bluuebunny/crossref_metadata_2025 huggingface dataset:
Strain‐structure‐property relation in Co‐super tetragonal
BiFeO3
heterojunctions
Authors: Nukala Pavan , Sanders Anke , Carretero Cecile , Fusil Stephane , Barthelemy Agnes , Dkhil Brahim , Garcia Vincent , Bibes Manuel
Published: December 2016

Bismuth ferrite (BiFeO
3
) has received a lot of attention as a rare multiferroic material showing simultaneous ferroelectric and antiferromagnetic orders at room temperature. Strain engineering offers a new avenue to tune the coupling between various orders, and provides access to some new crystalline phases, which are not stable in bulk BiFeO
3
(BFO). The super tetragonal phase (T‐phase) with a giant axial ratio (c/a~1.27) is one such strain engineered phases, obtained through a biaxial compressive strain on BFO (>‐4%) [1]. This phase exhibits a large polarization (~150 µC/cm
2
) that was exploited for applications such as ferroelectric tunnel junctions with colossal electroresistance [2]. The interface between ultrathin films of T‐phase BFO and the bottom electrode of (Ca,Ce)MnO
3
has been thoroughly investigated recently [Marinova et al. Nano Lett. 15, 2533‐2541 (2015)]. However, the interface between the ferroelectric and the top electrode of Co used in devices remains to be investigated.

In this context, we synthesized Co‐BFO‐CCMO heterostructures using pulsed laser deposition on the (001) surface of three different substrates (NdGaO
3
, LaSrGaO
4
, YAlO
3
). Structural details were probed and compared through high resolution STEM imaging in both annular bright field (ABF) modes to obtain information about lighter atoms, and in high angular annular dark‐field modes. Furthermore, by correlating local chemical information obtained from STEM EELS, with ABF STEM images, we detect features that are consistent with the existence of oxygen vacancies in ultrathin T‐phase BiFeO
3
. We also investigated the interface quality between Co and T‐BFO using EELS and EDX, and correlate these observations with our transport measurements.
SAMF
ire ‐ a smart adaptive fitting algorithm for multi‐dimensional microscopy
Authors: Ostaševičius Tomas , de la Peña Francisco , Midgley Paul
Published: December 2016

The large amounts of high‐quality “multi‐dimensional” data generated by modern microscopes open new avenues for quantitative nano‐characterization. Quantitative analysis of spectra and images often involve fitting a model to experimental data and, indeed, the literature is rich in applications; examples include atom counting [1], time resolved microscopy [2], electron energy loss [3] and cathodoluminescence [4] spectroscopy. However, using conventional methods to fit large datasets is challenging and when applied to multi‐dimensional models, they may become ill‐suited. The dominant and most common problem is that conventional methods struggle with any non‐linearity in the model and often require an estimate of the starting parameters that are close to the true values. Here we present a Smart Adaptive Multi‐dimensional Fitting algorithm (SAMFire) designed to ease the task of fitting such data by automatically generating best estimates for the parameters as the fitting progresses. SAMFire can fit multi‐dimensional spectra, images and data of higher dimensionality and will be available in open‐source software package HyperSpy v1.0.0 [5].

SAMFire enables quantitative analysis of large multi‐dimensional datasets that would be very challenging—if not impossible—to analyse by other means. It provides multiple fitting strategies that consist of pixel selection and parameter estimation, each tailored to different data structures. Example pixel selection orders are shown in Figure 1(a) for conventional fitting algorithms and in Figure 1(b) for one of the SAMFire strategies. The “raster” order is only viable for unusually stable and constrained models. In contrast, SAMFire follows the “path of least resistance”, learned from already fitted parts of the data and hence is applicable to a much broader range of problems.

As an example of a complex electron microscopy data analysis problem that can be easily addressed with SAMFire, Figure 2 shows a single spectrum and the result of EELS elemental and bonding quantification by curve fitting from a tilt‐series of spectrum‐images of a mixed phase nanoparticle. The model consists of eleven components to accurately describe the five elements and a background. Due to the complexity of the model, the geometry of the particle and the low signal‐to‐noise‐ratio, the outcome of fitting individual pixels was highly dependent on the starting parameters, making the analysis very challenging using conventional fitting routines. In contrast, SAMFire was able to fit the whole tilt‐series with minimal user input.

Since SAMFire enables highly sophisticated models to be fitted to large multi‐dimensional datasets significantly faster and more easily than previous algorithms, we anticipate it will become standard analysis practice, especially when quantitative analysis is required. Examples that we are currently considering include tracking motion in a time series and quantification of both light and trace elements in multiple‐domain structures.
Development of a novel straining holder for TEM compatible with electron tomography
Authors: Sato Kazuhisa , Miyazaki Hiroya , Gondo Takashi , Miyazaki Shinsuke , Murayama Mitsuhiro , Hata Satoshi
Published: December 2016

Electron tomography (ET) has been introduced in materials science in the past decade and it has opened a new prospect: the technique can retrieve three‐dimensional (3D) structural information [1]. However, a major roadblock exists to combine the
in‐situ
experiments with electron tomography, which is expected to reveal real time 3D structural changes [2]. Towards dynamic 3D (
i.e
., “4D‐ET”) visualization of material's microstructures under various straining conditions with a time scale of a few minutes or less, we designed and developed a new specimen holder compatible with tensile test and high‐angle tilting, termed as “straining and tomography (SATO)” holder [3].

Figure 1 shows a schematic illustration of a newly designed specimen holder (a) and a cartridge‐type blade on which a specimen is glued (b). The area of gluing is marked by gray. The basic concept of this development is a single tilt‐axis holder with a tensile mechanism and also being capable of electron tomography. To achieve straining and high‐angle tilting simultaneously, we developed a novel mechanism as shown in Fig. 1(a). A linear motion actuator deforms a newly designed cartridge‐type blade on which a specimen is glued. Deformation velocity of the blade is designed as 1/3 of that of the actuator. Figure 1(b) explains the motion of blade. The trajectory (dotted line) is an arc but the radius of curvature (R) is so large (3 mm) that the tensile axis is perpendicular to the holder for a nanometer‐ scale object whose center is located at O.

Figure 2 shows (a) an appearance of the developed specimen holder and (b) a magnified photo of the cartridge‐type blade. The holder motion is fully computer‐controlled via graphical user interface developed for this system. We measured the deformation velocity of the blade and deduced the strain rate. The minimum and the maximum values obtained were 1.5×10
−6
and 5.2×10
−3
s
−1
. The blade as well as the holder is robust and multiple acquisitions raise no technical problem at all. This result demonstrates stability and reliability of the holder as a novel
in‐situ
experimental instrument for 4D‐ET. We also confirmed that the maximum tilt angle of the specimen holder reaches ±60
°
with a rectangular shape aluminum specimen.

Figure 3 shows an example of
in‐situ
tensile test using the newly developed holder. The material is an Al‐Mg‐Si alloy with a conventional 3 mm diameter disk shape prepared by electropolishing (Fig. 3(a)). When the actuator moved 9.87 µm from the initial position, slip bands were suddenly introduced (Fig. 3(b)). With increasing the tensile stress, slip bands were discontinuously but incrementally introduced in several parts of the field of view until a crack was introduced elsewhere. It should be mentioned that the drift of a field of view was negligible throughout the
in‐situ
tensile experiment. The new specimen holder will have wide range potential applications in materials science.
Characterisation of ordering in the A‐site deficient perovskite
Ca1‐xLa2x/3TiO3
using
STEM
/
EELS
Authors: Danaie Mohsen , Kepaptsoglou Demie , Ramasse Quentin , Ophus Colin , Whittle Karl , Lawson Sebastian , Pedrazzini Stella , Young Neil , Bagot Paul , Edmondson Philip
Published: December 2016

Perovskite structures based on the formulation Ca
1‐x
La
2x/3
TiO
3
have been extensively studied across a wide range of possible applications, such as anodes for solid oxide fuel cells [1], dielectric resonators [2], high density memory storage devices [4], and as host matrices for inert matrix nuclear fuels and as containment media for high‐level nuclear waste forms [5–6]. Understanding the crystallographic ordering at the atomic scale and the nature of present defects is essential in order to successfully utilize this class of perovskites across the multitude of applications.

We have studied the vacancy ordering behaviour of the A‐site deficient perovskite system, Ca
1‐x
La
2x/3
TiO
3
, using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy‐loss spectroscopy (EELS), with the aim of determining the role of A‐site composition changes. At low La content (x = 0.2), this system adopts Pbnm symmetry, with no indication of long‐range ordering. Atomic resolution high‐angle annular dark‐field (HAADF) STEM image, acquired along [010]
p
pseudo‐cubic zone axis, Figure 1(a), shows varying intensities indicating changes in La
3+
/ Ca
2+
ratio across the field of view. Elemental intensity maps from characteristic core‐loss edges, shown in Figure 1(b), demonstrate anti‐correlated Ca versus La intensities. Domains, with clear boundaries, were observed in bright‐field (BF) imaging, but were not immediately visible in the corresponding high‐angle annular dark‐field (HAADF) image. These boundaries, with the aid of polarisation maps from A‐site cations in the HAADF signal, are shown to be tilt boundaries.

At the La‐rich end of the composition (x = 0.9), adopting Cmmm symmetry, long‐range ordering of vacancies and La
3+
ions was observed, with alternating La‐rich and La‐poor layers on (001)
p
planes, creating a double perovskite lattice along the c axis. One such ordered region is imaged in Figure 2(a) along the [100]
p
zone axis, in conjunction with EELS elemental maps shown in panel (b), showing the alternating La‐rich and La‐poor atomic planes. These highly‐ordered domains can be found isolated within a random distribution of vacancies / La
3+
, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, e.g. the area imaged in Figure 3, these highly‐ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries, as shown in panels (a) and (b), respectively. The occurrence and characteristics of these ordered structures will be discussed and compared with similar perovskite systems.
Atomic structure of the ultrathin amorphous aluminium oxide barrier in
Al/AlOx/Al
Josephson junctions
Authors: Zeng Lunjie , Tran Dung Trung , Tai Cheuk‐Wai , Svensson Gunnar , Olsson Eva
Published: December 2016

Amorphous materials are widely used in many technologically important devices and structures. In Josephson junctions, amorphous aluminium oxide film with a thickness of a few nanometers is used as tunnel barrier [1]. The atomic structure of this thin amorphous oxide film determines the tunnelling properties of the junction. However, the disordered nature, the miniaturized dimension and the close proximity to the adjacent crystalline contacts of the oxide barrier are challenging aspects of the direct investigation of the atomic structure of the tunnel barrier layer.

In this work, we have unveiled the microscopic structure of the ultrathin (<2 nm) aluminium oxide barrier in Al/AlO
x
/Al Josephson junctions via transmission electron microscopy. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were used to reveal the detailed atomic structure and the chemical bonding in the junction. By combining nano‐beam electron diffraction (NBED) and pair distribution function (PDF) analysis with reverse Monte Carlo (RMC) refinement [2] we unravelled the atomic structure of the nanosized AlO
x
barrier. The technique of combining NBED‐PDF and RMC enabled us to treat the crystalline Al contacts and the amorphous AlO
x
barrier layer as integrated parts in the system and take the interfacial interaction between Al and AlO
x
into account while retrieving the atomic structure information.

The atomic structure of the AlO
x
tunnel barrier layer resembles the structure in bulk Al
2
O
3
in terms of bond length distribution, bond angle distribution, and connectivity of the elemental structure units. However, the average coordination number of the Al atoms in the barrier is around 3.4, which is lower than that in other amorphous aluminium oxide systems [3]. The combined NBED and RMC simulation showed that the miniaturized dimension of the oxide barrier and the presence of the interfacial interaction between crystalline Al and amorphous AlO
x
give rise to an oxygen deficiency at the metal/oxide interfaces in the junction. This is of importance since atomic defects such as oxygen vacancies at the interfaces are possible origins of the two‐level systems that contribute to the decoherence and noise in superconducting quantum circuits based on Josephson junctions.
3D‐
TEM
studies of hierarhical graphene‐composite aerogels for ultra‐long‐life Li‐ion batteries
Authors: Sologubenko Alla , Zeng Guobo , Tervoort Elena , Gramm Fabian , Niederberger Markus
Published: December 2016

Among all potential application fields of graphene, most important are still the electrochemical energy‐storage devices, in particular the Li‐ion batteries (LIB). Despite of all the efforts, the performance of the graphene‐based Li‐ion batteries is still far from satisfactory, considered their insufficient electrical capacity, rate performance and long‐term stability (1). In the present work, we present the technique allowing the production of hierarchical graphene‐based composite aerogels as binder‐free anodes for the ultra‐long‐life Li‐ion batteries (LIBs). Our approach is to increase an active area of the composite aerogel for the Li‐ion uptake (or adsorption)by the introduction of spacers between the graphene sheets. The spacers are the Mo
x
S
y
particles of three different size ranges: sub‐nanometer (iii), a few nanometers (ii) and several hundreds of nanometer large amorphous carbon balls (i) filled with nanometer‐sized Mo
x
S
y
(see schematic in Figure 1). The incorporation of these poly‐dispersed particles as spacers between the graphene sheets results in the hierarchically porous aerogel. Such structures as anodes in LIBs possess high capacity, 1069 mAh/g at 0.35 A/g, rate performance, 425 mAh/g at 10 A/g and 304 mAh/g at 50 A/g, and show ultra‐long stability.

In the present work, Mo
x
S
y
‐particle loaded aerogels before and after the Li‐ion uptake were studied by HRTEM / HRSTEM. Morphological and elemental analyses of the particle populations were performed (Figure 2). Moreover, all three groups of the spacers were analyzed for their spatial arrangement in relationship to graphene sheets by HRTEM and HRSTEM tomography, revealing the fully homogeneous spatial distribution of two groups of the small‐sized clusters on the sheets and the hierarchical nature of the largest spacers consisting of the polymer balls uniformly filled with the small clusters. TEM analyses confirmed that all three kinds of the spacers are very effective in the preventing the graphene sheets from coalescence. Both, graphene sheets and the spacers can intake Li‐ion, contributing to the charge‐discharge cycles and are as such responsible for the long‐life electrochemial performance.
Direct detection and electron counting ‐ A beginning of a new era for electron microscopy
Authors: Pan Ming
Published: December 2016

Over the last decade, great progress has been made in the instrumentation of transmission electron microscopy (TEM) through the introduction of aberration correctors, electron energy monochromators, and a wide variety of TEM specimen holders designed for in‐situ applications. With these new capabilities, a wealth of advanced experimental data can be easily generated by advanced TEM systems. However, the development of a revolutionary image capture device has been lacking; this effectively makes imaging detectors the main bottleneck when striving to achieve the full potential performance of an advanced TEM.

Traditional image detectors use scintillator and optical transfer path (fiber‐coupling or lens) to convert high energy electrons to photons that are subsequently transferred to the imaging sensor to form an image. One of the disadvantages of this indirect detection approach is the loss of image resolution and sensitivity during the electron‐photon conversion and the photon transfer as additional noise sources that significantly degrade the signal‐to‐noise ratio (SNR) of the image detector.

Very recently direct detection imaging devices have been successfully developed based on technology advancement made in CMOS (complementary metal oxide semiconductor) design and manufacturing, high speed data architectures, vastly increased memory densities and speed, etc. The elimination of scintillator and subsequent optical transfer path has significantly improved the detective quantum efficiency (DQE) – a critical measure of SNR for resolution and sensitivity of an imaging device. The state‐of‐the‐art direct detection imaging device further boosts the DQE and image quality under extremely low beam conditions by electron counting at high speed (e.g. 400 fps @ 4k x 4k resolution) to eliminate the sensor readout noise and minimize the electron scattering noise. Figure 1 shows the comparison of DQE measurement at 300 kV for K2 direct detection cameras operated in electron counting (solid blue) and linear (dotted blue) mode, and scintillator/fiber optical coupled cameras (purple). It is clear that electron counting has significantly restored the DQE performance of direct detection cameras for all frequencies.

This new generation of direct detection imaging device has revolutionized the field of cryo‐electron microscopy (cryoEM) in structural biology and is starting to impact many applications in electron microscopy of materials science, for example,
in‐situ
microscopy, 4D STEM, imaging beam sensitive materials, quantitative measurement of radiation damage or quantitative electron microscopy, etc. Direct detection and electron counting are poised to advance electron microscopy into a new era.
New Opportunities in multi‐frame
STEM
Spectroscopy & Fractional Beam‐current
EELS
Authors: Jones Lewys , Varambhia Aakash , Kepaptsoglou Demie , Ramasse Quentin , Freer Robert , Azough Feridoon , Lozano‐Perez Sergio , Beanland Richard , Nellist Peter
Published: December 2016

Electron energy‐loss spectroscopy (EELS) and energy‐dispersive x‐ray spectroscopy (EDX) are two of the most common means of chemical analysis in the scanning transmission electron microscope (STEM). While the instrumentation hardware has progressed markedly in recent years, the way that microscopists operate these spectrometers has changed very little. In general small areas are scanned just using coarse pixilation, slow scan‐speeds and high beam‐currents. Finally, once acquired these chemical signals are usually expressed in only relative or arbitrary units. This stands in stark contrast with modern best‐practice in STEM imaging, where wider fields of view are surveyed utilising multi‐frame acquisitions, faster scan speeds, finer pixel sampling and lower electron doses. More recently it has also become common place to express STEM image data in units of fractional‐beam‐current facilitating direct comparison with simulation.

We will present EELS and EDX results where best‐practice techniques from STEM imaging have been repurposed to improve chemical map quality. Multi‐frame spectrum‐image data were recorded with simultaneous EELS and EDX spectra, before using non‐rigid data registration [1] in the spatial domain, Figure 1. This not only reduces scan‐distortion but also unlocks tools such as; digital super‐resolution, strain mapping performed directly on chemical maps, and the digital accumulation of weak signals such as those from monochromated EELS. We also present an ‘equal fixed‐dose’ fractionation study where sample damage was reduced drastically using a fast‐scanning multi frame approach compared with its single scan equivalent.

EELS spectrum data fidelity was improved by energy‐drift tracking and correction [2,3]. After this, the limiting performance in multi‐frame spectra becomes the spectral‐noise and was found to depend strongly on the quality of the available dark‐reference, Figure 2. To mitigate this artefact, separate dark‐references and full‐beam gain‐references were recorded along with wholly un‐processed experimental spectra. Further improvement in the noise performance was released by stepping the EELS spectra in energy between each scan before un‐stepping them in post‐processing [4]. Dark‐correction was performed offline before dividing by the gain‐reference to finally express the EELS spectra in terms of fractional beam current [5].

It is expected that leveraging all these individually small improvements collectively will deliver more precise chemical maps while minimising sample damage and experimental time overheads.
Investigation of the cathode material Li9V3(P2O7)3(
PO
4)2 for Li‐batteries using Cs‐corrected
HRTEM
Authors: Geiger Dorin , Balasubramanian Prasanth , Mancini Marilena , Axmann Peter , Wohlfahrt‐Mehrens Margret , Kaiser Ute
Published: December 2016

New cathode materials for Li‐ion batteries are investigated assiduously to achieve higher performance in terms of energy density life time and safety. In pursuit, a polyanion‐based lithium vanadium monodiphosphate, Li
9
V
3
(P
2
O
7
)
3
(PO
4
)
2
[LVPP] was identified. This layered material poses distinct advantages like higher energy density, thermal stability over the more commonly used metal oxides, owing to the strong binding of the phosphate anions. Theoretically the extraction of maximum 6 Li ions per unit cell is possible, thus giving out a theoretical capacity of 173mAhg
‐1
through complete oxidation of vanadium from its initial +3 to final +5 state. A facile synthesis method of LVPP has been designed. To comprehend the structure and its functionality, transmission electron microscopy (TEM) with image Cs‐corrector was used (FEI Titan 80‐300kV), allowing atomic resolution even at lower accelerating voltages [2] (in this case at 80kV), where lower knock‐on damage is expected.

To preserve as good as possible the original quality of the synthesized powder material, the TEM specimen was prepared without additional grinding using three methods: simply spread of powder particles on the carbon foil, sonicate powder in alcohol and fishing particles on the carbon foil and spread of particles, which were crashed after cooling in liquid nitrogen, on the carbon foil.

The crystallographic structure of LVPP corresponds to the trigonal symmetry (space group P‐3c1) with a = b = 90°, g = 120°,
a
=
b =
9.724 Å
, c =
13.596 Å [3]. VO
6
octahedra and PO
4
tetrahedra, connected to the Li atoms, are organized in two layers generating a 2D ionic conductivity. Fig.1d) shows the simulated structure in [0001]‐ and [‐2021]‐projections where V atoms are red, P atoms blue, O atoms light rosa and Li atoms yellow.

HRTEM images of thin particles edges at 80 kV and 300kV are shown in Fig. 1 a‐c). To get interpretable TEM micrographs usually the crystalline specimen has to be aligned during imaging in a particular pre‐selected crystal orientation. However LVPP is very sensitive to the electron beam with the result that practically the crystal could not be oriented. This behaviour was found independent of the accelerating voltage of 80kV or 300kV. Fortunately, some particles had by chance a favorable crystal orientations and qualitatively interpretable TEM images were obtained (see Fig. 1). We confirmed the expected crystallographic structure for the synthesized particles by corresponding image simulations using JEMS. Moreover we found no significant volume change of the structure during electron‐beam driven delithiation.
Defect Investigation by Atomic‐Resolution
STEM
of
III
‐V Horizontal Nanowires grown via Template‐Assisted Selective area Epitaxy
Authors: Bologna Nicolas , Knoedler Moritz , Borg Mattias , Cutaia Davide , Erni Rolf , Riel Heike , Marta Rossel D.
Published: December 2016

Scaling of silicon microelectronics is reaching fundamental physical limitations related in particular to the power consumption. A possible solution is represented by III‐V semiconductors integrated on Si which have much higher electron mobility and injection velocity. The possibility to grow III‐V nan‐owires allows also the creation of new transistor concepts for Tunnel Field‐Effect Transistors [1], which could lead to an increase in the efficiency of the circuits by reducing the supply voltage and thus the power consumption.

The growth of III‐V nanowires by Template Assisted Selective Epitaxy (TASE) (Fig. 1a) [2] has enabled the direct integration of heterostructures on Si (100) for TFET applications. The performance of the de‐vice may however be affected by the presence of defects (twins, dislocations, stacking faults, anti‐phase boundaries) along the nanowire since every single discontinuity in the crystal can lead to a mod‐ification in the electrical properties of the material. The understanding, control and suppression of such defects have always been a very challenging task and a crucial point to obtain well performing devices.

Here, we report the investigation of different III‐V compounds, by means of atomic‐resolution STEM, starting from GaAs homostructures to InAs‐GaSb heterostructures (Fig.1b), allowing detailed charac‐terization of defects, strains and compositions. We make use of the large degree of freedom of growth parameters in the TASE technique (temperature, V/III ratio, molar flux of precursors) to prepare nan‐owire samples (Fig.1b) with the aim to find the right parameters combination to reduce the defects density. Our analysis confirms a process‐dependent defect density in nanowires and shows that de‐fect‐free nanowires can be indeed obtained by optimizing the growth conditions (Fig.2).
Determination of the platelet structure in natural diamond by
ADF
‐
STEM
Authors: Korneychuk Svetlana , Turner Stuart , Abakumov Artem , Verbeeck Johan
Published: December 2016

Electron microscopy is well known for its capability of determining the structure of many materials down to the atomic scale. A heavily studied material is natural diamond, but its rather small interatomic distance has made it a challenging material for TEM investigation. In this work we will demonstrate how novel TEM techniques can give fresh insights to this heavily studied material. Imperfections in natural diamond range from point lattice defects to dislocations and platelets. Platelet defects were first observed by means of TEM in the 1960s [1, 2] and nitrogen was detected making use of EELS in the 1980s [3]. Nevertheless, the exact crystalline structure of the platelets in natural diamond remains, perhaps surprisingly, still unclear.

In this work, we demonstrate our approach to determine the structure of platelet defects making use of annular dark field scanning transmission electron microscopy (ADF‐STEM) combined with advanced image processing. Combining experimental data with image simulations we are able to offer a consistent crystallographic model of the defect. Furthermore, using monochromated energy loss spectroscopy technique we suggest the type of nitrogen embedding into the defect plane.

TEM samples were prepared by a focused ion beam lift out procedure from natural type Ia diamond. ADF‐STEM and EELS experiments were carried out on FEI Titan microscope equipped with probe aberration corrector. We studied more than 50 platelets from different parts of the sample. In order to reduce scan distortions and damage to the defect plane a series of images with fast dwell time was obtained. The images were treated in the Smart Align software [4] to reduce the scan noise, correct the sample drift and a final summed image was obtained. Improved signal to noise ratio in this image allowed us to estimate the atomic positions and create a model of the platelet. Based on this model STEM image simulations were carried out. They are found to be in the good agreement with the experimental data. Using STEM‐EELS we prove the presence of nitrogen exactly in the platelet plane. Comparing DFT calculations of different nitrogen centres in diamond and the experimental spectra extracted from the defect plane we suggest the model for the nitrogen coordination in the platelet. Using this detailed information on the platelet defect, we will discuss the possible mechanisms of platelet formation.
Simulation of elastic and inelastic electron scattering in
EF‐HRTEM
mode by back‐propagation of electron beam
Authors: Tyutyunnikov Dmitry , Lubk Axel , Rusz Jan
Published: December 2016

In this work we present a new approach for simulation of energy‐filtered HRTEM images by back‐propagating the electron beam. Here, the back‐propagation means that we consider the fast electrons going in the opposite way through the microscope: from a detector through a specimen, where they undergo an energy‐gain process, and then they continue to the source. We show that this time‐reversed approach is computationally feasible and can bring consistent results. In this new approach, real collection angle becomes a convergence angle of the simulated probe and the real convergence angle becomes the simulated collection angle, as shown in the scheme below. In this new simulation setup we use small collection angles in order to mimic nearly parallel illumination in HRTEM. The “probe”, on the contrary, has a large convergence angle, because for HRTEM image simulation wide objective apertures are used. The contrast transfer function of the imaging system is inverted by changing the sign of aberrations. This reciprocal approach is well suited to generalize the in‐house code developed for energy‐filtered STEM simulations using combined Multislice/Bloch‐waves approach [1]. We can now switch between different simulation modes, e.g., energy‐filtered TEM and STEM, by employing different sets of parameters in the code. It allows to avoid multiple multislice evaluations at each atom position, instead it requires separate calculation for each pixel of the desired EF‐HRTEM image, which is often a smaller computational cost. The mathematical proof of the concept will be outlined. Simulated energy‐filtered HRTEM images of selected materials, such as STO or LSMO based on this approach will be presented as well.
Atomic structure and magnetic circular dichroism of antiphase boundary defects in
NiFe2O4
thin films
Authors: Wang Zechao , Zhong Xiaoyan , Jin Lei , Yanagihara Hideto , Kita Eiji , Jiang Hanbo , Dunin‐Borkowski Rafal E
Published: December 2016

The complex and interesting properties of ferrimagnetic spinel ferrite thin films are of great fundamental interest, as well as being of practical importance for applications in spintronic devices and ultra‐high‐density recording media. The presence of antiphase boundary (APB) defects is responsible for reduced spin polarization and magnetism in spinel ferrites
[1,2]
. There is also considerable discussion about the relationship between the atomic structures of APBs and their magnetic properties. Whereas atomic structures of many APBs have been determined using high‐resolution transmission electron microscopy and high‐angle annular dark field (HAADF) imaging
[3]
, local measurements of magnetic properties at APBs on the nm scale have not yet been achieved.

Electron magnetic circular dichroism (EMCD) was demonstrated experimentally in 2006 for a specific diffraction geometry
[4]
. Since then, the spatial resolution of EMCD has been improved to approximately 1 nm using nanobeam diffraction
[5]
. In 2013, we developed a site‐specific EMCD method for magnetic structure determination and achieved EMCD spectra with high signal‐to‐noise ratio
[6]
. Here, we combine site‐specific EMCD in nanobeam diffraction mode with high‐resolution HAADF imaging, in order to simultaneously determine the magnetic circular dichroism and atomic structure of APBs in NiFe
2
O
4
thin films.

We find the atomic structures of APBs that are formed on {111} planes by a crystallographic translation of 1/4a[0‐11] using HAADF imaging. The EMCD signals at such defects were obtained using an electron beam with a diameter of ~1 nm and compared with signals obtained from a perfect single crystalline region under the same illumination and acquisition conditions. We demonstrate experimentally that the strength of the magnetic circular dichroism at APBs is suppressed significantly when compared with that in the perfect area. The capability of EMCD at 1 nm spatial resolution enable us to correlate our experimental magnetic circular dichroism spectra from local defects with corresponding structural and chemical information recorded at the atomic scale, opening the door to experimental investigations of the relationship between atomic structure and magnetic properties of local defects in materials.
Atomic Resolution Tomography of Nanoparticles Reconstructed from Exit Wave
Authors: Chen Fu‐Rong , Dyck Dirk Van , Kisielowski Christian , Helveg Stig , Calderon Hector , Kirkland Angus
Published: December 2016

Resolution and sensitivity of the latest generation aberration‐corrected Transmission Electron Microscopes allow imaging the vast majority of single atoms from the Periodic Table of Elements with sub‐Ångstrom resolution and determining their locations in an image plane with a precision that exceeds the 1.9 pm wavelength of 300 kV electrons. Further progress that exploits this ability requires addressing at leading‐edge: (1) there is the need to recover structural information in three dimensions from the two dimensional projection (2), a most noticeable bottleneck to reconstruct 3D structural information are electron beam‐induced sample alterations , since a large accumulated electron dose in the range of 10
4
− 10
5
e/Å
−2
s
−1.
is typically required to achieve a needed resolution around one Ångstrom and single atom sensitivity. We present, in the talk, a general tomographic method to recover the 3D shape of a crystalline particle from high‐resolution images of a single projection without need for sample rotation. The method is compatible with low dose rate electron microscopy, which improves on signal quality while minimizing electron beam‐induced structure modifications even for small particles or surfaces. We apply it to MoS
2
, NiO, CeO
2
, Ge
1
, Au
1
and MgO
1
particles and achieve a depth resolution of 1−2 Å, which is smaller than inter‐atomic distances. The algorithm of 3D reconstruction method at atomic resolution is outlined followed.

Determination of the true “z” height (focus) of the exit surface of a column from an image plane with the maximum peak intensity (MPI) criterion by wave propagation along “defocus circles” as shown in Fig. 1
1
.

Refining the “z” height using the Big‐Bang scheme
2
.

Correcting the focus of each column wave by back‐propagation, and bring the exit wave back to “mass” circle
1
.

Measuring the phase of the normalized focus corrected wave (at mass circle) from the center of mass circle. The column mass is given by the phase angle θ' between the normalised focused corrected wave and the vacuum wave. See Fig. 1

The fig. 2 shows for MoS
2
and CeO
2
nanoparticles the phases of electron exit wave functions that are used to reconstruct the corresponding reconstructed atomic resolution tomograms.
The use of Correlation Coefficient maps to enhance visibility of internal structure for nanocrystalline thin foils
Authors: RAUCH Edgar , KISS Akos , LABAR Janos
Published: December 2016

Orientation and phase are routinely determined with the automated ACOM tool developed for Transmission Electron Microscopes [1]. With this attachment, the beam is scanned over the area of interest and all the diffraction patterns are collected and kept in memory for further off‐line analysis. The present work concerns a novel approach that makes use of the memorized data to compute a structural image of the sample through a straightforward post‐processing algorithm that consists in weighting the similarities between the neighbor diffraction patterns [2].

The successive diffraction patterns acquired within a given crystal are anticipated to be nearly identical, while an abrupt change is expected when the beam is crossing a grain or a phase boundary. Plotting the value of a correlation coefficient that compares the intensities of every pixels of the neighbor diffraction patterns produces a contrasted picture in which all structural features that modify the local diffracting conditions are highlighted.

Fig. 1 gives a typical example where grain boundaries for a polycrystalline sample are retrieved. Of particular interest is the fact that the grain boundary contrasts are directly related to the boundary inclination. Indeed, the sharp changes in diffraction patterns expected for boundaries parallel to the electron beam are associated to a strong contrast. A weak and extended contrast indicates qualitatively a gradual modification of the diffracting signal as expected for inclined boundaries. Quantitative evaluation needs different processing [3].

The correlation coefficient is sensitive to any structural component that modifies the diffracting conditions. This is valid for dislocations, too. Two of them appear in the upper grain in figure 1.

Moreover, the correlation coefficient is less sensitive to non‐visibility conditions. This is because it is constructed on the difference between the intensities of all reflections including the faint ones. In particular, if the sample is in a so‐called two beam condition, the main reflection g remains unchanged when the beam is crossing a dislocation line whose Burgers vector is normal to g. This reflection will be dominant in the bright field image and the dislocation will not be visible. Being constant, g will not contribute to the correlation coefficient. By contrast faint reflections that always exist in the diffraction pattern ‐ even in two beam conditions ‐ will be affected by the distortion around the defect line. Figure 2 compares the virtual bright‐field image and the correlation coefficient map for a deformed ferritic steel sample. The thin foil is slightly bended (less than 2°, mainly from left to right) so that the contrast conditions are not homogeneous in the micrograph and part of the structural information is missing. The correlation coefficient is less sensitive to the exact illumination conditions and consequently additional dislocations appear in the map.
Detection of magnetic circular dichroism in amorphous materials utilizing a single‐crystalline overlayer
Authors: Zhong Xiaoyan , Lin Jie , Cheng Song , Rusz Jan , Xin Huolin , Cui Bing , Vancho Kocevski , Han Lili , Zhu Jing
Published: December 2016

Electron energy‐loss magnetic chiral dichroism (EMCD) in a transmission electron microscope allows the quantification of the magnetic structure of crystalline materials down to the nanometer scale
[
1‐3]
. However, restricted by a confined diffraction geometry applied in EMCD experiments
[4]
, no experiments or theories have yet been performed to obtain EMCD signals for amorphous materials, due to their lack of long range ordering.

In this work, we for the first time demonstrate it is possible to detect element‐specific magnetic signals in amorphous materials utilizing a single‐crystalline overlayer as an EMCD beam splitter. The approach is applied to a bilayer sample where a very thin amorphous magnetic FeO
x
layer is grown on a single‐crystalline Yttrium‐stabilized ZrO
2
substrate. We found that both experimental results and theoretical calculations lead to unprecedented EMCD signals. The quantitative orbital to spin magnetic moment ratio of Fe in amorphous FeO
x
layer has been acheived.

Our approach allows us to break through the constraint of crystal formats in EMCD spectra measurements, providing new prospects of detecting EMCD signals from amorphous and ultrathin materials at the nanometer scale. This approach might also be extended to the magnetic quantitative analysis of other heterogeneous materials at high spatial resolution.

This work may open a door to meet the challenge of exploring magnetic states and behaviors of amorphous films, and have important consequences for revealing the magnetic structures of magnetic materials in various crystal forms at the nanoscale using transmission electron microscopy.
Fractals in the nucleus ‐ understanding chromatin organisation with
3D SMLM
Authors: Lynch Christophe , Botchway Stanley , Webb Stephen , Robinson Ian
Published: December 2016

Abstract

Chromatin presents a unique challenge in structural biology. Its fundamental units, DNA and the nucleosome, are well‐characterised at the Ångstrom level.
1
Yet, the higher‐order organisation of the chromatin ensemble, particularly during interphase, remains largely to be determined. Single molecule localisation microscopy (SMLM) appears uniquely positioned to answer many remaining questions about chromatin structure – it is possible to determine structures to accuracies of better than 20 nm,
2
and analysis of spatial point data has been used by some groups to determine the scaling behaviour and fractal dimension of chromatin.
3
This is of particular interest when considering nuclear molecular transport and gene activation: it has been suggested that cells use the very structure of chromatin to regulate transcription and expression.
4

Whilst imaging has been able to shed light on the general organisation of chromatin and other fibrous structures in the cell, there is a lack of methods to fully characterise their folding and looping behaviour. To date, most SMLM analysis has focussed on clustering‐based algorithms such as Ripley's K function or more recently Bayesian clustering.
5
It is clear, however, that different approaches are needed to tackle the analysis of elongated or fibrous structures within cells, and in three dimensions.

Here, we demonstrate the power of 3D pair correlation analysis combined with careful labelling of regions of interest in chromatin, showing that the fractal dimension of chromatin can be determined across the entire nucleus for structures 30 nm in size upwards. This provides a platform for analysis complementary to imaging, describing differential folding behaviour across the nucleus, yielding results comparable to Hi‐C and other recent methods. In addition to applying spatial statistics to probe chromatin structure on the nanoscale, we incorporate photon statistics into the analysis. This allows us to draw conclusions about chromatin structure with greater confidence by adding a weighting to reduce the influence of data points localised with lower precision than others.

Understanding how chromatin fibres fold at all stages in the cell cycle is crucial to the future development of drugs and novel treatments. More and more, drugs target specific loci yet in many cases lack the ability to be directed to the target of interest with great accuracy.
6
We hope that by understanding the organisation of different regions of the genome, drugs can be designed to be delivered to their targets in ‘smarter’ ways.
Transmission
K
ikuchi Diffraction: effective nano‐scale analysis using conventional
EBSD
hardware
Authors: Goulden Jenny , Mansour Haithem , Bewick Angus
Published: December 2016

SEM‐based Transmission Kikuchi Diffraction (SEM‐TKD) [1,2] is an extension to the conventional technique of reflection EBSD (using bulk samples) to transmision EBSD using thin samples. The main advantage of SEM‐TKD over the conventional method is a large improvement in spatial resolution of point‐analysis from small particles and of mapping data from thin films.

This improvement is primarily a consequence of examining thin, electron‐transparent TEM samples in transmission, which reduces the effective diffraction and escape volume, as well as the use of zero‐ to low sample tilts which reduces anisotropic beam‐spreading effects. The technique is used to produce 2D datasets from flat, thin sample areas. This data is then used for microstructural analysis including crystallographic orientation, grain size, phase distribution, and grain boundary character and distribution. It is especially useful in characterizing highly‐strained materials and materials with grain sizes under 50nm.

A significant attraction of TKD is that these improvements can be realised using conventional EBSD hardware, in its conventional position on the SEM, without any modification. However, for TKD the sample position, pattern centre (high up or above the detector screen) and sample tilt (close to horizontal) are very different to those required for conventional EBSD (just above the screen centre line, and 70° respectively). Thus the TKD‐mode imposes a more extreme case of the gnomonic projection which is already inherent to the capture of conventional electron backscatter patterns.

The most notable effects of the TKD projection geometry are that horizontal bands near to the bottom of the screen are imaged wider than normal, and that the non‐symmetric intensity across these bands is highlighted (as illustrated in Figure 1).

Here we discuss how these distortions affect analysis using a conventional EBSP solving engine and some new technology developed to improve performance.