, Y.-K. Tzeng, L. Yang, Y.-Y. Chen, Y. Y. Hui, C.-Y. Fang, and
H.-C. Chang, Adv. Mater. 22, 843 (2010).
21
L. Shao, R. Liu, M. Zhang, A. V. Shneidman, X. Audier, M. Markham, H.
Dhillon, D. J. Twitchen, Y.-F. Xiao, and M. Lončar, Adv. Opt. Mater.
4(7), 1075–1080 (2016).
22
J. Riedrich-M€
oller, S. Pezzagna, J. Meijer, C. Pauly, F. M€ucklich, M.
Markham, A. M. Edmonds, and C. Becher, Appl. Phys. Lett. 106, 221103(2015).

3. Toward wafer-scale diamondnano- and quantum technologies
Richard Nelz, Johannes Görlitz, Dennis Herrmann, Abdallah Slablab, Michel Challier,Mariusz Radtke, Martin Fischer, Stefan Gsell, Matthias Schreck, Christoph Becher, andElke Neu
APL Materials 7, 011108 (2019)
doi: 10.1063/1.5067267

This paper was selected as an Editor's Pick.

The above stated manuscript shows the potential of a novel diamond material to upscalediamond-based technologies.
Richard Nelz (R.N.) designed and realized the homebuilt confocal microscope that wasused to characterize NV centers in the investigated novel, wafer-sized, heteroepitaxial single crystal diamond supplied by the company AuDiaTec (Augsburg Diamond Technology
GmbH, Augsburg, Germany). R.N. performed all optical and spin measurements on NV
centers presented in the manuscript. In addition, he evaluated the density of native NV
centers and compared it to the density predicted by the gas composition during the growthof the material showing a good accordance. R.N. suggested using SiV centers in the material as a probe for internal strain. The corresponding measurements were performed by J.
Görlitz and D. Herrmann. R.N. applied ndings from previous work to proof the predicted
relaxation of strain in the diamond. Following implantation of shallow NV centers, nanostructures were fabricated by M. Challier and M. Radtke. Subsequently, R.N. investigatedthe properties of the NV ensemble implanted into the top layer of the material and theNV centers in the fabricated nanostructures to further show the material's suitability forsensing applications. Lastly, R.N. outlined and wrote the rst draft of the manuscript andthe supplementary material. In addition, he designed all gures and actively contributed
to all major revisions of the article and to all discussions throughout the collaboration on
appropriate experiments and data evaluation.

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Toward wafer-scale diamond nanoand quantum technologies
Cite as: APL Mater. 7, 011108 (2019); doi: 10.1063/1.5067267
Submitted: 16 October 2018 • Accepted: 2 January 2019 •Published Online: 28 January 2019
Richard Nelz,1Johannes Görlitz,1 Dennis Herrmann,1 Abdallah Slablab,1 Michel Challier,1 Mariusz Radtke,1
2
Martin Fischer, Stefan Gsell,2 Matthias Schreck,3
Christoph Becher,1
and Elke Neu1,a)AFFILIATIONS
1

Universität des Saarlandes, Fakultät NT, Physik, 66123 Saarbrücken, Germany

2

Augsburg Diamond Technology GmbH, 86159 Augsburg, Germany

3

Universität Augsburg, Institut für Physik, 86135 Augsburg, Germanya)Electronic mail: elkeneu@physik.uni-saarland.de

ABSTRACT
We investigate native nitrogen vacancy (NV) and silicon vacancy (SiV) color centers in a commercially available, heteroepitaxial,
wafer-sized, mm thick, single-crystal diamond. We observe single, native NV centers with a density of roughly 1 NV per µm3
and moderate coherence time (T2 = 5 µs) embedded in an ensemble of SiV centers. Using low temperature luminescence of SiV
centers as a probe, we prove the high crystalline quality of the diamond especially close to the growth surface, consistent with areduced dislocation density. Using ion implantation and plasma etching, we verify the possibility to fabricate nanostructures withshallow color centers rendering our material promising for fabrication of nanoscale sensing devices. As this diamond is available
in wafer-sizes up to 100 mm, it offers the opportunity to up-scale diamond-based device fabrication.
© 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5067267

Diamond nanostructures are of significant importancefor various applications in science and industry including
nanomechanical devices,1 photonics2 and sensing.3 A major
challenge for most of these applications is the scalabilityof the fabrication process predominantly due to a lack oflarge area single-crystal diamonds (SCDs) with good crystalline quality and high purity. Manufacturing synthetic SCD
on a wafer scale has been an active field of research4,5 leading to the commercial availability of SCD with a diameterof ≈ 100 mm recently. This progress opens the road toward
up-scaling the fabrication of SCD nanostructures especiallyfor diamond related sensing applications.6 Color centers in
SCD, in particular, the negatively charged nitrogen vacancy(NV) center in nanostructures, have been extensively used tosensitively measure, e.g., magnetic fields in the last decade.
Single color centers allow for sensing with high spatial resolution and offer bright, photostable photoluminescence (PL).
In addition, NV centers provide highly coherent, controllable
spin states3 and show optically detected magnetic resonance

APL Mater. 7, 011108 (2019); doi: 10.1063/1.5067267© Author(s) 2019

(ODMR) enabling to read out their spin states via PL detection.
As a consequence, even single NV centers can serve as
quantum-enhanced sensors. Magnetic field imaging using NV
centers has various applications ranging from material characterization in superconductors7 or magnetic materials for
spintronics8 to life science applications where nuclear magnetic resonance detection of single proteins is of interest.9
Recently, silicon vacancy (SiV) centers emerged as alternative
enabling all optical sensing of temperatures using their narrowelectronic transitions.10
We here demonstrate the basic applicability of commercial, wafer-sized SCD for quantum technology applications. To this end, we demonstrate coherent manipulation of single native NV center spins in the SCD, while
we use low-temperature spectroscopy of SiV center PL asa probe to prove the high crystalline quality of the material. In addition, we implant shallow NV centers (depth≈ 10 nm), indispensable for nanoscale sensing, and investigate their suitability for quantum sensing applications.

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Furthermore, we apply reactive ion etching to the material to illustrate routes toward up-scaling of diamond-related
nanofabrication.
We use (100) oriented SCD from AuDiaTec (Augsburg Diamond Technology GmbH) synthesized in a microwave plasma
enhanced chemical vapor deposition process based on amethane/hydrogen (CH4 /H2 ) gas mixture at a temperature>1000 ◦ C. H2 was purified by passing it through a AgPd membrane, while the used CH4 had a purity of 99.9995%. As aconsequence, we estimate the nitrogen (N2 ) concentration in
the gas phase to be <0.5 ppm. The Ir/YSZ (yttria stabilized
zirconia)/Si substrate was removed by chemical etching andgrinding before the diamond wafer was cut into pieces of 3 mm
× 3 mm using a pulsed IR laser. The SCDs are 1.3 mm thick.
For our experiments, we use the crystals with the as grownsurface. SCD synthesis at AuDiaTec is based on the technology developed at the University of Augsburg. As describedin Ref. 5, successful scaling of heteroepitaxial nucleation andgrowth to 100 mm wafer-size was achieved by the use of themultilayer substrate Ir/YSZ/Si. During heteroepitaxial growthof thick (mm) SCD, the dislocation density decreases proportional to the inverse of the SCD layer thickness indicatinghigh crystalline quality for this material at the growth surface.5 Previous studies5 furthermore reveal a high homogeneity of the crystalline quality using Raman scattering and X-ray
diffraction.
To investigate PL and perform spin manipulation of NV
centers, we use a home-built confocal microscope (numerical aperture NA 0.8, 532 nm laser excitation) where confocal
filtering of the PL is ensured by using a single mode optical fiber. To quantify the PL intensity, we use highly efficient photon counters (Excelitas SPCM-AQRH-14, quantum
efficiency ≈ 70%). In addition, we use a grating spectrometer (Acton Spectra Pro 2500, Pixis 256OE CCD) to record the
PL spectrum. To measure PL lifetimes, we employ time correlated photon counting (PicoQuant, PicoHarp 300) and pulsed
laser excitation (NKT EXW-12, pulse length ≈ 50 ps, wavelength 527-537 nm). The setup is equipped with a microwave
source and an amplifier (Stanford Research Systems, SG384
and Mini Circuits, ZHL-42W+) for spin-manipulation of NV
centers. To investigate SiV centers, we use a second confocal
microscope (numerical aperture NA 0.9) embedded into a liquid helium flow cryostat employing a titanium-sapphire laser(Matisse, TX) at 690 nm as an excitation source. For confocal filtering, a single mode optical fiber is used. PL spectra of
SiV centers are recorded using a grating spectrometer (Jobin
Yvon, Horiba, Grating: 1800 g/mm).
We now discuss the PL results from our SCD under continuous laser excitation at 532 nm. Close (<5 µm) to the nucleation side, we observe very bright PL consisting of a broad
background as well as distinctive NV and SiV PL. By contrast, PL maps recorded in a wavelength range from 680 nm
to 720 nm from planes parallel and closer to the growth surface show bright, distinguishable spots [see Fig. 1(a)]. The spots
clearly show PL spectra of NV centers [see Fig. 1(b)], constituting the first observation of individual native NV centersin heteroepitaxial SCD. Additionally, these PL spectra reveal
the presence of homogeneously distributed SiV centers which,

APL Mater. 7, 011108 (2019); doi: 10.1063/1.5067267© Author(s) 2019

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FIG. 1. (a) PL map of the SCD (detected wavelength range 680 nm–720 nm) showing ≈25 bright spots which all have NV PL. (b) Room temperature PL spectrum of
one of these NVs. Bright SiV zero phonon line (ZPL) is visible leading to the background measured in the PL map in (a) despite spectral filtering. The narrow PL line
observed at 805 nm is most probably connected to the SiV ensemble,14,15 while
the peak at 610 nm corresponds to the second order Raman scattering of diamond.
(c) g(2) -measurement of one of the NVs. A clear anti-bunching is visible. g(2) (0)is only limited by the background of the SiV ensemble (see the supplementarymaterial). (d) ODMR measurement of single NV showing a background-corrected
contrast C above 10% (external magnetic field approximately 11 G).

despite spectral filtering, influence observation of single NV
centers as background.
To test the applicability of the material for NV-based
quantum technologies, we investigate single, native NV centers in the highest-quality region, namely, the first micrometers below the growth surface. To prove the observation of
single NV centers, we measure second order photon correlations g(2) [see Fig. 1(c)]. We achieve a signal-to-backgroundratio of less than 4, limited by SiV PL. Nevertheless, these measurements clearly show single photon emission of NV centers,
while the SiV ensemble PL limits g(2) (0) (see the supplementarymaterial). From PL maps recorded at different positions of thesample [see Fig. 1(a)], we determine a homogenous NV densityof ≈1 NV per µm3 equivalent to a concentration of ≈0.005 ppb.
This value is in reasonable accordance with the density estimated from the nitrogen impurity concentration in the feedgases (for details, see the supplementary material). Subsequently, we perform photoluminescence lifetime imaging ofindividual NV centers (for data, see the supplementary material). The NV centers show on average a lifetime of 12.3(6) ns
which agrees well with the lifetime in bulk SCD.11 This finding rules out quenching, e.g., due to structural defects associated with the incorporation of silicon impurities12 as wellas quenching via potential near-field energy transfer between
NV and SiV centers.13 Furthermore, we perform ground state
ODMR measurements of single NVs. We obtain the ODMRspectrum in Fig. 1(d) with narrow resonances (an external
magnetic field of 11 G applied) and a contrast exceeding 10%.
The zero-field-splitting of 2.876 GHz indicates low strain close

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to the growth surface which would otherwise shift or split theresonance. Spin-Echo measurements of single NV centers at
randomly chosen positions in the sample consistently show acoherence time of T2 = 5(1) µs, proving homogenous propertiesof the NV centers in the SCD.
To further characterize the crystalline quality of the SCD,
we study the SiV ensemble in detail. Hereby, we focus on thefine structure of the SiV zero phonon line (ZPL) which is ameaningful measure of the crystalline quality as strain shifts
and broadens the fine structure components in the ensemble. Consequently, we measure the ZPL fine structure at 10 K
in different depths along the growth direction (see Fig. 2).
We observe a clear fine structure close to the growth surface, while ZPL broadening completely masks the fine structure deeper in the SCD toward the nucleation side where thegrowth started. We expect the ZPL to broaden linearly withdislocation density.16 To verify this fact, we fit the linewidth
using a 1/layer thicknessn dependence. We obtain n ≈ 1 whichis in good agreement with the predicted dislocation density
evolution during growth which should lead to an estimateddensity of dislocations of ≈107 cm−2 close to the growth surface.5 In this area, we observe a fitted linewidth of 61.2(5) GHz(for details, see the supplementary material). Our observed
linewidths are thus only roughly a factor of 6 higher than in
high-quality, thin, homoepitaxial SCD films17 witnessing highcrystalline quality.
Efficient nanoscale sensing using color centers in SCD
typically requires shallow NV centers (<50 nm below thesurface) incorporated into dedicated nanostructures, e.g.,
nanopillars.6 To test the usability of our novel SCD material, we shallowly implant nitrogen ions with an implantationenergy of 6 keV and doses of 1.5 and 3 × 1011 cm−2 to forma NV layer 10 nm below the SCD surface. After annealing
the sample, PL maps reveal homogenous creation of the NV
layer across the sample for both doses. PL spectra from theimplanted layer [see Fig. 3(a)] show strong NV PL, while weaker
PL below 640 nm corresponds to phononic side bands originating from neutral NV centers; strong SiV PL is still visible. Due to the enhanced number of NVs, their PL is more
pronounced, whereas the SiV PL remains mainly unchanged.

FIG. 2. Probing strain in the SCD material via SiV PL: (a) PL spectra of the SiV
ensemble at 10 K. Close to the growth surface, a clear fine structure is visible(black curve A), while the ZPL continuously broadens along the growth direction.
Closer to the substrate, broadening completely masks the fine structure components (gray curve B). (b) Linewidth of the transition C along the growth direction of
the diamond. Inset: SiV level scheme according to Ref. 18.

APL Mater. 7, 011108 (2019); doi: 10.1063/1.5067267© Author(s) 2019

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FIG. 3. (a) PL spectrum of the implanted NV ensemble. A clear NV− as well as
minor NV0 spectrum is visible, while SiV PL remains unchanged. (b) ODMR spectrum without external magnetic field strongly broadened due to strong microwave
pumping of the transition (unbroadened linewidth 10.67 MHz, C = 20%). (c) Spinecho-measurement of the implanted NV centers, fitted employing the formalism
derived in Ref. 22. The coherence time is T 2 = 5.2(3) µs. (d) Scanning electron
microscopy image of nanopillars etched into the growth surface of the SCD. The
cylindrical pillars have a diameter of ≈200 nm and a height of ≈1 µm.

The PL lifetime of the implanted NV centers is longer than
the native NV centers’ lifetime due to their proximity to thesurface. We find τ = 17 ns, typical for shallowly implanted
NV centers.6 To demonstrate the usability of the NV center
ensemble for magnetic sensing, we measure ODMR without
applying an external magnetic field which shows a contrast of
C = 20% [see Fig. 3(b)] at 2.871(2) GHz. Power broadening for
the ODMR resonance occurs because of the strong microwave
field necessary to saturate the ground state spin transitionin the ensemble. To estimate the unbroadened linewidth, we
follow Ref. 19 and obtain a linewidth of 10.67 MHz (see thesupplementary material). Rabi oscillations of a sub-ensembleof the implanted NV centers using a bias magnetic field of
38 G confirm coherent manipulation of the spins. The spin
coherence time T2 = 5.2(3) µs of the implanted NV centers
is comparable to T2 for the native NVs [see Fig. 3(c)] andconsistent for positions several mm apart. Consequently, we
assume that T2 is limited by the properties of our SCD. Often,
the concentration of substitutional nitrogen [N]s limits T2 .3
However, as the estimated concentration of [N]s is in the
ppb range for our SCD, we would expect T2 to be at least
an order of magnitude higher than observed. We also exclude
minor concentrations of compensated boron acceptors as thesource of decoherence. A possible spin bath reducing T2 forthe observed native NV centers might arise from various paramagnetic, silicon-containing defects in the material including neutral SiV centers and optically inactive silicon-hydrogencomplexes.20 Furthermore, vacancy complexes especially divacancies have been identified as a source of decoherence for
implanted NV centers recently.21 We tentatively suggest that

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vacancy complexes forming at dislocations might reduce T2 in
our material.
Another important test for the material is to manufacture
well-defined and stable nanostructures by reactive ion etching like, e.g., nanopillars or scanning probes.6 Such photonic
nanostructures significantly enhance photon rates from single
color centers, consequently boosting sensor sensitivity.23 We
use the pristine growth surface (2.2 nm rms roughness, area 2µm × 2 µm) to etch nanopillars in an inductively coupled reactive ion etching plasma (Ar/O2 , 50 sccm each, 18.9 mTorr, 500
W ICP, 200 W RF power) enabling highly anisotropic etchingof SCD. The nanopillars have an almost cylindrical shape witha diameter of ≈200 nm and a height of ≈1 µm [see Fig. 3(d)]closely resembling scanning probes optimized for high sensitivity magnetometry.23 The surface roughness remains mainly
unchanged (<4 nm rms roughness, area 2 µm × 2 µm) with noetchpits forming. We note that considering the estimated density of dislocations, the probability to find a dislocation within
an individual pillar is only ≈1% indicating the suitability of ourhigh quality heteroepitaxial SCD for the production of quantum technology devices. Thus, our wafer-sized SCD opens up
the way for up-scaling nanofabrication of NV center-related
structures for various applications.
In conclusion, we show that our wafer-sized, heteroepitaxial SCD contains single, native NV centers with a homogenous density of roughly 1 NV per µm3 embedded in an ensemble of SiV centers. Native NVs show moderate coherencetime of T2 = 5 µs and are suitable for ODMR-related sensing applications. A clear fine structure of the SiV ZPL close
to the growth surface witnesses low strain and high crystalline quality. Additionally, we shallowly implant NV centers as required for nanoscale sensing and demonstrate
the fabrication of nanopillars into the pristine SCD growthsurface.
The outstanding advantage of the present material system is its commercial availability and superior size, potentiallyenabling up-scaling of nanostructure fabrication for sensing and consequently reducing manufacturing cost for SCDbased devices. Due to wafer-scale growth, also the cost per
carat can be reduced by potentially one order of magnitude compared to present high-purity SCD. Consequently,
the material has the potential to eliminate a bottle neck in
SCD-based technologies. Challenges arise from moderate NV
coherence times in the material and the detrimental influence of the SiV PL on NV spin readout limiting the usabilityof NV centers for sensing. To reduce the incorporation of silicon, we envisage overgrowing millimeter-thick SCD wafers
after removal of the Ir/YSZ/Si substrate since the Si-wafer
is the main source of silicon in the process. For many applications, only thin active layers (<10 µm) are required. As
a consequence, a slow growth rate and process conditions
for optimized crystal quality can be chosen. We anticipate
that this approach will considerably improve NV coherencein the active zone. Nevertheless, the observed native NV centers are already useful for sensing using ODMR resonances
shifts to detect magnetic fields with moderate strength, e.g.,when performing failure analysis of electric circuits24 or imaging of domain walls in thin ferromagnetic films applicable in

APL Mater. 7, 011108 (2019); doi: 10.1063/1.5067267© Author(s) 2019

ARTICLE

scitation.org/journal/apm

spintronics.8 Similarly, implanted ensembles of NV centers are
suitable for wide-field imaging of magnetic particles in living
cells.25
See supplementary material for details about nitrogen incorporation, background determination of the g(2) measurements, fitting of the SiV PL, PL lifetime maps, andestimation of the power broadening.
This research has been funded via the NanoMatFutur
program of the German Ministry of Education and Research(BMBF) under Grant No. FKZ13N13547. We acknowledge funding via a PostDoc fellowship of the Daimler and Benz Foundation. We thank Jörg Schmauch for recording SEM images
as well as Dr. Rene Hensel for the use of the ICP-RIE andassistance.

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7, 011108-5

4. Reliable Nanofabrication of
single-crystal diamond photonic
nanostructures for nanoscalesensingMariusz Radtke, Richard Nelz, Abdallah Slablab, and Elke Neu
Micromachines 10, 718 (2019)
doi: 10.3390/mi10110718

The above mentioned manuscript details an optimized fabrication process for single crystal
diamond scanning probes containing single NV centers.
Richard Nelz (R.N.) monitored the NV centers' properties throughout the whole fabrication
process including the performance of the nal devices. To this end, a homebuilt confocal
microscope was used which was designed and realized by R.N. The continuous monitoringof the NV center properties was highly important to the process development: one of theprocessing steps employed in the rst version of the process was damaging the diamondsurface rendering shallow NV centers unstable.

Consequently, R.N. contributed to the

optimization and reliability of the process via characterizing NV centers throughout theprocess. In addition, R.N. participated in all discussions during the project. He wrote thecorresponding part of the manuscript and designed the appropriate gure. R.N. actively
contributed to all major revisions of the article.

micromachines
ArticleReliable Nanofabrication of Single-Crystal Diamond
Photonic Nanostructures for Nanoscale Sensing
Mariusz Radtke † , Richard Nelz

, Abdallah Slablab

and Elke Neu *Faculty of Natural Sciences and Technology, Saarland University, Physics, Campus E2.6,
66123 Saarbrücken, Germany* Correspondence: elkeneu@physik.uni-saarland.de† Current address: Department of Organic and Macromolecular Chemistry, Ghent University Krijgslaan 281,
building S4, 9000 Gent, Belgium.
Received: 26 September 2019; Accepted: 17 October 2019; Published: 24 October 2019


Abstract: In this manuscript, we outline a reliable procedure to manufacture photonic nanostructuresfrom single-crystal diamond (SCD). Photonic nanostructures, in our case SCD nanopillars on thin(<1 µm) platforms, are highly relevant for nanoscale sensing. The presented top-down procedureincludes electron beam lithography (EBL) as well as reactive ion etching (RIE). Our method introduces
a novel type of inter-layer, namely silicon, that significantly enhances the adhesion of hydrogensilsesquioxane (HSQ) electron beam resist to SCD and avoids sample charging during EBL. In contrast
to previously used adhesion layers, our silicon layer can be removed using a highly-selective RIE step,which is not damaging HSQ mask structures. We thus refine published nanofabrication processes toease a higher process reliability especially in the light of the advancing commercialization of SCD
sensor devices.
Keywords: top-down nanofabrication; single-crystal diamond; HSQ; electron beam lithography;
inductively coupled-reactive ion etching (ICP-RIE)

1. Introduction
In recent decades, the use of optically active point defect, i.e., color centers, in single-crystal
diamond (SCD) as atom-sized, solid-based quantum systems has emerged in various fields [1,2].
Applications span from quantum metrology (temperature [3], strain [4], electric [5] and magnetic
fields [6]) to using color centers as spin qubits in quantum computing [7] and single photon sources
for quantum communication [8,9]. The outstanding color center in diamond is the NV− center due toits optically readable spin [10] and usage as sensor. For many of these applications, color centers will
be incorporated into photonic nanostructures e.g., nanopillars [9] to ease fluorescence detection fromthe color centers and to enable, e.g., scanning a color center close to a sample surface [6].
SCD’s wide indirect bandgap of ∼5.45 eV makes undoped SCD a good insulator [11]. Moreover,
SCD shows a high chemical inertness. Both properties render fabricating SCD nanostructureschallenging: Top-down methods for nanofabrication will use lithography, typically electron beamlithography (EBL), as well as etching. As the high chemical inertness of SCD prevents wet etching,
only plasma etching, typically inductively coupled reactive ion etching (ICP-RIE), is applicable.
Moreover, the insulating nature of SCD renders EBL highly challenging due to uncontrolled sample
charging and the resulting deflection of the electron beam. A peculiarity of SCD nanofabricationarises also from the fact that only certain materials can efficiently serve as an etch mask in thehigh-bias, high-density plasmas necessary for anisotropic SCD etching [12]. The now state-of-the-art
masks for SCD nanostructuring are EBL written structures consisting of hydrogen silsesquioxane(HSQ). HSQ is stable in anisotropic etch plasmas used for SCD etching; it etches an order ofMicromachines 2019, 10, 718; doi:10.3390/mi10110718

www.mdpi.com/journal/micromachines

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magnitude slower than SCD using typical etching recipes [12]. In general, HSQ enables creating
very small mask structures down to 20 nm [13]. SCD structures etched using HSQ masks showsmooth sidewalls [12]. Smooth sidewalls ensure low light scattering from photonic structures anddefined waveguide properties. Consequently, HSQ masks enable etching almost cylindrical pillars with
optimized shape and well-defined photonic properties [6,9,12,14,15]. On the other hand, we find that
HSQ has a non-optimal adhesion to SCD. In previous work, this challenge has often been addressedusing metallic inter-layers between HSQ and SCD, e.g., titanium [16]. However, even very thin(<1 nm) metallic residuals on SCD surfaces strongly disturb color centers placed shallowly below
the surface [17]. Consequently, any metallic residues are detrimental for the process and a metal-freeprocess is highly desirable. Often, removing the metallic layer also requires wet chemical removal [16]or the use of toxic etch gases such as chlorine [18]. The first can leave trace amounts of the etchant on
the SCD surface and the second is technically demanding considering safety and reactor corrosion.
In this manuscript, we present a method to overcome two previously not satisfactorily addressedchallenges in SCD nanofabrication, namely sample charging as well as non-optimal resist adhesion.
We used the optimized process to fabricate SCD scanning probes, namely nanopillars on thin holding
platforms [6,16]. We thus reliably fabricated SCD nanostructures easing, e.g., commercial fabricationof SCD scanning probes. Figure 1 depicts the steps of our nanofabrication process. We start witha clean SCD sample with a shallow NV− layer (details on sample pre-treatment see Section 2) as
mandatory for high resolution sensing. We use electron beam evaporation of silicon on SCD to forma de-charging and adhesive layer (Figure 1b, Section 3). This layer will enable highly reliable spin
coating of HSQ (Figure 1c) as well as EBL (Figure 1d, Section 3). Subsequently, we remove the siliconadhesion layer selectively using ICP-RIE without damage to the HSQ mask (Figure 1e) and perform
ICP-RIE of SCD to form the desired structures (Figure 1f, Section 4). Our method eases manufacturing
complex structures, e.g., in our case, nanopillars (diameter 200 nm), on top of SCD platforms (size of theplatform ∼ 3 × 20 µm): In the first structuring step, we form the platforms (Figure 1f). The siliconadhesion layer survives the subsequent wet-chemical removal of the HSQ mask (Figure 1g) and canbe reused for a second round of processing (Figure 1h–l). In this second processing, we form thepillars. We note that, when etching the platforms, the HSQ mask protects NV centers in the wholearea of the micrometer-sized platform. During pillar etching, only NVs protected by the pillar mask
survive the process and will be used as nanoscale sensors. The method presented here has been filed
for a patent (EP19198772.6).

Micromachines 2019, 10, 718

3 of 10Figure 1. (a) The process starts with an SCD sample with a shallow layer of NV centers. Subsequently,
we deposit (b) a silicon adhesion layer and (c) spin coat HSQ. (d) We use electron beam lithography(EBL) to structure the HSQ resist and obtain masks for the platforms. Using ICP-RIE, (e) we remove
the silicon layer in-between the HSQ mask and (f) perform the structuring etch for SCD. (g) We finalize
platform structuring via removing residual HSQ. We now (h) spin coat HSQ onto the remaining siliconlayer and (i) perform EBL again. We repeat the ICP-RIE procedure, firstly (j) removing the silicon layer
and then (k) etching the pillars into SCD. In a last step, (l) we remove residual HSQ as well as silicon to
obtain clean SCD devices.

2. Sample Pre-Treatment
We purchased high-purity, (100)-oriented, chemical vapor deposited, SCD from Element Six(electronic grade quality, [N]s < 5 ppb, B < 1 ppb). As we aimed for free standing SCD devices
consisting of nanopillars on platforms, the SCD plates (size 2 × 4 mm2 ) were polished down tothickness of 50 µm (Delaware Diamond Knives, Wilmington, DE, US). The SCD surface showed an
initial roughness of Ra < 3 nm. As the mechanical polishing of the SCD can leave highly contaminatedsurfaces, we first wiped the sample surface using clean-room wipes and performed cleaning in an
ultrasonic bath (solvents: isopropanol and acetone). We then cleaned the sample in boiling acids(1:1:1 mixture of sulfuric acid, perchloric acid and nitric acid, 5 mL each).
Mechanical polishing is suspected to introduce damage that potentially extends several
micrometers deep into the SCD material [19,20]. To remove this potentially damaged and strainedmaterial, we applied ICP-RIE to our SCD samples. We avoided the use of toxic or corrosive gases in
the process following our previously published routine [21]. We used a Plasmalab 100 ICP-RIE reactor(Oxford instruments, Abington, UK) and removed the topmost 3–5 µm of SCD from each side. We

Micromachines 2019, 10, 718

4 of 10

used a combination of SF6 ,O2 , Ar biased plasmas with mixed RF and ICP discharges. Following recent
approaches [22,23], we terminated the etching using low-damage, 0 V bias plasma with pure oxygen.
The use of such soft etching was motivated by the potential close-to-surface damage due to highlybiased ICP etching [24]. We typically obtain very smooth surfaces with an rms roughness of ∼1 nm.
Using the above described procedure, we avoided creating NV centers in potentially damaged
SCD. We formed a homogeneous layer of NV− centers by implanting nitrogen ions with a density of
2 × 1011 ions/cm2 and an energy of 6 keV. During the implantation, the sample was tilted by 7◦ withrespect to the ion beam to avoid ion channeling. The SCD sample was then annealed in vacuum at
800 ◦ C followed by an acid clean. This treatment will typically leave our sample with a mixed oxygen
termination on the surface [25]. We found a contact angle for water of 67◦ [26], indicating a hydrophilic
surface. As the electron beam resist we want to apply to the SCD was dissolved in methyl isobutyl
ketone, a polar molecule, the resist’s solvent had high affinity to hydrophilic surfaces. Despite the,in principle, fitting surface termination of the SCD sample, we observed non-reliable adhesion when
applying HSQ to the SCD surface.
3. Deposition of Adhesion Layer and HSQ Mask Structuring
Motivated by the lack of reliable adhesion of HSQ to clean SCD surfaces, we explored silicon as
an inter-layer. We expect this layer to foster adhesion between polysilicate HSQ resist and the nativeoxide (SiO2 ) on the layer. To deposit the silicon adhesion layer, we used electron beam evaporation at
a pressure of 10−6 Torr and 10 kV acceleration voltage with elliptical beam scanning mode in an electron
beam evaporator “Pfeiffer Classic 500 L” machine. For the present work, we chose a thickness of thesilicon layer of 25 nm. We note that we also found sputtered silicon layers to efficiently foster adhesion
between SCD and HSQ. However, the SCD surface was attacked during the sputtering process. This in
our case led to excess blinking and bleaching of NV− centers in the final sensing devices and rules out
this approach for our application. We also note that we tested spin coating Ti-prime as an adhesion
promoter but did not obtain reliable results. We furthermore tested chromium layers as alternative
to quickly oxidizing titanium layers [16]. Using this approach, we faced micromasking effects mostprobably arising from the incomplete, non-reliable wet-chemical removal of chromium layers. We notethat our silicon layers still enable efficient HSQ adhesion weeks after deposition and storage underambient conditions. We consequently conclude that the formation of a native oxide layer on the silicon,
which will occur during storage at ambient conditions, is not detrimental. Thus, technically speaking,evaporation of silicon layers can be performed in batch processes for several SCD samples, whicheases the fabrication workflow and reduces machine time. The evaporated silicon layers are very
uniform and show a low roughness, as evidenced by AFM. We note that adhesion of the silicon layer to
SCD was very reliable and we never observed any hints of cracking or peeling throughout the wholeprocess, deduced from AFM and SEM microscopy. We have processed more than 10 samples using thehere described method and no SEM images showed peeling or cracking of the silicon layer. To avoid
any damage of the surface caused by contact with the tip (AFM) or amorphous carbon deposition(from SEM chamber), no routine checks were performed prior each fabrication step. We note we also
observed the surface topography of samples covered with HSQ layers. In case of any silicon peeling,
this would be evident by folding of spin-coated HSQ.
To manufacture etch masks based on HSQ, we used Fox 16 resist (Dow Corning, Midland, MI,
USA), which we spin-coated onto the SCD plate. To ease handling of our small SCD plates, we glued
them to silicon carrier chips using crystalbond adhesive. We note that the silicon carrier can be removed
at the end of our nanofabrication process using acetone to dissolve crystalbond without damage to the
SCD nanostructures. Prior to spin coating, we heated the SCD sample on the silicon carrier for 10 min
at 120 ◦ C to remove any moisture from the surface. We applied roughly 0.3 mL of Fox 16 solution
to the SCD plate and spin-coated it at 1000 rpm for 10 s then increasing rotation speed to 3300 rpm
for 60 s. Subsequently, we pre-baked the sample at 90 ◦ C for 5 min. We note that great care has beentaken to not exceed the shelf life of the Fox 16 resist. As a result of the small size of our samples as

Micromachines 2019, 10, 718

5 of 10well as the spin coating on already etched structures in the latter stages of our fabrication process,we can only estimate the thickness of the HSQ, which shows a significant variation from sample tosample. From SEM images of pillar masks on platforms, we estimate a HSQ layer thickness of ∼0.9 µm.
Consequently, considering a pillar diameter of ∼200 nm, we demonstrate reliable adhesion of HSQ
masks with an aspect ratio of 4.5.
We inserted the SCD plate including the silicon carrier chip into our EBL machine (cold-cathodeSEM, Hitachi S45000, Chiyoda, Japan, equipped with RAITH Elphy software). We note that EBL of thespin-coated HSQ layer has to be done directly after spin coating to avoid any reaction of HSQ with air.
We performed EBL at 30 kV acceleration voltage and 20 µA extracting current. The working distance iskept at 15.3 mm for 400 × 400 µm2 fields. During our device fabrication, larger structures, namely therectangular holding platforms (size ∼ 3 × 20 µm2 ) as well as masks for nanopillars (diameter 200 nm),were of interest. We wrote platforms using longitudinal writing mode and pillars using concentric
writing modes. The doses for large structures were established to be optimal as 0.49 mC/cm2 andfor pillar structures, with a variation with thickness of the HSQ layer, between 2.24 (planar SCD) and7 mC/cm2 (pre-structured SCD with, e.g., platforms).
We developed the HSQ in 25% TMAH solution without swirling the solution. After 20 s, the SCD
sample was placed in ultra-pure 18 MOhm cm MiliQ water and subsequently immersed several times
in acetone and isopropanol. We note that the development has to take place directly after removing
the SCD plate from the EBL vacuum chamber.
4. Selective ICP-RIE of Adhesion Layer and SCD Structuring
A dedicated ICP/RIE plasma sequence based on O2 -based etching of SCD, preceded by a shortpulse of SF6 plasma, was designed. This sequence first selectively removes the silicon layer
between the HSQ-based mask structures and subsequently enables highly anisotropic etching of
SCD. The parameters of the plasmas are summarized in Table 1. In the final process, we ran the above
mentioned sequence without removing the sample from the ICP-RIE reactor in-between the plasma
steps to avoid any contamination.
Reliably removing the silicon layer without any residuals is vital to our process: We observeda partial etch stop as well as strong micromasking when applying the O2 -based plasma without
applying the SF6 pulse (see Figure 2a). We deduced a complete and reliable removal of the siliconadhesion layer from two facts: First, in SEM images taken directly after the SF6 pulse (see Figure 2b),a clear contrast between etched and non-etched areas is visible. We also investigated the SF6 -based
etching process by means of optical emission spectroscopy shown in Figure 2c. In the spectrum a seriesof emission lines corresponding to fluoride (F− ) was observed [27]. We attributed the etching of silicon
to this F− ions. Second, knowing that the O2 plasma used to etch SCD was not etching the silicon layer,
the absence of micromasking and very smooth surfaces in-between the etched structures (see Figure 3a)
proved the complete removal of the silicon layer. We note that using pure SF6 is vital to arrive at thisresult, as introduction of other gases (Argon or Oxygen) at this stage generated severe micromasking.
Our SF6 plasma removes the silicon layer while maintaining a 1:20 selectivity in favor of the HSQ
mask. For our process, this means that, during removal of the 25 nm thick silicon adhesion layer, lessthan 2 nm of the HSQ mask, which in our case is several hundreds of nanometer thick, will be lost.
This result corresponds well to similar plasmas obtained in different systems showing highly selective
silicon etching while conserving SiO2 (in our case HSQ) [28]. We furthermore observed no or minor
etching of SCD during the SF6 pulse and no roughening of the exposed SCD surface. We confirmed
full etching of the silicon layer using Raman spectroscopy and XPS, whereas the latter only showed
C1s and O1s peaks. Using EDX, we checked that there is no silicon contamination on the etched SCD
after the O2 -based plasma (see Figure 4).

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Table 1. Etching plasma parameters. SF6 pulse used to selectively remove the evaporated silicon layer(5 s including plasma ignition step). The O2 plasma is subsequently used as an anisotropic etch for
SCD to form the platforms as well as the pillars (10–15 min etching time).
PlasmaICP Power
W

RF PowerW

Gas Flux
sccm

Etch Rate
nm/minPressurePa

SF6 pulse
O2 plasma

300
500

100
200

SF6 :25
O2 : 50

Si: 1072 HSQ: 52
104

1.3
1.5

Figure 2. (a) Pattern of holding platforms etched with O2 plasma without applying the SF6 pulse plasma
to remove the silicon layer, the SEM image shows the bare SCD structures. Strong micro-masking andcorresponding roughening as well as a partial etch stop are visible (b) Scanning electron microscope(SEM) image of SCD surface with HSQ structures, here platforms, after SF6 pulse. A part of theplatforms is covered with a quartz plate (marked in the image) during the SF6 pulse. The strongly
reduced brightness of the etched surface in contrast to non-etched surface indicates the complete
removal of the silicon adhesion layer. We furthermore observe no or minor etching of SCD during
the SF6 pulse and no roughening of the exposed SCD surface. (c) Optical emission spectrum of theSF6 pulse plasma step indicating presence of fluoride (F− ) species responsible for selective removal ofsilicon from the SCD surface.

Figure 3. (a) Devices at intermediate stage of the process with etched-in platforms. Note that the SEM
image shows the bare SCD structures that have been obtained via removing the adhesion layer using
the SF6 pulse followed by anisotropic O2 RIE and subsequent cleaning. (b) HSQ masks for pillars
etching written by EBL. The masks are residing on SCD platforms coated by a freshly evaporated
silicon layer. Note that the silicon layer is not discernible in the SEM images.

After successfully structuring our SCD platforms, we removed HSQ residuals using HF-based
buffered oxide etch by immersion of SCD for 20 min in the solution. Although this step removed thenative oxide from our silicon adhesion layer, the layer itself survived the process, as clearly discernible
from the EDX imaging in Figure 4b. Consequently, it can be re-used for consecutive steps. We thenspin-coated HSQ again, which in our case formed a layer on top as well as in-between the platforms.
We then re-emploedy EBL to create pillar masks on the platforms (see Figure 3b). We repeated theetching to transfer the pillar mask into the SCD platform creating almost cylindrical pillars.

Micromachines 2019, 10, 718

7 of 10Figure 4. Energy dispersive X-ray (EDX) spectroscopy mapping of SCD cantilevers prepared by thepresented method. The EDX mapping has been performed after the SF6 plasma, the O2 plasma etching
the SCD structures (parameters see Table 1) and the wet chemical removal of residual HSQ: (a) carbon
signal; and (b) silicon signal. The two maps show complementary images, clearly indicating that,in-between the platforms, we find bare SCD (carbon) with no silicon signal while on the platform,
the silicon adhesion layer survived and is ready to be used in the next processing step.

5. Final Devices and Device Characterization
To obtain clean SCD devices, we removed all HSQ residuals using HF-based buffered oxide
etch. We immersed the SCD into buffered oxide etch for 20 min which removes the HSQ as well as
any native oxide on the silicon layer. Afterwards, we immersed the SCD sample in 3M potassiumhydroxide at 80 ◦ C for 30 min to remove the silicon adhesion layer and revealed the clean SCD
structures. After this process, we repeated the three-acid cleaning described above before characterizing
the photoluminescence (PL) of NV− centers in the SCD nanostructures. Figure 5a displays devices
obtained using this process. We note that, to obtain free standing devices, which we mounted to quartz
capillaries as holders (see Figure 5b), the SCD plate has to be thinned from the non-structured side
until the devices are fully released. For more details on the mounting, see Reference [16]. To thisend, we employed previously published deep-etching routines [21] which are beyond the scope of
this manuscript.

Figure 5. Scanning probe devices manufactured using the described nanofabrication process. (a) Pillars
on platforms. The shown devices still need thinning of the SCD plate from the backside to release thedevices. (b) Mounted SCD scanning probe. On the right hand side, a quartz capillary is visible thatserves as a holder to mount the device to a scanning probe microscope.

To investigate the photonic properties of our SCD nanostructures, we used a custom-built confocal
microscope (numerical aperture 0.8). Details of the setup are given in References [22,29–31].
We first measured confocal PL maps of the structures (see Figure 6a,b) excited at 532 nm witha power of 500 µW. We clearly observed intense PL (∼100 kcps) originating from single NV− centersin the pillars (see Figure 6a). We estimated the maximum achievable PL of the NV− centers to be>300 kcps comparable to previous work [16]. In addition, we investigated the background PL fromthe etched surface (∼1 kcps), which is negligible compared to the NV− center PL from the pillar(see Figure 6b).

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Keeping background PL from etched surfaces low is important as it limits the signal-tobackground ratio and consequently the achievable magnetic field sensitivity [32]. Figure 6c shows an
exemplary optically detected magnetic resonance (ODMR) measured on single NV− centers in the
structures. Here, we measured an ODMR contrast of ∼15%. By investigating the coherence of theNV− centers in the nanostructures, we found a coherence time of T2 ≤10 µs. We attributed this to theNV− centers’ proximity to the surface which is in a good agreement with results from other groups
measuring the coherence of shallow NV− centers in three-acid-cleaned SCD [33]. Considering thealready low T2 , we cannot fully exclude a negative influence of the structuring on T2 .
(a)

(b)

(c)

Figure 6. (a,b) PL map of SCD platforms with individual nanopillars fabricated with the processdescribed in Section 4. The outer edge of the holding platform is indicated using a dashed line,
while the pillars appear as bright spots. To enable comparing the background PL from the platforms
with the PL of a few NV− centers in the pillars, we show the same PL map with two different scaling:
individual NV− centers in the pillars show PL countrates above 125 kcps at an excitation powerof 500 µW at 532 nm (a). (b) It is clearly discernible that the cantilevers show only a weak PL of∼ 1 kcps. The enhanced background in-between the platform arises from a slight roughening between
the structures. We detect NV− PL in the wavelength range >650 nm. (c) An exemplary opticallydetected magnetic resonance (ODMR) of one of the NV− centers in the pillar. The resonance (without
an external magnetic field) at 2.87 GHz is clearly visible and has a contrast of ∼15%.

6. Summary and Conclusions
In this paper, we present a reliable technology for nanofabrication of SCD structures. We used our
method to manufacture SCD scanning probes with shallowly embedded negative nitrogen vacancies.
The method introduces an evaporated silicon adhesion layer on the SCD surface to ease adhesion and
EBL with spin-coated HSQ-based Fox 16 resist. We present a methodology for the selective removalof our silicon adhesive/decharging layer with SF6 plasma. In areas protected by the HSQ mask,silicon layer survives etching as well as wet chemical removal of the residual HSQ mask and can be
re-used for further nanofabrication, in our case for pillars on SCD platform. The shallowly implanted
NV− centers survived the nanofabrication process. We found this method to be reliable, which isa considerable advancement in SCD nanofabrication technology that can be expanded to various kindsof SCD structures including SCD cantilever or cavity structures (e.g., photonic crystals).
Author Contributions: Funding acquisition, E.N.; Investigation, M.R., R.N., A.S. and E.N.; Methodology, M.R.,
R.N., A.S. and E.N; Supervision, E.N.; Writing—original draft, M.R., R.N. and E.N.; Writing—review and editing,
M.R., R.N., A.S. and E.N.
Funding: This research was funded by NanoMatFutur grant of the German Ministry of Education and Research(FKZ13N13547).
Acknowledgments: We would like to acknowledge Sandra Wolff (TU Kaiserslautern, Germany) for help withelectron beam evaporation, Jörg Schmauch (INM, Germany) for his help with acquiring high-quality SEM images
and Rene Hensel (INM, Germany) for granting access to the ICP/RIE reactor. We acknowledge Michel Challier for
his assistance and Andreas Ruh (Saarland University, Germany) for his help with EDX. We gratefully acknowledge
cooperation and fruitful discussion with QNAMI (Basel), especially with Felipe Favaro De Oliveira and Patrick
Maletinsky on the evaluation of our novel nanofab method. We note that the results presented in this study are
filed for a patent, application number: EP19198772.6.

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Conflicts of Interest: The authors declare no conflict of interest.

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5. Plasma treatments and photonic
nanostructures for shallow
nitrogen vacancy centers indiamond
Mariusz Radtke, Lara Render, Richard Nelz, and Elke Neu
Opt. Mater. Express 9, 4716-4733 (2019)
doi: 10.1364/OME.9.004716

Plasma treatments are at the heart of single crystal diamond processing and nanofabrication: They thin and structure single crystal diamond but might also have detrimentaleects like damaging the surface. The above stated manuscript evaluates in details dierent plasma treatments of single crystal diamond.
Richard Nelz (R.N.) investigated the material after the pre-treatment of the diamond toinvestigate background photoluminescence arising from dierent plasmas which is crucial
for the investigation of single NV centers. Following NV center creation, R.N. investigatedthe NV ensemble's optical and spin properties in the dierent regions to identify the optimal plasma-treatment. Hereby, a homebuilt confocal microscope, designed and realized
by R.N. was employed. After the fabrication of nanostructures in this area by M. Radtke
and L. Render, R.N. selected the appropriate structures to characterize.

He supervised

L. Render, a bachelor student of the group, investigating the NV centers in the structures and helped to evaluate the data. Further, he wrote the corresponding parts of themanuscript and designed the corresponding gures. In addition, R.N. actively contributed
to all discussions during the project and all major revisions of the manuscript.

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Plasma treatments and photonic nanostructures
for shallow nitrogen vacancy centers in diamond
M ARIUSZ R ADTKE , L ARA R ENDER , R ICHARD N ELZ , AND E LKE
N EU *
Saarland University, Department of Physics, Campus E2.6, 66123 Saarbrücken, Germany* elkeneu@physik.uni-saarland.de

Abstract: We investigate the influence of plasma treatments, especially a 0 V-bias, potentially
low damage O2 plasma as well as a biased Ar/SF6 /O2 plasma on shallow, negative nitrogenvacancy (NV− ) centers. We ignite and sustain our 0 V-bias plasma using purely inductive
coupling. To this end, we pre-treat surfaces of high purity chemical vapor deposited single-crystal
diamond (SCD). Subsequently, we create ∼10 nm deep NV− centers via implantation andannealing. Onto the annealed SCD surface, we fabricate nanopillar structures that efficiently
waveguide the photoluminescence (PL) of shallow NV− . Characterizing single NV− inside thesenanopillars, we find that the Ar/SF6 /O2 plasma treatment quenches NV− PL even consideringthat the annealing and cleaning steps following ion implantation remove any surface termination.
In contrast, for our 0 V-bias as well as biased O2 plasma, we observe stable NV− PL and low
background fluorescence from the photonic nanostructures.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1.

Introduction

Single-crystal diamond (SCD) with high purity, high crystalline quality and well-controlled
surface properties is an enabling material for quantum technologies including quantum sensing[1,2]. SCD is transparent for visible light and supports light confinement and guiding in photonic
nanostructures (refractive index 2.4 visible range, indirect electronic band gap 5.45 eV, [3]).
SCD hosts more than 500 optically active point defects termed color centers [3]. Considering
quantum technologies, the currently mostly investigated color center is the negative nitrogenvacancy complex (NV− ). NV− centers provide controllable, optically readable spins [4] and arestable electrical dipoles that emit single photons [5,6] or react to optical near-fields [7,8]. Theirelectronic spin degree of freedom renders them sensitive to magnetic fields [9], electric fields[10], temperature [11] and strain [12] in the diamond matrix. In this context, the key to sensitive
measurements with simultaneously nanoscale spatial resolution is to employ individual NV−
centers placed shallowly (typically <10 nm) below SCD surfaces. To enable nanoscale imaging
of various samples, it is mandatory to embed the sensing NV− in a SCD scanning probe tip [9,13].
In addition to enable scanning of a sample, the SCD tip’s photonic properties direct the NV−
photoluminescence (PL) to the collection optics and thus enhance sensitivity [9,14]. The mostprominent candidates for SCD scanning probes are pyramidal tips [15,16] and cone-shaped or
cylindrical pillars (on platforms) [9,13,14]. The latter require sophisticated top-down processes
to sculpt the structures from SCD.
Top-down nanofabrication processes reliably create tailored sensing devices potentiallyenabling highly efficient sensing. However, these processes simultaneously endanger shallow
NV− centers: To ensure reliable processing, typically smooth (roughness several nanometers),
mostly commercially available SCD plates are used. Mechanical polishing of ultra-hard SCD
used to obtain these smooth surfaces, however, may create subsurface damage and stress extending
several micrometers into SCD [17,18]. Consequently, SCD surfaces which are highly suitedfor nanofabrication might not be optimal to host shallow NV− . To circumvent this challenge,
typically several micrometer of SCD are removed using inductively coupled plasma reactive ion#379297
Journal © 2019https://doi.org/10.1364/OME.9.004716
Received 14 Oct 2019; revised 8 Nov 2019; accepted 8 Nov 2019; published 21 Nov 2019

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etching (ICP-RIE) [13]. Typical ICP-RIE plasmas, however, have been shown to damage theuppermost few nanometers of the etched SCD [19]. This damage will also potentially affectshallow NV− created under this etched surfaces [20]. Moreover, e.g. chlorine gas in the etchplasma is very helpful to keep SCD surfaces smooth during etching [21,22] but might attach
to the surfaces, potentially de-activating shallow NV− [23]. Low damage plasmas in which
the etching species are not accelerated towards the etched SCD (0 V-bias plasmas) have been
investigated as a solution to the challenge of ICP-RIE induced damage [20]. Using ICP-RIE hereis the method of choice as the density of activated species in the plasma and the acceleration
towards the etched substrate can be controlled independently of each other by controlling thepower inductively coupled to the plasma (ICP power) and the power coupled capacitively to theplasma (often termed radio frequency (RF) power or platen power) [24–26]. Previous work [20]employed the 0 V-bias plasma after creating shallow NV− . Consequently, the plasma treatment
removes SCD containing shallow NV− centers and alters the NV− density. Uncertainties in the
etch rate of the 0 V-bias plasma thus transform into uncertainties in NV− density rendering thereliable creation of individual NV− in nanostructures challenging.
In this manuscript, we present a process to manufacture photonic nanostructures namely
nanopillars with shallow, individual NV− . We, for the first time, include a 0 V-bias O2 plasma
as pre-treatment for NV− creation and SCD nanopillars fabrication thus avoiding changing
the NV− density by our plasma treatment. Figure 1 summarizes our nanofabrication process.

Fig. 1. Schematic of the employed nanofabrication process. The manuscript is structured
into the sections according to these schematics.

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Following our previous work [27], we restrict the used etch gases to non-corrosive, non-toxic
gases namely Ar, SF6 and O2 . Starting from polished, commercial SCD plates [Fig. 1(i)], we
use high bias ICP-RIE to remove micrometer thick potentially damaged layers [stress relief
etch, Fig. 1(ii)] while conserving smoothness. We finish this processing step with different
plasma treatments, especially a potentially low damage O2 plasma step [pre-etch, Fig. 1(iii)].
Following this sample pre-treatment, we use established techniques to create NV− centers[Figs. 1(iv)–1(vi)]. We use an optimized process involving electron beam lithography (EBL) and
ICP-RIE to structure nanopillars with single NV− [Figs. 1(vii)–1(ix)]. We characterize single
NV− using PL spectroscopy, PL saturation, PL lifetime, photon correlation measurements aswell as optically detected magnetic resonance (ODMR).
2.

Experimental setup and methods

We use a custom-built confocal scanning microscope (numerical aperture 0.8) to characterize theNV centers’ properties in the SCD where confocal filtering is ensured using a single mode fiber.
To acquire confocal PL maps, we excite NV centers with a continuous diode-pumped solid-state(DPSS) laser with a wavelength of 532 nm. To investigate the charge state of NV centers, we usean additional DPSS laser with a wavelength of 594 nm. We detect the PL signal through a 650 nm
longpass-filter and use highly-efficient photon counters (Excelitas SPCM-AQRH-14, quantum
efficiency ∼ 70 %) to quantify the PL intensity. In addition, we can send the light to a grating
spectrometer (Acton Spectra Pro 2500, Pixis 256OE CCD). We use a tuneable (450-850 nm),pulsed laser (NKT EXW-12, pulse length ∼ 50 ps) equipped with a filter system (NKT SuperK
Varia) and correlation electronics (PicoQuant, PicoHarp 300) to perform time-resolved PL
analysis for e.g. lifetime measurements. To perform spin manipulation on NV− centers in the
SCD, we equip the setup with a microwave source (Stanford Research Systems, SG 384) and an
amplifier (Mini Circuits, ZHL-42W+) to deliver microwaves through a 20 µm thick gold wire.
To perform ICP-RIE, we employ an Oxford Plasmalab 100 ICP RIE system. The plasma lab 100
uses a helical coil ICP configuration. We analyze the plasma composition by means of opticalemission spectroscopy (Ocean Optics USB 2000+ coupled by fiber optics to the ICP-RIE system).
We employ atomic force microscopy (AFM) measurements (Bruker Fastscan, tapping mode,silicon carbide cantilevers) to deduce the surface roughness of our etched surfaces. To create NV−
centers, ion implantation has been performed at Augsburg University (Ion-Implantation-System
NV 3206, Axcelis Technologies). Samples were annealed at 800 ◦ C under 1.5 × 10−7 mbar
vacuum using a home-built annealing oven incorporating a heater plate (Tectra Boralectric). We
employ a cold-cathode scanning electron microscope (SEM) (Hitachi S45000), equipped with
RAITH Elphy software to perform electron beam-lithography (EBL).
3.

Sample pre-treatment: stress relief- and pre-etch

In this study, we use commercially available SCD grown by chemical vapor deposition ofelectronic grade purity (nitrogen, [N0s ] <5 ppb and boron [B]<1 ppb, Element Six, UK). The
SCD is polished to form plates with 50 µm thickness by Delaware Diamond Knives, US. Usingsuch thin SCD plates is motivated by our goal to manufacture free-standing SCD scanning probe
devices that require thinning the plate to a thickness <5µm [13]. Here, a thickness of 50 µm
is a good compromise between mechanical stability and process time for thinning. To easehandling during nanofabrication, we fix the SCD plate to a silicon carrier using small amountsof crystalbond adhesive. The plates have a roughness of Ra =3 nm according to manufacturer
specifications and lateral dimensions of 2x4 mm. The received diamonds were cleaned bya boiling tri-acid mixture (1:1:1 v/v of H2 SO4 , HNO3 , HClO4 ) and subsequent washing in
acetone/isopropanol. We check surface cleanliness in a stereo microscope (50 x magnification)and find no visible residues over the entire SCD surface.

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Subsequent to cleaning, we use an etching recipe published previously [27] to remove thetopmost 2.3 µm of our SCD plate. This stress relief etch [see also Fig. 1(ii)] is applied uniformly
to the full surface of the SCD plate and uses an Ar/SF6 /O2 plasma as final step (parameters see
Table 1). This recipe avoids surface roughening, despite the fact that we are removing potentiallydamaged SCD material [27]. As indicated by previous work, adding a fluorine containing etchgas to the process aids in avoiding micromasking on the SCD arising from potential silicon
contamination in typical ICP-RIE chambers [26].
Table 1. Plasma parameters for stress relief and pre-etch. The etch rate of the 0 V-bias O2 plasma
varied between 8 to 10 nm/min and was thus reasonably consistent.
PlasmaICP Power(W)

RF Power(W)

DC Bias (V)Gas Flux(sccm)

Etch Rate(nm/min)Pressure(mTorr)roughness(nm)

435 V O2

500

200

435

O2 : 50

Ar/SF6 /O2

700

100

150

O2 22

98

11

0.7

87

13

1

8

12

0.7

SF6 :7
Ar:15
0 V O2550

00O2 : 50

While the stress relief etch procedure reliably removes even tens of micrometers of SCD
keeping the surface smooth [27], highly energetic etch species in the plasma potentially damage
the first nanometers of the SCD [19]. In this study, we thus systematically investigate the effect ofadditional plasma steps, termed pre-etch plasmas, which follow the stress relief etch and by which
we terminate the removal of material [see Fig. 1(iii)]. We aim to compare different pre-etch
treatments applied to the same SCD plate. We thus rule out that sample quality changes influence
our findings. To this end, we protect not to be etched areas using a 100 µm thin quartz plate andcreate three areas in the SCD plate [see Fig. 2(a)]. In two areas, we apply additional pre-etch
plasmas (0 V-bias O2 area I, 435 V-bias O2 area III) prior to creating shallow NV− centers (seeSec. 4). In contrast, area II is not treated by a pre-etch plasma and was consequently only etchedby the Ar/SF6 /O2 plasma of the stress relief etch.
Table 1 summarizes the parameters of the employed etch plasmas. We note that a larger varietyof 0 V-bias plasmas has been tested but only the 0 V-bias O2 plasma enabled etching without
excess surface roughening (see Appendix 7.1). We also take great care to ignite the 0 V-bias
plasma without bias building up in its starting phase by careful adjustments of capacitors in
the reactor and without using capacitively coupled radio frequency power. As discernible from
Table 1, the biased plasmas reach etch rates of almost 100 nm/min, while the 0 V-bias O2 plasma’s
etch rate is an order of magnitude lower. As indicated above, 2.3 µm of SCD were removed in the
stress-relief etch (and consequently in area II). In area I, we additionally remove ∼200 nm using
the 0 V-bias O2 plasma. In area III, we additionally remove 2 µm using the 435 V-bias O2 plasma.
After the pre-etches, we again clean the SCD sample as described above and measure thesurface roughness in areas I, II and III using AFM. The results of these measurements are depicted
in Figs. 2(b)–2(d) and summarized in Table 1. For all areas, we find an rms roughness below
1 nm, consequently all regions are usable for the fabrication of nanopillars with shallow NV− .
Smooth surfaces are mandatory for high spatial resolution, scanning probe sensing using NV− as
any surface roughness will transform into uncontrolled stand-off distances between NV− sensor
and sample under investigation.
In the following, we discuss the etch mechanisms underlying the different plasma processes.
While all surfaces show low roughness, remarkably the surface etched with the 0 V-bias O2 plasmashows the lowest roughness. 0 V plasmas have been found to etch isotropically [28,29] via chemical
etching effects. This situation might induce micromasking due to e.g. residual impurities/dust on
the SCD surface that cannot be removed by this soft plasma or lead to preferential etching ofResearch Article

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Fig. 2. (a) Sample layout for the characterization of different plasma treatements. The
whole polished SCD plate first undergoes the stress relief etch. Subsequently, we apply
different pre-etches to form three distinct areas (plasma type and depth of etch given in thesketch). The ordering of the plasma treatments for the pre-etch step is as follows: We coverareas II and III with a quartz plate (thickness 100 µm) and apply the 0 V O2 plasma to area I.
By moving the quartz plate and covering areas I and II, the 435 V O2 plasma was applied toarea III. Subsequently, we fabricate nanopillars in all areas. The pillars are not to scale andfor illustration only. (b)-(d) AFM scans to determine the surface roughness in areas I,II and
III. The given roughness is the rms value obtained from 5x5 µm2 tapping mode scans.

defective areas [26]. However, we do not observe pronounced micromasking in our work. We
use optical emission spectroscopy to compare the composition of reactive species in our plasmas(see Fig. 3). Observing the molecular transitions depicted in the insets of Figs. 3(b) and 3(c), we
analyze the concentration of ionized oxygen O+2 . In a biased O2 plasma, O+2 is the main etchingspecies as the charged ions are effectively accelerated towards the SCD sample and will induce
physical etching [30,31]. We find a 50 times higher concentration of O+2 in the 435 V-bias O2
plasma compared to the 0 V-bias O2 plasma (see Fig. 3). In contrast, the etch rate of the 0 V-bias
O2 plasma is only roughly one order of magnitude lower than for the 435 V O2 plasma indicating
an additional etching species in the 0 V-bias plasma. We suggest this additional etching species
to be (atomic) oxygen radicals. Observing the transitions 3s5 S0 → 3p5 P and 3s5 S0 → 3p3 P
in the oxygen radical system, we find almost the same concentration of oxygen radicals in theVol. 9, No. 12 / 1 December 2019 / Optical Materials Express

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0 V-bias and 435 V-bias plasmas. For the 0 V-bias plasma, chemical etching based on the oxygen
radicals is dominant which leads, as expected [32], to much lower etch rates (see Table 1). We
expect chemical etching induced by non-accelerated radicals to introduce less damage to the
SCD surface compared to the etching via ionized species accelerated under a DC bias. However,chemical etching is isotropic [28,29] and therefore not applicable for nanostructures that require
steep sidewalls e.g. nanopillars.

Fig. 3. Optical emission spectra of (a) Ar/SF6 /O2 , (b) O2 435 V and (c) O2 0 V (350 W
ICP) plasmas with corresponding Boltzmann plots used to extract electron temperatures Tel .

To additionally characterize the employed O2 plasmas, we investigate their electron temperature
Tel . Electrons in the plasma will collide with the molecules/atoms of the etch gas exciting thegas. Consequently, higher electron densities transfer into a higher population of excited states inmolecules/atoms. In the absence of a DC bias, the electron temperature can be an upper estimate
for the thermal energy of the etching species in the plasma. It should be noted that the electrons
might not fully transfer their thermal energy to the etching species but are still a clear indicationof the plasma density [33]. In contrast in the case of a biased plasma, the energy of the etchingspecies will be mainly governed by the DC bias [25]. We use the measured optical emission
spectra of the plasmas to extract Tel . First, we consider that
ln

Iλ
Ek
=−
+ C.
gk A
kTel

(1)

Here, λ is the wavelength and I the measured intensity of each transition. gk is the statisticalweight of the upper excited level, k the Boltzmann constant, Ek the energy of the excited upper
level, A the Einstein coefficient for the respective transition and C is the constant of integration.
We plot ln gIλas a function of Ek and thus the slope measured is equal to − kT1el and Tel can bekA
extracted. The electronic transitions taken into account to determine Tel are marked in Fig. 3.
We find Tel =6000 ± 2000 K for the 0 V O2 plasma, Tel =10000 ± 1400 K for the 435 V O2
plasma and Tel =16000 ± 9000 K for our Ar/SF6 /O2 plasma. For the O2 plasma especially the000following transitions were chosen: 5 P3 →3 S2 (544.04 nm),5 P1 →5 S2 (543.06 nm),3 P1 →5 S2001(646.18 nm), 3 D2 →3 P2 (615.83 nm), 3 D1 →3 D0 (599.45 nm),3 D1 →3 D0 (601.19 nm). These
values clearly indicate the higher density of the biased plasmas.
After determining the surface roughness of our SCD samples, we also analyze the PL
originating from the etched surfaces. Broadband PL from processed SCD surfaces is detrimental

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in experiments with individual NV− as it will lead to low signal-to-background ratios for single
color center observation. Here, we find a spatially homogeneous background PL (detected
at wavelengths > 650 nm, excited using 532 nm laser light) in the order of 30 kcps/mW for
both O2 -based pre-etches (areas I and III). The observed PL is stable under continuous laser
illumination. In addition, we show that the background PL arises purely from the SCD surface
as no PL is observed upon focusing the laser deeply (> 5 µm) into the SCD material. Thus, incontrast to previous work [20] that compared a biased Ar/O2 plasma with a 0 V-O2 plasma, we
do not find a reduction of PL background for the 0 V-O2 plasma. We note that the observed
background PL from these surfaces is in the same order of magnitude as the PL from single NV−
centers. We find similar background levels after creating shallow NV− centers (see section 4).
However, the fabrication of photonic nanostructures allows us to efficiently reduce the influenceof background PL (see section 5).
In stark contrast, the SCD surface exposed to the Ar/SF6 /O2 plasma (area II) shows an intense,
fast-bleaching background PL. The background that bleaches within ∼ 100 ms is in the orderof more than 7.5 Mcps/mW. For examples of the PL maps recorded on these surfaces, see
Appendix Fig. 8. For diamond films, typically broadband PL in the red-spectral range is observed
and attributed to highly defective material (sp2 inclusions, disorder, amorphous carbon [34]).
Significant broadband PL in the red spectral range thus indicates the presence of damaged
material close to the surface which could not be removed using the acid clean. Interestingly, thisbackground PL is removed after NV creation steps (see Sec. 4). We suspect that this is connectedto the annealing process which in conjunction with acid cleaning aids in removing damaged
layers at SCD surfaces [19].
4.

Creation of NV centers

Subsequently, we implant the whole surface of our SCD plate using 14 N+ ions (Energy 6 keV,
2 × 1011 ions/cm2 ). We create NV− centers via annealing at 800 ◦ C for 4 hours. We note that
directly after this procedure, we typically do not observe any NV− PL and the emission spectrumof the SCD surface shows non-diamond Raman lines. Re-cleaning the sample in tri-acid mixture
activates the NV− PL which we attribute to the removal of non-diamond, graphitic phases fromthe SCD surface. We note that alternatively thin layers of NV− centers can also be introduced
using δ-doping, reducing crystal damage due to implantation [35].
We first characterize the influence of the plasma treatments on NV− centers by investigating theimplanted NV− ensembles. To obtain illustrative results in the different areas, we investigate NV−
ensembles in micrometer-sized marker structures (crosses) that we obtained during nanofabrication(see Sec. 5). These structures do not significantly alter the collection efficiency for NV− PL or
the properties of NV− ensembles due to their size in the micrometer range. However, they give us
the possibility to clearly distinguish NV− PL from background PL of the SCD surface as well as
to clearly compare the different areas. First, we record PL maps of crosses and the corresponding
PL spectra in areas I, II and III [see Figs. 4(a)–4(c)]. As clearly discernible from Fig. 4, in areas I
and III, the NV− ensemble has similar PL brightness and the recorded PL spectra clearly reveal
the spectral features of NV− PL. In contrast, in area II treated using the Ar/SF6 /O2 etch, we do
not find any NV− PL nor any evidence for neutral NV0 centers. We note that we do not expect
this effect to result from surface termination: First, fluorine terminated surfaces are supposed tostabilize NV− [37]. Second, any surface termination would have most probably been removed
during vacuum annealing and replaced by oxygen functional groups during the acid cleaning.
The complete absence of an NV− fingerprint supports the assumption of highly-damaged layers
which prevented the creation of NV− centers or strongly quench their PL.
In addition, we measure the excited state lifetime τNV − [see Fig. 4(d)] of the NV− ensemble inareas I and III. We obtain comparable results for both areas with τNV − =17(1) ns for area I andτNV − =16(1) ns for area III, respectively. Here, τNV − is longer than the bulk lifetime (12 ns) due toResearch Article

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Fig. 4. NV− center ensembles in areas exposed to different pre-etches: (a)-(c) show PL
maps and the corresponding PL spectra. It is clearly visible that in the area exposed to the
Ar/SF6 /O2 stress-relief etch only [Area II, part (b)] we do not observe any NV− PL. The
measured spectrum only slightly differs from the background PL (light gray). Both areas
treated with pure O2 plasmas [Area I (unbiased):(a); Area III (biased):(c)] clearly show NV−
PL with a minor NV0 signal. The peak at 574 nm is the first-order Raman line of SCD. Dueto experimental constraints (laser linewidth, spectrometer resolution) the Raman line here
does not allow to deduce the level of strain in the SCD material. (d) Excited state lifetime
τNV − of the NV− ensemble as recorded in areas I and III. Both ensembles show similar
average τNV − 17(1) ns in area I and 16(1) ns in area III. τNV − is thus prolonged compared tobulk SCD due to the NV− ’s proximity to the surface [36]. In addition, the measurements
clearly show an absence of quenching. (e) Saturation behavior of the NV− ensembles area I
and III. Discussion see text.

the NV− ’s proximity to the surface [36]. However, we exclude quenching which would reduceτNV − .
To further characterize the influence of the plasma-treatments onto the brightness of the NV−
ensemble, we measure PL saturation curves in areas I and III. Here, we note that we investigate a
NV− ensemble with a low density of ∼7 NV− centers statistically distributed in the laser focus of
our confocal microscope (density estimate see Sec. 5). Consequently, the observed saturation
behavior and the parameters extracted from it are only estimates as few NV− centers experience
different laser intensities as well as different collection efficiencies. We find saturation powers
Psat which are in the same order of magnitude (1.60(5) mW area I, 0.74(3) mW area III) as wellas comparable background PL rates (∼60 kcps/mW area I, ∼40 kcps/mW area III) and count rates(I∞ =407(7) kcps area I, I∞ =315(5) kcps area III) in both O2 -etched areas. We also note thatthe background PL level estimated here agrees with the level measured in area I and III before
creating NV− centers (see Sec. 3).

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By applying microwave-driven spin manipulation to the NV ensembles in areas I and III,
we are able to measure their coherence time T2 <10 µs limited mainly by the proximity of theNV− centers to the surface which agrees well with other measurements of shallow NV− centers
under tri-acid cleaned surfaces [38]. In contrast to previous work [20], we do not observe an
enhancement of T2 as a result of the 0 V-bias O2 plasma treatment. We consequently conclude
that T2 for our NV− centers is mainly governed by noise due to surface termination and we are
not yet able to reveal positive effects of the 0 V-bias O2 plasma treatment. As we do not observe
significant strain-induced splittings of ODMR resonances, we conclude that our NV− centers are
not experiencing significant strain.
5.

NV− in photonic nanostructures

To asses the stability as well as brightness of individual NV− centers created under the 0 V-bias
O2 plasma treated surface, we fabricate SCD nanopillars in the shape of truncated cones. We aim
for diameters of the pillars’ top facet that contains the NV− center in the range of 200 nm. To
this end, we use a refined process compared to previously published processes [6,13,39], detailsof the process are published elsewhere [40]. The steps in the process are:• Deposition of a silicon adhesion layer onto SCD.
• Spin coating of hydrogen silsesquioxane (HSQ)-based negative tone EBL resist (Fox 16,
Dow Corning).
• EBL to create pillar masks, development of resist.
• ICP-RIE removal of adhesion layer and subsequent etching of SCD to form pillars, details
see Appendix 7.3 and Ref. [40]• residual mask stripping in buffered oxide etch and sample cleaning (acid clean).
To demonstrate reliable nanofabrication on all plasma treated surfaces, we generate various
patterns of nanopillars on the SCD sample. Using our optimized fabrication process, we reliably
create large fields of pillars with high yield [see Fig. 5(a)]. Due to the novelty of our 0 V-bias O2
plasma pre-treatment, we focus on area I. For consistency, we also check pillars with individual
NV centers in area III, confirming stable, bright PL in accordance with previous work [13]. Fromvarious pillars fields written in area I using different EBL doses (1.96-2.52 mC/cm2 ) and etchedusing different plasmas (Ar, Ar/O2 pure O2 , for details see Appendix 7.3), we focus on two
fields in which we were straightforwardly able to identify single NV− in the pillars. The tapered
pillars written with an EBL dose of 2.2 mC/cm2 show top diameters of 120 nm and 180 nm [seeFigs. 5(b) and 5(c)].

Fig. 5. Scanning electron microscope images of SCD nanopillars. (a) overview of large
pillar field, (b) pillars with top diameter of 120 nm (c) pillars with top diameter of 180 nm
Images recorded using a Jeol arm200 microscope at 20 kV acceleration voltage with noconductive layer applied.

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The investigated pillars have been etched using a pure O2 plasma (500 W ICP power, 50 W
radio frequency (RF) power, 50 sccm O2 , pressure 1.5 Pa). We found O2 plasmas to be mostreliable for etching our SCD pillars and we obtain an etch rate of 65 nm/min. We observe smoothsurfaces in-between the pillars without any indication of micromasking. In contrast to the O2
plasma, mask erosion and micromasking was strong using a pure argon plasma due to excess
sputtering of material. We note that the investigated pillars have a stronger taper angle than wetypically aim for [14]. The novel fabrication process used here includes a SF6 -based plasma to
remove the silicon adhesion layer which could potentially attack HSQ masks [40]. We however
find that this plasma etches our thin (25 nm) silicon adhesion layer 20 times faster than the HSQ
mask and should not cause mask erosion and strong tapering [40]. We consequently suspect

Fig. 6. (a) and (b) Fluorescence maps and lifetime τNV − maps of a pillar field (top diameter:
180 nm, length: ∼460 nm) in area I (0 V O2 plasma). (a) shows the measured PL rate
under pulsed excitation at 530-540 nm with a pulse energy of 15 nJ and a repetition rate of
8 MHz while (b) shows the corresponding lifetime map. It is nicely visible that bright pillars
have a consistent lifetime corresponding to τNV − measured for the ensemble in Sec. 4. (c)Exemplary second order correlation measurement g(2) of a NV− center in the pillars clearly
showing single-photon emission with a reasonable signal/background ratio. (d) Summary of
PL saturation of several pillars containing single NV− centers in the fields with top diametersof 120 nm and 180 nm. While most measured NV− have comparable saturation powers, the
180 nm-pillars offer slightly higher countrates compared to the 120 nm-pillars as expectedfrom their photonic properties [14]. However, these results indicate that even 120 nm-pillars
act as waveguides, potentially improving AFM operation of NV− -based scanning probe
devices.

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an additional effect causing the tapering. One possibility might be a low thickness of the HSQ
layer as the mask structures were not imaged using SEM prior to etching the pillars to avoid
contamination. Also strong mask faceting might have occurred that is known to cause tapered
sidewalls [41].
To characterize NV− centers in our photonic nanostructures, we first measure PL and lifetime
maps of the pillar fields [see Fig. 6(a) and 6(b)]. Here, we clearly observe a comparable lifetime
for most of the pillars containing NV− centers. In addition, the average lifetime of τNV − =17(2) ns
agrees with τNV − we find for the NV− ensemble in Sec. 4. By measuring second order correlation
functions g(2) [see Fig. 6(c)], we estimate a NV− probability per pillar. We then use the pillars’ top
diameter from SEM images (Fig. 5) to calculate the NV− density. Subsequently, we compare thiswith the implantation dose and extract an implantation yield of 0.01 NV− /implanted N which isin good agreement with literature [13,42]. To estimate the PL enhancement in our nanostructures,
we perform several PL saturation measurements on pillars containing single NV− centers. These
measurements are summarized in Fig. 6(d). Here, we compare NV− centers in pillars with alength of ∼ 460 nm and diameters of 120 nm and 180 nm, respectively. As discernible from
Fig. 6(d), most measured NV− centers show similar saturation power. The 180 nm-pillars offer
slightly higher PL rates as the 120 nm-pillars. Simulations suggest a much stronger decreasein PL rate from pillars thinner than 180 nm [14]. However, the here observed brightness of
single NV− PL also indicates a sufficient usability of 120 nm-pillars for sensing applications. For
most scanning probe based sensing applications, it would be advantageous to use pillars with asmall top diameter, as a thinner scanning probe tip will be able to follow the topography of asample more closely and keep the NV− center closer to the surface under investigation e.g. when
scanning over a step in the sample [13]. We note that we were not able to deduce NV− and NV0
probabilities for our NV centers [43–45] due to inconclusive results (see Appendix 7.4).
6.

Conclusions

In the presented study, we investigate a chemically etching, pure ICP discharge, 0 V-bias O2
plasma as a pre-etch for the fabrication of SCD photonic structures. We compare this treatment
to a biased O2 plasma as well as a Ar/O2 /SF6 plasma. We analyze plasma compositions andetch rates. While the biased plasmas have etch rates exceeding 100 nm/min, the 0 V-bias plasmashows an order of magnitude lower etch rate. While etching in the biased O2 plasma is dominatedby oxygen ions, oxygen radicals dominate in the 0 V-bias case. Remarkably, we only succeed increating shallow NV− under the O2 treated surfaces, while all NV− centers are deactivated underthe Ar/O2 /SF6 treated surface. We investigate single NV− centers in the nanopillars with 180 nm
as well as 120 nm top diameter. We find stable, bright emission from single NV− rendering ourstructures suitable for nanoscale sensing using single NV− . We have shown that 0 V-bias plasmas
can be integrated into the fabrication of SCD photonic nanostructures as a pre-treatment, opening
the route towards low damage treatments of SCD surfaces within SCD nanodevice fabrication.
7. Appendix
7.1. Testing of 0 V plasmas and sample layout
To check the applicability of 0 V-bias plasmas to pre-etch our SCD surfaces, we run different
plasmas. Figure 7 summarizes the tested 0 V plasmas. We select only the 0 V O2 plasma for themain study due to the observed large surface roughening of 0 V Ar/SF6 and Ar plasmas (rms ≥10 nm). We furthermore expect no significant etch rate in a 0 V SF6 plasma and do thus also
not use this plasma. Figure 7 shows the bias drop in time for each plasma after the ignition.
This drop could be eliminated by optimizing the matching circuit parameter conditions- which
required a run of pre-tests before actual etching prior to each process. The bias drop was found to
be comparable in time for each plasma and was dependent on the chamber pressure and gas flux.

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For the studies presented in the manuscript, we used the optimized parameters for the matchingcircuit and consequently avoid any buildup of bias.

Fig. 7. Bias drop in time in generation of 0 V-bias plasmas using O2 , Ar, SF6 and their
mixtures.

7.2. Characterization of the pre-etched areas
As described in the main text, we characterize the three pre-etched areas after etching and cleaning
before implanting nitrogen ions into the SCD.
Here, we see a similar behavior of the two oxygen etched areas leading to a spatiallyhomogeneous background PL of 30 kcps/mW [see Figs. 8(a) and 8(c), area I and III]. In contrast,
the Ar/SF6 /O2 -etched area II shows a different behavior: Here, we clearly see a high backgroundPL in the order of 7.5 Mcps/mW which is rapidly bleaching within roughly 100 ms. This isclearly visible in Fig. 8(b). Here, we first measure a smaller PL map and afterwards a larger PL
map around it. This second PL map clearly shows the bleached square of the first measurement.
By further focusing the laser onto a single point (as can be seen in the lower left and upper rightcorner of the smaller square) we are able to further bleach the PL to PL rates comparable withthe oxygen-etched areas. In addition, we scan the focus of our confocal laser microscope throughthe SCD surface (z-scans). Z-scans in all areas clearly show that the background PL originates
only from the SCD surface. Note, that z-scans in area II [Fig. 8(b)] do not show a clear peak due
to the bleaching behavior which is triggered by focusing onto the surface.

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Fig. 8. Optical characterization of the three pre-etched areas: (a) unbiased O2 , (b)Ar/SF6 /O2 , (c) biased O2 . For further information, see text.

7.3. Details on pillar fabrication
7.3.1. Technical details on electron beam lithography

Diamond plates immobilized on a silicon chip and spin coated with negative tone, HSQ-based
resist FOX-16 were inserted into cold-cathode SEM (Hitachi S45000), equipped with RAITH
Elphy software. The electron beam is calibrated on a Si chip containing nanoparticles and thewriting process is performed at 30 kV acceleration voltage and 20 µA extracting current. The
beam current was measured always prior the lithography session and applied as correction for thewriting dose in the software. The z-distance was kept constant at 15.3 nm for 400x400 µm fields.
The writing process occurs at 100x magnification. The writing mode for long structures is kept
longitudinal and for small structures concentric. The area dose is kept as 1400 µAs/cm2 . The
dose was established to be 2.24 mC/cm2 for the pillar structures. Varying the dose in the electron
beam lithography, influences the shape of the resulting nanopillars as well as their diameter.
7.3.2. Details on pillar etching

Areas I, II and III were divided into three sub-parts each. We fabricated various pillars masks
using a variety of EBL parameters out of HSQ that were treated with various oxygen and argon
plasmas to elucidate the effects of DC bias, RF/ICP power and gas flux on the geometry of thepillars.
In order to remove the adhesive layer used at the SCD/HSQ interface, a specialized SF6 -based
etching plasma was applied prior to etching the SCD pillars for 5 seconds. Figure 9 showsdetailed scanning electron microscope images of etched SCD nanostructures. Figure 9(a) shows
large pillar fields as an indication of the reliability of the employed nanofabrication process.
Figure 9(b) gives examples of nanopillars written with different EBL doses and a nominal
mask diameter of 180 nm. As clearly discernible from Fig. 10, increasing the dose increases
the effective mask size and consequently the top diameter of the pillar. All SEM images wereacquired at 20 kV acceleration voltage without the use of conductive layer. The mask erosion
and resulting micromasking was especially visible with use of pure Ar plasma probably due tosputtering effects and we observed the generation of needles and damaged surface. This effectwas lowered, while mixing argon with other gases (SF6 , O2 ) and totally removed by using only
biased oxygen plasma for the etching. We achieved the best control over the pillar shape using a

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O2 plasma at 750 W ICP power and 50 W RF power, 90 ±5 nm etch rate, 2.2 mC/cm2 writing
dose for the FOX-16 mask (compare Table 2).

Fig. 9. SEM images of SCD nanopillars: (a) overview image of large pillar fields etched
into the diamond by O2 plasma (b) High magnification scanning electron microscope (Jeolarm200, 20 kV acceleration voltage, no conductive layer applied).

Fig. 10. Correlation between the EBL dose and the resulting top diameter of the nanopillars.
The investigated pillars have been etched using a pure O2 plasma with 500 W ICP power
and 50 W RF power.

7.4. Charge state detection
To further study the influence of the different pre-etches on single NV− centers, we investigate
the switching between NV− and NV0 for our centers according to [43–45]. Typically, thisVol. 9, No. 12 / 1 December 2019 / Optical Materials Express

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Table 2. Parameters for different etch plasmas. Due to excess mask erosion etch rates of the pureAr plasmas have to be considered coarse estimates.
PlasmaDC biasRF power (W)ICP Power (W)etch rate (nm/min)

O2 /Ar

191

50

500

–

O2

170

50

500

65

Ar

149

50

500

20

O2 /Ar

157

50

750

65

O2

144

50

750

90

Ar

119

50

750

10

measurement shows two Poisson distributions whereas the distribution at low count rates
corresponds to NV0 and the second one to NV− PL. We here find one distribution broader thanusual [see Fig. 11(a)]. This could be explained by the following: First, both distributions arejoint hindering a clear distinction of both distributions [see Fig. 11(b)]. This could be mainly
arising from enhanced background PL. This enhanced background might arise from residuals
that were created during repeated cleaning of our sample before the charge state investigations.
Second, these residuals could also cause a change of the electrical environment of the NV
centers potentially increasing the charge state switching rates drastically. When this rate exceeds
1/read-out time, both distributions start to merge and eventually become one distribution centeredaround the mean PL of both charge states [see Fig. 11(c)]. Due to the limited amount of photons
we detect under weak excitation with the 594 nm laser, we are not able to optimize our read-out
time for fast charge state switching rates.

Fig. 11. (a) Exemplary measurement of the charge state distribution we obtain on the NV
centers in the nanostructures. Here, we use the following parameters: 532 nm laser: 1 ms at
100 µW, 594 nm-laser: 1 ms at 15 µW, read-out time: 1 ms. While using excitation powers
ranging from 1 µW to 250 µW for both lasers and integration times up to 25 ms, the meanvalues of the distributions are just shifting without a significant change of the distribution.
(b-c) Calculated photon statistics for illustrating the behavior for special cases described in
the text.

To further explain the behavior of the charge state detection measurement, we first have tobriefly introduce the photon statistics model used to describe the process. Here, we follow thederivation of [44,45].
By allowing the charge state of the NV center to switch during the measurement, the resulting
photon probability changes from two pure Poissonian distributions to a more complex distribution
which involves a sum over an infinite number of Poisson distributions weighted by the probability
for switching the charge state [44,45]. This leads to a complex behavior:First, we describe the case of the Poissonian distributions lying closely together. This belongs
to the case of a strong background PL or a weak NV− PL as shown in Fig. 11(b). Both panels

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in Fig. 11(b) show the same mean count-rate of the background and NV− , respectively, withdifferent read-out times (1 ms in the lower panel, 20 ms in the upper). Here, it is clearly visiblethat even by significantly increasing the read out time of the charge state detection it is notpossible to distinguish two distributions when the difference of background PL and NV− PL is
too low. Note, we use a negligible charge state switching rate for this calculation.
Second, we describe the case of a fast switching between the charge states of the NV centercompared to the read-out time. This is shown in Fig. 11(c). Here, we have two clearly
distinguishable distributions for the pure NV charge states (blue for NV0 and olive for NV− ). By
calculating the complex photon statistics with ionization rates exceeding 1/read-out time, we
get the black curve. This shows that even distinguishable mean photon rates can result in an
indistinguishable photon distribution in the case of fast charge state switching.
Funding
Bundesministerium für Bildung und Forschung (FKZ13N13547).
Acknowledgments
We would like to acknowledge Dr. Matthias Schreck and Wolfgang Brückner (Augsburg
University, Germany) for performing nitrogen ion implantations, Dr. Ing. Sandra Wolff (TU
Kaiserslautern, Germany) for help with electron beam evaporation, Jörg Schmauch (Saarland
University, Germany) for his help with acquiring high-quality SEM images and Dr. Rene Hensel(INM, Germany) for granting access to the ICP RIE reactor. We acknowledge Michel Challier
for his assistance.
Disclosures
We note that the nanofabrication method applied in this study is filed for a patent, application
number: EP19198772.6. The authors declare no conflicts of interest.
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6. Nanoscale sensing based on
nitrogen vacancy centers in single
crystal diamond andnanodiamonds: achievements andchallenges
Mariusz Radtke, Ettore Bernardi, Abdallah Slablab, Richard Nelz, and Elke Neu

Nano Futures, in press (2019)
doi: 10.1088/2399-1984/ab5f9bThis article is a topical review.

The above mentioned manuscript is a review article summarizing the recent progress on
manufacturing and structuring diamond as well as achievements and challenges in NV
sensing. In addition, it gives an outlook on recently developed applications.
In contrast to standard contributions to a review, Richard Nelz (R.N.) contributed theresults of a proof-of-principle experiment which strongly support one novel approach tohigh-resolution imaging using color centers outlined in the review: R.N. investigated theinteraction of NV centers with graphene akes in their proximity to show the occurrenceof FRET-like processes for NV centers in single crystal diamond. Furthermore, the experimental data strongly supports the applicability of NV centers as multifunctional sensors
simultaneously undergoing FRET processes but retaining their spin-properties and magnetic sensing capabilities. In addition, R.N. contributed to the development of a model todescribe the interaction of an ensemble of shallow NV centers with graphene. R.N. summarized all information necessary to integrate the experimental results in the manuscriptand designed the corresponding gure.
during writing and revising the review.

He actively contributed to all major discussions

Nanoscale sensing based on nitrogen vacancy centersin single crystal diamond and nanodiamonds:
achievements and challenges
M Radtke1 , E Bernardi1 ‡, A Slablab1 , R Nelz1 , and E Neu1
1

Faculty for natural sciences and technology, Physics, Saarland University, 66123
Saarbrücken, Germany
Abstract. Powered by the mutual developments in instrumentation, materials andtheoretical descriptions, sensing and imaging capabilities of quantum emitters insolids have significantly increased in the past two decades. Quantum emitters insolids, whose properties resemble those of atoms and ions, provide alternative ways
to probing natural and artificial nanoscopic systems with minimum disturbance andultimate spatial resolution. Among those emerging quantum emitters, the nitrogenvacancy (NV) color center in diamond is an outstanding example due to its intrinsic
properties at room temperature (highly-luminescent, photo-stable, biocompatible,
highly-coherent spin states). This review article summarizes recent advances andachievements in using NV centers within nano- and single crystal diamonds in sensing
and imaging. We also highlight prevalent challenges and material aspects for different
types of diamond and outline the main parameters to consider when using color centers
as sensors. As a novel sensing resource, we highlight the properties of NV centers
as light emitting electrical dipoles and their coupling to other nanoscale dipoles e.g.
graphene.

‡ Present address: Istituto Nazionale di Ricerca Metrologica, Strada delle cacce 91, 10137 Torino (To),ItalyNanoscale sensing based on nitrogen vacancy centers

2

1. Introduction
During the past two decades, color centers in diamond emerged as sensors for various
relevant quantities like electric, magnetic and optical near fields as well as local
temperature and strain [1–5]. As point defects with tightly-localized electrons, color
centers in diamond resemble atomic-scale quantum systems. Thus, color centers open
up routes towards nanoscale sensing technologies in which the spatial resolution is nolonger limited by the sensor’s volume [6]. However, ultimate spatial resolution will only
be attainable using individual defect centers. This approach simultaneously limits theintensity of the photoluminescence (PL) light which is mainly being used as the sensor
read-out. Consequently, sacrificing ultimate spatial resolution, color center ensembles
can provide enhanced sensitivity, speed up measurements and lower experimental
complexity. In addition to being atomic-scale sensors, color centers are highly-sensitive,
potentially quantum-enhanced sensors providing coherent superpositions of spin states
as sensing resource. Diamond as a host material has a multitude of outstanding
characteristics ranging from high optical transparency, to mechanical hardness andchemical inertness. Diamond also has the rare property that its crystal lattice isnaturally almost free of nuclear spins as the most abundant carbon isotope (12 C,
98.9%, [7]) has nuclear spin zero. Consequently, the lattice has low magnetic noise(given that the concentration of paramagnetic impurities like e.g. nitrogen is low) andelectronic spins of color centers can have long coherence times reaching the millisecond
range even at room temperature. However, diamond’s high refractive index renders
light extraction from color centers challenging and often demands structuring diamond
to yield photonic structures or to use nanodiamonds (NDs). Due to this fact, the fields
of diamond photonics and nanofabrication are strongly connected to sensing using color
centers.
In 1997, the first observation of spin manipulation of an individual negativelycharged nitrogen vacancy (NV) center was reported [8]. In 2000, stable, roomtemperature single photon emission from individual NVs added to their potential for
quantum technologies [9]. The NV defect consists of a nitrogen atom replacing a carbon
atom and a vacancy on a neighboring lattice site. For simplicity, NV center refers to thenegatively-charged defect throughout this review. Since then, other defects have beencharacterized as single emitters. Among these novel defects there are several groupIV-based centers namely silicon vacancy (SiV) [10], germanium vacancy (GeV) [11],tin vacancy (SnV) [12] and even lead-related defects [13]. In contrast to NV centersthat show an almost 100 nm broad emission band, group IV-based defects concentratealmost all PL into their only a few nanometer wide zero phonon lines (ZPLs) even atroom temperature. Temperature dependent shifts of these ZPLs have been exploited
for all optical temperature sensing (see e.g. Refs. [14, 15]). Despite recent progressin sensing using group IV defects, negatively-charged NVs remain outstanding in thefield of sensing. Their superior usability for sensing applications stems from the factthat the NV center’s electronic spins can be optically polarized using convenient, non-Nanoscale sensing based on nitrogen vacancy centers3resonant excitation at room temperature while they can be manipulated using microwave
radiation in an easily accessible frequency band (2.88 GHz). Internal population
dynamics of the NV center lead to different PL intensities for different spin states.
Consequently, the spin state can be read out using this PL difference (optically-detectedmagnetic resonance, ODMR). However, sensing using NV centers is not limited to using
the spin degree of freedom: Potential sensing resources include the NV’s charge state,
modulations of excited state lifetimes and absorption. The latter two exploiting the
dipolar nature of the NV center’s transitions.
In this paper, we will review recent advances in sensing with NV centers. We willsummarize main breakthroughs and drawbacks in the field. The review is structured
as follows: Section 2 presents basics of diamond material synthesis and structuring forsensing applications. Section 3 starts with a short introduction of NV properties andthen details the main parameters characterizing NV sensors. In Section 4, we summarize
the spatial resolution attainable with NV sensors in different sensing modes. Section 5summarizes selected applications to illustrate recent progress using NV sensors.
2. Manufacturing and structuring of nano- and single crystal diamonds
2.1. Manufacturing of diamond
The major factors influencing diamond’s properties and its potential applications are
purity and crystallinity. The following retrospective summarizes recent developments
within diamond growth and synthesis according to those parameters. In theory, diamond
is a crystal consisting only of sp3 hybridized covalently bound carbon atoms, whichbuild a repetitive unit cell made of fused hexagonal chair cyclohexane conformations
as depicted in Fig. 1 [16]. The Hermann Mauguin notation classifies diamond into the
Fd3m-07h face centered cubic group. The unique arrangement of carbon atoms within
diamond makes it not only the hardest material in Mohs scale, but also causes it to
be inert towards chemical modifications without use of harsh conditions. Diamond’s
wide bandgap (indirect gap 5.45 eV) results in optical transparency for light up to≈230 nm. The naturally almost spin free lattice of diamond (12 C, 98.9%, I=0, [7])has been engineered by isotopically purifying diamond (residual 13 C, 0.3%) as presented
in Ref. [7].
There are many types of diamonds and their classification is based on several factors.
One classification system has its foundation in the origin of the diamonds: naturally
occurring diamonds, diamond created via synthetic hydrothermal high-pressure hightemperature (HPHT) protocols, shock wave synthesis (e.g. through detonation) andlow pressure pyrolysis of hydrocarbon gases in chemical vapor deposition (CVD). The
latter includes microwave assisted CVD (MWCVD) and hot filament CVD (HFCVD)
deposition, whereas MWCVD is most suitable to grow high purity diamonds [19].
Fig. 2 summarizes this classification. For CVD diamond growth, the most common
precursor gases are methane CH4 and hydrogen H2 . The detonation method only

Nanoscale sensing based on nitrogen vacancy centers

4Figure 1. (a) Structures of nanodiamond supercells from various crystallographic
axes. Reprinted with permission [17]. (b) Diamond phase diagram based on [18], Courtesy of AGU.
forms nanodiamonds (NDs) of about 5 nm size [20]. Figure 1 summarizes thetemperature/pressure regimes for diamond synthesis through shock wave, metastable
CVD diamond phase generation or HPHT protocols. In both HPHT and CVD synthesis,by tailoring the additives in the reaction chamber (e.g. insertion of N2 ), diamondswith various color centers are obtained. For detonation diamonds (DNDs), creating
luminescent NDs is challenging. However, recently density control of NV centers in
5 nm-sized DNDs was reported [21]. Usually DNDs require special post-treatment toremove graphitic sp2 layers, e.g. through immersion in a boiling mixture of oxidizing
mineral acids (HNO3 , H2 SO4 and HClO4 ) and thermal annealing in vacuum or underoxygen for adequate stabilization of spectral properties [22]. Various synthesis methods
generate diamond with tailored properties. For example DNDs are commonly used asabrasives in industry, while HPHT and CVD (especially single crystal) diamonds areoften more suitable for quantum applications.
For HPHT and CVD methods, diamond growth is typically initiated on a substrateor seed. For CVD growth, this can be attained by directly growing on a diamond(homoepitaxy) or via using a non-diamond substrate (heteroepitaxy) which needs tobe prepared suitably. Substrate pretreatments for CVD span from seeding with NDs
to grow polycrystalline diamond to creation of nuclei using bombardment with carbon
ions (bias enhanced nucleation for heteroepitaxy) [23]. Homoepitaxy limits the sizeof grown SCDs due to the size of available substrates. This can be overcome using
heteroepitaxy [23].
The smallest possible seeds for diamond growth are so called diamondoids.
Diamondoids mainly combine one or more adamantane molecules (C10 H16 ), the smallest
unit cage structure of the diamond crystal lattice. Using the smallest possible
seed is among other reasons motivated by minimizing the influence of the seed on
the resulting ND’s properties. Refs. [16, 24] demonstrate the synthesis of nanoscale’diamond molecules’ (higher diamondoids, e.g. tetramantanes, less than 1 nm in size).
Diamondoids were recently employed to enable the growth of high quality NDs in a
HPHT process [25] and in a CVD process [26]. Ref. [27] reports that diamondoids serve
as nuclei in CVD if they contain more than 26 carbon atoms. Another approach toNanoscale sensing based on nitrogen vacancy centers5

Figure 2. Classification of diamond, details see text.

molecular sized diamonds is the use of meteoritic diamonds [28]. For this material class,fluorescent SiV centers have been observed in NDs with only 1.6 nm size. However,this results has never been obtained using man-made NDs, thus significantly lowering
applicability. A novel route to manufacturing an extreme diamond nanomaterial is therecently observed transformation of a bilayer of graphene into a single layer of (fluorine
terminated) diamond [29].
The second system that classifies diamond, in addition to the origin of the diamond,is based on the crystallinity. Here diamonds are classified as single crystal (SCD),
polycrystalline and (ultra-)nanoscrystalline diamonds according to their crystallite size.
For grain sizes below 10 nm the diamond is typically called ultrananocrystalline, between
10 nm and 50 nm nanocrystalline, and from 50 nm to 500 µm micro- or polycrystalline.
If the crystallite size exceeds 500 µm, the diamond is termed single crystalline and a
specified crystallographic growth plane can be assigned [30].

Nanoscale sensing based on nitrogen vacancy centers

6

Figure 3.
Timescale in period between 2013-2018 showing the evolution of
the searching phrase "single crystal diamond growth", based on the database
WorldCat.org.

The amount of publications detailing the phrase ’single crystal diamond growth’in WorldCat.org. in the time period between 2013-2018 yields 3.575 results (Fig. 3),
illustrating the significance of the field.
Free standing wafer-sized polycrystalline diamond films are currently commerciallyavailable with diameter up to 140 mm (Element Six, UK). Recently, Ref. [23] presented a
mm-thick SCD wafer with 98 mm diameter grown by heteroepitaxy. The basic usabilityof this material for sensing with NV centers has been demonstrated [31].
The most common growth direction in CVD is (100). However, for many
applications it is advantageous to control the orientation of NV centers with respect
to the SCD surface. Thus, alternative growth of (111) [32] diamond and (113) [33]has been realized that orients the dipoles of NV centers more optimal with respect tophotonic structures manufactured into the SCD surface [34]. While single SCD allows
for the highest purity material with the lowest background PL, poly- and nanocrystalline
forms will always suffer from broadband PL occurring at grain boundaries [19].
The third classification has been historically established for natural diamonds andclassifies them according to their optical absorption [35]. Figure 2 summarizes thisclassification. Type IIa diamonds were classified by a lack of optical absorption [35], so
this were the purest diamonds in this classifications. The absorption of type I diamond
is nowadays assigned to nitrogen [36]. Type Ia diamonds contain nitrogen in aggregatedform; while in type Ib diamonds, it is present in its isolated form substituting single
carbon atoms (substitutional nitrogen) [36]. Most CVD diamonds are of IIa type, while
HPHT diamonds are often type Ib [37]. In contrast, type I diamonds are known tooccur most commonly in nature. Type IIb contains boron in small quantities and has
p-doped character. This historical classification doesn’t sufficiently classify diamonds

Nanoscale sensing based on nitrogen vacancy centers7

typically used for sensing with color centers: These diamonds are mostly so purethat they are all class IIa but need further classification. So typically, the amountof substitutional nitrogen and boron is directly given. Pushing purity to the limit,electronic grade diamonds with less than 5 ppb nitrogen and less than 1 ppb boron are
nowadays commercially available from ElementSix and are the basis for most sensing
experiment especially using individual NV centers. In addition, 12 C isotopically-purified
diamond has been demonstrated and termed quantum grade diamond [7].
When impurities in diamond are present in high concentration, they induce acoloration of diamonds. For sensing, typically low concentrations of defects are used,which does not necessarily lead to a coloration of diamond.
Color centers can be introduced into diamond using two approaches. The first
approach creates color centers in the growing diamond via using selected precursors[38–40]. Often NV centers are also created due to background nitrogen in the process[34, 40–42] . However, for sensing applications it is important to carefully control color
center density and depth below the diamond surface (see also Sec. 4). Creating thinlayers containing NV centers has been attained in diamond CVD using δ-doping (e.g.
Ref. [38, 43]): in a CVD process with low growth rate, nitrogen gas is introduced fora short time thus inducing an only 6 nm thick nitrogen-doped layer [38]. Moreover,nitrogen doped layers were obtained by overgrowing nitrogen-terminated diamondsurfaces [44]. Using δ-doping, conversion of nitrogen impurities to NV centers is often
fostered via irradiation e.g. using helium ions [45]. One advantage of forming color
centers directly during diamond growth is that they can show preferential alignment:
instead of aligning along all equivalent directions in the crystal, color centers only form
with one orientation leading to a perfectly aligned ensemble of centers (see Refs. [39–41]).
Moreover, NV centers created using conversion of grown-in nitrogen to NV centers havebeen found to have superior properties compared to centers arising due to implanted
nitrogen [46, 47].
The second approach creates impurities after diamond manufacturing has finished.
It utilizes irradiation (by electrons, neutrons or protons [46, 48]) and ion implantation[49–54]. It allows to control the fluence, which determines the density and the energy(ranging from keV to MeV) to set the implantation depth [55]. The creation of color
centers can be also controlled in lateral dimension by implanting through a pierced AFM
tip [56], using nanomasks [57] or focused ion beams [58]. Ion implantation in diamondrequires post-treatment annealing (mostly at temperatures exceeding 600◦ C) in order toinduce vacancy migration, repair implantation damage and form color center complexes
like NV centers.
2.2. Nanostructuring of diamond
To efficiently extract color center PL from diamond, photonic structures e.g. nanopillar
waveguides [34, 59], optical antennas [60], solid immersion lenses [61], bulls eye gratings[62] or pyramids [63–65] are highly-desirable (for reviews on diamond photonics, seeNanoscale sensing based on nitrogen vacancy centers

8

Refs. [66–69]). Figure 4 displays some examples of diamond nanostructures. Nanopillar
waveguides channel the light into conveniently collectible emission angles (NA <0.95)
and circumvent total internal reflection [70]. Here, the high refractive index of diamond(2.4) aids in confining the light in the waveguide. In contrast, for unstructured diamond,it limits the collection efficiency to only a few percent [71]. While optical antennas, inprinciple, allow up to unity collection efficiency [60], tip-like structures as pillars or
pyramids might simultaneously serve as a tip for a scanning probe microscope [72] thus
enabling to scan a color center in close proximity to a sample under investigation. It
is noteworthy that nanopillars have been produced incorporating not only NV centers[34, 70, 73] but also SiV centers [59] and lead-related defects [74]. Additionally, cavity
structures like one or two-dimensional photonic crystals [56,75] or microdisk (whispering
gallery mode) resonators [76] allow the coupling of color centers to cavity modes withhigh Q-factor. Mostly, photonic structures are used to extract light from single color
centers [59]. However, very recent studies also indicate the usefulness of pillar arraysfor sensing with NV ensembles [73].
To harness the full potential of photonic structures, nanofabrication processes must
be carefully controlled: First, the shape of the nanostructure will strongly influenceits performance e.g. changing the taper angle of a nanopillar [see Fig. 4] changes theefficiency of light extraction [71, 77]. Second, nanofabrication, especially on example ofplasma etching, might damage diamond surfaces [78]. This damage was estimated toreach several nanometers into the diamond and found to deteriorate the properties ofshallow color centers [79]. Reduced damage and enhanced properties of NV centers havebeen reported by using plasmas with zero bias voltage and thus minimal accelerationof ions in the plasma [79] and with careful combinations of etching processes [46]. As
diamond is highly chemically inert, wet chemical etching methods are not an alternative
to plasma-based processing. Focused ion beam (FIB) milling leaves gallium impurities
in the diamond that need to be removed using sophisticated post-processing [80]. So
the most common processing techniques utilize standard nano-lithography techniqueswith electron beams, laser or 2-photon lithography (2PP) for mask generation andtransfer the patterns into diamond using reactive ion etching (RIE), mostly in theform of inductively coupled reactive ion etching (ICP-RIE) [81, 82]. Plasma-etchingof diamond requires special gases as well as resists for the masks (negative and positivetone). Most commonly oxygen/argon or chlorine-based mixtures are used. Masks areeither based on PMMA with hard mask techniques (Al, Al2 O3 , W) or directly on thenegative tone resist hydrogensilsequioxane (HSQ) [34, 83].
Nanostructuring of diamond can be classified into traditional nanotechnology
approaches: bottom up and top down approaches. For example nanodisc resonatorswith quality factor exceeding 300000 may be generated by top-down techniques involving
electron-beam lithography, laser lithography and ICP-RIE [76].
In bottom-up approaches, nanostructures are directly formed during diamond
growth thus potentially forming many structures at the same time and avoiding potential
damage due to plasma-etching [64, 84]. However, top-down approaches have theNanoscale sensing based on nitrogen vacancy centers9Figure 4. Current state of the art examples of successful nanofabrication performedon diamond. Fabricated structures include waveguides and cavities. As examples ofnanowaveguides, part (a) shows nanopillars (reprinted with permission from Ref. [77])and part (c) displays pyramids (reprinted with permission from Ref. [63]). As examplesof nanocavities, part (b) shows a microdisk cavity (reprinted with permission from Ref.
[76]), whereas part (d) shows nanobeams (reprinted with permission from Ref. [75]).

advantage, that color centers can be created while the diamond is still in its bulk form,
which eases annealing and cleaning followed by fabrication of the nanostructures (e.g.
Ref. [72]).
3. Achievements, improvements and challenges in NV sensing
The strengths of sensing using color centers in diamond are its spatial resolution and its
sensitivity (for a tutorial on NV magnetometry see e.g. Ref. [85]). Moreover, dependingon the application, it is important to consider in which modality the diamond sensor
should be operated. This especially includes the host diamond’s geometry (bulk, singlecrystal diamond (SCD), nanostructures or nanodiamonds (NDs)) and how the color
center is brought into interaction with the sample (e.g. NDs entering cells, SCD tip
scanning over the sample, sample transferred to SCD surface). Moreover, a decisionneeds to be rendered if single color centers should be used or if ensembles are moresuitable. Ensembles can provide better sensitivity whereas single centers, in combination
with scanning probe operation, provide ultimate spatial resolution. In the following, we
give a very short introduction to the physics of NV centers. We are not aiming forcompleteness nor we will describe any theoretical background here but just introduce
the basic level scheme and effects needed to follow the discussed sensing topics. For
details on the NV center see e.g. Ref. [86].
3.1. NV Basics
The NV center consists of a nitrogen atom that substitutes a carbon atom in the
diamond lattice and a neighboring vacancy. Negatively-charged NV centers display
a system of singlet and triplet levels. A strong optical transition (lifetime 12.9 ns, [87])Nanoscale sensing based on nitrogen vacancy centers

10Figure 5. Basic level scheme of the negatively-charged and the neutral NV center.
Explanation see text.

occurs between the 3 A2 ground state and the 3 E excited state. This transition leads tothe zero-phonon-line (ZPL) at 637 nm at room temperature. Due to phonon assistedtransitions, the narrow ZPL is accompanied by a broad phonon sideband spanningalmost 100 nm spectral width [88]. The 3 A2 → 3 E transition, which induces the visible
photoluminescence (PL) of the NV center, is represented by two orthogonal transition
dipoles. These dipoles are placed in the plane orthogonal to the NV’s high-symmetry
axis that connects the nitrogen and the adjacent vacancy. This axis is oriented along
one of the equivalent [111]-directions of the diamond lattice [89, 90].
In the triplet ground state of NV− , three spin states with ms =-1, 0 and +1 areobserved. Between the ms =0 level and the degenerate ms =-1,+1 levels a zero-field
splitting of 2.87 GHz occurs. These levels show spin-dependent PL: A NV center inthe ms =0 cycles between 3 A2 and 3 E and produces bright PL. In contrast, a NV centerin the ms =-1 or +1 state undergoes an intersystem crossing to the singlet states withhigher probability. As the center will reside in these states for typically around 200
ns [91], the visible PL of the center is reduced. The PL difference between the two spinstates under continuous, non-resonant laser excitation (e.g. using 532 nm laser light) can
be up to 20% [2]. Simultaneously, non-resonant laser excitation polarizes the NV center
into the ms =0 state. Following this initialization, transitions between the spin levels
can be driven using microwaves.
Between their 1 A1 and 1 E levels, NV centers display a transition in the infrared
spectral range (1042 nm) [88]. As a result of non-radiative processes, the PL due tothis transition is typically four orders of magnitude weaker than the visible PL [92].
Nevertheless, transitions in the singlet system are a valuable resource for sensing (seeSec. 3.2).

Nanoscale sensing based on nitrogen vacancy centers

11

If one electron is removed from the NV− center, it converts to a neutral NV0 center.
This occurs during optical excitation of NV− when a NV− center which resides in the
excited state absorbs an additional photon. As a consequence, an electron is excited
to the conduction band and via an Auger process, a free electron is created, while thecenter converts to NV0 [93]. NV0 centers display a ZPL at 575 nm, thus, light withwavelengths shorter than 575 nm is needed to excite NV0 . As a consequence, light withwavelengths between 637 nm and 575 nm only excites NV− but not NV0 . This can beused to detect the charge state of NV centers [42, 94] even without changing the charge
state: only if the center is NV− , under excitation with e.g. 594 nm light PL occurs,
while no PL is observed in the NV0 case. To convert NV0 back to NV− it is necessary
to excite NV0 : If an electron from the lower orbitals of NV0 is excited, an electron fromdeep lying orbitals in the valence band can be promoted to the NV0 and re-charge thecenter to NV− [93]. It should be noted that the NV charge state in an ensemble canstrongly depend on previous laser excitation of the NVs [95].
3.2. Sensitivity
Sensing using NV centers can be classified into three broad categories, considering thephysical quantity to be detected and the degree of freedom of the NV used:• Detection of the (electronic) spin resonance frequencies (magnetometry, temperature sensing, conductivity measurement)• Detection of NV PL intensity and excited state lifetime (near-field energy transfer,FÃűrster-Resonance Energy Transfer (FRET) imaging, near field sensing)• Detection of the charge state of NV centers (sensing of electrostatic environment)
Measurements based on the spin degrees of freedom are most mature, while nearfield based sensing and using the NV charge state for sensing are rather novel. Regarding
electronic spin-based sensing, we introduce the sensitivity η for DC magnetometry [2]:
h ∆ν
√
(1)
gµB I0 C
where I0 is the NV PL rate, C is the ODMR contrast, ∆ν the linewidth of the ODMR
resonance, g the Landé factor and µB the Bohr magneton. Considering equation (1),improving sensitivity can be obtained by:η=• Increasing I0 via increasing the collected photon rates or the collection efficiency
for NV PL [2, 34, 39, 40, 60, 62, 96–98]• Decreasing ∆ν, which is connected to increasing the coherence time T?2 [43, 44, 49–
52, 79]• Increasing the efficiency of the spin readout (in Eq. 1 represented via C) [50,94,99–
105].
Equation (1) is valid for a single NV,√ if we use an ensemble including N NV centers,
the sensitivity improves by a factor of N . Indeed, to date, optimal sensitivities in the

Nanoscale sensing based on nitrogen vacancy centers
12√pt/ Hz are reached using ensembles of N ∼ 1011 NVs [48]. For a recent review focusing
on enhancing the sensitivity of NV ensemble-based sensors see Ref. [106].
For sensing schemes based on the NV charge state and FRET-based sensing andimaging, it is much more challenging to define a sensitivity. However, due to opticalread-out, enhancing photon collection will enhance sensing capabilities. For FRETbased sensing, the quantum efficiency QE of the NV transitions, that is the probability
for a radiative transition between excited and ground state, also plays a major role.
Recent work indicates that the QE for shallow NVs, required for FRET-based sensing(4.5 nm below surface), amounts to 0.7 while defects deeper in the diamond reach QE
of 0.86 [53].
In the following, we review the above mentioned methods and the main parameters
influencing their sensitivity.
3.2.1. Collection efficiency As introduced in Section 3.1, the PL of NV centersoriginates in a semiclassically description from two emitting dipoles in the planeorthogonal to the NV axis. Two main factors limit the PL collection from NV centers
in SCD:• Refraction effects at the SCD-air interface. Due to the high refractive index of
diamond (2.4), a significant fraction of the PL undergoes total internal reflection
limiting the collected fraction of PL often to a few percent (see also Section 2.2).
• Dipole orientation and directionality of NV radiation: Optimal collection of PL
is obtained for dipoles parallel to the SCD surface [34]. However, the mostcommon CVD SCDs have (100) surfaces. Consequently, the NV axis lies underan oblique angle (54.7◦ ) to the SCD surface, leaving the NV’s dipoles in a nonoptimal orientation. Consequently, a fraction of PL even if it escapes the SCD
might not be collected because it is emitted into angles that exceed the numericalaperture of the collection optics.
Optimal orientation of NV dipoles can be obtained using SCD with (111)-oriented
surfaces [34]. (111)-oriented CVD growth can even provide NV centers that all alignalong one [111] direction, namely the growth direction of the crystal [39–41]. No
NVs in other equivalent directions are formed and a perfectly aligned ensemble canbe created. As (111)-oriented CVD growth is technically very challenging, researchers
also investigate similar growth directions like (113) that allow for an almost optimal
orientation of NV centers [33].
Regarding refraction effects, PL collection can be improved further via shaping
SCD into nanostructures as reviewed in Section 2.2.
3.2.2. Coherence time Shallow NVs typically used for sensing, present degradedcoherence properties compared to NVs well below the surface [107,108]: Their coherence
times T2 and T?2 reduce, leading, among other drawbacks, to an increased ODMR

Nanoscale sensing based on nitrogen vacancy centers

13

linewidth ∆ν and reduced sensitivity for DC magnetic fields (see Eq. 1). The main
sources of noise leading to the reduced coherence are:
• Surface modified phononic-coupling [108]• Surface electronic spin bath [108, 109]

• Paramagnetic defects due to implantation [51, 52]
Methods to improve coherence in shallow NVs are e.g. reviewed in Ref. [110].
As shallow NVs are often created using low energy (keV) ion implantation, noise dueto the implantation process is crucial. Paramagnetic defects responsible for magnetic
noise are vacancy clusters or chains created during post-implantation annealing of
the diamond [49, 52]. Ref. [49] identifies the neutral di-vacancy complex V2 as themain source of decoherence and avoids its formation via charging the vacancies in thespace charge larger of a p-i-junction using boron-doped diamond [49]. Other work
employs annealing at elevated temperatures to anneal out vacancy complexes [51, 52].
Alternatively, shallow NV centers can be produced using the δ-doping technique (seealso Section 2.1). The subsequent creation of vacancies e.g. using a focused electron
beam followed by annealing, introduces in general lower levels of magnetic noise andenhanced coherence times have been demonstrated [111].
The noise due to the surface of the diamond strongly depends on the pre-treatmentof the surface as many functional groups can be present on the surface [20] as well as e.g.
damage due to plasma treatment. Improvement of single NV coherence was observed
after wet oxidizing chemistry and annealing in oxygen environment [50], however, othergroups were not able to reproduce the enhancement for shallower NVs [112]. Considering
plasma treatments, Ref. [79] shows increased coherence times for NVs below surfacesthat have been treated using an oxygen plasma with low acceleration of ions towards
the etched diamond (low-bias, soft plasma).
3.2.3. Readout and charge state For most sensing experiments, PL-based read-out of
the NV electronic spin is performed (ODMR, see Section 3.1). Though being technicallysimple and versatile, this method has its limitations in the fact that the read-out laser
re-polarizes the spin to the ms =0 state and low collection efficiencies render single-shotread-out of the electronic spin impossible at room temperature.
Improving ODMR-based spin read-out is feasible using:• Repetitive readout [50]• Phase sensitive (lock-in) methods [104, 105]

• Noise reduction by mutual referencing of discrete readout windows [99]For a repetitive read-out, the NV electronic spin state is transferred into the nuclear
spin state of 14 N. In contrast to the electronic spin, the nuclear spin can be repeatedlyprobed via quantum non demolition measurements [113]. The advantage here is that
the NV state is not reset to ms = 0 during readout. Ref. [104] presents a techniquemodulating the microwave frequency used for spin manipulation or of the amplitude

Nanoscale sensing based on nitrogen vacancy centers

14of static magnetic fields; subsequently demodulating the PL response using lock-in
amplification suppresses noise at low frequency (typically below 30 kHz). Another way
to reduce noise due to uncertainties in microwave frequency or power is to subtract
PL signals retrieved during subsequent pulse repetitions [99]. Using this approach,microwave noise with a correlation time exceeding the duration of a single pulse sequenceis filtered out.
In recent years, novel approaches to read out NV electronic spins beyond ODMR
arose, which we summarize here briefly. For a more detailed review see Ref. [114]. Novel
read-out methods that partly bypass the challenge of low collection efficiency include:• Absorption of infrared (IR) light due to transitions within the NV singlet system[101, 102]
• Laser threshold-related magnetometry [115, 116]• Spin to charge conversion for more efficient spin-readout [94, 103]
• Readout based on the detection of photo-currents [117]The absorption via the singlet transition 1 E→ 1 A1 (level scheme see Section 3.1)
depends on the population of the involved levels. As these levels are mainly populated
from the ms = ±1 states in the triplet (assuming the NV is pumped e.g. using green laser
light), the IR absorption monitors the spin resonances in the triplet and consequently
allows the measurement of magnetic fields [101, 102]. To measure the weak absorption
on the 1 E→ 1 A1 transition, it is advantageous to enclose the diamond in a cavity or atleast to use a multipass configuration to enhance
the signal. Using miniaturizes Fabry√
Perot cavities sensitivities down to 28 pt/ Hz have been shown [101].
Another approach to avoid the bottleneck of low PL detection
is laser threshold√
magnetometry, which promises sensitivities down to the ft/ Hz regime [115]. In thisapproach, NV centers serve as gain medium for a laser. A change in spin state andthus in PL intensity translates into switching on and off the laser, which operates
very close to the laser threshold. However, this requires stimulated emission andpopulation inversion in the NV triplet levels. Despite the fact that stimulated emission
has been observed [118], ionization dynamics counteracting stimulated emission render
this approach challenging. Combining the previously mentioned approaches, Ref. [116]presents a hybrid laser threshold magnetometer. The laser gain is supplied by an
external laser material lasing at the 1 E→ 1 A1 transitions wavelength. The diamond isinserted into the cavity of the external laser. Enhanced absorption on the IR transition
increases the losses for the laser and stops laser emission.
In spin-to-charge conversion, the spin state of the NV center is mapped to a charge
state (NV− or NV0 ) [94, 103]. In brief, the technique is based on the fact that NV−
centers are easily ionized from the triplet excited state 3 E. In contrast, a transition to
the singlet, which is more likely to occur if the center is in the ms = ±1 state, protects
the center from ionization. Consequently, ms = 0 is mapped to NV0 and ms = ±1 ismapped to NV− . Following the ionization, the charge state is read-out as described inNanoscale sensing based on nitrogen vacancy centers

15

Section 3.1. Using this read-out method, enhanced sensing with a 25-fold increase inimaging speed has been demonstrated [103].
Each cycle of ionization and re-charging of a NV center under optical illumination
creates an electron in the conduction band which can be detected in the form ofa photocurrent if an electric field is applied using electrodes on the SCD surface.
Photocurrents in the pA are observed for a single NV center and the contrast of thephotoelectrical read-out is comparable to the ODMR based read-out [117].
3.2.4. Ensemble density and alignment of NVs for wide field imaging In wide fieldmagnetic imaging, a thin layer of NV centers is used. To enable imaging a larger sample
area, NV PL is collected using a camera. Typically, hundreds to 10000 of NV centers
contribute to the signal in every pixel [119, 120]. In this way, every pixel encodes theproperties of the local magnetic field.
Two properties of the NV layer determine the magnetic field sensitivity:• The density of NVs• The alignment of NVs
The most common way to increase the NV density is to increase the nitrogen
implantation dose. This has two effects on sensitivity:√On the one hand, it increases the
PL rate and thus improves the sensitivity scaling with N , where N is the number of NV
defects. On the other hand, it increases the density of implantation-related defects (seealso Section 3.2.2) and decreases the coherence [121]. These two counteracting effects
have been found to results in a sensitivity which can be independent of the implantationdose [51]. The spin coherence time T2 influences the range of AC frequencies to which
the NV center is sensitive [122]. Consequently, the optimal NV density is determinedby the specific needs of each sensing application. For a recent review focusing on thesensitivity using NV ensembles see Ref. [106]For wide field imaging, also the orientation of NV centers has to be taken into
account. NV centers oriented along different equivalent directions sense the projected
field onto their high symmetry axis. Consequently, for an ensemble and an arbitrarilyaligned magnetic field, 4 pairs of ODMR resonances are observed. Using these 4 ODMR
resonance pairs, the magnetic field vector can be reconstructed. However, the ODMR
contrast for each of the resonances is strongly reduced as only 25% of the NV centers
contribute to each resonance line. For high-sensitivity experiments, consequently theuse of an NV ensemble that is aligned along only one of the equivalent directions isadvantageous. Using diamond oriented in [111] direction and creating NV centers during
growth, perfectly aligned ensembles with high density (d = 1019 cm-3 ) and high ODMR
contrast, comparable to the one reported for single NVs, have been demonstrated [39].

Nanoscale sensing based on nitrogen vacancy centers

16

4. Spatial resolution in NV sensing
Using ensembles of NV centers often involves wide field imaging, where a larger diamondarea is illuminated and NV PL is detected using a camera [123–127]. This techniquetypically leads to a spatial resolution on the order of 1 µm [123–125, 127]. This spatial
resolution allows e.g. to investigate microwave devices [127], integrated circuits [125] ormagnetic features in cells [124]. In wide field imaging, the signal on each pixel of thecamera involves hundreds√ to 10000 NV centers [119,120]. DC magnetic field sensitivities
in the order of µT / Hz have been quoted [120]. Enhancing the sensitivity can beobtained via increasing the number of NV centers participating in the signal creation.
However, as discussed in Section 3.2.4, enhancing the volume density of NVs in SCD does
not necessarily enhance the sensitivity. An alternative route enhances the number of
NV centers via increasing the effective SCD volume contributing to the sensing signal:
Ref. [96] employs light trapping in a SCD that forms a multi-pass waveguide (SCDdimensions 3 mm x 3 mm x 0.3 mm to increase the number of interacting NV centers).
Ref. [101] uses NV infrared absorption in a cavity realizing a probed volume√ of 390 µm
2x 4500 µm . Both approaches √lead to sensitivities in the order of 10 pT / Hz range.
Ref. [128] demonstrates 15 pT / Hz in a volume√ of 13 µm x 200 µm x 2000 µm, while
Ref. [99] extends sensitivity even to the sub-pT / Hz using optimized diamond material
and optimized pulse sequences.
Using a single NV center for sensing, the probed volume is in principle given by
the spatial extent of the electronic wave-function of the NV center. Recent simulations
show that the electrons bound to the NV defect are closely localized within a few
lattice constants [129] rendering the NV center a truly atomic-scale sensor. However,the spatial resolution will be limited by the distance of the sensing NV center to thesample under investigation which is ultimately limited by the distance of the NV
center to the diamond surface. NV centers have been observed to be stable 2 nm
below a SCD surface [130]. In addition to this distance to the SCD surface, technicallimitations of the scanning probe imaging have to be considered which lead to an
additional stand-off distance: Using SCD scanning probes, NV-to-sample distances
around 25 nm [131, 132] at ambient conditions and distances around 10 nm in vacuumat cryogenic temperature [133] have been demonstrated yielding spatial resolutions inthe same order of magnitude[132]. Typically, scanning probe microscopy using single√
NV centers reaches√ µT / Hz sensitivity for DC magnetic fields, while sensitivities on
the order of nT / Hz have been shown for AC magnetometry [131, 132]. While thesesensitivities are significantly lower than obtained using ensemble magnetometry, one has
to take into account that the color center is placed in the high field region of the sample.
Considering that e.g. dipolar magnetic fields decay with the third inverse power of the
distance to the sample [2], the lower sensitivity is at least partially compensated by theclose proximity of single NVs to the sample in a scanning probe geometry.

Nanoscale sensing based on nitrogen vacancy centers

17

5. Selected sensing/imaging applications for each type of diamondApplications of color centers in diamond have been summarized in several recent reviews
focusing on magnetometry [2, 110], NV sensing as a probe for solid state physics [134],
single spin magnetic resonance [135] and the coupling of spins and mechanical oscillators[136]. In the following section, we select a few recent and novel applications using
different diamond types to illustrate the points discussed above.
In addition to demonstrating novel applications, a trend has been arising tominiaturize NV-based sensing devices to enable mobile use and potential industrial
applications [137]: via connecting the diamond to micro-optics (graded index lenses),
integrating microwave driving on printed circuit boards and using an LED instead ofa laser for excitation, the volume of a diamond sensing device has been reduced toless than 3 cm3 . This small footprint in combination with a low power consumption of
1.5 W allows for highly-flexible field-use of the diamond device opening the route toward
novel applications. Additionally, recent work presents the first integration of diamond
sensors into complementary metal-oxide-semiconductor (CMOS) technology, paving theway toward chip-scale diamond quantum sensors [138].
5.1. Single-crystal, bulk diamond and Förster Resonance Energy Transfer (FRET):
novel approaches to NV sensing to enhance versatility of NV sensors
While NVs in high-purity, single-crystal, bulk diamond (SCD) typically have superior
properties compared to NVs in often less pure NDs, probing sample dynamics withhigh spatial resolution requires to transfer the sample under investigation onto the
SCD surface and spatial resolution is typically limited by diffraction to around 0.5 µm.
Nevertheless, shallow NVs in SCD have been used for various applications probing
micron-scale dynamics especially in the context of novel materials and current imaging.
The almost back-action-free detection of currents in graphene, where currents in theµAmpere regime are measured with a resolution of 0.5 µm [139], and the usage ofNV centers to investigate failure of integrated circuits [125] illustrate their potential
to analyze currents on the micron scale. Very recently, NV centers measured the band
bending inside a diode like device in-situ [54]. Whereas in Ref. [140] shallow NV centers
are used as sensor to perform nuclear magnetic resonance spectroscopy of atomicallythin hexagonal boron nitride layers. Thus NV centers allow to bring the technology ofmagnetic resonance truly to the nanoscale. For probing liquid samples, Kehayias et al.
structured SCD to form a microfluidic chip and flow the substance under investigation
through the diamond device [141].
In contrast to the above mentioned applications which all rely on the spin-based
sensing capability of NVs, sensing relying on the dipolar nature of the NV center is lessadvanced. There have been first demonstrations using nearfield-based energy transfer(Förster Resonance Energy Transfer, FRET) to detect dye molecules attached to thesurface of NDs [142,143]. Additionally, a scanning ND attached to an AFM tip has beenused to image graphene flakes with nanoscale resolution [3]. Other authors reported non-Nanoscale sensing based on nitrogen vacancy centers

18

successful attempts to establish FRET using NDs potentially due to a lack of control overthe surface properties of NDs [144]. Consequently, it would be highly-advantageous tobroaden FRET-based sensing to shallow color centers in SCD. For sensing and imaging
approaches using the dipolar nature of NV centers, it is significant to consider the role
~ and Y~ as we discuss in detail in section 7. Asof the two electric transition dipoles, X
a first step toward FRET-based imaging using color centers in SCD, we here present
quenching due to graphene deposited onto the SCD surface.
We use commercial, high-purity, chemical vapor deposited, electronic grade SCD
from Element Six ([N]s <5 ppb, B<1 ppb). To create shallow NVs, we implant the sample
with 6 × 1011 cm−2 nitrogen ions (Innovion, USA). Via annealing to 800 ◦ C in vacuum
and cleaning in boiling acids (nitric, sulfuric, perchloric acid, 1:1:1), we create a layerof NV centers on average 7 nm below the SCD surface. From this clean surface, we
observe homogeneous NV PL. We estimate the number of NV centers in the laser focus
of our confocal microscope to be on the order of < 10. The NV ensemble shows aspatially consistent lifetime of 15.5±1 ns, which is slightly higher than the bulk lifetimeof 12.9±0.1 ns [87] due to a decrease in effective refractive index close to the surface.
We now apply graphene flakes from solution (pristine graphene monolayer flakes fromgraphene supermarket with an average flake size of 550 nm (150-3000 nm), dispersed inethanol). We spin coat this solution as-received with 600 rpm onto the SCD surface.
We observe a slight trend to agglomeration of graphene. While in principle not desired,these agglomerates enable to straightforwardly observe the interaction of the NV centerswith the graphene: We identify agglomerates via white light illumination of the SCD
surface as well as Raman spectroscopy revealing a G Band at 1586 cm−1 . At the positionof graphene agglomerates, we observe a reduced PL intensity [see Fig. 6(a)] as well as aclearly decreased NV lifetime [see Fig. 6(b)]. We check our results on several positions
on the sample and find typical results as shown in Fig. 6(a)-(c). From the decrease inlifetime and the graphene Förster radius of 15 nm [3], we estimate the average distanceof the graphene to the NV centers to be 19 nm which is higher than the average depth of
the NV centers of 7 nm. We attribute this finding to a strong quenching of NV centersvery close to the surface. These centers consequently do not contribute significantly
to the measured PL signal which is dominated by the NVs deeper inside the diamond.
Using the distance to the graphene, we estimate a reduction to 75% of the non-quenched
PL. We observe a stronger reduction to 50%. However, our simplified consideration does
not take into account the unknown absorption of the excitation laser as well as the NV
PL due to the graphene agglomerate. We now test if NV ensembles below graphene
agglomerates still show the typical spin properties of NV centers. To this end, werecord an ODMR of an NV ensemble [see Fig. 6(d)] below one of the agglomerates,
which shows the characteristic ODMR resonance. This result thus strongly indicatesthat NV centers can serve as multifunctional sensors, which probe the presence of other
dipoles as well as the magnetic environment simultaneously.

Nanoscale sensing based on nitrogen vacancy centers19

Figure 6. (a) PL map of the NV ensemble with graphene agglomerates (darker area)on the SCD surface (PL detected at wavelengths above 650 nm, excitation 0.7 mW at
532 nm. (b) Lifetime map of the same area like in (a). The excited state PL lifetimeτ decreases up to 4 ns under the agglomerates. (c) NV ensemble lifetime measurement
under the graphene (grey, τ =14.5 ns) and next to a graphene agglomerate (black,τ =16.4 ns). (d) ODMR measurement of the quenched NV centers under a graphene
agglomerate.

5.2. Nanodiamonds for sensing and imaging
NDs are versatile tools in the life sciences as they can enter living cells and organisms.
Their applications do not only cover sensing and imaging but also extend to drug delivery
and tissue engineering. For recent reviews on these applications see Refs. [145–152].
Color centers in NDs first serve as non-bleaching fluorophores and additionally sense
temperatures and magnetic fields inside cells. Moreover, novel approaches using the NV
center’s charge state as a resource allow to sense the electrostatic environment of theNV center [153,154] and potential changes as small as 20 mV have been measured using
this technique.
Considering electronic devices, NDs with NV ensembles enable the simultaneous
measurement of local temperature and electric current distribution [155]. These
quantities are relevant in the context of high electron mobility electronics, where
local heating occurs and potentially induces device degradation [see Fig. 7(a)]. Bothparameters can be measured in parallel as the local temperature affects the NV’s zerofield-splitting, while the local current density influences the splitting of the ms =-1 andms =1 states as a result of the local magnetic field. Despite a certain scatter in the

Nanoscale sensing based on nitrogen vacancy centers

20

Figure 7. (a) Schematic of temperature and current (magnetic field) imaging using
ND coated onto a transistor. (b) Zero-field-splitting statstics of NDs with NV
ensembles. Reprinted from Ref. [155].

zero-field-splitting of individual NDs [see Fig. 7 (b)], the relevant temperature increases
can be measured. For this applications, the ability to deposit NDs on almost all desired
substrates and devices is highly advantageous.
5.3. Nanostructured diamond: pushing spatial resolution to the limit
Nanostructured SCD especially in the form of scanning diamond pillars has beenused to image magnetic features with nanoscale resolution revealing magnetic features
like superconducting vortices [133], skyrmions [156, 157] and magnetic properties in
antiferromagnets [158]. Moreover, scanning NVs locally imaged the magnetic fieldarising due to microwave currents in a stripline [131]. As a novel field of application,
NV centers in scanning tips have been used to probe the local conductivity of metallic
structures [103] and have proven to be useful for the investigation of the magnetic
field of a hard-disk write head [159]. To avoid the need for scanning pillars, also thesample under investigation e.g. a cell can be scanned over a stationary pillar. Using
this approach, clusters of iron-storage molecules (Ferritin) have been imaged in a single
cell [160].
Recently, the discovery of ferromagnetic behavior in two-dimensional van der Waals
materials has triggered significant research interest. An example for such a material is
CrI3 . Using scanning NV magnetometry [see Fig. 8(a)], fundamental questions on themagnetism in CrI3 have been addressed [161]: The measurements show that flakes withan even number of layers show no resulting magnetization in contrast to flakes with an
odd number of layers [see Fig. 8(b)-(d)]. Due to the high spatial resolution and the
low back-action in this sensing approach, it is possible to observe direct connections
between structural and nanoscale magnetic properties of the material. Consequently,
magnetic imaging using a scanning single NV center can help to understand magnetic
two-dimensional materials which are candidates for future spintronic devices.

Nanoscale sensing based on nitrogen vacancy centers21

Figure 8. (a) Schematics of single NV scanning magnetometry (b) Optical micrographof the CrI3 flake with regions with 2 and 3 layers. (c) Magnetic stray field as imaged
using a single NV center. (d) Calculated magnetization of the flake. Reprinted withpermission from Ref. [161].

6. Conclusion and Outlook
In recent years, sensing using color centers has been applied to many novel fields ofapplications. Novel sensing and imaging modes are under development taking into
account e.g. near-field interactions of color centers. Using such near field interactions,color centers will be efficient probes for the presence of other light emitting systems
like e.g. quantum dots, molecules or two-dimensional materials [162]. Novel approaches
to read-out the spin state of NV centers have made magnetic sensing faster and more
efficient. Sensing capabilities of color centers have been extended not only to spin-based
temperature sensing [4], but also to all optical temperature sensing [15]. The controlled
coupling of color centers to strain and thus vibrational modes in nanoscale devices
opens the route towards new control schemes in sensing and quantum information [163].
Moreover, the charge state of NV centers close to diamond surfaces has been foundto depend strongly on the treatment of the surfaces [164]. Consequently, charge-statechanges might be used as a future sensing resource. These ongoing developments willrender color centers truly multi-functional sensors and the full potential of combining
these recent results in sensing is still mainly un-explored.
Developments of new materials are ongoing and for example manufacturing of
smaller and smaller nanodiamonds with active color centers will be pave the way towards
enhanced usage of nanodiamonds inside cells [165]. Integrated sensors pave the way
towards industrial applications and color center sensors have enabled new insights into

Nanoscale sensing based on nitrogen vacancy centers

22

the nanoworld e.g. in the investigation of novel two-dimensional magnetic materials [161].
Challenges still arise from decoherence due to surfaces as well as implantationrelated effects [47], and improvements and deeper understanding of the underlying
effects could trigger a significant enhancement in NV sensor performance. A very recent
example of novel approaches towards mitigating negative effects of nanofabrication isRef. [46] in which a combination of electron-irradiation, high-temperature annealing
and optimized etching lead to the creation of optically coherent NV centers. Similar
approaches might be applied to sensing devices, enhancing their performance.
7. Appendix
7.0.1. Influence of the orientation of NV centers on near-field based sensing For
sensing and imaging approaches using the dipolar nature of NV centers, it is significant~ and Y~ . The detection of nearto consider the role of the two electric transition dipoles, X
field processes typically involves measuring the lifetime of the interacting systems. The
following calculation illustrates how the observed lifetime is influenced by the existence~ and Y~ .
of two dipoles X~ and Y~ are orthogonal to each other and located in the plane perpendicular to theX
NV axis (see Section 3.1, [89, 90]), while local strain determines the in plane orientation~ and Y~ . Without local strain and the NV axis along [111], X~ and Y~ orient as Xk[1̄1̄2]
X
and Y k[11̄0] [90].
Following a semiclassical approximation, the radiative decay rate Γrad associatedto a transition from an excited state to a ground state is given by [89]
P
.
(2)
h̄ω
Here, P is the power dissipated by the transition dipole µ~ . The lifetime τ for thistransition is given as the inverse of the radiative decay rate Γrad (assuming a quantum
efficiency of unity).
In a homogeneous medium with a dielectric constant 1 , we can write:
√
Phom
Γrad
,
Phom (~µ2 ) = µ~ 2 1 ω 4 /3c3 .
(3)
hom =
h̄ω
Assuming a homogenous dielectric is suitable for an NV center buried deeply indiamond. In this case, the observed decay rate (and hence the lifetime) will be given by~~ ,radY
the average of ΓX,radhom and Γhom :Γrad =

1 X,rad~~ ,radYΓrad(4)hom = (Γhom + Γhom ).
2
To model a NV center close to the diamond surface, we have to use a a layered
system, with a NV center buried at a depth h. We decompose the power dissipated by~~XY
the two dipoles Play, Playin components parallel and orthogonal to the surface:µ~ 2k,X~µ~ 2⊥,X~
~2
~2
~
!
X
X
Play = 2 Play,⊥ + 2 Play,k = aX
⊥ Play,⊥ + ak Play,k ,
µ
~ X~
µ
~ X~

(5)

Nanoscale sensing based on nitrogen vacancy centers23µ~ 2k,Y~µ~ 2⊥,Y~~2~2~!
YPlay = 2 Play,⊥ + 2 Play,k = aY⊥ Play,⊥ + aYk Play,k ,
µ
~ Y~
µ
~ Y~

(6)

where Play,⊥ (Play,k ) is the power dissipated by the dipole component orthogonal(parallel) to the surface. We note that Play,⊥ and Play,k as well as all deduced quantities
depend on the depth h of the NV center. Equations defining quantities and coefficient!
are highlighted using =.
~~Considering Eqns. 2, 3, Eq. 5, we find the decay rate ΓX,rad for X:
lay

~
X
Play
~
~2
~2
!
X,rad
rad
X
X
Γlay == FX~ Γradhom = (a⊥ F⊥ + ak Fk )Γhom ,

h̄ω

(7)

with:
!

F⊥ =
Similarly for Y~

Play,⊥! Play,k, Fk =
.
Phom
Phom

(8)

~

~

ΓYlay,rad =

Y
Play
~2
~2
!
Y
rad
Y
= FY~ Γradhom = (a⊥ F⊥ + ak Fk )Γhom .
h̄ω

(9)

To obtain the total decay rate, we use Eq. 4:
1 X,rad~~Γrad+ ΓYlay,rad ).
lay = (Γlay2(10)

Considering Eq. 7 and Eq. 9,
1 rad
~2
~2
~2
~2
Y
rad
2
2
rad
X
Y
X
Γrad
lay = Γhom [F⊥ (a⊥ +a⊥ )+Fk (ak +ak )] = Γhom [a⊥ F⊥ +ak Fk ] = Γhom Flay ,(11)
2
with:
!

Flay = a2⊥ F⊥ + a2k Fk ,

(12)

and
~2
~2
X
Y
2 ! a⊥ + a⊥
a⊥ =

2

,

~2
~2
Y
aX
k + ak
2 !
ak =
.

2

(13)

~ and Y~ in (100)-oriented SCD
Considering in detail the possible orientations of X
which is the most common SCD orientation, we find that that a2⊥ and a2k do not depend~ and Y~ in the plane orthogonal to the NV axis, nor on which ofon the orientation of X
the equivalent directions the NV occupies and a2⊥ = 13 and a2k = 23 .
Consequently, Γradlay is the same for all equivalent orientations of NV axis and~orientations of X and Y~ . Γradlay and the lifetime of the NV will depend only on the
NV depth h. FRET will add a non-radiative channel to the system. In this case thetotal decay rate Γtot
lay for the layered system is:
rad
nr
rad
nr
Γtot
lay = Γlay + Γlay = Γhom Flay + Γlay .

(14)

Nanoscale sensing based on nitrogen vacancy centers

24

Consequently, we find a unique decay rate Γtot
lay for an ensemble of NV centers at
the same depth h. Hence, the observed PL decay will be monoexponential characterized
by a lifetime:
τ=

1
.
Γtot
lay

(15)

Consequently, shalllow NV centers in (100) oriented diamond may form highly-suitablesensors for FRET, despite their non-trivial transition dipoles.
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7. Near-eld energy transfer between
a luminescent 2D material andcolor centers in diamond
Richard Nelz, Mariusz Radtke, Abdallah Slablab, Zai-Quan Xu, Mehran Kianinia, Chi Li,
Carlo Bradac, Igor Aharonovich, and Elke Neu
Adv. Quantum Technol. 1900088 (2019)
doi: 10.1002/qute.201900088

The above detailed manuscript shows near-eld transfer between two stable, luminescentsolid state systems: NV centers in diamond and two-dimensional WSe2 .
Richard Nelz (R.N.) aided in pre-characterizing shallow NV ensembles in the single crystal
diamond sample.

The co-authors from University of Technology Sydney (Z.-Q. Xu, M.

Kianinia, C. Li, C. Bradac and I. Ahoronovich) used a process developed at their institution
to transfer WSe2 akes to the diamond sample.

Pre-characterization of the WSe2 hasbeen carried out at UTS. After the transfer, R.N. performed extensive photoluminescence
mapping, lifetime imaging and spin measurements of the NV centers in proximity of WSe2 .
These measurements constitute the main body of the manuscript. The employed confocal
microscope was designed and realized by R.N. Evaluating the obtained data, R.N. foundclear evidence for a resonant energy transfer, namely FRET, between the NV centers andthe excitons in the WSe2 akes.

R.N. developed a model which very well describes theinteraction of the shallow NV ensemble with a monolayer WSe2 ake. Applying this model,
he estimated the FRET radius for the observed process.

In addition, R.N. developed ameasurement scheme to show the inuence of the FRET process on the excitation of WSe2
excitonic photoluminescence. By establishing lifetime-gated photoluminescence detection,
R.N. showed that NV centers coupled to WSe2 via FRET retain their magnetic sensing
capabilities thus constituting multifunctional sensors.

R.N. outlined and wrote the rst

draft of the manuscript including the design of all gures and was actively involved inall discussions during the collaboration.
manuscript.

He actively contributed to all revisions of theFULL PAPER
www.advquantumtech.com

Near-Field Energy Transfer between a Luminescent 2DMaterial and Color Centers in Diamond
Richard Nelz, Mariusz Radtke, Abdallah Slablab, Zai-Quan Xu, Mehran Kianinia, Chi Li,
Carlo Bradac, Igor Aharonovich,* and Elke Neu*
excitation pathway, thus not inducing lifetime changes but potentially enhancing
acceptor luminescence. Realized for avariety of heterogeneous donor–acceptor
systems—quantum
dots,
molecules,
constichromophores,
etc.[2] —FRET
tutes the basis of a wide range of both
fundamental eﬀects and practical applications
includingsingle-molecule(bio)sensing and (bio)imaging, superresolution ﬂuorescence microscopy, as well
as FRET-enhanced photodetection and lightharvesting.
Evidently, the characteristics of thedonor–acceptor systems are key to thepractical realization of any FRET-based
application. As ﬂuorescent dye molecules
traditionally used in FRET may bleach, recent work has focused on exploring resonant energy transfer processes in luminescent solid-state systems. These show superior stability andmuch broader versatility for many of the proposed sensing, imaging, and optoelectronic applications. Luminescent point defects
in diamond,[3] including the well-studied nitrogen-vacancy (NV)center,[4] are long-term stable, ﬂuorescent probes with established nanoscale sensing capabilities for magnetic[5] and electric ﬁelds,[6] as well as temperature.[7] Furthermore, FRET processes involving NV centers in nanodiamonds have already been
demonstrated with organic molecules[8,9] and graphene.[10,11] Simultaneously, there is an entire family of ﬂuorescent probes inatomically thin 2D transition metal dichalcogenide (TMDs) materials which exhibit ultra-bright luminescence. 2D-TMDs are leading candidates for emerging applications in optoelectronics, photodetection, and valleytronics[12–14] rendering them signiﬁcant forquantum technologies.[15] So far, FRET involving luminescent
2D-TMDs has been demonstrated with organic dye molecules[16]and colloidal quantum dots.[17,18] Notably, electrical gating of 2DTMDs enables control over the eﬃciency of the energy transfer process.[19] It is noteworthy that FRET has been straightforwardly established between systems of diﬀerent dimensionalities (e.g., 1D: molecules, quantum dots, color centers; 2D:TMDs)
and involving dipole transitions of fundamentally diﬀerent nature (atom-like transitions in color centers and molecules, excitons in quantum dots and TMDs).[10,16–19] While the FRET mechanism itself is quite universal, the distance dependence of theprocess will reveal the nature of the participating dipoles. Conventionally, the rate of energy transfer between a pair of point
dipoles such as molecules and chromophores scales with the distance z between the dipoles like z16 . In contrast, FRET between an

Energy transfer between ﬂuorescent probes lies at the heart of many
applications ranging from bio-sensing and bio-imaging to enhanced
photodetection and light harvesting. In this work, Förster resonance energy
transfer (FRET) between shallow defects in diamond—nitrogen-vacancy (NV)centers—and atomically thin, 2D materials—tungsten diselenide (WSe2 )—isstudied. By means of ﬂuorescence lifetime imaging, the occurrence of FRET in
the WSe2 /NV system is demonstrated. Further, it is shown that in the coupledsystem, NV centers provide an additional excitation pathway for WSe2
photoluminescence. The results constitute the ﬁrst step toward the realizationof hybrid quantum systems involving single-crystal diamond and 2D materials
that may lead to new strategies for studying and controlling spin transfer
phenomena and spin valley physics.

1. Introduction
Förster resonance energy transfer (FRET) is the near-ﬁeld
transfer of energy due to the dipole–dipole interaction in adonor–acceptor pair system. FRET transfers energy from an
excited donor to an acceptor which is initially in the groundstate. The transfer does not involve the exchange of photons
and occurs at a rate determined by the overlap integral, that is,
the frequency-dependent oscillator strengths of the quantum
spectroscopic (ﬂuorescence/absorption) transition dipoles aswell as their distance.[1] For the donor, FRET provides a new decay channel and consequently reduces its excited state lifetime.
In contrast, for the acceptor FRET constitutes a non-radiative
R. Nelz, Dr. M. Radtke, Dr. A. Slablab, Dr. E. Neu
Universität des Saarlandes
Fakultät NT, Physik, 66123 Saarbrücken, Germany
E-mail: elkeneu@physik.uni-saarland.de
Dr. Z.-Q. Xu, Dr. M. Kianinia, C. Li, Dr. C. Bradac, Prof. I. Aharonovich
School of Mathematical and Physical Sciences
Faculty of Science
University of Technology Sydney
Ultimo 2007, Australia
E-mail: Igor.Aharonovich@uts.edu.au
The ORCID identiﬁcation number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/qute.201900088© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim. This is an open access article under the terms of theCreative Commons Attribution License, which permits use, distribution
and reproduction in any medium, provided the original work is properly
cited.

DOI: 10.1002/qute.201900088

Adv. Quantum Technol. 2019, 1900088

1900088 (1 of 9)© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 1. a) Schematics of the NV/WSe2 hybrid system illustrating the crystal structure of WSe2 as well as the structure of the NV color center. The
transfer of WSe2 ﬂakes to the SCD surface is described in Section 2. b) Schematics of the FRET process investigated here: negative NV centers serve as
donors, monolayer WSe2 ﬂakes as the acceptor in the FRET process. The strong overlap of the absorption band of the WSe2 ﬂakes with the emission
band of the NV centers establishes the main pre-requisite for FRET.

exciton delocalized in 2D, as observed in graphene or in our casein WSe2 , and a quantum dot or an atomic-scale quantum system
like a color center scales with z14 .[10,20]
In this work, we report FRET between two solid-state, stable,
luminescent quantum systems—an ensemble of shallowly implanted NV centers in single-crystal diamond (SCD) and a 2D
WSe2 monolayer. The investigated hybrid system is depicted in
Figure 1a. Although the observation of FRET between NV centers and molecular donors or 2D materials like graphene has already been demonstrated for nanodiamonds,[9,10] this is the ﬁrst
demonstration of FRET for shallowly implanted NV centers in
SCD. In the process, NV centers act as donor dipoles which nonradiatively transfer their excitation energy to excitons in WSe2 .
WSe2 is an optimal FRET partner for NV centers, as its broad absorption band (500–750 nm)[21,22] largely overlaps with the NV
photoluminescence (PL) band between 640 and 750 nm. Fig-

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1900088 (2 of 9)

ure 1b established the schematics of the donor–acceptor pair
NV/WSe2 . We employ ﬂuorescence lifetime imaging to measure
lifetime changes due to FRET between WSe2 ﬂakes and NV centers. We also observe enhanced excitation of WSe2 via FRET
processes. We estimate the FRET radius for the NV/WSe2 pair
to be 13 nm, and we show that the spin-based magnetic sensing
capabilities of NV centers are conserved when FRET takes place.

2. Experimental Section
High-purity, (100)-oriented, synthetic, SCD from Element Six(electronic grade quality, [N]s < 5 ppb, B < 1 ppb) was used. The
SCD sample (size 2 × 4 mm2 ) was polished to a roughness of
Ra < 3 nm by Delaware Diamond Knives. Reactive ion etching[23]was ﬁrst employed to remove the top 15 µm of the SCD. Creating NV centers in the diamond top layers which are potentially© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 2. a) Transfer process of the WSe2 ﬂakes onto the SCD surface as described in the main text. b) Atomic force microscope image of a WSe2 ﬂake
on top of a SiO2 substrate. c) Scanning electron microscope image of WSe2 ﬂakes on an SCD surface.

damaged as a result of the mechanical polishing was thus
avoided. A homogeneous layer of NV centers is then formed byshallowly implanting nitrogen ions with an implantation densityof 4 × 1011 (nitrogen ions) cm−2 and an energy of 4 keV. During
the implantation, the sample was tilted by 7◦ with respect to theion beam to avoid ion channeling. The SCD sample was then annealed in vacuum at 800 ◦ C and cleaned in boiling acids (1:1:1
mixture of sulfuric acid, perchloric acid, and nitric acid). Using
Monte Carlo simulations,[24] a resulting depth of the NV centersof (6.5 ± 2.7) nm below the SCD surface was estimated. As thegeometry of the implantation process avoided ion channeling,
SRIM simulations were a valid approximation for the NV depth
distribution in this case. Using PL measurements, the creation ofa spatially homogeneous ensemble of NV centers was shown. Assuming typical creation yield in the order of 1%, the above given
implantation density leads to a sparse NV ensemble with < 10NV centers in the focus of the confocal microscope. Reactive ion
etching onto a target area of the SCD to remove selectively all NV
centers (area size ≈ 1 × 0.4 mm2 ) was also employed. Using thisarea, the properties of the WSe2 ﬂakes on SCD independently ofinteractions with NV centers were characterized.
WSe2 monolayer ﬂakes were synthesized on a sapphire (0001)
substrate via chemical vapor deposition following Ref. [22].

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1900088 (3 of 9)

The ﬂakes were transfered onto our SCD sample using themethod described below and depicted in Figure 2a. Poly(methyl
methacrylate) (PMMA; Microchem, A4, MW 495K, solvent:anisole) was ﬁrst spin coated at 3000 rpm for 1 min onto thesapphire substrate with the ﬂakes. The sample was subsequently
cured by heating it up on a hotplate at 180 ◦ C for 1 min. The
PMMA-coated substrate was then immersed in 2 m NaOH solution at 80 ◦ C for 1 h to reduce the adhesion between the PMMA
and the sapphire substrate. Subsequently, the PMMA layer was
peeled oﬀ together with the WSe2 ﬂakes exploiting the surface
tension between them. The PMMA/WSe2 composite layer wasrinsed three times in deionized (DI) water and placed onto theclean SCD surface (or onto a SiO2 substrate for further characterization, see below). Heating on a hot plate for 20 min at 150◦
C enhanced the adhesion of the WSe2 ﬂakes to the new substrate. Finally, the SCD sample was immersed in hot acetone for10 min followed by ﬂush washing it in isopropanol (IPA) and DI
water to remove the PMMA layer. The method had a high transfer yield over a millimeter-sized area, as conﬁrmed by the densityof WSe2 ﬂakes on the SCD being similar to that on the sapphire
before transfer. The WSe2 ﬂakes mostly retained their triangularshape after transfer onto diﬀerent substrates (see Figure 2b,c).
Atomic force microscopy to measure the thickness of our WSe2

© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Table 1. Experimental parameters at a glance.
NV−

WSe2

460–630

500–750[22]Emitted PL [nm]637–750760 ±20

Depth [nm]6.5 ± 2.7

Absorption [nm]Thickness [nm]1.3(4)

Detection [nm]680–720>650

Excitation [nm]cw: 532

cw: 532

Pulsed: 460–600

Pulsed: 460–600

ﬂakes was used. For a reference WSe2 ﬂake transferred onto a
SiO2 substrate, a thickness of 1.3 nm and a root mean square(rms) roughness of 0.4 nm was found. It was noted that this transferred ﬂake was 0.6 nm thicker than our as-grown ﬂakes. Thisobservation was attributed to intercalated water in between the
ﬂake and SCD as well as potential residuals from the transfer on
top of the WSe2 ﬂake. It was emphasized that the strong PL of the
WSe2 ﬂakes at 760 nm presented in Section 3 together with the
AFM-based thickness measurements clearly identiﬁed the ﬂakes
as monolayers.
A custom-built confocal scanning microscope (numericalaperture 0.8, pinhole size 50 µm) was used to investigate theinteraction between the NV centers in the SCD and the WSe2
monolayer. We used a tuneable (450–850 nm), pulsed laser (NKT
EXW-12, pulse length ≈50 ps) equipped with a ﬁlter system (NKT
SuperK Varia) as excitation source enabling pulsed PL and lifetime measurements. To acquire standard confocal microscopy PL
maps, the sample was continuously excited with a diode-pumped
solid-state laser at a wavelength of 532 nm while the collected PL
was detected through a 650 nm longpass ﬁlter. The collected PL
signal was either sent to highly eﬃcient photon counters (Excelitas SPCM-AQRH-14, quantum eﬃciency ≈ 70 %) or to a grating spectrometer (Acton Spectra Pro 2500, Pixis 256OE CCD).
A single-photon counting time correlator (PicoQuant, PicoHarp
300) was employed to perform time-resolved PL analysis. The
measured instrument response function (IRF) of the setup wasmodelled by a Gaussian function with a full width at half maximum (FWHM) of 326 ps. When ﬁtting PL decay curves, exponential decays convoluted with the IRF were used and consequently
corrected the obtained PL lifetimes for the IRF. When performing PL lifetime imaging of NV centers, the PL in a spectral window of 680-720 nm was ﬁltered. Consequently, the NV lifetime
measurements did not reﬂect contributions from neutral NV centers but only from negative NV centers. The setup was equippedwith a microwave source (Stanford Research Systems, SG384)
and an ampliﬁer (Mini Circuits, ZHL-42W+) which allowed forthe delivery of microwaves through a 20 µm thick copper wire
to enable electronic spin manipulation for NV centers. Table 1
summarizes all important experimental parameters.

3. Photoluminescence and Lifetime Measurements
First, we characterize the hybrid NV/WSe2 system depicted in
Figure 1a under continuous excitation. The inset in Figure 3b
shows a typical PL map of a triangular WSe2 ﬂake which weAdv. Quantum Technol. 2019, 1900088

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localize due to its strong excitonic PL at 760 nm (see Figure 3b,
gray curve). We ﬁnd very similar PL spectra for our WSe2 ﬂakes
transferred onto SiO2 (see Figure 3b, red curve) and on the bare
SCD (see Figure 3b, black curve). The observed PL is characteristic for WSe2 monolayers as previously reported.[22] Despite
the dominant WSe2 PL signal, we identify a zero-phonon line
at 637 nm originating from NV centers located underneath the
ﬂakes (Figure 3a, gray curve). As expected, in between individual
WSe2 ﬂakes, we only observe PL due to implanted NV centers.
We note that for both cases, NV ensemble below WSe2 ﬂakes aswell as bare NV ensemble, we do not observe any PL due to neutral NV centers. We consequently infer that the presence of WSe2
does not signiﬁcantly alter the NV charge state.
To further investigate the hybrid NV/WSe2 system, we perform
PL lifetime measurements. In the area in which all NV centers
have been removed, we ﬁnd a lifetime of the WSe2 exciton recombination PL of 𝜏WSe2 = 0.41(5) ns. This is consistent with 𝜏WSe2
for pristine WSe2 ﬂakes before transfer. For our NV centers as
donors, FRET will provide a new, additional decay channel. Consequently, their excited state lifetime (𝜏NV ) will reduce. In contrast, for the acceptor, FRET establishes a (non-radiative) excitation pathway. Thus FRET is not expected to change 𝜏WSe2 which isin accordance with our observations for pristine ﬂakes, ﬂakes on
SCD not coupled to NV centers and ﬂakes coupled to NV centers(see Figure 4d and the discussion below).
Figure 4b shows the PL decay recorded on a WSe2 ﬂake placedon the SCD surface with the shallowly implanted ensemble ofNV centers. We ﬁt two main components: a fast decay with atime constant of 0.42(3) ns and a slower decay with a time constant of 5.1(3) ns. While the fast decay clearly arises due to theWSe2 PL (𝜏WSe2 ), we attribute the slower decay to the NV ensemble interacting with the WSe2 ﬂake (𝜏NV ). In contrast, in areas not covered by WSe2 ﬂakes we consistently measure a muchbulk
≈ 12(1) ns (seelonger NV lifetime with an average value of 𝜏NV
Figure 4c), typical for NV centers in bulk diamond.[25] For shallowly implanted NVs, slightly longer lifetimes of ≈ 16–17 ns havebeen previously reported.[26] We point out that, in our experiment,
residuals from the transfer process might slightly reduce the NV
lifetime in between the ﬂakes. Excitation with short (< ns) laser
pulses potentially reveals internal spin dynamics of NV centers
including spin-dependent lifetimes.[27] The here observed valuebulk≈ 12(1) ns corresponds to the lifetime expected for theof 𝜏NVms =0 state.[27] Indeed the high repetition rate (8 MHz) pulsed excitation that we use here will polarize the NV centers into ms =0
as the time interval between subsequent pulses is several ordersof magnitude shorter than typical NV T1 times. For NV centers
coupled to WSe2 ﬂakes, we ﬁnd that 𝜏NV is halved compared to
the non-coupled NVs in our sample. This ﬁnding proves the occurrence of non-radiative energy transfer—FRET—between NV
centers in SCD and excitons in the WSe2 ﬂake. The WSe2 ﬂakes
provides the NV centers with a non-radiative decay channel—
mediated by dipole–dipole interaction—which reduces 𝜏NV .
To further investigate FRET between WSe2 and NV centers, we
perform PL lifetime imaging of diﬀerent areas of the SCD sample (see Figure 4a,c,d). We ﬁt a double exponential decay to themeasured data and extract 𝜏NV and 𝜏WSe2 . We consistently observe𝜏NV < 6 ns in areas where the SCD surface is covered by a WSe2
ﬂake, as discernible from comparing the PL map in Figure 4abulk
on
and the lifetime map in Figure 4c. In contrast, we ﬁnd 𝜏NV© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 3. PL spectrum of the NV ensemble measured between WSe2 ﬂakes [black curve in (a)] showing the NV zero-phonon-line (at 637 nm) and phonon
sideband. Gray curves in (a) and (b) show the PL spectrum of a WSe2 ﬂake with underlying NV ensemble. In (a) we normalize both spectra to the NV
ZPL to facilitate comparison. In (b), we compare WSe2 PL spectra on bare SCD (black) and SiO2 (red) substrates as well as on the NV center ensemble
in SCD (gray). All spectra have been normalized to their peak value. All spectra clearly reveal the characteristic PL of WSe2 monolayers centered around≈ 760 nm with maximum peak shifts of ≈ 10 nm. In contrast, the PL of a bilayer would occur centered around ≈ 815 nm.[22] The inset shows a PL map(scale bar 2 µm) of a transferred WSe2 ﬂake (bright area) on top of the NV center ensemble (blue) recorded detecting wavelengths above 650 nm using
continuous laser excitation at 532 nm (exc. power 700 µW).

all other positions. The pattern of the WSe2 ﬂakes is furthermore
conﬁrmed when plotting 𝜏WSe2 (see Figure 4d).
Below, we interpret these results in detail and highlight the peculiarities of the FRET process between an ensemble of NV centers and a WSe2 ﬂake. In our case, the FRET process is non-trivial
as FRET strongly depends on the distance between the donor andthe acceptor and our NV centers show a spread of depths z in
the SCD. Assuming the ﬂake is in direct contact with the SCD
surface, each speciﬁc NV center of the ensemble lies at a diﬀerent distance z from the WSe2 . We thus would expect a complicated multi-exponential PL decay corresponding to a spread of
𝜏NV . Experimentally, however, we ﬁnd that the NV PL decay isvery well described by a single exponential decay with time constant 𝜏NV . To simulate the PL decay expected from the NV ensemble coupled to the WSe2 ﬂake, we ﬁrst calculate 𝜏NV as a function of z. To this end, we need to determine the non-radiative
decay rate 𝛾non-rad due to the FRET process. We assume thatthe exciton is fully delocalized in the WSe2 ﬂake and the ﬂakes
have inﬁnite size compared to the atomic-sized NV centers; weﬁnd[10]𝛾non-rad (z) = 𝛾rad

R4
z4

(1)

where R is the Förster radius, that is, the distance at which theeﬃciency of the FRET mechanism is 50%. The quantity R depends on the quantum eﬃciency of the donor and on the spectral
overlap between the donor’s emission and the acceptor’s absorption, as well as their dipole moments. We furthermore assume
that the radiative decay rate 𝛾rad is constant for all NV centers andbulk −1) . For each NV center
equal to the reference bulk value (𝜏NV
at a speciﬁc depth z, we should ﬁnd a mono-exponential decay
with 𝜏NV (z). Equation (1) shows that FRET is more eﬃcient if z
is smaller; consequently NV centers very close to the surface will

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be strongly quenched and emit less photons. We calculate the PL
intensity I(z) of a NV center at a depth z asI(z) = I0𝛾rad𝛾rad + 𝛾non-rad (z)(2)

where I0 is the non-quenched PL intensity, and 𝛾rad and 𝛾non-rad
are the radiative and non-radiative decay rates, respectively.
To obtain the resulting PL decay of the ensemble, we weight
each of the mono-exponential decay curves for each depth z with
the intensity I(z) (Equation (2)) and with the depth distribution
D(z) of NV centers resulting from the implantation process. We
extract the depth distribution D(z) using Monte Carlo Simulations (SRIM), which produce a depth proﬁle of (6.5 ± 2.7) nm.
As we observe all NV centers in the ensemble simultaneously,
we integrate over the whole implantation proﬁle to retrieve theobserved PL decay.
Notably, the calculated PL decay of the NV ensemble indicatesa decay which is well represented with a mono-exponential funceﬀ(assuming R > 5 nm) in full agreetion with an eﬀective 𝜏NVment with our experimental observations. Figure 4e shows theeﬀ
for 5 nm < R < 30 nm—which we now use
expected value of 𝜏NV
to estimate R for the NV/WSe2 pair and ﬁnd RNV∕WSe2 = 13 nm.
To further conﬁrm the agreement between the measured dataand our model, we reduce the data to the time range in which
the NV’s mono-exponential decay is dominating. We do not include data from the ﬁrst 3 ns—as this signal mainly represent
PL from WSe2 —and we do not consider the long time tail-that is,
all the data points with count rates below 1 % of the NV centers’
peak value. This data treatment leads to the black line in Figure 4f
which agrees very well with the calculated PL decay (red line), aseﬀ
= 5.2 ns. We notesuming RNV∕WSe2 = 13 nm, and resulting in 𝜏NVthat 𝜏NV = 5.2 ns in this situation is expected for NV centers at a
depth of ≈ 12 nm. Our value for RNV∕WSe2 is comparable to that© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. a) PL map of WSe2 ﬂakes on NV ensemble. Here we use pulsed excitation (≈ 9 nJ, 8 MHz repetition rate, 2 s integration time per pixel, excitation
bandwidth 530–534 nm). b) Exemplary lifetime measurement of the ﬂakes and the underlying NV centers. The contributions of the instrument response(IRF), the WSe2 PL, and the NV center PL are drawn separately. c,d) Lifetime maps using 𝜏NV and 𝜏WSe2 corresponding to the PL map in (a). We
bulk andconsistently observe quenching of NV centers underneath the WSe2 ﬂakes with 𝜏NV < 6 ns. In contrast, between the ﬂakes, we consistently ﬁnd 𝜏NV
a lower overall PL level (blue areas in (a) and (c)). As we only detect NV center PL in these areas, we ﬁt the corresponding data with a mono-exponential
eﬀ
decay. Consequently, in these areas, no values for 𝜏WSe2 are obtained which we represent in (d) in white color. e) Observable NV ensemble lifetimes 𝜏NV
extracted from the simulation as a function of the FRET radius R. From our measurements, we extract a FRET radius of R = 13 nm. f) Simulated PL
decay (red line) for the NV center ensemble using a Förster radius R = 13 nm in comparison to the observed PL decay (black line). Model and observed
data agree very well.

reported for the NV/graphene system, RNV∕graphene ,[10] strongly
supporting the hereby observation of FRET between NV centers
and WSe2 .
To further investigate the energy transfer mechanism between
NV centers and WSe2 ﬂakes, we study how the WSe2 PL intensity
depends on the excitation wavelength 𝜆exc . Negatively charged
NV centers act as donors for WSe2 . Consequently, FRET fromexcited NV centers constitutes an excitation path for WSe2 PL
which adds to laser excitation of WSe2 PL. Especially when theAdv. Quantum Technol. 2019, 1900088

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direct laser excitation of the acceptor is ineﬃcient, FRET enhances the PL of the acceptor (and simultaneously reduces the
PL of the donor). Recording PL spectra of ensembles of NVs at
positions not covered by WSe2 ﬂakes, we clearly observe PL fromnegatively charged NV centers for 𝜆exc > 465 nm. We howevernote that we cannot infer a ratio of neutral and negative NV centers for excitation with blue laser light as the employed setup
does not permit observing spectral features due to neutral NV
centers (in contrast to the setup we used for 𝜆exc = 532 nm, see© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 5. a) Median PL rate of the WSe2 ﬂakes for diﬀerent excitation wavelengths 𝜆exc . Black dots: PL of WSe2 coupled to NV centers. WSe2 PL rates
are corrected for background as well as for the PL of the underlying NV ensemble. Blue stars and green squares: PL of WSe2 on bare SCD as recordedin two areas (1,2) separated by more than 0.5 mm. These measurements demonstrate that FRET from NV centers constitutes an additional excitation
pathway for WSe2 PL (for details see text). b) NV lifetime recorded along a line perpendicularly crossing the edge of a WSe2 ﬂake. We ﬁt a Gaussian
to the transition from bulk lifetime to the quenched lifetime (red curve). We obtain a FWHM of 620(30) nm which is comparable to the point spreadfunction measured for our confocal setup.

Figure 6. Optically detected magnetic resonance of the nitrogen vacancy centers ensemble a) without and b) with an external magnetic ﬁeld of 25 G
applied indicating the potential for NV centers as multi-functional sensors.

Figure 3). However, as we clearly observe negatively charged NV
centers, they will contribute via FRET to the excitation of WSe2
PL for 𝜆exc > 465 nm. We thus investigate this excitation wavelength range in detail. We investigate the PL rate of the WSe2
ﬂakes as a function of 𝜆exc (see Figure 5a) and aim to compare
the situation where WSe2 ﬂakes are placed on the NV ensemble
and the situation in which the WSe2 ﬂakes are placed on SCD
regions where all NV centers have been removed via etching (seeSection 2). To account for variations in the PL intensity for diﬀerent ﬂakes, we compare the median count rate of WSe2 in areasof 20 by 20 µm2 . To check for consistency, we furthermore investigate two areas without NV centers separated by more than
0.5 mm. We also correct the WSe2 PL intensity for NV center andbackground PL. As we compare the WSe2 PL intensity with andwithout NV centers, changes in the excitation laser power whenchanging 𝜆exc aﬀect both measurements in the same way. Consequently, we did not correct for variations of the laser power aswell as a potentially wavelength-dependent transmission of ourAdv. Quantum Technol. 2019, 1900088

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setup. We note that as a consequence of this, our measurements
do not necessarily reveal the wavelength-dependent PL excitation
probability of NV centers. Figure 5a displays a clear tendency ofan enhanced excitation of WSe2 PL for ﬂakes coupled to the shallow NV ensemble. We note that the enhanced excitation cannot
be due to the absorption of NV PL by the WSe2 ﬂakes as the PL
is too weak to induce the observed enhancement. The observed
excitation enhancement of WSe2 in the presence of NV centersconstitutes additional strong evidence for FRET between NV centers and WSe2 .
We now investigate the electronic spin properties for NV centers under the WSe2 ﬂakes. We use lifetime gating to separate
NV PL and WSe2 PL to enhance the measurement contrast when
performing optically detected magnetic resonance (ODMR) measurements under pulsed laser excitation. Figures 6a,b show the
ODMR of the NV ensemble underneath the WSe2 ﬂake in the absence and presence of an external magnetic ﬁeld, respectively. We
observe an ODMR contrast of 10 % without external ﬁeld, which© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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is typical for shallow NV centers, and which proves clear separation of NV and WSe2 PL. Observing ODMR with a magneticﬁeld-dependent splitting for NV centers undergoing FRET indicates that they can serve as multi-functional sensors: while using FRET processes to monitor the presence of other dipoles, NV
centers can simultaneously sense magnetic ﬁelds. This observation renders NV centers promising as multi-functional sensors
in biological systems or for the investigation of novel materials
where they can operate as nanoscale probes for nuclear magnetic
resonance spectroscopy[28] that simultaneously couple to excitons
via FRET.
We ﬁnally investigate how precisely we can localize the edge ofa WSe2 ﬂake using the spatial variation of 𝜏NV . We measure 𝜏NV
along a line perpendicularly crossing the edge of a WSe2 ﬂake
(see Figure 5b). We ﬁt 𝜏NV using a Gaussian function approximating the point spread function (PSF) of our setup. We ﬁnd
a FWHM of 620 nm which is closely matching the PSF of oursetup which we estimated by imaging single color centers in nanodiamonds. Consequently, localizing the edge of a WSe2 ﬂake islimited by the setup’s PSF only indicating the possibility for highresolution imaging using the FRET process investigated here. We
now address the imaging speed of the PL lifetime imaging. Typical detected PL rates of our NV ensemble amount to 70 kcps fora pulse energy of ≈ 10 nJ (𝜆exc = 530–534 nm, repetition rate 8MHz). Following ref. [29], we estimate the minimum number ofphotons needed to reliably determine 𝜏NV to be 1000. This leads
to a minimum integration time per pixel of <15 ms.

and 2D materials are potential candidates for spin transfer andspin valley physics. The latter has triggered intense research in
TMDs and has potential for quantum information and sensing
applications.[33] The FRET process investigated here can potentially aid to determine the depth of shallow color centers in SCD
given the FRET radius of the WSe2 /NV pair is precisely known.
One advantage of this approach would be its applicability to color
centers that do not posses an electronic spin.

Acknowledgements
This research has been funded via the NanoMatFutur program of theGerman Ministry of Education and Research (BMBF) under grant number FKZ13N13547. The Australian research council (via DP180100077,
DP190101058 and DE180100810), the Asian Oﬃce of Aerospace Researchand Development Grant FA2386-17-1-4064, and the Oﬃce of Naval Research Global under Grant N62909-18-1-2025 are gratefully acknowledged.
The authors acknowledge funding via a PostDoc fellowship of the Daimler
and Benz foundation. The authors also thank Jörg Schmauch for recording
SEM images as well as Dr. Rene Hensel for use of the ICP-RIE and Michel
Challier for help with the RIE etching of the sample.

Conﬂict of Interest
The authors declare no conﬂict of interest.

Keywords
4. Summary and Outlook
In conclusion, we have demonstrated FRET between shallow NV
centers in SCD and WSe2 ﬂakes with an estimated Förster radiusof 13 nm. The FRET process strongly reduces 𝜏NV to around 6 ns,
whereas the coupling to the NV centers enhances the excitation of
WSe2 for 𝜆exc below 500 nm. We show that NVs undergoing FRET
retain their ODMR and are applicable as multi-functional sensors.
In the future, we will investigate the transfer of WSe2 ﬂakes
onto SCD photonic structures, for example, nanopillars with single NV centers. The ﬁrst tests conducted during this work provethat typical SCD nanopillars are robust in the applied transfer
process. Using such photonic structures will enhance the PL
rates from single NV centers and will also allow for the modiﬁcation of the excitonic properties of 2D materials via inducing
local strain.[30] While traditionally FRET pairs are formed by attaching FRET partners to larger molecules or nanoparticles[8,31]
or directly within a biological specimen,[31] the extension to stable solid-state systems could enable the realization of scanningdevices where FRET is established between a single quantum
probe scanning the system under investigation. Consequently,
the distance between sample and probe can be varied continuously, which allows for in-depth characterization of the FRET process and imaging the sample on the nanoscale. Such techniques
termed FRET-Scanning near-ﬁeld optical microscopy (FRETSNOM)[32] will highly proﬁt from stable probes like NV centers in
SCD. The ﬁrst demonstration of FRET-SNOM enabled nanoscale
imaging of graphene ﬂakes using a scanning NV center in a
nanodiamond.[10] Moreover, hybrid systems involving NV centers

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diamond, Förster resonance energy transfer, nitrogen vacancy centers, twodimensional materials
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© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

8. Conclusion and Outlook
In this thesis, we optimized nanoscale sensing based on NV centers in single crystal diamond (SCD). We address the whole process of building optimized SCD sensors, including
novel materials, optimized nanofabrication and novel sensing schemes (see Fig. 8.1).

novel materials
100mm

Pyramidal scanning probes- bottom-up growth of tips with in-situ
created color centers- moderate coherence → ensemble
magnetometry- photonic enhancement

NV

Wafer-scale SCD- heteroepitaxial SCD with good
crystalline quality- single, native NV centers embeddedin SiV ensemble- suitable for fabrication of scanningprobe devices

optimized nanofabrication

HSQ mask
Si layer

ICP-RIE plasma

Reliable nanofabrication

Plasma treatment- silicon lead to better adhesion of
HSQ mask- removable with SF6 pulse- NV centers survive nanofabrication
using novel approach- comparison of biased and unbiased O2 plasma- found Ar/O2/SF6 plasma to hinder NV creation- nanostructuring enable stable NV centersnovel sensing applications
C

Se
W
Se

Interaction with graphene

FRET with WSe2

- spin-coat colloidal graphene on NV
ensemble- first demonstration of near-fieldinteraction of NV centers in SCD- evidence for multifunctional sensing- transfer monolayer WSe2 on diamond- clear FRET between NV centers and WSe2
- sensing magnetic field while FRET occur→ proof for multifunctional sensing capability

Figure 8.1.:

Scheme of the progress made in this thesis with a summary of all main ndings ofthe publications.

Due to the very complex and time consuming fabrication procedure for appropriate SCD
nanostructures usable for sensing applications (see Sec. 1.2), we investigated novel material systems potentially enable upscaling of sensor manufacturing. Here, we compared104

8. Conclusion and Outlook

a bottom-up approach where the structure is already formed during SCD growth withconventional top-down approaches fabricating devices after the growth. Further, we characterized wafer-sized SCD material and tested its applicability for manufacturing sensors
on the wafer-scale. This would overcome one signicant limitation for fabricating nanostructures in large quantities as so far, high purity SCD is only available in limited sizes
in the order of few millimeters. The outstanding advantage of bottom-up and top-down
approaches on the wafer-scale are their potential to provide NV-based scanning probe tips
in large quantities.
Afterwards, we optimized the nanofabrication process itself by introducing new steps toimprove its reliability and reproducibility.

Here, a new approach using silicon as a thinadhesion layer for the HSQ etch mask signicantly enhanced the fabrication yield. Novel
plasma treatments were investigated to improve the quality of SCDs' surface as well asof shallow NV centers. Here, we characterized the photoluminescence (PL) of single NV
centers in photonic nanostructures as well as their spin properties to identify the eect of
the optimization.
Lastly, we studied novel concepts for NV-based sensing: We investigated near-eld interaction in form of Förster Resonance Energy Transfer (FRET) as a source for sensing andimaging applications. We studied the near-eld interaction of a NV ensemble shallowly implanted into SCD with two-dimensional materials applied to the surface. Here, especially
the work on the interaction with a luminescent monolayer of WSe2 enabled deep insightsof the NV center's usability as a donor for FRET-based applications. During this, we also
found the NV center in SCD to be an extraordinary candidate for multifunctional sensing.
While we can use FRET-based approaches for detecting nearby dipoles, we can use theretaining spin properties of the NV center to sense a variety of physical quantities like e.g.
local magnetic elds.
In the following, we discuss and summarize all publications included in this thesis whose
main ndings are summarized in Fig. 8.1 followed by an outlook on how NV centers in
SCD can be employed in the future.

8.1. Materials for sensing applications
In Chap. 2 and 3, we investigated novel material systems applicable for NV-based sensing.
Here, especially approaches allowing to upscale nanostructure fabrication are of major interest to eciently use complex and time consuming fabrication routines.
In the publication of Chap. 2, we focused on a bottom-up approach used to create SCD
tips with a pyramidal shape which are commercially available.

While our conventional

top-down process is only able to fabricate a few hundred tips in parallel, the main ad5
vantage of this bottom-up approach is the large amount of tips (> 10 ) producible in asingle growth process. Here, a complex chemical vapor deposition (CVD) process is used
to selectively grow in (100) direction while other crystal facets grow highly defective. This
leads to an evolving micro-pyramid with a square (100)-oriented basal plane surrounded bynano-crystalline material removable by air annealing. After the growth, the structures are105

8.1. Materials for sensing applications

attached to cantilevers using epoxy and oer a good usability as ultra-hard AFM tips dueto their small radius of curvature of 2-20 nm. To investigate the usability of these structures
for sensing applications, we characterized native NV and SiV color centers created during
the growth process by using our homebuilt confocal microscope. Characterizing several diamond pyramids, we found that they consistently show intense NV center PL in the orderof 500 Mcps at a reasonable excitation power of P

< 400 µW. This PL rate corresponds

to an emitted power of ∼ 100 pW easily measurable with common photodiodes. By investigating the PL originating from various planes parallel to the basal plane, we found that
SiV centers are only present close to the apex of the pyramid. We were able to explain thisby the growth process itself: The plasma involved in the CVD process etches the silicon
substrate used to grow the diamond lm on. The etched silicon is then incorporated into
the growing diamond and forms SiV centers. As soon as the substrate is fully covered withdiamond, the incorporation of silicon rapidly decreases leading to an occurrence of SiV centers only at the apex of the diamond pyramid. In addition, we found an increased PL of
NV centers close to the apex despite constant incorporation of nitrogen during the growth.
Simulating the photonic properties, we estimate PL intensity that agree very well with themeasured one. Here, we were able to identify a clear photonic enhancement of detected
PL from color centers close to the apex.

However, by characterizing the color centers'

lifetime along the growth direction, we found a quenching of NV centers close to the apex.
Hereby, quenching describes a process decreasing the PL intensity as well as excited state
lifetime due to opening an additional non-radiative decay channel. This can be caused bye.g. the near-eld interaction with nearby dipoles or other structural defects. To explain
this quenching, we rst established a model estimating the mean distance between SiV and
NV centers and found a mean distance between a NV center and the closest SiV center of< 10 nm. Thus, dipole-dipole interaction in form of energy transfer from the NV centers to
the SiV centers is feasible. In addition, the quenching of NV centers might be explainable
by a lower quality at the apex. Further, we characterized the spin properties of the NV
centers present at the pyramid apex and compared them to the NV centers in the bulk-like
SCD close to the basal plane of the pyramid. Using Spin-Echo measurements, we foundmoderate coherence times of T2 = 7.2(1) µs for NV centers at the apex and T2 = 7.8(2) µs
close to the basal plane. Despite the strong doping with silicon and nitrogen during the
growth,T2 is almost an order of magnitude better than in typical industrial diamonds(HPHT). However, they are much lower than in high purity CVD diamonds conventionallyused for NV-based sensing approaches. We found good sensitivities η for DC and AC mag-√netic elds of up to ηDC ≈ 131 nT/√

Hz and ηDC ≈ 23 nT/

Hz. The present pyramidal

tips oer a comparable low spatial resolution for sensing approaches due to the spatiallyspread NV ensemble.

To overcome this, it would be necessary to drastically reduce thenitrogen and silicon incorporation. However, the present pyramidal tips are applicable forapplications where scanning the sensor over the sample is mandatory and spatial resolution
in the micrometer-range are sucient.
Since the doping of the investigated diamond pyramids was too strong to identify single
color centers, we also followed another approach to upscale the fabrication process:In

Chap. 3 we investigated commercial SCD material available up to 100 mm wafer-size and106

8. Conclusion and Outlook

its suitability for quantum technologies. The material is grown in a microwave-enhanced
CVD process at high temperatures based on the successful scaling of heteroepitaxial nucleation resulting in a 100 mm wafer-size [101]. First, we characterized color centers close
to the growth surface using a homebuilt confocal microscope and observed for the rsttime individual native NV centers in a heteroepitaxial SCD. However, we found these NV
centers embedded in an ensemble of native SiV centers incorporated during the growth
process. As the dislocation density reduces during growth [101], we expected the highest
material quality close to the growth surface of thick samples. To conrm this, we used the
SiV center ensemble as a strain sensor.

We exploited the ne structure of the SiV zerophonon line as strain caused by dislocations would lead to a shift and broadening of thene structure components. By measuring the linewidth of the SiV centers' ne structure
along the growth direction, we were able to conrm the expected reduction of dislocation
density.

Consequently, we investigated the native NV centers' properties in this region.
3(≈ 0.005 ppb) which

Here, we found single NV centers with a density of ≈ 1 NV per µm

is in accordance with the nitrogen concentration in the feed gases of the CVD reactor.

By measuring NV center excited state lifetimes, we found an average lifetime of 12.3(6) ns
agreeing well with reported bulk lifetimes of NV centers [84]. In addition, this nding also
excludes any possible quenching of the NV centers due to structural defects or near-eld interaction with the SiV centers in the surrounding in contrast to the bottom-up approach in
Chap. 2. To investigate the NV centers' spin properties, we performed Spin-Echo measurements nding a coherence time of T2 = 5(1) µs. By shallowly implanting NV centers into
the growth surface, we found spin properties comparable to the native NV centers mainly
limited by the material potentially caused by surrounding silicon-containing defects.

Inaddition, we showed that the fabrication of nanopillars is analogous to the commonly used
SCD rendering the material applicable for scanning probe devices.

However, the main

challenge of using the present material for high-sensitivity sensing applications emerges
from the limited coherence times of the NV centers. To overcome this limitation, it might
be necessary to tweak the growth to drastically reduce silicon incorporation. This could
be achieved by overgrowing the SCD wafers in a silicon-free CVD process. Here, alreadythin layers of overgrown material will be sucient for most applications.
In general, the publications addressed here show a vast potential to upscale the fabricationof NV-based sensors.

This would lead to a strong reduction of the cost per device.

As

there are already a few existing start-up companies, this upscaling might help to open novel
elds of application. However, for both material systems reducing the defect concentration
is mandatory to be able to sense elds with a good spatial resolution and sensitivity.

8.2. Optimized nanofabrication processes
In the publications of Chap. 4 and 5, we optimzed a procedure to fabricate nanostructures
usable in scanning probe based applications. Here, our main goal was to establish a reliable
and most important a reproducible process. Further, we studied the inuence of reactive
ion etching on shallow NV centers in SCD.

8.2. Optimized nanofabrication processes

107

In the publication in Chap. 4, we improved the reliability of a top-down nanofabrication
process. Here, the most important step enhancing the reliability was to use a silicon layer
to improve the adhesion of the HSQ etch mask and to nd the critical parameters usedin the plasma etching.

We used our homebuilt confocal microscope to characterize thePL of NV centers in structures fabricated with the newly implemented method. Here, we
observed an intense PL of the NV centers as well as a low background photoluminescencefrom the etched surfaces. We found that the background intensity (< 1 kcps) is negligible
compared to the NV center intensity (∼ 100 kcps) which is important as the signal-to-

background ratio fundamentally limits the achievable sensitivity to detect magnetic elds
using NV centers. Further, we were able to show a good contrast (C ∼ 15 %) in ODMR
comparable to other shallow NV centers [58]. We also found a quite short coherence timeT2 ≤ 10 µs mainly limited by the NV center's proximity to the three acid cleaned surface

matching observations in the literature [102]. As we found stable NV centers in the fabricated structures, we exclude any negative eect of silicon on the NV centers. In addition,
we showed that the usage of a silicon adhesion layer for HSQ improves the reliability of
the fabrication signicant rendering it advantageous for nanofabrication-based technology.
The publication in Chap. 5 contains a study of the inuence of novel plasma treatments onshallow NV centers. Here, we compared three dierent plasmas: a 435 V O2 plasma, a 0 V
O2 plasma and an Ar/SF6 /O2 plasma. The bias voltage is hereby a parameter dened by
the applied RF power of the reactive ion etching controlling the anisotropy of the plasma.
We studied the inuence of each plasma as the nal step of the process needed to remove
mechanical stress induced by polishing. By shallowly implanting NV centers in each etched
area, we characterized the inuence of the plasma on the NV centers. While both O2 plasmas lead to similar results in terms of emission spectra, coherence and intensity of the NV
centers, the Ar/SF6 /O2 plasma inuenced the SCD negatively such that we do not nd any
signature of NV centers present in this area. We exclude any eect of surface terminationby the plasma as uorine should stabilize the negative charge state of the NV center andvacuum annealing after implantation most probably removes any surface termination. Dueto the complete absence of any NV centers, we suspect the Ar/SF6 /O2 to strongly damage
the surface preventing NV centers creation. Although the bias-free O2 plasma should prevent subsurface damage compared to the biased O2 plasma [96], we were so far not able
to reveal any positive eect of the unbiased plasma on the NV centers. To further study
the intensity and stability of NV centers under a bias-free etched surface, we fabricatednanopillars with a variety of diameters. First, we map the NV center lifetime in various
pillar elds as a damaged surface would quench NV centers and consequently shorten the
lifetime.

We observed a consistent lifetime in all pillars as well as the ensemble before

pillar fabrication. By comparing single NV centers in pillars with a top-diameter of 120 nm
and 180 nm, we found that the 180 nm-pillars oer slightly higher photoluminescence rates
than the 120 nm-pillars as expected from simulations [61]. In contrast to the simulations,
we found a signicant photonic enhancement for NV centers' photoluminescence in the

120 nm-pillars. This would be advantageous for scanning probe based applications as thinner scanning probe tips would be able to keep the NV center closer to the surface underinvestigation because the tip follows the sample topography more closely.

108

8. Conclusion and Outlook

In conclusion, the publications discussed here constitute important steps towards the optimization of the nanofabrication procedure used to sculpt scanning probe devices with NV
centers embedded as a sensor.

8.3. Novel applications
In the publications of Chap. 6 and 7, we investigated novel concepts to sensing using NV
centers in SCD based on the near-eld interaction with nearby dipoles.

In parallel, weinvestigate the spin properties of shallow NV centers undergoing a near-eld interaction.
Here, dierent material systems have been employed: while graphene mainly quenches theNV centers' emission without producing any luminescence by itself, we also used a luminescent two-dimensional material WSe2 which oers promising applications in quantum
technologies [103].
In Chap. 6, we studied the inuence of graphene on NV centers in SCD. Based on literature [27], it was known that near-eld interaction with graphene signicantly quenches NV
centers. However, this observation was so far restricted to NV centers in nanodiamonds. To
demonstrate this interaction also for NV centers in SCD, we spin coated colloidal graphene
onto the surface of a shallow NV ensemble in SCD. Hereby, the akes formed agglomerates
on the SCD surface easily visible by light microscopy in our homebuilt confocal microscope.
To study the inuence of the graphene agglomerates on the NV centers, we measured their
excited state lifetime and photoluminescence and identify a strong quenching in presenceof the graphene. We clearly showed near-eld interaction of color centers in SCD and theirbasic applicability for imaging applications based on this process. We, for the rst time,present evidence that the NV center oers multifunctional sensing capabilities by investigating the preservation of the spin resonance while interacting with graphene.
In the publication of Chap. 7, we investigated the interaction of a NV ensemble with amonolayer of two-dimensional WSe2 . Here, we rst prepared a SCD by creating shallow NV
centers. Collaborators from University of Technology Sydney then transferred monolayer
WSe2 akes on this SCD. We identied the strongly uorescent WSe2 akes based on confocal PL mapping. Afterwards, we studied the interaction between the NV ensemble andthe WSe2 . We clearly showed that FRET opens an additional non-radiative decay channel
for the NV centers as their lifetime drastically reduced in the proximity of WSe2 to 5.1 ns
instead of the bulk lifetime of 12(1) ns.

By exciting the FRET pair at a wavelength at

the edge of the absorption of WSe2 and by investigating the WSe2 photoluminescence, we
showed that FRET opens an additional excitation path for the excitons in WSe2 . Thesendings constituted a clear proof for the occurrence of FRET between NV centers andmonolayer WSe2 .

Combining the estimated depth prole of our implanted NV centerswith the distance-dependency of FRET, we identied a FRET radius of 13 nm.

This iscomparable to the results reported for a FRET process between NV and graphene [27].
Using the calculated FRET radius, we calculated the expected PL decay for the NV ensemble which agrees very well with our measurements.

To estimate the potential speedof FRET-based imaging, we have to determine the minimal integration time needed to8.4. Outlook

109

measure the lifetime of the NV centers. By using a previously reported model [100], wefound an integration time of 15 ms rendering the NV center promising to use FRET as a
tracking tool for moving particles. To proof the multifunctional sensing capabilities of theNV center, we measure an external applied magnetic eld while the NV centers undergo
FRET with the WSe2 akes. Since the emission of the WSe2 excitons is more intense thanthe NV centers' emission, we implemented an advanced detection scheme. Here, we synchronized the detection with the pulsed excitation of the system. By an appropriate choiceof the detection time, we separated the WSe2 and NV photoluminescence and sensed theexternal magnetic eld with a good signal-to-background ratio rendering the NV center in
SCD applicable in multifunctional sensing schemes.
By investigating FRET, we introduced a novel sensing resource usable with NV centers in
SCD. With this publications, we open routes towards novel possible sensing schemes basedon near-eld interaction and to use NV centers in SCD for nanoscale imaging of dipoles in
e.g. molecules or solids.

In conclusion, the publications contained in this thesis represent important steps for NVbased sensing in general by addressing some of the major challenges related to fabricationof nanostructures based on SCD. We were able to show the potential for upscaling theprocesses by using wafer-scale SCD and by developing novel procedures rendering nanofabrication more reliable. Further, we showed a potential new sensing resource for NV centers
in SCD in form of FRET. In particular, the multifunctional sensing capability by combining FRET with spin-based sensing approaches has to be highlighted. This might lead to
new applications for NV-based sensors in the future.

8.4. Outlook
Recently, we started to optimize NV-based sensing schemes by exploiting the charge state
properties of the NV center in SCD. During a bachelor project [104], we built a setup
usable to detect the charge state of the NV center. Here, we employed a complex pulse
sequence containing green (532 nm) and yellow (594 nm) laser pulses. As the yellow laser
pulse excites only the negative charged NV center, it is possible to distinguish both charge
states by photon counting [105]. This can be used to investigate e.g. the inuence of different surface treatments on shallow NV centers. In addition, by including charge state
detection in other pulsed schemes like Rabi measurements, it is possible to increase thecontrast between the dierent spin states of the NV center's ground state. Here, the charge−
state is checked prior to the spin manipulation to exclude an insucient NV initialization
0
caused by a charge conversion to NV [106]. The charge state properties can be further
used to optimize the eciency to readout the spin state of the NV center [107, 108]. During
the time in which the ms = ±1 state undergo the transition to the singlet state system

and is kept there, it is possible to ionize the ms = 0 state by an intense red laser pulse(637 nm). This then leads to a conversion of the spin state into a charge state which can

110

8. Conclusion and Outlook

0
be detected by a yellow laser pulse. Here, detecting NV corresponds to the ms = 0 state−
while the detection of NV corresponds to the ms = ±1 state. This approach potentiallyreduces the time needed to e.g. measure the T1 time of NV centers by a factor of 25 due
to an increased signal-to-noise ratio [109].

In the future, NV centers in nanostructured SCD used as a scanning probe or in bulk SCD
will enable completely new applications in other scientic elds. For instance, NV-basedmagnetometers have become a common tool called

quantum diamond microscope for ana-lyzing the magnetic properties of geological samples [110]. However, further optimizing thenanofabrication seems mandatory. Based on our experiments on wafer-scale SCD, it is potentially possible to overgrow the wafer-sized SCD with a silicon-free high purity diamondlayer using a homoepitaxial CVD process.

This would allow to grow high quality SCD

comparable with the commercially available electronic-grade standard. This would enableupscaling nanofabrication to the wafer-scale potentially reducing the fabrication cost per
device by an order of magnitude. This will attract more scientist from other research areas
to employ NV centers to their applications.
In addition, it will be necessary to further optimize the properties of shallow NV centers
in SCD. This could be reached by appropriate surface treatment of the devices.

Here,rst promising results have been achieved in our group by wet-chemically terminating the
SCD surface using uorine [111]. A stabilized negative charge state of the NV center could
be advantageous for sensing applications due to an enhanced signal-to-background ratio.
This enhancement is caused by reducing the amount of measurements not containing any
sensing informations as the NV center converted to its neutral charge state during themeasurement. In addition, the results of our studies show a hydrophilic instead of a hydrophobic surface normally created by acid-cleaning the SCD [111]. This could lead to abetter implementation into biological relevant conditions and will enable novel applications
studying the magnetic uctuations and currents in cells and tissues.
Due to the NV centers' potential for bio-applications, we also started a collaboration withthe group of Marc Schneider at Saarland University. Here, we want to use our NV-scanningprobes to investigate the properties of magnetic nanoparticles used for magnetic heatingin cells. Hereby a clear characterization of the magnetic eld of the particle is necessary
to be able to eciently design particles for in-vivo use them inside human tissue.
By employing scanning-FRET, it will be possible to study the interaction of NV centerswith dierent kinds of acceptor systems in detail.

Here, concerning life science applica-

tions, especially the interaction with a variety of luminescent particles and dye molecules
used as bio-markers is of major interest. By employing the multifunctional sensing capabilities of NV centers, it enables a spatially resolved in-depth look on processes usuallyoccurring in cells. However, scanning-FRET also allows the development of a novel technique to determine the depth of NV centers under the SCD surface.

By approaching a

FRET-partner with a known FRET radius to a shallow NV center, it is possible to use thedistance-dependency of the interaction to precisely extract the depth of the NV center: By
attaching a dye molecule to the scanning probe tip of our AFM setup and approaching it
to a shallow NV center in bulk SCD as well as by approaching a NV center in a scanningprobe device to a FRET partner placed in the AFM, it is possible to precisely determine

111

8.4. Outlook

the depth of NV centers in scanning probe devices as well as in bulk SCD. In contrast to
the conventional approach of bringing a spin ensemble in form of an immersion oil onto thesurface and using NMR techniques [112], using scanning-FRET to determine the depth isadditionally applicable using color centers without a spin.
In addition, the NV center in SCD is a viable research tool for nanomaterials. For instance,
the magnetic eld sensing capabilities of the NV center might lead to new approaches tocharacterize magnetic nanoparticles. In particular, characterizing magnetic nanoparticles
with a uorescent shell might benet from the good spatial resolution achievable by combining magnetic eld sensing and scanning-FRET using a NV center as a scanning probe.
Furthermore, the possibility to detect small magnetic elds present on the surface of thinlms will enable to study e.g. the evolution of nanoscale antiferromagnetic domains andthe transition between paramagnetic and antiferromagnetic phases [71] or to study the
coupling between single antiferromagnetic layers [74]. Additionally, using NV-based magnetometry aid to enable studies on Skyrmions in ultrathin magnetic lms [69, 113].
In particular, the NV center's potential to sense physical quantities with an extraordinary
sensitivity combined with an exceptional spatial resolution under ambient conditions will
potentially revolutionize many applications. Here, we mention for example a project under
the coordination of Prof. Jörg Wrachtrup called

BrainQSens [114]. The aim of this projectis to use the NV center in SCD to sense neuronal magnetic elds. The signatures of theseelds are supposed to be used as an interface between humans and machines potentiallyenabling a better control of e.g. the movement of prostheses.
By establishing appropriate surface treatments ensuring bio-compatibility and combining
scanning-FRET with techniques enabling to sense temperatures and magnetic elds orcurrents simultaneously [115], the potential of NV-based sensing will lead to a completely
new insight of mechanisms and their occurrence in biological systems. This could lead toa better understanding of biological processes in general.

A. Supplementary informations
In the following sections, we will provide the supplementary informations for the publications in Chap. 2 and Chap. 3.

A.1. Chap. 2: Color center uorescence and spin
manipulation in single crystal, pyramidal diamond
tips
Color center uorescence and spin manipulation in single crystal, pyramidal
diamond tips

Chapter 2:

Richard Nelz, Philipp Fuchs, Oliver Opaluch, Selda Sonusen, Natalia Savenko, Vitali
Podgursky, and Elke Neu
Applied Physics Letters 109, 193105 (2016); doi: 10.1063/1.4967189

Supplementary information for manuscript: Color center fluorescence andspin manipulation in single crystal, pyramidal diamond tips
Richard Nelz,1 Philipp Fuchs,1 Oliver Opaluch,1 Selda Sonusen,1 Natalia Savenko,2 Vitali Podgursky,3 and Elke
Neu1, a)1)

Universität des Saarlandes, FR 7.2 Experimentalphysik, 66123 Saarbrücken,
Germany2)
Artech Carbon OÜ, Jõe 5, 10151 Tallinn, Estonia
3)
Tallinn University of Technology, Department of Materials Engineering, Ehitajate tee 5, 19086, Tallinn,
Estonia(Dated: 6 October 2016)

This supplementary material summarizes technical details on the experimental setup. We also present additional photoluminescence (PL) spectra and 3-dimensional PL maps of the diamond tips together with further
information on the numerical simulation of photonic properties. The supplementary material also gives further
details on the measurement and evaluation of optically detected magnetic resonance (ODMR) spectra andcolor center lifetimes. Finally, we here summarize details on the plasma treatments as well as the respective
electron microscopy images and PL spectra.
I.

EXPERIMENTAL SETUP

We analyze the photoluminescence (PL) of the diamondtips by mounting them onto a three-axes piezoelectricscanner (Attocube ANS xyz100) in our home built confocal microscope. We use a 532 nm DPSS laser (Integrated Optics, Matchbox 532L-21B) and an acousto-optic
modulator (AOM, Crystal Technologies, 3200-146, 11 ns
rise-time) to realize pulsed and continuous excitation of
our samples. A 100× microscope objective (Olympus,
LMPLFLN100X) with a numerical aperture (NA) of 0.8
focuses the excitation light onto the sample and collects the PL. Two dichroic mirrors (cut in wavelength:
560 nm) and a dielectric longpass filter (cut in wavelength: 650 nm) separate the excitation light from theemitted PL photons. We subsequently couple the PL toa single mode fiber (Thorlabs SM600, MFD 3.6 - 5.3 µm,
NA 0.1 - 0.14) which serves as a pinhole.
The single mode fiber directs the PL of the sample toour detection setup. This detection setup contains a
Hanbury-Brown Twiss interferometer (HBT) consistingof two avalanche photodiodes (APD, Excelitas SPCMAQRH-14) to obtain the time resolved PL rate of thedefects and a grating spectrometer from Princeton Instruments (Acton SpectraPro SP-2500 with Pixis 256OE
CCD) to measure the spectral properties of the PL.
Using microwave electronics (SpinCore PulseBlaserESRPRO, Stanford Research Systems SG384, MiniCircuits
ZASWA-2-50DR+) together with a moveable nearfieldantenna,1 we are able to perform optically detectedmagnetic resonance (ODMR) measurements of nitrogenvacancy (NV) centers as well as coherent manipulationof NV electronic spins.
To quantify the color centers’ excited state lifetime, we
perform pulsed excitation using a supercontinuum lasera) Electronic mail: elkeneu@physik.uni-saarland.de(NKT Photonics, EXW-12, pulse length ≈ 50 ps) with afilter system (Varia) to select a defined wavelength range.
Together with a time tagging module (PicoQuant, PicoHarp 300), this allows for time correlated (single) photon
counting with high temporal resolution.

II.

3-DIMENSIONAL PL MAPPING AND SPECTRA

Figure S1 summarizes 3-dimensional PL mapping of thetips and is obtained via the following method: First, wemeasure the PL on each point in the xy-plane. Subsequently, we change the focus to a diﬀerent z-position of
the pyramid and repeat the measurement. Thus, we construct a 3-dimensional PL map. Together with the PL
spectra at each z-position, we deduce information about
the density of color centers at the respective positions in
the pyramid. For the interpretation of the spectra, see
the main manuscript.
In the lower row of Fig. S1, a bright background emission is visible in a distance of about 2 µm to the pyramidbase. This background probably results from a contamination on the Si cantilever and is thus not related tothe diamond device itself. The epoxy used for gluing thetips to the cantilever only produces minor, broadband
PL observed when focusing the laser to the base of the
diamond pyramid.

III.

PL SIMULATIONS

To simulate the color centers’ emission characteristics inside the pyramidal tip, we use a commercial Finite Diﬀerence Time Domain (FDTD) software (Lumerical FDTD
Solutions). The software allows for broadband simulations, nevertheless we restrict our analysis to a single
wavelength of 700 nm, which corresponds to the center ofthe used filter window for NV sideband detection. The
pyramidal tip is abstracted as a frustum with a square2Countrate (Mcps)
10

(b)

2.76
2.42

8
2.07
1.73

6

1.38
4

1.03
0.690

2
0.345

Intensity (a.u.)

4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5

0.00

0
0

2

4

6

8

0.0
550

10

X-Position (µm)

700

750

800

850

900

1.0

Countrate (Mcps)
1.83
1.60
8
1.37
1.14

6

0.915
4

0.686
0.458

2
0.229

Intensity (a.u.)

Y-Position (µm)

650

Wavelength (nm)

10

0.00

0
0

2

4

6

8

0.8

0.2

600

650

700

750

800

850

900

Wavelength (nm)1.0Countrate (Mcps)
10

0.866
0.743
0.619

6

0.495
4

0.371
0.248

2
0.124

Intensity (a.u.)

0.990

8

0.8

Figure S2 shows the saturation measurement done at
the apex of the pyramid detecting PL in the wavelength
range from 680 to 720 nm. This detection window has
been chosen to separate the NV PL from the SiV PL. We
find a saturation power Psat = 377(6) µW and a maximal
countrate I∞ = 5.52(3) Mcps. The data are fitted with

0.6

0.4

0.2

0.00

0
0

2

4

6

8

0.0
550

10

X-Position (µm)

600

650

700

750

800

850

900

Wavelength (nm)FIG. S1. Mapping of NV fluorescence (680-720 nm) (a) andPL spectra (b) at the bottom (lower), the middle (middle) andthe apex (upper) of the tip. The SiV PL is rapidly decreasing
compared to the NV PL. Note that the countrates are measured through an OD1 neutral density filter. The spectra arenormalized on the maximum in NV PL.

base of 5 × 5 µm2 that narrows to 20 × 20 nm2 at thetop, a height of 10 µm and a refractive index of n = 2.4.
An electric point dipole, placed in the horizontal centerof the pyramid, represents the color center. Its vertical
position is varied over the height of the pyramid, corresponding to an experimental excitation in diﬀerent focal
planes (z-positions). The whole simulation area is surrounded by perfectly matched layers and is meshed with
15 × 15 × 15 nm3 Yee cells. A monitor, placed 100 nm
above the pyramid’s top, records the fields emitted towards the upper half-space. This allows us to calculate
the far field and thereby the power radiated into a specific NA.
As sensing applications often demand to maximize thephoton flux into the detection optics, our figure of meritis the ratio of the radiated power into a specific NA to
the radiated power in homogeneous diamond, here called
collection factor ξ.
ξ=

SATURATION

0.4

0.0
550

10

IV.

0.6

X-Position (µm)

Y-Position (µm)

600

the diamond. The refractive index n = 2.4 is determinedby the host material. Thus, the collection factor quantifies the power or the photon flux, respectively, into the
NA in a more handy and comparable way than absolute
powers do. Note that a high collection eﬃciency, usually defined with the denominator being the total timeaverage power radiated by the dipole in the specific dielectric environment, does not necessarily indicate a highphoton flux.

Countrate (Mcps)

Y-Position (µm)

(a)

PNA
Phom

(S1)

Note that the denominator Phom = n x P0 , with P0 the
total time-average power radiated by an electric dipole invacuum, is independent from the dipole’s position inside

5
4
3
P

2

sat

= 377(6)µW= 5.52(3) Mcps

1
0

0

500

1000

1500

2000

2500

Power (µw)

FIG. S2. Saturation measurement at the apex fitted with Eq.
S2.

the following function of the excitation power P including
a linearly increasing background PL B:
I(P ) = I∞ x

P
+ B xP
P + PSat(S2)

With this, we calculate an estimation of the PL countrate emitted into the first lens of our confocal microscope. First, we estimate the fraction of the whole NV
spectral intensity we detect to be roughly one third of
the full intensity. Knowing the transmission of our confocal setup (T = 44 %) and the detection eﬃciency of
our avalanche photodiodes (70 %) as well as the transmission of the OD1 filter, we estimate the countrate emitted
into the first lens to be 500 Mcps. This is easily measurable with a common commercial photodiode as thiscorresponds to PL in the 100 pW regime.
V.

LIFETIME MEASUREMENTS

We measure the excited state lifetimes of NV τNV and SiV(silicon-vacancy) τSiV along the z-axis of the pyramid.
By choosing suitable filters (680 - 720 nm for NV and
735 - 745 nm for SiV), we aim at separating the PL of
NV and SiV centers. Fitting the NV PL decay with a

3

1

(b)
NV

= 11.4(1) ns

Intensity (a.u.)

Intensity (a.u.)

(a)

0.1

0.01
0

10

20

30

40

50

SiV

0.1

= 11.5(2) ns= 0.81(3) ns

0.01

60

0

10

Time (ns)

20

30

40

50

60

Time (ns)FIG. S3. Excited state lifetime measurements plotted using
logarithmic scaling for the intensity for NV (a) fitted with asingle exponential decay and SiV PL (b) fitted with a double
exponential decay.

VI.

ODMR-SPECTRATo determine the suitability of the tips as magnetic sensors and to prove that the tips are single crystals, we firstmeasure ground state ODMR spectra of the embedded
NV centers in a PL detection window between 680 and⃗
720 nm. Applying a randomly-oriented magnetic field B
gives us ODMR-spectra with eight dips, comparable tothe measurement shown in Fig. S4. These eight dips corresponds to the four equivalent NV ⟨111⟩ orientations in
the diamond lattice. The occurrence of four pairs of resonances witnesses the single crystal nature of the tip as
any misoriented, polycrystalline grains would lead to additional resonances or blurring of the ODMR-spectrum.
For spin manipulations, it is helpful to align the mag-Intensity (a.u.)

1.01

1.00

0.99

0.98

0.97

0.96
2.7

2.8

2.9

3.0

(b)

1.02

1.01
1.00

Intensity (a.u.)

1.00
0.98
0.96
0.94
0.92
0.90
0.88

0.99
0.98
0.97
0.96
0.95

Rf = 2.804(2) GHz
FWHM = 12.9(5) MHz
Contrast = 7.4%

0.94
0.93

0.86
2.75

2.80

2.85

2.90

2.95

Frequency (GHz)

3.00

0.92
2.76

2.78

2.80

2.82

2.84

Frequency (GHz)⃗ aligned
FIG. S5. (a) Full ODMR-spectrum of a tip with B
onto one of the NV axes. (b) Zoom onto the ms = 0 → −1
transition at a frequency of 2.8 GHz; we find a linewidth of
12.9(5) MHz. The contrast is C = 7.4 %.

1

NV

(a)
Intensity (a.u.)

single exponential decay yields very good agreement withthe measured data [see Fig. S3(a)] and we extract τNV
on every z-position. For τSiV , fitting the measured datawith a double exponential decay is necessary [see Fig.
S3(b)] because of the NV sideband PL which also partly
falls in the wavelength range 735 - 745 nm. The double
exponential fit reveals τSiV and τNV , where the latter isconsistent with the independent determination of τNV in
the wavelength range 680 - 720 nm.

3.1

Frequency (GHz)

FIG. S4. Full ODMR-spectrum of the tip with randomly⃗aligned B.

⃗ onto one of these NV axes and to use thenetic field B⃗ is aligned. This is shown
sub-ensemble onto which B
in Fig. S5(a). The obtained ODMR-spectrum reveals acontrast of C = 7.4 % [see Fig. S5(b)]. The linewidth ofthe dip is 12.9(5) MHz at a central frequency of 2.8 GHz.
Assuming a T2∗ limited linewidth, this would indicate
T2∗ ≈ 13 ns. However, Ramsey type measurement reveal
T2∗ = 0.24(7) µs, indicating significant power broadening
in the ODMR spectra.
As visible in Fig. S5, there is still a splitting in the central dip of 8 MHz despite the alignment onto one of the
NV axes. This splitting could occur due to non-perfect⃗ because wealignment of the external magnetic field B,
use a permanent magnet mounted in a rotation stage ona three-axes linear translation stage. However, this splitting is also measurable when no external magnetic field
is applied. Therefore, we suspect strain in the diamonditself causing this splitting.2 However, we can not fully
rule out that this splitting results from residual magnetic
fields due to magnetized components in our setup whichis still present after removing the magnet.

VII.

PLASMA TREATMENTS OF DIAMOND TIPS

As a first step towards optimization of the tips formagnetometry, we tested cleaning of the diamond tip
using a plasma cleaning device (Diener Femto, 0.3 mbar,
4 min O2 plasma with 80 W RF power at 13.56 MHz).
We intended to remove possible surface contaminationor highly defective regions at the tip surface. This
treatment was not successful, but rather introduced
additional broadband luminescence (possible due toresidual contamination in the cleaning plasma), so thisapproach was not further investigated.
In a next step, we use an Ar/O2 plasma for etching
the diamond in accordance with previous approaches.3,4Similar plasma recipes have been found to provide
highly-anisotropic etching of diamond as well as an
etch rate almost independent of crystal orientation.
Such a plasma can also smoothly etch polycrystalline
diamond and does not selectively attack grain boundaries and defects (see Ref. 4 and the corresponding
supplementary material). We use Ar/O2 gases, 50 sccm
each, 18.9 mTorr, 500 W ICP, 200 W RF power in an

4(b)

The yield to form color centers (0.1 % for N→NV6 andIntensity (a.u.)

(a)

(b)

4.0

1.0

3.5

Intensity (a.u.)

(a)

3.0
2.5
2.0
1.5
1.0

0.8

0.6

0.4

0.2

0.5
0.0
550

◦

FIG. S6. 90 SEM picture of the tip after the second (a) andthird (b) etch step.

Oxford Plasmalab inductively coupled reactive ion
etcher (ICP-RIE), while the wafer is cooled to 20 ◦ C.
From etching of diamond pillars in (100) diamond using
Fox16 (Dow corning) electron beam resist as mask, wedetermine an etch rate of ≈ 110 nm/min and verify
anisotropic etching. Whereas ICP-RIE processes allow
for smooth, high-rate etching of diamond and other inert
materials, they simultaneously bear the risk of sample
heating in the plasma. The epoxy used to mount the
tip is stable up to 200 ◦ C (information from supplier).
To not exceed this temperature range, we restrict the
maximum duration of the plasma steps (see below). The
tip has been subjected to the following etch steps:
1. 90 s etching
2. 4 cycles of 60 s etching, 300 s cooling under Ar after
each etch step
3. 3 cycles of 60 s etching, 300 s cooling under Ar after
each etch step
From SEM images taken under an angle of 90◦ [see Fig.
S6], we estimate the rate, with which the height of the
pyramid is reduced during the etching to be 120 nm/min.
We note that despite significant removal of diamond thesurface roughness is still comparable and the pyramid
still shows a sharp apex. The PL measurements revealthat we are able to significantly reduce the SiV density
at the apex of the pyramid. This is shown in Fig. S7. In
Fig. S7, a clear change in the spectra is visible. The SiV
PL is drastically decreasing while the NV PL is nearly
unchanged (compared by 3-dimensional PL maps). Thus,
we are able to change the color center composition at theapex by our established plasma treatment. Please notethat all these etch steps are realized while the pyramid isglued to its cantilever. Thus, we implemented a tool totailor ready made devices.
VIII.

DISTANCE BETWEEN COLOR CENTERS

From previous work, we estimate 60 ppm silicon5 , while
our measurement tentatively suggest ≈ 10 ppm nitrogen.

600

650

700

750

800

Wavelength (nm)

850

0.0
550

600

650

700

750

800

850

Wavelength (nm)FIG. S7. PL spectra recorded at the pyramid apex before(a) and after (b) all etch steps. A significant change in SiV
density is visible. This means, that we are able to actively
change the color center composition at the apex of the tip.
Note that the spectrum in (b) has been measured using ahigher dispersion grating.

15 % for Si→SiV7 ) indicates the parameters from Tab. SI.
This means that in average one out of 232 unit cells con-TABLE SI. Parameters for the estimation of the mean distance between the color centers.
Density (cm−3 /ppm)
Conversion to color center
Color center dens. (ppm)
number of unit cells (1D)Distance between two (nm)NV
1.8 x 1018 /10
0.001
0.01
232
83

SiV
1019 / 57
0.15
≈8
28
10

tains a NV center and one out of 28 unit cells a SiV center. This gives an approximate mean distance between
centers < 10 nm at the apex. Thus, FRET between NV
and SiV centers might be feasible.

1 P. Maletinsky,

S. Hong, M. S. Grinolds, B. Hausmann, M. D.
Lukin, R. L. Walsworth, M. Loncar, and A. Yacoby, Nat. Nanotechnol. 7, 320 (2012).
2 J. Teissier, A. Barfuss, P. Appel, E. Neu, and P. Maletinsky,
Phys. Rev. Lett. 113, 020503 (2014).
3 P. Appel, E. Neu, M. Ganzhorn, A. Barfuss, M. Batzer, M. Gratz,
A. Tschöpe, and P. Maletinsky, Rev. Sci. Instrum. 87, 063703(2016).
4 E.
Neu, P. Appel, M. Ganzhorn, J. Miguel-Sánchez,
M. Lesik, V. Mille, V. Jacques, A. Tallaire, J. Achard,and P. Maletinsky, Appl. Phys. Lett. 104, 153108 (2014),
http://dx.doi.org/10.1063/1.4871580.
5 J. Barjon, E. Rzepka, F. Jomard, J.-M. Laroche, D. Ballutaud,
T. Kociniewski, and J. Chevallier, physica status solidi (a) 202,
2177 (2005).
6 J. R. Rabeau, A. Stacey, A. Rabeau, S. Prawer, F. Jelezko,
I. Mirza, , and J. Wrachtrup, Nano Lett. 7, 3433 (2007).
7 A. Bolshakov, V. Ralchenko, V. Sedov, A. Khomich, I. Vlasov,
A. Khomich, N. Trofimov, V. Krivobok, S. Nikolaev, R. Khmelnitskii, and V. Saraykin, physica status solidi (a) 212, 2525 (2015).

118

A. Supplementary informations

A.2. Chap. 3: Toward wafer-scale diamond nano- andquantum technologies
Chapter 3:

Toward wafer-scale diamond nano- and quantum technologies

Richard Nelz, Johannes Görlitz, Dennis Herrmann, Abdallah Slablab, Michel Challier,Mariusz Radtke, Martin Fischer, Stefan Gsell, Matthias Schreck, Christoph Becher, andElke Neu
APL Materials 7, 011108 (2019); doi: 10.1063/1.5067267

Supplementary information for manuscript:Towards wafer-scale diamondnano- and quantum technologies
Richard Nelz,1 Johannes Görlitz,1 Dennis Herrmann,1 Abdallah Slablab,1 Michel Challier,1 Mariusz Radtke,1Martin Fischer,2 Stefan Gsell,2 Matthias Schreck,3 Christoph Becher,1 and Elke Neu1, a)
1)

Universität des Saarlandes, Fakultät NT, Physik, 66123 Saarbrücken, Germany
Augsburg Diamond Technology GmbH, 86159 Augsburg, Germany
3)
Universität Augsburg, Institut für Physik, 86135 Augsburg, Germany
2)(Dated: 7 December 2018)

This supplementary material summarizes details on the estimation of the NV concentration in the diamond.
In addition, we estimate the power broadening of the NV optically-detected magnetic resonance and present
further data on NV spin manipulation and the fitting of SiV PL lines.
I. ESTIMATION OF NV DENSITY AND
CONCENTRATION OF SUBSTITUTIONAL NITROGEN

To estimate the expected number of NV centers, we use
the nitrogen concentration in the gas phase of ≈ 0.5 ppm
N2 . Ref. 1 finds for a concentration of 10 ppm N2 in thegas phase a concentration of substitutional nitrogen of≈ 55 ppb and a concentration of negatively-charged NV
centers of ≈ 0.2 ppb. Assuming similar incorporation andconversion efficiencies in our case, we expect a substitutional nitrogen concentration of ≈ 2.75 ppb and a concentration of negatively-charged NV centers of ≈ 0.01 ppb.
Now, we compare this value with the number of NVs weobserve in the diamond material.
Countrate (cps)
5560
4923

8
4285

[NV− ]measured ≈ 0.005 ppb
Thus, the observed density of NV centers agrees well withthe expected density.

II.

PL LIFETIME IMAGING

To characterize the lifetime of the native NV centers in
the diamond, we measure lifetime maps of different areasof the diamond. In Fig. S2, we show an exemplary measurement of the lifetime in a 10×10 µm2 large area of thesample. On average, we obtain a lifetime of 12.3(6) ns of
the NV centers.

3648

6

3010

4

1735

2
1098
460.0

0
0

1

2

3

4

5

6

7

8

9

10

x-Position (µm)

FIG. S1. xy-scan of a 10 × 10 µm2 -sized area of the diamond.
With a depth-to-focus of about 1 µm, we can estimate thedensity of NVs to 1 NV/µm3 .

We estimate the density of NV centers from the PL map
visible in Fig. S1. Hereby, we obtain ≈ 100 NV centersin an area of 10 × 10 µm2 . By taking into account thedepth-of-focus of the used confocal microscope (≈ 1µm),a) Electronic mail: elkeneu@physik.uni-saarland.de

Lifetime (ns)

10

2373

y-Position (µm)

y-Position (µm)

10

we find an observed NV density of 1 NV per µm3 . By
comparing this to the number of unit cells in this volume
and the number of carbon atoms per unit cell, we cancalculate the measured NV center concentration to

16.00
14.88

8

13.75

6

12.63
11.50

4

10.38
9.250

2
0

8.125

0

2

4

6

8

10

7.000

x-Position (µm)

FIG. S2. PL lifetime image of individual NV centers in our diamond. From the fitted data, we obtain a lifetime of 12.3(6) nsof the NVs.

2
III.

ION IMPLANTATION OF SHALLOW NV CENTERS

For several sensing-related applications and fabrication
procedures, it is important to form NV centers close to
the surface of the diamond.2 To obtain individual NV
centers in nanostructures, typical NV densities in the order of 109 cm−2 are necessary.2 Therefore, we use ion implantation to create such shallow NV centers. Here, we
use an implantation energy of 6 keV and doses of 1.5 and3 × 1011 cm−2 which leads to a formation of NV centers
on average 10 nm below the surface. After annealing, we
first move the implanted layer along the optical axis (zaxis) through the laser focus of our confocal microscope,
while recording the PL intensity. The recorded PL trace
is visible in Fig. S3. A clear PL peak coinciding withthe diamond surface is visible. The width of the peakis given by the optics of our confocal microscope. The
slight asymmetry of the PL trace arises due to the PL of
the SiV center ensemble in the diamond material which,despite careful spectral filtering, is still discernible whenobserving NV PL in our diamond material.

Countrate (cps)

30000
25000
20000

with τ1 and τ2 describing the anti-bunching and bunching time constants; a is a measure for the impact of the
shelving state.
We now take background contributions into account bydefining pf = s/(s+b) with the signal- s and backgroundcountrate b, leading to:(2)

gdet (τ ) = 1 + p2f { − (1 + a) exp(− |τ − τ0 | /τ1 )+ a exp(− |τ − τ0 | /τ2 )}This means, that the minimum achievable g (2) (0) is determined byg (2) (0) ≥ 1 − p2f = 1 −s2(s + b)2

In the main text, we show a g (2) measurement of a single
NV center in the presented diamond material. The investigated NV center has a photon countrate of s = 2870 cps
while the background is b = 719 cps determined from thePL map depicted in the main text. This leads to a mini(2)mum gmin (0) = 0.36, assuming that the background consists of fully uncorrelated emission. The observed background is attributed to PL from the SiV ensemble. Despite spectral filtering, minor parts of the SiV PL overlap
with our window for NV observation 680 − 720 nm.

15000

V. SPIN MANIPULATION OF IMPLANTED NVENSEMBLE

10000
5000
0
-5

-4

-3

-2

-1

0

1

2

3

4

distance from surface (µm)

FIG. S3. PL trace of the implanted layer of NV centers. A
clear PL peak is visible at the surface.

To show the basic usability of the implanted NV ensemble
layer for quantum technologies, we first measure an Optically Detected Magnetic Resonance (ODMR). To further
show the usability for more complex sensing schemes, we
coherently manipulate the spin states of the NVs. This,
we show by performing Rabi oscillations of a subensembleof the NV centers as depicted in Fig. S4.
(a)

(b)

1.04

1.02

To prove observation of single NV centers, we measuresecond order correlation functions g (2) (τ ). In general,g (2) is defined as
g (2) (τ ) =hI(t)I(t + τ )ihI(t)i2with the intensity I(t) and h·i indicates the time average.
By using a 3-level model to describe the NV center, themeasured g (2) function can be fitted by:3
g (2) (τ ) = 1 − (1 + a) exp(− |τ | /τ1 ) + a exp(− |τ | /τ2 )Intensity (a.u.)

IV. MEASUREMENT OF INTENSITYAUTOCORRELATION OF NATIVE NV CENTERS

Intensity (a.u.)

1.00
1.02

1.00

0.98

0.96

0.94

0.92
2.7

0.98
0.96
0.94
0.92
0.90
0.88
0.86

2.8

2.9

3.0

3.1

0

200

400

600

800

1000

Frequency (GHz)

Pulse duration (ns)FIG. S4. (a) ODMR measurement of the NV ensemble. The
different resonances belong to the different equivalent orientations of the NV center. (b) Coherent spin manipulation of
the NV subensemble resonant to 2.768 GHz, clear Rabi oscillations are discernible.

3
VI. ESTIMATION OF POWER BROADENING OFODMR LINEWIDTHTABLE S1. Parameters for the estimation of unbroadened
ODMR linewidth.

Fig. S5 shows the ODMR measurements obtained from asingle, native NV and an ensemble of shallowly-implanted
NVs. To calculate the linewidth of the ODMR resonances
(b)

1.04

1.00
0.98
0.96
0.94
0.92

Ensemble
15.46
5.81
0.8
7.94
10.67

0.95

0.90

0.85

0.80

0.90
2.75

single NV
7.9
8.6
0.8
1.9
7.7

1.00

1.02

Intensity (a.u.)

Intensity (a.u.)(a)measured linewidth (MHz)7Γ∞c (10 1/s)
s = Popt /Psat
Rabi frequency (106 1/s)
calc. unbroadened linewidth (MHz)

2.80

2.85

2.90

2.95

Frequency (GHz)

3.00

2.70

2.75

2.80

2.85

2.90

2.95

3.00

it would be possible to decrease the excitation power toaround s = 0.2. This would lead to a reduction of thelinewidths by a factor of ≈ 2.6. On the other hand, thiswould also decrease the obtainable contrast of the resonance.

Frequency (GHz)

FIG. S5. ODMR measurements of a (a) single NV center and(b) of an ensemble of NV centers in the diamond. Both lines
have a comparably broad linewidth due to power broadening.
In the text, we estimate the unbroadened linewidth.

∆ν we use the formula developed in Ref. 4
s
2
Ω2
Γ∞
s
c
∆ν =
+ ∞ R∞
2π1+sΓp Γcwith the Rabi frequency ΩR , the rate of optical cycles
at saturation Γ∞c which is given by the inverse excitedstate lifetime, the polarization rate Γ∞p which is givenby the inverse lifetime of the metastable singlet state (inthe order of 200 ns at room temperature) and the ratio
between the optical pumping power and the saturation
power of the transition s. Table S1 summarizes the estimated data. To further narrow the ODMR linewidth,

VII.

FITTING OF SIV PL LINESTo determine the linewidth of the SiV ZPL components,
we fit Voigt profiles to the measured PL lines. To accountfor the instrument response function of the spectrometer,
we used a fixed Gaussian width of wG = 0.0452 nm. The
Lorentzian width of the line is determined using a leastchi-square fitting algorithm implemented in Origin (Origin Lab Corpoation).
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87, 063703 (2016).
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Lett. 85, 290 (2000).
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List of publications
Refereed journal publications
 Color center uorescence and spin manipulation in single crystal, pyramidal diamond

tips

Richard Nelz, Philipp Fuchs, Oliver Opaluch, Selda Sonusen, Natalia Savenko, Vitali
Podgursky, and Elke Neu
Applied Physics Letters 109, 193105 (2016); doi: 10.1063/1.4967189

 Nanoscale Sensing Using Point Defects in Single-Crystal Diamond: Recent Progresson Nitrogen Vacancy Center-Based Sensors

Ettore Bernardi, Richard Nelz, Selda Sonusen, and Elke Neu
Crystals 7(5), 124 (2017); doi: 10.3390/cryst7050124

 Toward wafer-scale diamond nano- and quantum technologies 1
Richard Nelz, Johannes Görlitz, Dennis Herrmann, Abdallah Slablab, Michel Challier, Mariusz Radtke, Martin Fischer, Stefan Gsell, Matthias Schreck, Christoph
Becher, and Elke Neu
APL Materials 7, 011108 (2019); doi:10.1063/1.5067267

 Spin Measurement of NV Centers Coupled to a Photonic Crystal Cavity
Thomas Jung, Johannes Görlitz, Benjamin Kambs, Christoph Pauly, Nicole Raatz,
Richard Nelz, Elke Neu, Andrew M. Edmonds, Matthew Markham, Frank Mücklich,
Jan Meijer, and Christoph Becher
accepter for publication in APL Photonics (2019) Reliable Nanofabrication of single-crystal diamond photonic nanostructures for nanoscalesensingMariusz Radtke, Richard Nelz, Abdallah Slablab, and Elke Neu
Micromachines 10, 718 (2019); doi: 10.3390/mi10110718

 Nanoscale sensing based on nitrogen vacancy centers in single crystal diamond andnanodiamonds: achievements and challenges

Mariusz Radtke, Ettore Bernardi, Abdallah Slablab, Richard Nelz, and Elke Neu
Nano Futures, in press (2019); doi: 10.1088/2399-1984/ab5f9b

 Near-eld energy transfer between a luminescent 2D material and color centers indiamondRichard Nelz, Mariusz Radtke, Abdallah Slablab, Zai-Quan Xu, Mehran Kianinia,
Chi Li, Carlo Bradac, Igor Aharonovich, and Elke Neu
Adv. Quantum Technol. 1900088 (2019); doi: 10.1002/qute.201900088

1 This paper was selected as an Editor's Pick Plasma treatments and photonic nanostructures for shallow nitrogen vacancy centersin diamond

Mariusz Radtke, Lara Render, Richard Nelz, and Elke Neu
Opt. Mater. Express 9, 4716-4733 (2019); doi: 10.1364/OME.9.004716

Refereed conference proceedings
The proceedings marked with?

were presented by the author of this thesis.

 Optimized nano-fabrication of diamond scanning probes for novel imaging techniques
Selda Sonusen, Amandeep Kaur, Richard Nelz, Ettore Bernardi, and Elke Neu
Hasselt Diamond Workshop 2016, Hasselt (Belgium), March 2016

 Single crystal, pyramidal diamond tips as sensors for scanning probe magnetometry
Richard Nelz, Philipp Fuchs, Oliver Opaluch, Selda Sonusen, Natalia Savenko, Vitali
Podgursky, and Elke Neu
Hasselt Diamond Workshop 2017, Hasselt (Belgium), March 2017

 Toward Nitrogen Vacancy Center based Scanning Near Field Microscopy
Ettore Bernardi, Richard Nelz, Alexander Meyer, Selda Sonunsen, Michel Challier,and Elke Neu
Quantum 2017, Torino (Italy), May 2017

 Fluorescent single crystal, pyramidal diamond tips as photonic structures for scanningprobe magnetometry ?

Richard Nelz, Philipp Fuchs, Oliver Opaluch, Selda Sonusen, Natalia Savenko, Vitali
Podgursky, and Elke Neu
CLEO Europe, Munich (Germany), June 2017

 Optimizing nanoscale sensing using color centers in diamond
Richard Nelz, Alexander Meyer, Ettore Bernardi, Philipp Fuchs, Oliver Opaluch,
Selda Sonusen, Michel Challier, Natalia Savenko, Vitali Podgursky and Elke Neu
COST Action MP1403 Nanoscale Quantum Optics, Single Photons Single Spins conference II, Troyes (France), August 2017

 Lifetime study of single nitrogen vacancy (NV) centers in nanostructures: toward

NV-based scanning near eld microscopy

Ettore Bernardi, Richard Nelz, Alexander Meyer, Michel Challier, Selda Sonusen,
and Elke Neu
DIADEMS and SIRTEQ Quantum Technologies Workshop 14-15 September 2017

 Towards Energy Transfer-Based Sensing and Imaging with Color Centers in Single-

Crystal Diamond ?

Richard Nelz, Alexander Meyer, Michel Challier, Selda Sonusen, Ettore Bernardi,
and Elke Neu
MRS Fall Meeting 2018, Boston (USA), November 2017 Color Centers as Atomic-sized Sensors for Nanoscale Fields
Mariusz Radtke, Abdallah Slablab, Richard Nelz, Michel Challier, Philipp Fuchs,
Selda Sonusen, Ettore Bernardi and Elke Neu
COST Action MP1403 Nanoscale Quantum Optics Workshop, Prague (Czech Republic), February 2018

 Towards Energy Transfer-based Sensing and Imaging with Color Centers in Single-

Crystal Diamond ?

Richard Nelz, Michel Challier, Ettore Bernardi, and Elke Neu
Groupreport at DPG Spring Meeting 2018, Erlangen (Germany), March 2018

 Towards optimized scanning probe sensors with shallow nitrogen vacancy color centers
Michel Challier, Richard Nelz, Philipp Fuchs, Benjamin Kiendl, Frank Müller, SamuelGrandthyll, Anke Krueger and Elke Neu
Hasselt Diamond Workshop 2018, Hasselt (Belgium), March 2018

 Optical near-eld probe based on individual NV color centers
Abdallah Slablab, Richard Nelz, Mariusz Radtke, Michel Challier, and Elke Neu
International CECAM Workshop: Crystal defects for qubits, single photon emittersand nanosensors, Bremen (Germany), July 2018

 Color centers in diamond: single spin quantum sensors for nanoscale imaging
Abdallah Slablab, Richard Nelz, Mariusz Radtke, Michel Challier, and Elke Neu
Joint European Magnetic Symposia Conference, Mainz (Germany), September 2018

 Diamond as sensing tool:surface treatments for controlling the charge state of nitrogenvacancies in nanophotonic devices

Mariusz Radtke, Abdallah Slablab, Richard Nelz, Michel Challier, Benjamin Kiendl,
Frank Müller, Anke Krueger and Elke Neu Hasselt Diamond Workshop 2019, Hasselt(Belgium), March 2019

 Toward coupling color centers in single crystal diamond to two-dimensional materials
Oliver Opaluch, Richard Nelz, Michel Challier, Mariusz Radtke, Abdallah Slablab,
Zaiquan Xu, Igor Aharonovich, and Elke Neu
CLEO Europe, Munich (Germany), June 2019

 Diamond as Sensing Tool: NV− Color Centers in Ultrasensitive Devices ?
Mariusz Radtke, Abdallah Slablab, Richard Nelz, Michel Challier, Nimba Oshnik,
and Elke Neu
ICMAT 2019, Singapore (Singapore), June 2019

Workshops and schools
During the work on this thesis, the author participated at the following schools and workshops:

 DIADEMS Diamond and Spins School, Cargèse (France), May 2016 International School of Physics Enrico FermiCourse on Nanoscale Quantum Optics, Varenna (Italy), July 2018

 WE-Heraeus Seminar on Diamond Quantum Techologies, Bonn (Germany), March
2019

Danke!
An dieser Stelle möchte ich mich bei den vielen Menschen bedanken, die das Gelingendieser Arbeit ermöglicht haben.

An erster Stelle möchte ich mich bei meiner Betreuerin Elke Neu-Rung für die Möglichkeit
zur Promotion bedanken. Ich möchte mich insbesondere für das mir entgegengebrachte Vertrauen und für die bereichernden Beiträge zu meiner Arbeit bedanken. Ebenfalls möchte
ich mich für das Korrekturlesen meiner Publikationen und dieser Dissertation bedanken.
Ebenso bin ich dankbar für die Möglichkeit, meine Arbeit auf vielen Konferenzen an interessanten Orten vorstellen zu dürfen.

Ich muss mich ebenfalls dafür bedanken, dasssie mir bei der Entwicklung der experimentellen Aufbauten sowie der Verwirklichung geplanter Projekte und Messungen weitestgehend freie Hand gelassen hat, sodass ich mirmeine eigene Expertise auf dem Gebiet der quantenbasierten Sensoren erarbeiten konnte.
Uwe Hartmann danke ich ganz herzlich für die Bereitschaft, diese Arbeit als Zweitgutachterzu lesen.

Ein groÿes Dankeschön gilt natürlich auch den aktuellen aber auch allen ehemaligen Kollegen unserer Arbeitsgruppe. Ohne die vielen ergiebigen Diskussionen während meiner Zeit
als Doktorand wären viele Ideen zu der Weiterentwicklung der Setups und zur Verwirklichung neuer Experimente nie entstanden. Ebenfalls konnte ich durch die Interdisziplinarität unserer Gruppe auch einmal über den Tellerrand hinaus schauen.
In diesem Zusammenhang muss ich auch den Studenten danken, denen ich während meiner
Promotion helfen durfte, ihren eigenen Weg zu nden.

Durch die vielen interessierten

Nachfragen und Kommentare haben sie mir geholfen, neue Impulse für unsere Experimente zu nden. Alexander Meyer danke ich bei der groÿen Unterstützung beim Aufbau
des AFM-Confocals. Lara Render danke ich für die Unterstützung bei der Arbeit an derPlasmabehandlung von Diamant. Ein groÿer Dank gilt Marie Niederländer für die Hilfe
bei der Realisierung der Charge-State-Detection, aber auch für die Abende, die wir zum
Entspannen in der Boulderhalle verbracht haben.

Ebenfalls ein groÿer Dank gilt Oliver

Opaluch, dessen Weg ich vom Studienanfänger über Bachelor- und Masterstudent bis hin
zum Promotionsstudenten begleiten durfte. Ebenfalls würde ich die vielen Sitzungen an
den Wochenenden, bei denen wir stundenlang gezockt, haben nicht missen wollen.

Natürlich gilt auch ein Dank den Kooperationspartnern, ohne die eine Vielzahl von Experimenten in dieser Arbeit nicht möglich gewesen wären. Matthias Schreck und AuDiaTec
danke ich dabei für die Stücke des wirklich auÿergewöhnlichen Diamantwafers. I would liketo thank Vitali Podgursky and Artech Carbon for the pyramidal diamond scanning probes.
In addition, I want to thank Igor Aharonovich and his group for the excellent collaboration
on FRET with WSe2 .

Ein weiterer Dank gilt auch Christoph Becher, dessen Betreuung meiner Bachelor- undMasterarbeit geholfen hat, meinen Weg in die Wissenschaft zu nden.

Seiner Gruppe

möchte ich für die vielen Diskussionen während meiner Abschlussarbeiten und meiner Promotion danken. Insbesondere muss ich hierbei Andreas Lenhard, Christian Hepp, BenjaminKambs und Matthias Bock dafür danken, dass sie mich in die Kunst der Justage von optischen Systemen eingeführt haben. Der Aussage von Andreas Wer einen cw-OPO justierenkann, kann alles justieren, kann ich rückblickend nur zustimmen. Ebenfalls möchte ich Johannes Görlitz und Alexander Bommer für die vielen nützlichen, aber auch für die weniger
seriösen Diskussionen danken. Ein besonderer Dank gilt Philipp Fuchs und Anna Breunig
für die vielen netten Abende auÿerhalb der Uni.

Auÿerhalb des Labors ist die fordernste und komplexeste Aufgabe während der Promotion
den Umgang mit der Bürokratie zu meistern.

Bei der Unterstützung dabei und für diestets freundliche Auskunft bei bürokratischen Fragen möchte ich Elke Huschens danken.
Ebenfalls möchte ich der mechanischen Werkstatt danken, ohne die unsere optischen Tischewahrscheinlich noch recht leer wären. Bei Stefan Loew möchte ich mich für das Anfertigen
einer Vielzahl an Netzteilen etc. bedanken.

Meinen ehemaligen Lehrern Julia Christians und Burkhard Wolf möchte ich danken, dasssie mein Interesse an den Naturwissenschaften gefördert haben, ohne das ich wohl nie begonnen hätte zu studieren.

Aber der allergröÿte Dank gilt meiner Familie und meiner besseren Hälfte Kerstin. Ohneeure Unterstützung während meines bisherigen Lebens hätte ich nie den Weg bis hierhingeschat. Ich danke euch dafür, dass ihr immer für mich da seid, mir Kraft gibt und es
immer schat, mich von der Uni abzulenken.
Ohne euch wäre mein Leben nicht annähernd so schön, wie es ist.