Home

SearchCollections

Journals

About

Contact us

My IOPscience

Recovery of nanomolecular electronic states from tunneling spectroscopy: LDOS of lowdimensional phthalocyanine molecular structures on Cu(111)This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2013 Nanotechnology 24 395704(http://iopscience.iop.org/0957-4484/24/39/395704)
View the table of contents for this issue, or go to the journal homepage for more

Download details:
IP Address: 129.13.74.93
The article was downloaded on 06/09/2013 at 07:53

Please note that terms and conditions apply.

IOP PUBLISHINGNANOTECHNOLOGY

Nanotechnology 24 (2013) 395704 (11pp)

doi:10.1088/0957-4484/24/39/395704

Recovery of nanomolecular electronic
states from tunneling spectroscopy:
LDOS of low-dimensional phthalocyanine
molecular structures on Cu(111)
Y Yamagishi, S Nakashima, K Oiso and T K Yamada
Graduate School of Advanced Integration Science, Chiba University, 1-33, Yayoi-cho, Inage-ku,
Chiba-shi 263-8522, Chiba, Japan
E-mail: toyoyamada@faculty.chiba-u.jp

Received 14 May 2013, in final form 6 August 2013
Published 5 September 2013
Online at stacks.iop.org/Nano/24/395704
Abstract
Organic nanomolecules have become one of the most attractive materials for new
nanoelectronics devices. Understanding of the electronic density of states around the Fermi
energy of low-dimensional molecules is crucial in designing the electronic properties ofmolecular devices. The low dimensionality of nanomolecules results in new electronic
properties owing to their unique symmetry. Scanning tunneling spectroscopy is one of themost effective techniques for studying the electronic states of nanomolecules, particularly near
the Fermi energy (±1.5 eV), whereas these molecular electronic states are frequently buriedby the tunneling probability background in tunneling spectroscopy, resulting in incorrect
determination of the molecular electronic states. Here, we demonstrate how to recover
nanomolecular electronic states from dI/dV curves obtained by tunneling spectroscopy.
Precise local density of states (LDOS) peaks for low-dimensional nanostructures (monolayer
ultrathin films, one-dimensional chains, and single molecules) of phthalocyanine (H2 Pc)
molecules grown on noble fcc-Cu(111) were obtained.
(Some figures may appear in colour only in the online journal)

1. Introduction

been carried out and new electronic (spin) properties havebeen exhibited by single molecules [5–11], single-molecular
chains [12], and monolayer (ML) molecular films [13, 14] on
metal substrates. In particular, the local density of states ofnanomolecules varies greatly owing to their unique symmetry,
i.e., surface or interface effects mainly determine thenanodevice properties [5, 15]. Precise LDOS measurementsof low-dimensional molecular structures (single molecules,
one-dimensional (1D) molecular chains, and ultrathin films)
are expected to open a new pathway toward nanoscale organic
molecular electronics. A molecular film with a thicknessof 10–100 nm has the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital (LUMO)
states, which are known as bulk states. The bulk states willchange when the film thickness is decreased to 1–10 nm. Such

Organic molecules have enabled the realization of cheap,
flexible, and printed electronic devices, which are nowadays
widely used for organic solar cells [1], organic electroluminescence devices [2], organic displays [3], and organic
light-emitting diodes [4]. So far, organic molecular films
with a thickness of 10–100 nm grown on a substrate
have been used, and electron or hole transfer throughmolecular junctions has determined the electronic propertiesof devices. Minimization of the device size is an important
challenge for near-future electronics to realize low-cost
devices with low power consumption and high functionality,
which is why studies on nanoelectronics and nanoscience
have rapidly developed in the last decade. Fundamental
studies on nanomolecules of size 1–10 nm have recently
0957-4484/13/395704+11$33.00

1

c 2013 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 24 (2013) 395704

Y Yamagishi et alfilms are usually treated as interfaces and it is well known
that interfaces have different electronic states. However, so far
the electronic states and molecular orbitals at interfaces havenot been studied in depth, since present molecular devices
use bulk states and the interface effect is a minor problem.
However, at the scale of nanomolecules (of size 1–10 nm),interface or surface effects become a major issue since
they will determine the electronic properties of near-futurenanomolecular electronic devices.
One of the most effective techniques for precisely
measuring the LDOS of 1-nm-size molecules is tunnelingspectroscopy with a scanning tunneling microscopy (STM)
setup, known as scanning tunneling spectroscopy (STS).
This technique measures the so-called I(V) curves or dI/dV
curves, which are the tunneling conductivity and differential
tunneling conductivity as functions of the sample bias voltage,
respectively. The bias voltage VS = 0 corresponds to the
Fermi energy, negative (VS < 0) and positive (VS > 0)
voltages indicate energies below and above the Fermi energy,
respectively. Since the dI/dV curves are a convolution of
the surface LDOS and the tunneling probability function(T background) [16], the LDOS can be obtained by thenormalization of dI/dV curves using the T background,i.e., (dI/dV)/T below the Fermi energy (VS < 0) indicatesthe occupied LDOS (HOMO) and (dI/dV)/T above theFermi energy (VS > 0) indicates the unoccupied LDOS(LUMO) [15–18]. However, owing to the time-consuming
nature of this normalization, dI/dV or even I(V) curves havemainly been used to study the LDOS. For metal surfaces withstrong and sharp surface-state peaks, it is possible to study
the LDOS without the above normalization [22, 25], but for
samples with a broad, weak, or complicated LDOS near the
Fermi energy, it is impossible to recover precise LDOS peaks(for example, [18]).
So far, molecules have been used as semiconductormaterials in electronic devices, and therefore the HOMO
and LUMO states are the most important key parameters
determining the electronic properties. The LDOS near the
Fermi energy has not been focused on since it lies inside thegap. However, it has recently been found that nanomolecules,
especially π -conjugated single phthalocyanine molecules, on
metal surfaces have metal-like properties (a large conductivity
even at the Fermi energy as well as a magnetoresistanceeffect) [5–8], which is expected to open a new pathway
toward using nanomolecules as new conductive and magnetic
materials. The electronic properties of 1-nm-size single
molecules are strongly affected by the electronic states of thesubstrate electrode, which shift the HOMO/LUMO states ofthe molecules and are likely to close the band gap and switch
the electronic properties from semiconductor-like to metallic
properties. Therefore, obtaining a better understanding of the
LDOS near the Fermi energy of nanomolecules on metal
substrates is now very important for the development of next
generation nanomolecular electronics.
In this paper, we show that a normalization techniqueusing a correlated fitted tunneling probability functionc ) is a powerful technique for recovering nanomolecular(Tfitelectronic states from dI/dV curves obtained by tunnelingspectroscopy. This normalization method is described inc curves obtained for low-dimensional
details. (dI/dV)/Tfit
phthalocyanine (H2 Pc) molecules (monolayer films, onedimensional chains, and single molecules) on Cu(111)successfully recovered the molecular LDOS (HOMO and
LUMO) peaks near the Fermi energy (±1.5 eV), which showsthat H2 Pc nanomolecules have metallic properties (i.e., noband gap).

2. Experimental details
Scanning tunneling microscopy (STM) and spectroscopy(STS) experiments were performed at 4.6 and 300 K in
ultrahigh vacuum (UHV, base pressure <8 × 10−9 Pa). STS
measurements were performed by opening the feedback loopof the tunneling current, i.e., fixing the tip–sample separation(z). The tunneling current was measured at each pixel byvarying the sample voltage within ±1.5 V to obtain I(V)curves. In the case of metal surfaces, it has been reported
that dI/dV curves up to ±3 V are required for normalization[17, 18]. However, in the case of molecular surfaces, higher
voltages may damage the molecules. Thus, we only apply
a bias up to 1.5 V. Differential conductivity (dI/dV) curves
were obtained by numerical differentiation of the I(V) curves.
Although a lock-in amplifier was used to obtain the dI/dV
signals in our previous papers [5, 7], here we do not use alock-in amplifier so that we can distinguish between real andpseudogaps around the Fermi energy.
Tungsten (W) tips were used as STM tips, which were
electrochemically etched from W wires (φ = 0.3 mm, purity
99.9%) in air using KOH aq., while the etching process wasmonitored by optical microscopy. Subsequently, the tip wasrinsed with hot water and acetone, then transferred into theload lock chamber of our STM setup. The apex of the W tip
was sputtered with Ar+ in UHV and annealed at temperaturesof up to 2000 K to obtain a clean apex.
Cu(111) and Ag(001) single crystals (purity 99.999%)
were used as conductive substrates. A clean and atomicallyflat surface was obtained after several sputtering and annealing cycles in UHV. Phthalocyanine (H2 Pc) molecules wereused. Commercial H2 Pc powder (Alfa Aesar, purity 95%)
was purified by sublimation at 653 K and recrystallization
at 473 K at a pressure of 10−3 Pa (yield 30%). Nuclear
magnetic resonance (NMR) and infrared spectra confirmed
the absence of impurities after the purification. The clean
H2 Pc powder was placed in a crucible, which was set
in a molecular chamber and heated to 550 K. The H2 Pc
powder was deposited on the clean Cu(111) single crystals at
300 K by opening the shutter valve between the preparationand molecular chambers. The deposition rate on Cu(111) was
0.03 ML min−1 , calibrated by STM images.

3. Results and discussions
Organic molecules are now widely used as semiconductormaterials for electronic devices. Minimization of the devices
is one of the key issues in realizing high functionality, lowcost, and low power consumption. Recently, nanomolecules2Nanotechnology 24 (2013) 395704

Y Yamagishi et al

11 Å in figure 1) 1 , we obtain different dI/dV curves even on
the same sample surface as shown in figure 1(b).
dI/dV measurements have led to the discovery of many
surface states on metal surfaces including Cu(111) [20],Au(111) [21], Fe(001) [22, 23], Co(0001) [24], Mn(001)[17, 18], and Cr(001) [25]. These spectroscopy measurements
on metal surfaces are typically performed at a tunneling
resistance of R = 109 –1010  (e.g. VS = 1 V, I = 0.1–1.0 nA).
However, on molecular surfaces, care should be taken when
performing tunneling spectroscopy measurements.
Figures 1(c) and (d) show an example how H2 Pc
molecular structures are modified during STM measurements.
Here, we deposited about 0.3 ML H2 Pc molecules on aAg(001) surface at 300 K. The molecules diffuse on thesurface and accumulate at the steps, resulting in step-flow
growth. Molecules are observed as bright spots in thesefigures. We scanned the same area several times using the
STM tip with R = 2 × 1010 . Figures 1(c) and (d) show
STM images taken before and after the scan, respectively. It
is clear that now the molecular structures change as a result of
scanning (see the arrows in figures 1(c) and (d)). When R =109 –1010 , strong tip–sample interactions appear, which
are likely to modify the molecular structures during eachscan. This is not ideal for obtaining stable and reproducible
tunneling spectra since z must be constant during the tunnelingspectroscopy measurement (i.e., opened feedback loop).
A simple means of reducing the strong tip–sample
interaction is to increase the tunneling resistance, i.e., byincreasing the value of z, although a large value of z markedly
decreases the value of T. Also, the LDOS on the molecularsurface rapidly decays in vacuum, resulting in a weak dI/dV
signal.
In figure 1(b), we have shown how the value of z affects
dI/dV. Although an LDOS peak can be clearly observedin the dI/dV(1) curve, it is difficult to observe details of
the molecular electronic orbital states in the dI/dV(3) curve.
This might lead to an incorrect conclusion, for example,
that there is a gap around the Fermi energy. The result of
this simple simulation indicates that dI/dV measurements
on molecular surfaces are more difficult than those on
metal surfaces. For tunneling spectroscopy measurement at atunneling resistance larger than 1010 , a good signal-to-noise
ratio (S/N) is necessary to detect small signals in I(V) curves
and normalization using T is necessary to precisely recover
the molecular LDOS (HOMO/LUMO).
A large tunneling resistance (R) can be obtained byincreasing the tip–sample separation. However, on molecularsurfaces, maximum voltage set to 1.5 V should be applied to
avoid damage. Also, a typical commercial scanning tunnelingmicroscope has a current noise of 5–20 pA. These facts
prevent I(V) measurements at a large tunneling resistance,
for example, R = 1 × 1011  with VS = 1 V and I =10 pA. To perform I(V) measurements on molecular surfaces,
low-current-noise is necessary. We used a low-current-noise
amplifier and cables with a low capacitance (50 pF m−1 ),(single molecules, chains of molecules, and ultrathin
films) [5–14] on metal substrates have been found to
exhibit new electronic properties, such as metallic highconductivity and magnetoresistance effect [5, 6]. The origin of
these new electronic properties is a previously undiscovered
electronic interface state generated by electronic coupling
between the molecular orbitals and the metal LDOS.
Such 1-nm-size ultrasmall molecules on a metal substrate
are likely to exhibit new electronic properties instead ofthose intrinsic to semiconductor molecules. The electronic
properties are dependent on the LDOS near the Fermi
energy (HOMO/LUMO). Precise understanding of the LDOS
around the Fermi energy is the key to realizing new
nanomolecular conductive electronic (spintronic) devices.
Tunneling spectroscopy with an STM setup is one of themost effective methods of precisely obtaining the LDOS
of nanomolecules on metal substrates. However, the results
obtained from tunneling spectroscopy include not only the
LDOS but also the T background. The observed dI/dV
characteristics greatly vary with the T background, which
frequently results in incorrect values of the LDOS. Here, weshow how to recover LDOS peaks from the dI/dV curves byc.
normalization using Tfit
First, we explain a tunneling spectroscopy. The tunnelingcurrent (I) is plotted as a function of the sample bias
voltage (VS ) under constant tip–sample separation (z), which
gives the so-called I(V) (nA) curves. Then, numericaldifferentiation gives the differential conductivity dI/dV(nA V−1 ) curves, which correspond to the product of the
LDOS and the tunneling probability function (T) within the
WKB approximation [16–18];(dI/dV)/T ∝ LDOS,(1)

T = T + + T −,h pi
T + = a+ exp −b φ − VS /2 ,ih p
T − = a− exp −b φ + VS /2 ,

(2)
(3)
(4)

q
where b = 2 2m/h̄2 z ' 1.02z (m: electron mass, h̄: Planck’s
constant/2π , VS : sample bias voltage) and the dimension ofz is Å. φ denotes the averaged work function of the tip andsample, which is typically from 3 to 6 eV. a+ and a− areproportionality coefficients for positive and negative voltages,
respectively. Negative and positive voltages indicate occupied
and unoccupied density of states, respectively.
Figure 1(a) shows simulated LDOS (solid line) peaks
and tunneling probability functions (T, dotted lines). The
convolution of the LDOS(1) and T(1) curves is the dI/dV(1)curve shown in figure 1(b), in which a peak can still beobserved. Note that the LDOS detected by STM is that at
the apex of the tip (i.e., LDOS of the sample in vacuum,
0.5–1.0 nm above the surface). The LDOS decays withincreasing z. Also, T changes with z. Therefore, at different
tip–sample separations ((1) z = 9 Å, (2) z = 10 Å, (3) z =1 A tip–sample separation (z) is zero when the tip contacts to the sample [19],where the resistance is approximately 104 . As R ∝ V exp(2κz), at R =109 , z is estimated to be about 0.8–0.9 nm.

3

Nanotechnology 24 (2013) 395704

Y Yamagishi et al

Figure 1. (a) Simulated LDOS peaks (solid lines) and tunneling probability functions (T: open circles). LDOS peaks are simulated by
Gaussian functions, whose peak amplitudes are 1.5, 1.0, and 0.5 for (1), (2), and (3), respectively. Tunneling probability functions are
simulated with a constant averaged barrier height (φ) of 4.5 eV with different tip–sample separations (z) of 9, 10, and 11 Å for (1), (2), and(3), respectively. (b) Simulated dI/dV curves with T and LDOS peaks in (a). (c) and (d) show STM topographic images obtained at thesame surface of 0.3 ML H2 Pc on stepped Ag(001) (20 × 20 nm2 , Vs = −1300 mV, I = 50 pA). Single molecules (bright spots) accumulate
at atomic steps. Before and after ((c) and (d)) scanning the same area with an STM tip, the molecular structures changed, which are marked
by arrows.

which markedly decreased the base current noise to∼ 5 fA Hz−1/2 (see figure 2(a)), whereas noise remained at
100 Hz (15 fA Hz−1/2 ).
Test spectroscopy measurements were first performed
at a large tunneling resistance (up to R = 2 × 1011 )
on a Cu(111) surface. A clean and atomically flat Cu(111)
surface prepared by several cycles of sputtering and annealing
is shown in figure 2(b-1). The atomic steps follow the
hexagonal symmetry of fcc(111). The terrace widths are
typically 50–100 nm. Figures 2(b-2) and (b-3) show a low
energy electron diffraction pattern and an atomically resolved
STM image obtained on this surface, respectively. Figure 2(c)shows an I(V) curve obtained at R = 1 × 1011  by averaging
100 single curves to reduce statistical errors. The I(V)curve was numerically differentiated and normalized using its
fitted T. Figure 2(d) shows the recovered surface-state peakof Cu(111) at different tunneling resistances of 1 × 1010 ,3 × 1010 , 1 × 1011 , and 2 × 1011 . In all cases, the(dI/dV)/T curves successfully recovered the occupied LDOS
peak at approximately −150 meV. We consider that the S/Nratio of the current is sufficiently high to perform dI/dV
measurements at large tunneling resistances of 1010 –1011 
on molecular surfaces.
To evaluate how well the T normalization techniquerecovers molecular surface states, we used H2 Pc nanomolecular structures (ultrathin films, 1D chains, and single
molecules) on Cu(111). H2 Pc molecules have been widelyused for fundamental research [5–8] as well as in molecularelectronics [1]. The Cu(111) substrate is also a typical fcc
close-packed noble metal surface, whose electronic structure
has been studied in detail [20].
Figure 3 shows STM results obtained on Cu(111)
surfaces with ((a), (b)) 0.7 ML and ((c), (d)) 1.5 ML H2 Pc
molecules. As shown in figure 3(a), single molecules were
found adhering to the Cu(111) surface along three different
directions: −15◦ , +15◦ , and +45◦ from the [1̄10] directionof fcc-Cu(111), i.e., molecules have stable sites of A, B, andC along directions with intervals of 30◦ . Figure 3(b) shows amodel of H2 Pc molecules on a Cu(111) surface, assuming the4Nanotechnology 24 (2013) 395704

Y Yamagishi et al

Figure 2. All measurements were performed on a clean and atomically flat fcc-Cu(111) surface. (a) Noise spectrum of tunneling current of
our STM setup. (b-1) shows an STM topographic image (150 × 75 nm2 , Vs = −2 V, I = 50 pA). Several terraces are observed. (b-2) A low
energy electron diffraction pattern with a kinetic energy of 113 eV. (b-3) An atomically resolved STM image (1.5 × 1.5 nm2 , Vs = −2 mV,
I = 2 nA). (c) An I(V) curve averaging 100 single curves at R = 6 × 1010 . (d) Normalized (dI/dV)/T curves correspond to LDOS atdifferent tunneling resistances of R = 1 × 1010 , 3 × 1010 , 1 × 1011 , and 2 × 1011 .

atoms to be spherical. 36, 30, and 25% of the single molecules
adhered on the surface at sites A, B, and C, respectively.
Figures 3(c)–(e) show STM topographic images obtainedon the surface of 1.5 ML H2 Pc molecules on Cu(111).
Molecules grew in a layer-by-layer mode (figure 3(c))
and formed a densely packed tetragonal lattice structure.
Three different domains were observed with alignments
corresponding to site A, B, and C of single molecules asshown in figures 3(d) and (e).
We performed tunneling spectroscopy measurements on
this densely packed film surface. Since the molecular film hasa higher stability than single molecules and molecular chains,
we obtained stable tunneling spectra at R = 1 × 1010 –2 ×
1011  and even at 300 K. An experimentally obtained dI/dV
curve (dotted line) is shown in figure 4(a). Owing to the large
R, the obtained dI/dV value at −1.5 V (0.05–0.10 nA V−1 ) ismuch smaller than dI/dV values measured on metal surfaces
at −1.5 V (typically 5–10 nA V−1 ) [17, 18]. Shoulders
and bumps are observed near the Fermi energy, whichindicate molecular LDOS peaks. Although T normalization
can recover the peaks, it should first be determined whether
there is a gap at the Fermi energy. If a dI/dV curve witha gap is normalized by T, a pseudopeak can appear in the(dI/dV)/T curve. The dI/dV curve in figure 4(a) has a valueof 0.003 nA V−1 at the Fermi energy (VS = 0), which meansthat the first H2 Pc layer on Cu(111) has no gap, i.e. it exhibitsa metallic property. We can now normalize the dI/dV curveusing T to recover the molecular LDOS.
An important point in the normalization is how to obtainthe T background from the experimentally obtained dI/dV
curve. T normalization on metal surfaces is carried out asfollows [17, 18]. At higher voltages, the main contribution
to dI/dV is the exponential T background instead of LDOS
signals, which means that T can be easily obtained by fitting
to dI/dV values at high voltages. dI/dV curves are measured
up to ±3 V. T+ (equation (3)) is obtained by fitting to thedI/dV values in the range of +2.5–+3.0 V. The average work
function φ was assumed to be about 4 eV. The two parametersa+ and b are obtained. In the same way, T − (equation (4))
is fitted to dI/dV values in the range of −2.5 to −3.0 V,
and a− is obtained. Finally, from the fact that T = T + + T −(equation (2)), the experimentally obtained dI/dV curve isnormalized. A (dI/dV)/T curve (equation (1)) is proportional
to the LDOS.
5

Nanotechnology 24 (2013) 395704

Y Yamagishi et al

Figure 3. (a) An STM topographic image obtained on 0.7 ML H2 Pc on Cu(111) at 4.6 K (14 × 10 nm2 ). Single H2 Pc molecules areobserved. (b) A model of single H2 Pc molecules on a fcc-Cu(111) surface. Spheres reveal atoms. Three absorption sites were observed.
(c)–(e) show STM topographic images obtained on 1.5 ML H2 Pc on Cu(111) at 300 K (20 × 20 nm2 for (c) and (e), 30 × 30 nm2 for (d)).
First and second layers are observed. Three different domains were observed, which alignments correspond to site A, B, and C of single
molecules.

We normalized dI/dV using the corrected fitted tunnelingc (thick gray line in figure 4(a),probability function Tfit
c curve
where c = 0.008 nA V−1 ). The obtained (dI/dV)/Tfit
successfully recovers the molecular surface-state peaks (seefigure 4(b)). Although in our previous studies the dI/dV curvemeasured on single H2 Pc molecules could not resolve suchmolecular states in detail even using a lock-in amplifier [5, 7],in this study, because of the improved S/N of the tunnelingc normalization, six LDOS
current as well as the use of Tfit
peaks (Gaussian fitting) below and above the Fermi energy
are confirmed, corresponding to HOMO states at −0.21,−0.38, and −0.93 eV, and LUMO states at +0.27, +0.54, and+1.21 eV.
On the same sample surface, we investigated how thec isparameters in T + and T − affect the normalization. If Tfit
too sensitive to the fitting parameters, slight changes in theparameters, such as, φ and z, can markedly change T, causingcdifferent LDOS peaks to be recovered. Thus, we obtained Tfit
with different work functions (φ of 3.5, 4.0, 4.5, and 5.0 eV)
as well as with different tip–sample separations (z of 10
and 11 Å), then normalized the dI/dV curve in figure 4(a).
c curves.
Figures 4(c) and (d) show the obtained (dI/dV)/Tfitc
In spite of using different fitting parameters, the (dI/dV)/Tfit
curves are almost identical. In particular, the peak energy
positions marked by dotted lines are exactly the same,i.e., slight differences in the parameters do not significantly

However, on molecular surfaces, it is difficult to measure
the tunneling spectra at high voltages which damage themolecules (e.g., figures 1(c) and (d)). In this study, we wereable to measure reliable tunneling spectra up to ±1.5 V while
keeping the tunneling resistance larger than R = 1 × 1010 (e.g. as indicated in figure 4(a), there was no damage toH2 Pc molecules after the measurements, whereas an applied
voltage of more than 2.5 V damaged the molecules even at
R = 1011 ).
In this study, T + in equation (3) was fitted to dI/dV+
values in the range of +1.0–+1.5 V, and Tfit
in figure 4(a)−(thin black line) was obtained. T in equation (4) was fitted−
to dI/dV values in the range of −1.0 to −1.5 V, and Tfit
in figure 4(a) (thin gray line) was obtained. Note that theshape of the dI/dV curve in figure 4(a) above +1 V andbelow −1 V appears to be linear rather than exponential.
+−
Therefore, Tfit = Tfit
+ Tfitfrequently becomes larger thanthe experimentally obtained dI/dV around the Fermi energy(Tfit > dI/dV). However, in principle, the T background mustbe smaller than dI/dV. Therefore, we need to correlate Tfit byintroducing an offset c (nA V−1 ),c(dI/dV)/Tfit∝ LDOS,

(5)

+
−
c
Tfit
= Tfit+ Tfit− c > 0,(6)c = T(VS = 0) − dI/dV(VS = 0).

(7)
6

Nanotechnology 24 (2013) 395704

Y Yamagishi et al

Figure 4. Tunneling spectroscopy measurements on a first H2 Pc layer on Cu(111) were performed with a tip condition of No. 1. (a) An+−
experimentally obtained dI/dV curve (open circles) with a tip condition of No. 1 at R = 2 × 1010  was fitted by Tfit
and Tfit, thencnormalized by Tfit
and the LDOS was recovered as shown in (b). In (b), a fit with Gaussian functions showed six LDOS peaks, which arecnamed HOMO (1)–(3) for a negative voltage side, and LUMO (1)–(3) for a positive voltage side. (c) and (d) exhibit (dI/dV)/Tfit
curvesnormalized with different averaged barrier heights (φ) of 3.5, 4.0, 4.5, and 5.0 eV in (c) and with different tip–sample separations (z) of 10
and 11 Å in (d). (e) The same dI/dV curve in (a) (solid line) and its fitted quadratic function (Qfit , dotted line). (f) A normalized(dI/dV)/Qfit curve obtained with dI/dV and Qfit in (e).

affect the recovered LDOS. From an experimental view point,
this is very convenient since we do not need to pay close
attention when selecting the values of φ and z.
Since the tunneling probability function is exponential,
we can also approximately normalize the dI/dV curve using
the quadratic function Q(V) = aV 2 + bV + c where a, b, andc are fitting parameters.
(dI/dV)/Q(V)fit ∝ LDOS
2(|VS | < 1V),

Q(V)fit = aV + bV + c.

(8)
(9)

The dI/dV curve in figure 4(a) was also normalized by
its fitted quadratic function, as shown in figure 4(e).
7Nanotechnology 24 (2013) 395704

Y Yamagishi et al

Figure 5. Tunneling spectroscopy measurements on a first H2 Pc layer on Cu(111). (a), (b) and (c), (d) were obtained with different tip
conditions of No. 2 and No. 3, respectively. (a) An experimentally obtained dI/dV curve (solid line) at R = 2 × 1010  with a tip conditioncccin (a). (c) An experimentally obtained dI/dV curve
curve obtained with dI/dV and Tfit
. (b) A normalized (dI/dV)/Tfitof No. 2 and fitted Tfit
9
c
curve
with a tip condition of No. 3 at R = 3 × 10 . The inset shows an enlarged dI/dV curve near the Fermi energy. (d) (dI/dV)/Tfit
obtained with the dI/dV in (c).
Table 1. Energy positions of LDOS (HOMO&LUMO) peaks of H2 Pc on Cu(111).
HOMO(1)(eV)c, tip No. 1(dI/dV)/Tfit(dI/dV)/Qfit , tip No. 1
c(dI/dV)/Tfit, tip No. 2
c(dI/dV)/Tfit , 1D chainc(dI/dV)/Tfit, single molecule

−0.21
−0.19
−0.20
−0.22
−0.17

HOMO(2)
(eV)
−0.38
−0.34
−0.40
−0.45
−0.34

HOMO(3)
(eV)
−0.93
−3.00
−0.99
−0.98
−0.78

The (dI/dV)/Qfit curve show LDOS peaks (figure 4(f))
corresponding to HOMO states at −0.19, −0.34, and−3.00 eV, and LUMO states at +0.25, +0.53, and +1.32 eV.
Comparing these peak positions with the peaks in figure 4(b)(see table 1), we found that HOMO(1), HOMO(2), LUMO(1),and LUMO(2) are almost the same, while HOMO(3) andLUMO(3) are shifted by more than 100 meV. This indicatesthat Q normalization is effective for recovering LDOS peaks
within ±1 eV around the Fermi energy. This Q normalization
is particularly useful when the dI/dV curve can only be

LUMO(1)
(eV)
+0.27
+0.25
+0.28
+0.33
+0.26

LUMO(2)
(eV)
+0.54
+0.53
+0.52
+0.83
+0.48

LUMO(3)
(eV)
+1.21
+1.32
+1.26
+1.18

measured within ±1 eV, for example, in the case that unstablemolecules start to move upon the application of a sample
voltage above ±1 V.
dI/dV curves vary slightly even when measured on thesame sample area using the same tip. This is caused by aslight change in the atomic structure at the tip apex during
the scanning owing to the high electric field between the
tip and sample, or molecules or atoms adhering to the apex.
This variation slightly changes the tip work function, leading
to different T and dI/dV curves. Figures 5(a) and (b) were8

Nanotechnology 24 (2013) 395704

Y Yamagishi et al

Figure 6. (a)–(c) STM/STS measurements of a one-dimensional H2 Pc molecular chain at the edge of a 1 ML molecular island. (a) An STM
topographic image (7.5 × 4.9 nm2 , Vs = −1.5 V, I = 100 pA). (b) A dI/dV map at +0.75 V obtained at the same area as (a).
c(c) A (dI/dV)/Tfit
curve (solid line), corresponding to LDOS, obtained at a single-molecular one-dimensional chain and fitted Gaussianfunctions (gray dotted lines). (d)–(f) STM/STS measurements of single H2 Pc molecules on a first H2 Pc molecular layer. (d) An STM
ctopographic image (10 × 7 nm2 , Vs = −1.5 V, I = 100 pA). (e) An I(V) map at −1.4 V obtained at the same area as (d). (f) A (dI/dV)/Tfit
curve (solid line), corresponding to LDOS, obtained at single molecules and fitted Gaussian functions (gray dotted lines).

energy (±1 eV). Although from figures 1(c) and (d) we
concluded that there was a strong tip–sample interaction at
R = 1 × 109 , we evaluated how dI/dV measurement isaffected by the interaction. Figures 5(c) and (d) were obtainedon the same sample area after obtaining figures 4(a)–(f) and
5(a), (b). The strong tip–sample interaction causes instability
in nanomolecules, which frequently causes spike-like noises
in the tunneling current, disturbing the dI/dV measurement.
Despite this, after several trials, we were able to obtain
the dI/dV curve shown in figure 5(c). (However, weobserved some changes on the molecular surfaces after
the spectroscopy measurements.) At R = 1 × 109 , thetip–sample separation is smaller than that in the case shown infigure 4(a), which markedly increases the T background (thedI/dV value at −1.5 V increases to ∼15 nA V−1 ). The dI/dV
curve in figure 5(c) appears to be an exponential function
without any features of the molecular density of states (noobtained at exactly the same area as figure 4(a). Only a slightchange in the tip apex during scanning results in a different
dI/dV curve (we named this tip condition ‘Tip No. 2’). We
normalized this dI/dV curve (solid line in figure 5(a)) usingc (dotted line in figure 5(a)). Since the sample LDOS
its Tfitis independent of the tip condition even when dI/dV curves
have different shapes, we recovered the identical LDOS peaks
c curve in figure 5(b) has
as figure 4(b). The (dI/dV)/Tfit
six LDOS peaks corresponding to HOMO states at −0.20,−0.40, and −0.99 eV, and LUMO states at +0.28, +0.52,
and +1.26 eV. Comparing these peak energy positions withthose in figure 4(b) (see table 1), all peaks are identical withinc normalization can reproducibly
60 meV. This proves that Tfit
recover the LDOS peaks.
c normalization as well as Q
Table 1 shows that Tfit
fit
normalization can recover the LDOS peaks with a highaccuracy, particularly HOMO/LUMO states near the Fermi
9

Nanotechnology 24 (2013) 395704

Y Yamagishi et alc normalization successfully recovered the molecular LDOS
Tfit
from the dI/dV curves taken at a large T. Qfit normalization
was also very effective in recovering the LDOS within ±1 eV
near the Fermi energy. Slight differences in fitting parameters
did not significantly affect the recovered LDOS. LDOS peaks
were observed below (−) and above (+) the Fermi levelat −0.21, −0.38, −0.93, +0.27, +0.54, and +1.21 eV on
the first H2 Pc layer, and at −0.22, −0.45, −0.98, +0.33,
and +0.83 eV on 1D H2 Pc chain molecules. Single H2 Pc
molecules had LDOS peaks at −0.17, −0.34, −0.78, +0.26,+0.48, and +1.18 eV, whose energy positions are very close
to that on the first H2 Pc layer.

clear peaks, shoulders, or bumps near the Fermi energy).
Without careful checking, it might be concluded that a gap
exists around the Fermi energy, but careful examination of
the dI/dV curve around the Fermi energy (see the enlarged
dI/dV curve in figure 5(c)) reveals small features (bumps andshoulders), i.e., at R = 109 , a large T can bury the signal ofa weak molecular LDOS.
Frequently, a lock-in amplifier is used to obtain dI/dV
curves. However, such a dI/dV signal is dimensionless.
Therefore, when a flat dI/dV signal is experimentallyobtained around the Fermi energy, it is difficult to distinguishbetween a gap and a buried LDOS peak in the T background.
To avoid misinterpretation, it is advisable to verify the dI/dV
curve by numerical differentiation of the I(V) curve todetermine whether dI/dV is zero at VS = 0.
c.
The dI/dV curve in figure 5(c) was normalized by Tfitc
The obtained (dI/dV)/Tfit curve recovers LDOS as shownin figure 5(d). Dotted lines in figure 5(d) show LDOS peak
positions in figure 4(b), which mostly fit the peak andcshoulder positions in figure 5(d). This also proves that Tfit
normalization is a very powerful technique for recovering
LDOS peaks.
c normalization, we studied the molecular
Using Tfit
electronic states of 1D molecular chains and single molecules.
Figure 6(a) shows an STM topographic image around theedge of a 1 ML molecular island on Cu(111), where
single molecules form 1D chains. In the topographic image(figure 6(a)), it is difficult to observe the molecular chains, but
a dI/dV map at +0.75 V clearly shows the molecular chains(figure 6(b)). The obtained dI/dV curves were normalizedc and the LDOS peaks corresponding to HOMO statesusing Tfit
at −0.22, −0.45, and −0.98 eV, and LUMO states at +0.33
and +0.83 eV (see figure 6(c) and table 1) were recovered.
The molecular chains have the same HOMO states as the film,
but the LUMO states are shifted to +0.83 eV with a small
peak at +0.33 eV.
The LDOS peaks of single molecules on the firstH2 Pc layer were also measured. Figures 6(d) and (e)show a topographic image and an I(V) map taken at−1.4 V, respectively. Bright dots correspond to the single
molecules. Figure 6(f) shows the recovered LDOS peaks
which correspond to HOMO states at −0.17, −0.34, and−0.78 eV, and LUMO states at +0.26, +0.48, and +1.18 eV(see table 1). Single molecules on the first H2 Pc layer wereconfirmed to have the same LDOS as the first H2 Pc layer(figure 4(b)).

Acknowledgments
We thank Professor Dr H Ishii, Dr F Shibahara, andDr Y Noguchi for assistance in the purification of ourmolecules. The low-current-noise was realized with thesupport of Professor Dr W Wulfhekel as well as the technicalsupport of Dr T Schuh. Vacuum conditions in our STM
setups have been improved by the support of Dr T Uchihashi,
Professor Dr I Arakawa, and Mr Y Shimokawa. We thank
Mr N Bachellier for carefully reading our manuscript. This
work was supported by JSPS KAKENHI Grant Numbers
22810005, 23681018, Japan Science and Technology Agency(JST)—Improvement of Research Environment for Young
Researchers: Chiba University Young Research-Oriented
Faculty Member Development Program in Bioscience
Areas, Chiba University Global COE Program: Advanced
School for Organic Electronics, Casio Science Promotion
Foundation, Asahi Glass Foundation, Hatakeyama Culture
Foundation, The Association for the Progress of New
Chemistry, Iketani Science and Technology Foundation, The
Noguchi Institute, Ozawa Yoshikawa Memorial Electronics
Foundation, Research Foundation for the Electrotechnology
of Chubu, The Nakajima Foundation, The Futaba Electronics
Memorial Foundation, and Shimadzu Foundation.

References[1] Walter M G, Rudine A B and Wamser C C 2010
J. Porphyrins Phthalocyanines 14 759[2] Nothaft M, Hohla S, Jelezko F, Fruhauf N, Pflaum J andWrachtrup J 2012 Nature Commun. 3 628[3] Jackson T N 1998 IEEE J. Sel. Top. Quantum Electron. 4 100[4] Tang C W, VanSlyke S A and Chen C H 1989 J. Appl. Phys.
65 3610[5] Schmaus S, Bagrets A, Nahas Y, Yamada T K, Bork A,
Evers F and Wulfhekel W 2011 Nature Nanotechnol. 6 185[6] Bagrets A, Schmaus S, Jaafar A, Kramczynski D,
Yamada T K, Alouani M, Wulfhekel W and Evers F 2012
Nano Lett. 12 5131
[7] Takacs A F, Witt F, Schmaus S, Balashov T, Bowen M,
Beaurepaire E and Wulfhekel W 2008 Phys. Rev. B
78 233404[8] Brede J, Atodiresei N, Kuck S, Lazic P, Caciuc V,
Morikawa Y, Hoffmann G, Blügel S and Wiesendanger R
2010 Phys. Rev. Lett. 105 047204[9] Miyamachi T et al 2012 Nature Commun. 3 938[10] Komeda T, Isshiki H, Liu J, Zhang Y F, Lorente N, Katoh K,
Breedlove B K and Yamashita M 2010 Nature Commun.
2 217

4. Conclusions
We demonstrated a technique for recovering the molecular
LDOS from dI/dV curves experimentally obtained byscanning tunneling spectroscopy (STS) on low-dimensional
phthalocyanine (H2 Pc) nanomolecules on a clean andatomically flat Cu(111) substrate. Owing to the weak bonding
between the molecules and the noble metal surface, and to
avoid the tip–sample interaction in tunneling spectroscopy,a large tip–sample separation is required, which necessitates
low-noise measurements and the hiding of LDOS signals.
10Nanotechnology 24 (2013) 395704

Y Yamagishi et al[19] Kuk Y and Silverman P J 1990 J. Vac. Sci. Technol. A 8 189[20] Sanchez O, Garcia J M, Segovia P, Alvarez J,
Vazquez de Parga A L, Ortega J E, Prietsch M and
Miranda R 1995 Phys. Rev. B 52 7894
[21] Nicolay G, Reinert F, Hufner S and Blaha P 2001 Phys. Rev. B
65 033407[22] Stroscio J A, Pierce D T, Davies A, Celotta R J and Weinert M
1995 Phys. Rev. Lett. 75 2960
[23] Bischoff M M J, Yamada T K, Fang C M, de Groot R A andvan Kempen H 2003 Phys. Rev. B 68 045422[24] Okuno S N, Kishi T and Tanaka K 2002 Phys. Rev. Lett.
88 066803[25] Kleiber M, Bode M, Ravlic R and Wiesendanger R 2000 Phys.
Rev. Lett. 85 4606

[11] Mugarza A, Robles R, Krull C, Korytar R, Lorente N and
Gambardella P 2012 Phys. Rev. B 85 155437[12] Tanaka Y, Mishra P, Tateishi R, Cuong N T, Orita H, Otani M,
Nakayama T, Uchihashi T and Sakamoto K 2013 ACS
Nano 7 1317
[13] Zhao A et al 2005 Science 309 1542
[14] Tsukahara N et al 2009 Phys. Rev. Lett. 102 167203[15] Gerhard L, Yamada T K, Balashov T, Takacs A F, Daena M,
Ostanin S, Ernst A, Mertig I and Wulfhekel W 2010 Nature
Nanotechnol. 5 792[16] Ukraintsev V A 1996 Phys. Rev. B 53 11176[17] Yamada T K, Bischoff M M J, Mizoguchi T and
van Kempen H 2002 Surf. Sci. 516 179[18] Yamada T K, Bischoff M M J, Heijnen G M M, Mizoguchi T
and van Kempen H 2003 Phys. Rev. Lett. 90 056803

11