bluuebunny/crossref_metadata_2025 huggingface dataset:
STEM
imaging of atom dynamics: novel methods for accurate particle tracking
Authors: Furnival Tom , Schmidt Eric , Leary Rowan , Knez Daniel , Hofer Ferdinand , Bristowe Paul D , Midgley Paul A
Published: December 2016

Developments in scanning transmission electron microscopy (STEM) have opened up new possibilities for time‐resolved imaging at the atomic scale. Recent examples include a study of the diffusion of dopant atoms in semiconductors [1] and, using environmental STEM, in situ studies of catalytic reactions [2]. Rapid imaging of single atom dynamics brings with it a new set of challenges. High frame rates and long total acquisition times mean novel methods are needed for handling and processing “big data” sets. Further, the need for short exposure times leads to severe problems with noise, but by exploiting the spatial and temporal correlations between frames, it is possible to considerably improve the signal‐to‐noise ratio using a method known as singular value thresholding [3]. Crucially, by employing robust procedures to automatically estimate the noise and motion characteristics, it is possible to optimize the process with little user input (Figure 1a,b). The identity and positions of individual atoms in the denoised data can then be determined using a newly‐developed intensity‐based classification algorithm. Building on the theme of automation, the classifier can be trained using simulated STEM images to robustly process long image sequences, where manual identification would be prohibitive.

As an example, we have applied these methods to investigate the diffusive behaviour of copper atoms on the (110) surface of silicon. The noise removal and atom identification steps are used along with particle tracking software [4] to extract a set of atomic trajectories from a series of annular dark‐field STEM images (Figure 1c‐e). The form of these trajectories can be related to the underlying silicon substrate, as in Figure 1f, which suggests the existence of preferred pinning sites for copper atoms. The interaction between adatoms and the substrate can be explored with unprecedented spatio‐temporal resolution using rapid imaging, and interpreted by modelling with density functional theory (DFT) calculations (Figure 1g). This highlights the potential for combining time‐resolved STEM with theory, forming a powerful approach to investigating and understanding the dynamic behaviour of materials at the atomic scale.
Combined
TEM/STEM
and In‐situ c‐
AFM
Characterization of
2D
Nanoflake‐like Heterostructures for Energy Storage and Conversion Applications
Authors: Tang Peng‐Yi , de la Mata María , Han Li‐Juan , Verdaguer Albert , Genç Aziz , He Yong‐Min , Zhang Xuan , Zhang Lin , Galán‐Mascarós José Ramón , Morante Joan Ramon , Arbiol Jordi
Published: December 2016

Honeycomb‐like hematite nanoflakes/branched polypyrrole nanoleaves heterostructures with a 3D complex structure have been synthesized and employed as high‐performance negative electrodes for asymmetric supercapacitors application. The detailed TEM‐STEM characterization and deep EELS chemical analysis at the nanoscale has been combined to elucidate the mechanisms underlying the formation and morphology evolution of core‐branch Fe
2
O
3
@PPy heterostructures.
[1]
In addition, we have studied the mechanism of converting MoO
2
nanoflakes into 2D free‐standing MoS
2
electrode by sulfurization process for water splitting. In this way, the atomic resolution aberration corrected HAADF STEM reveals the sulfurization mechanism in an unprecedented detail, together with EELS chemical maps. Furthermore, in‐situ electrical measurements have been performed by means of c‐AFM on the MoO
2
/MoS
2
nanoflakes surfaces in order to detect their electronic active sites.
SEM
and
FTIR
investigation of molecular composition changes in allergenic common mugwort (Artemisia vulgaris L.) and hazel (Corylus avellana) pollen induced by traffic pollution
Authors: Depciuch Joanna , Kasprzyk Idalia , Roga Elzbieta , Parlinska‐Wojtan Magdalena
Published: December 2016

Nowadays pollen allergies become an increasing problem for human population. Mugwort (
Artemisia vulgaris)
and hazel (
Corylus avellana
) are major herbaceous allergenic plants in Europe [1‐3]. In this study the effect of vehicle pollutants on the structure and chemical composition of mugwort and hazel pollen were investigated. For this purpose pollen of the respective plants were collected from three sites with different vehicle pollution level. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and curve‐fitting analysis of amide I profile was performed to assess the structural changes of mugwort and hazel pollen. SEM imaging did not reveal any differences in shape or any physical degradation of the hazel (Fig. 1a, b) and mugwort (Fig. 1c, d) pollen grains collected from the different sites. It was found that infrared spectra look the same for pollen collected at sites with high traffic pollution. Conversely, they differ a lot from spectra of respective pollen types collected from sites without pollution. Moreover, structural changes in proteins, observed in the second derivative of the FTIR spectra and in the curve‐fitting analysis of amide I profile, are a consequence of mutations occurring in the genetic material of pollen, which can be caused inter alia by air pollutants [4]. The results suggest, that mugwort and hazel pollen chemical composition may be a good indicator of air quality and FTIR may be applied in biomonitoring.
Advanced
TEM
study of homogeneous flow and size dependent mechanical behaviour in highly ductile
Zr65Ni35
metallic glass films
Authors: Idrissi Hosni , Ghidelli Matteo , Gravier Sébastien , Blandin Jean‐Jacques , Raskin Jean‐Pierre , Pardoen Thomas , Schryvers Dominique
Published: December 2016

Metallic glasses (MGs) exhibit outstanding mechanical and functional properties for numerous applications. However, although intensive research on the deformation and fracture mechanisms has been performed on metallic glasses for more than 20 years, the fundamental mechanisms governing the mechanical behaviour as well as the recently observed mechanical size effects in this class of materials are still not fully understood. Recently, amorphous Zr
65
Ni
35
(%at) freestanding thin film MGs (TFMGs) deposited by magnetron sputtering have been deformed using a “lab‐on‐chip” technique based on MEMS technology [1]. The results have shown that the ductility of the films is highly enhanced compared to bulk MGs and other TFMGs in the literature, and the plastic deformation occurs homogenously, i.e., without the observation of mature shear bands until fracture. In order to unravel the origin of these remarkable mechanical properties, the films have been investigated in‐depth using advanced transmission electron microscopy (TEM).

Quantitative nanobeam electron diffraction (NBED) was used to investigate the relationship between the local atomic order and the activation of “shear transformation zones” (STZs) [2]. The basic principle of NBED is shown in Figure 1a, consisting of a coherent electron beam with diameter of around 0.4 nm in order to produce two‐dimensional diffraction patterns from atomic clusters with comparable size. Figures 1b, 1c and 1d exhibit NBED patterns with strong Bragg reflections which is the signature of a locally ordered region (i.e., atomic clusters) while in the NBED pattern of Figure 1e, only a diffuse background with speckles without Bragg reflections can be observed. High resolution HAADF‐HSTEM and EELS revealed a heterogeneous microstructure with Ni‐rich and Zr‐rich regions exhibiting different atomic densities with characteristic length of 2‐3 nm (Figure 2). Such behaviour can be attributed to the sputter deposition process involving very high cooling rates compared to bulk MGs. The results raise several fundamental questions that will be addressed: Does the nucleation of the STZs occur preferentially in regions with specific enriched chemical composition and atomic density? How will this affect the interaction between the STZs? How such features can be used to explain the exceptional high plastic deformation levels, the absence of shear bands and the delay of fracture in the Zr
65
Ni
35
TFMGs used in the present work?
Atomic‐scale investigation of interface phenomena in two‐dimensionally Sr‐doped
La2CuO4
and
La2CuO4/ La2‐xSrxNiO4
superlattices
Authors: Wang Yi , Suyolcu Y. Eren , Sigle Wilfried , Salzberger Ute , Baiutti Federico , Gregori Giuliano , Cristiani Georg , Logvenov Gennady , Maier Joachim , van Aken Peter
Published: December 2016

Atomic‐scale investigation of interface phenomena in two‐dimensionally Sr‐doped La
2
CuO
4
and La
2
CuO
4
/ La
2‐x
Sr
x
NiO
4
superlattices

Y. Wang, Y. E. Suyolcu, W. Sigle, U. Salzberger, F. Baiutti, G. Gregori, G. Cristiani, G. Logvenov, J. Maier, and P.A. van Aken

Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany

Physics phenomena at interfaces of complex oxide heterostructures have stimulated intense research activity due to the occurrence of a broad range of electric and magnetic functionalities that do not pertain to the constituting single phases alone. Interface effects have been proven to be a powerful tool for improving or even inducing novel functionalities [1, 2]. In the case of interface superconductivity, the interatomic structure relaxation and charge transfer play a key role [3].

In this work, we combine atomic‐resolved quantitative STEM imaging with analytical STEM‐EELS/EDX analysis to investigate the interface effects in La
2
CuO
4
(LCO)‐based hetero‐structures, i.e. superlattices of Sr two‐dimensionally (2D) doped LCO, and LCO/La
2‐x
Sr
x
NiO
4
(LSNO) hetero‐structures, both exhibiting
T
c
up to 40 K, despite its non‐superconducting constituents. HAADF images of the structure are displayed in Fig.1 confirming high‐quality epitaxy. A detailed study of the cation distribution and concentration at the doped interfaces was performed by EDXS and EELS analyses. In the case of Sr‐2D doped LCO superlattices, the analysis shows that Sr cations undergo an asymmetric redistribution, as a result of the MBE growth process. Oxygen
K
‐edge fine structure analysis reveals that the hole concentration profile on the downward side of the nominal SrO layer is clearly decoupled from the Sr‐profile indicating an accumulation of positive charges compensating the negatively charged SrO planes (Fig.2).

These results were supplemented by quantitative analysis of atomic‐resolved high‐angle annular dark‐field (HAADF) and annular bright‐field (ABF) images, which were simultaneously acquired. From these images the local lattice and copper‐apical‐oxygen distortions at the interfaces were evaluated. The relation of these results with the observed
T
c
will be discussed.
Electron tomography: influence of defocus on the determination of reconstructed soot aggregates' surface and volume
Authors: Cabié Martiane , Gailhanou Marc , Ferry Daniel
Published: December 2016

Aerosols affect the climate system through various physical processes as they can scatter and absorb solar radiation, emit thermal radiation, or act as cloud condensation nuclei that modify the cloudiness coverage, changing then its albedo. Carbonaceous solid aerosols resulting either from anthropogenic processes or biomass burning are one of the most significant contributors to global climate change [1] with respect to their impact on radiative forcing. They are constituted by tiny primary spherules having a diameter typically ranging from a few nanometers to dozens nanometers. These spherules are aggregated to form particles of larger sizes (0.1 to 1 micrometre) showing a complex morphology (Figure 1) that plays an important role on their radiative and transport properties. These soot aggregates are commonly characterized from 2D transmission electron microscopy (TEM) images and their 3D shape is then deduced from TEM projections assuming geometrical models of spheres, cylinders or spheroids. In order to check the validity of the 2D to 3D transition of these geometrical models we investigated bright field (BF) TEM tomography and directly determine 3D soot morphological characteristics.

To be suitable for tomographic reconstruction, images of the tilt series must fulfill the projection requirement and BF TEM images of amorphous specimen, which are dominated by mass‐thickness contrast, can satisfy this assumption. We show in this work that attention must be paid on the influence of defocus, responsible of the appearance of Fresnel fringes, which do not answer the projection requirement and can lead to artefacts that do not ensure a realistic reconstruction of soot aggregates.

To do so, we analyse the effect of defocus both on real and numerical soot particles. Thus the same real soot aggregate is reconstructed (with the SIRT algorithm) from different tilt series obtained at different defocus values. Its surface area and volume are determined by using the Amira software suite after a segmentation step based on the method of Adachi et al [2]. The same procedure (reconstruction, segmentation, surface area measurement) is then applied on an amorphous numerical particle, which is generated using a tight binding model [3] processed with the Nanofabric software developed by Y.Lebouar. Projection series are simulated with the EMS software based on the multislice method [4]. Both experimental and numerical approaches show that defocus drastically modifies the intensity profile of primary particles along their diameter and subsequently affects the shape of the reconstructed aggregate (Figure 2) and leads to overestimated values of their surface area and volume.
A Correlative Methodology based on
SIMS
for Advanced Materials Characterization
Authors: Eswara Santhana , Hu Rong , Yedra Lluís , Audinot Jean‐Nicolas , Schwedt Alexander , Tasan Cem , Mayer Joachim , Raabe Dierk , Wirtz Tom
Published: December 2016

Development and innovations of modern high‐performance materials rely crucially on the repertoire of characterization methods available. While individual techniques provide information on a specific aspect of the investigated material, a comprehensive understanding requires a correlative approach in which complementary information are needed from exactly the same Region of Interest (RoI). Therefore, correlative methodologies combining several techniques are indispensable in a wide range of scientific disciplines including materials science [1].

In contrast to the traditional EDX and EELS spectroscopies, SIMS is well‐known for high‐sensitivity (ppm), high dynamic range and more interestingly, isotope analysis for all the elements of the periodic table. This presentation will highlight the benefits of correlating SIMS imaging with complementary analysis using Electron Back Scattered Diffraction (EBSD) and Atomic Force Microscopy (AFM). As the SIMS images do not reveal crystallographic orientations of the grains in the microstructure, studies that aim to correlate structural defects such as twinning and grain boundary orientation in a multiphase material require that the strengths of SIMS are coupled with that of EBSD to probe the local crystallographic characteristics. As a first step, we correlate the SIMS with EBSD imaging to complement specific chemical characteristics seen in SIMS with the corresponding crystallographic characteristic captured in EBSD.

To further characterize the material and for correct interpretation, the exact topography of the RoI is required. We do topographic analysis using an AFM on the exact location where SIMS and EBSD analysis are carried out. In this way, a three‐way correlative methodology is developed. As each of these analyses is done on separate instruments, specific methodological strategies are employed to overcome the challenges.

To illustrate this methodology, we selected the topic of hydrogen embrittlement in a TWinning Induced Plasticity (TWIP) steel [2]. As conventional techniques such as EDX are not capable of mapping hydrogen distribution, the SIMS approach brings a particularly strong advantage to this study. Prior to the SIMS analysis, the steel samples were electrochemically charged with deuterium to distinguish it from hydrogen naturally present in the sample. The SIMS images of H
‐
and D
‐
distributions are obtained from a Cameca NanoSIMS50 with a Cs
+
primary ion beam. Then, EBSD and AFM analyses are carried out on the exact location to correlate SIMS chemical characteristics with crystallographic defects such as twins, grain boundaries and sample topography.

In this presentation, we will introduce and demonstrate this correlative paradigm for materials characterization. We will also present the scientific and technical challenges and discuss the strategies to go beyond the current state‐of‐the‐art.
Mechanical twinning and microstructure refinement in metastable titanium alloys
Authors: Václavová Kristína , Stráský Josef , Harcuba Petr , Stráská Jitka , Veselý Josef , Polyakova Veronika , Semenova Irina
Published: December 2016

Metastable beta titanium alloys gain increasing interest due to their unique properties such as high strength, good hardenability, high fracture toughness and excellent corrosion resistance. In this study two different metastable beta titanium alloys, Ti‐15Mo and Ti‐6.8Mo‐4.5Fe‐1.5Al (TIMETAL LCB) were subjected to severe plastic deformation (SPD) by high pressure torsion (HPT) technique. The main purpose of SPD methods is the insertion of large plastic deformation to the material without a change of the original shape of the material. The decreasing grain size and increasing the number of high‐angle grain boundaries results in changes of mechanical and other physical properties. The main aim of this work is to analyse the effect of deformation on the microstructure and lattice defects. The initial stages of microstructure refinement were examined by electron backscatter diffraction (EBSD). The inverse pole map figure of Ti‐15Mo alloy after N = ¼ HPT turns is depicted in the Figure 1. It was shown that initial stages of deformation include misorientation within grains (see changing ‘colour’ along black line) and formation of low‐angle sub‐grain boundaries (black arrows). Moreover, it was proven that multiple twinning occurs in the material (Figure 2) which notably contributes notably to the microstructure refinement. Further microstructure refinement by HPT observed by transmission electron microscopy (TEM) and automated crystal orientation mapping (ACOM‐TEM) will be also presented.
Morphology and composition tailoring of cobalt ferrite nanoparticles
Authors: de Medeiros Indira Aritana Fernandes , Lopes‐Moriyama André Luís , Madigou Véronique , de Souza Carlson Pereira , Leroux Christine
Published: December 2016

Cobalt ferrite nanoparticles have scientific and technological interest due to their magnetic properties, good chemical stability and low cost, combined with catalytic properties. These factors allow their use in various applications, such as ferrofluid technology [1], catalysts [2] and gas sensors [3]. One way to improve the catalytic properties of the cobalt ferrite is to control the size and the morphology of the nanoparticles. Studies show that crystallites which expose only a particular family of crystallographic planes have enhanced catalytic activity [4].We already obtained nanooctahedron exposing only {111} facets, 20 nm in size, by a solvothermal method [5]. Another way to enhance catalytic properties is to control the composition of cobalt ferrites; previous studies showed that high amount of cobalt favors the catalytic conversion of methane [2]. The aim of this study is to synthesize cobalt ferrite nanoparticles with different compositions (CoFe
2
O
4
and Co
1.8
Fe
1.2
O
4
) and morphologies, in order to control and optimize the catalytic properties.

The powders were obtained by solvothermal synthesis using different solvents and precursors. The control of the shape was realised using different surfactants. For cobalt ferrite Co
1.8
Fe
1.2
O
4
cobalt nitrate and iron nitrate were used as precursors,water and ethylene glycol as solvents. The experiments showed that the solvent has a significant influence on the powder composition. When water or a mixture of water and ethylene glycol were used as solvents, two phases were identified in the final product: Co(OH)
2
and Co
1.5
Fe
1.5
O
4
(fig. 1). When only ethylene glycol was used, a pure phase with homogeneous composition was obtained: Co
1.8
Fe
1.2
O
4
with the spinel structure and size around 8 nm (fig. 2).

In order to obtain different morphologies of CoFe
2
O
4
, cobalt acetylacetonate and iron acetylacetonate with benzyl alcohol as solvent were used with different amount of oleic acid and oleylamine as surfactants. The cobalt ferrite powder produced by both oleic acid and oleylamine has a heterogeneous composition containing some crystals with cubic shape (fig. 3). Using only acid oleic as surfactant led to a CoFe
2
O
4
powder with homogeneous composition. The shapes of the particles are nearly cubic or octahedral (fig.4). These preliminary results indicate that shape of the nanoparticles is controled by the amount and nature of surfactant.
Quantifying
M
g doping in
AlGaN
layers
Authors: Amichi Lynda , Mouton Isabelle , Robin Eric , Delaye Vincent , Mollard Nicolas , Vennéguès Philippe , Matta Samuel , Brault Julien , Grenier Adeline , Jouneau Pierre‐Henri , Bougerol Catherine
Published: December 2016

Gallium nitride and its alloys, AlGaN and InGaN, are essential but also challenging materials for the development of optoelectronic devices, such as visible or UV LEDs. The optical and electric properties of nano‐objects such as 2D layers, quantum dots or nanowires are indeed directly affected by the concentration and the distribution of dopants. Mg is the most used p‐type dopant for nitrides, however its impact on structural and optical properties is still not fully understood. In particular it is of the upmost importance to determine if it is homogeneously distributed or not. In metal organic chemical vapor deposition or hydride vapour phase epitaxy grown layers, for concentrations higher than 10
18
cm
‐3
, Mg has been observed to segregate into a variety of defects, inversion polarity, and pyramidal inversion domains

[1][2][3]

. In molecular beam epitaxy (MBE) grown layers, the question is not solved for such low concentrations. However, higher concentration of Mg causes clustering or incorporation in interstitial sites

[4]

. For this purpose, we have used atom probe tomography (APT), EDX both in SEM and TEM, and EELS with the aim to compare and determine which one is the most appropriate technique for providing qualitative and quantitative information on Mg low doping. The obtained results will be presented.

Mg‐doped Al
0.2
Ga
0.8
N 2D layers grown by MBE have been studied with a concentration around 3x10
19
at.cm
‐3
according to SIMS measurements. The sample has been prepared by FIB either as a thin layer for EELS and EDX investigations, or as a sharp needle for APT experiments.

For EDX as well as for EELS experiments, the main difficulty arises from the position of the emission line (Kα) or respectively absorption line of Mg (Mg K‐edge), very close to that of Ga which is the main element of the sample. Adding to this, it has not been possible to observe Mg on TEM‐EDX elemental maps.

Thanks to the use of a Bruker FlatQUAD detector in SEM‐EDX mode with a very large solid angle data collection which ensures a very high counting rate

[5]

, it has been possible to detect Mg and its concentration has been estimated around 3.8 ±1.4 x10
19
at.cm
‐3
(Figure
[1]
). This is in very good agreement with SIMS experiments.

Finally, we have performed APT analyses on the same samples. The mass spectrum clearly shows the presence of Mg, its isotopes and also its alloys (Figure
[2]
). The concentration has been estimated around 3.2 ±2.5 x10
19
at.cm
‐3
after data treatment. The 3D reconstruction of Mg (Figure
[3]
) shows a homogenous distribution of Mg. Statistical analyses are currently performed using first and k‐nearest neighbor method in order to determine if the distribution is actually homogenous or not at small scale, and to try to have a more accurate measurement of the Mg content according to the method developed by Thomas Philippe et al

[6]

.
Structural quality of
CH
3
NH
3
PbI3
perovskites for photovoltaic applications analyzed by electron microscopy techniques
Authors: Fernández‐Delgado Natalia , Herrera‐Collado Miriam , Delgado‐González Francisco , Juárez‐Pérez Emilio , Mora‐Sero Iván , Suárez Isaac , Martínez‐Pastor Juan , Molina Sergio
Published: December 2016

Hybrid Halide Perovskites (HPVK) are novel materials that have attracted attention in the last few years as promising materials in the photovoltaic field,[1, 2] as efficiencies higher than 20% have been reported[3].The HPVK can have different compositions, which allows the control of lattice parameters[4] and bandgap values in the visible to near infrared region[5]. In particular, CH
3
NH
3
PbI
3
presents optimal electronic and optical properties such as direct optical band gap of 1.55 eV or long electron/hole diffusion length (100 nm)[6].

In this communication, we have analysed the structural quality of two CH
3
NH
3
PbI
3
samples by electron microscopy techniques. The samples have been grown using spin coating solution processing method. The details of the process can be found in[7]. The structure of the samples consists of a substrate of SiO
2
(2µm), a TiO
2
layer (40 nm), an active layer of HPVK and a poly‐methyl methacrylate (PMMA) capping layer (1µm) in order to protect the sample from oxidation. The two samples studied (S500 and S100) have different HPVK thicknesses, 500 and 100 nm respectively, obtained by changing the speed in the spin‐coating process [7]. The first analyses have been carried out by high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM), which is sensitive to the atomic number of the atoms in the material. Fig. 1 a shows a HAADF‐STEM image of the first sample, where the different layers of the structure can be observed. These layers have been studied by energy dispersive X‐ray analysis (EDX) as exhibited in Fig. 1 b, and the layers sequence (SiO
2
, TiO
2
and HPVK) has been confirmed. Thus, spectrum 3 in Fig. 1 b shows the presence of Pb and I, indicating that this layer corresponds to the HPVK, despite it shows a reduced size due to the ion bombardment during the sample preparation process for TEM. Electron diffraction patterns have been taken from the HPVK layer and, surprisingly, some areas have shown to be polycrystalline but others are amorphous. Previous analysis of the functional properties of these samples suggests that they are crystalline. Sample preparation to obtain the electron transparent specimen may cause a detrimental effect in the HPVK layer, producing the amorphization of a material. Because of this, the procedure of sample preparation is being optimized in order to analyse in detail the structural properties of this material.
Growth and degradation of advanced octahedral
P
t‐alloy nanoparticle catalysts for fuel cells
Authors: Heggen Marc , Gocyla Martin , Gan Lin , Strasser Peter , Dunin‐Borkowski Rafal
Published: December 2016

Octahedral Pt‐Ni nanoparticles are highly attractive as fuel‐cell catalysts due to their extraordinarily high activity for the oxygen‐reduction‐reaction (ORR). A deep understanding of their atomic‐scale structure, degradation and formation is a prerequisite for their use as rationally designed nanoparticle catalysts with high activity and long‐term stability.

Here we present an extensive microstructural study of the growth and degradation behavior of various octahedral Pt‐alloy nanoparticles using in situ transmission electron microscopy (TEM) and Cs‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) combined with electron energy‐loss spectroscopy (EELS) and energy‐dispersive X‐ray spectroscopy (EDX). We show that octahedral nanoparticles often show compositional anisotropy with Ni‐rich {111} facets leading to complex structural degradation during ORR electrocatalysis. The Ni‐rich {111} facets are preferentially etched, resulting in the formation of first concave octahedra and then Pt‐rich skeletons that have less active facets (Figure 1)[1]. Furthermore, we reveal element‐specific anisotropic growth as the reason for the compositional anisotropy and the limited stability. During the solvothermal synthesis, a Pt‐rich nucleus evolves into precursor nanohexapods, followed by the slower step‐induced deposition of Ni on the concave hexapod surface, to form octahedral facets (Figure 2)[2]. While the growth of Pt‐rich hexapod is a ligand‐controlled kinetic process, the step‐induced deposition of the Ni‐rich phase at the concave surface resembles a thermodynamically controlled process accomplished in much longer time. In order to tune the atomic‐scale microstructure of the octahedra for long‐term stability, we illustrate the effect of varying the growth conditions on morphology and compositional segregation by producing trimetallic PtNiCo nanooctahedra and comparing “one‐step” and newly‐developed “two‐step” synthesis routes [3]. Furthermore we demonstrate how Pt atom surface diffusion may produce a protective Pt surface layer on top of the Ni‐rich facets, resulting in advanced and more stable octahedral catalysts. Figure 3 shows a sequence of structural changes taking place on an octahedral nanoparticle during in situ heating up to 800°C using a MEMS chip heating holder (DENSsolutions, Delft, NL). It can be observed that Pt‐rich corner atoms diffuse and subsequently fill the concave Ni‐rich {111} facets, forming perfectly octahedral nanoparticles with flat Pt‐rich {111} surfaces (Figure 3) [4].
Direct holographic depth‐ and lateral‐ imaging of nanoscale magnets generated by ion impact
Authors: Röder Falk , Hlawacek Gregor , Wintz Sebastian , Hübner René , Bischoff Lothar , Lichte Hannes , Potzger Kay , Lindner Jürgen , Fassbender Jürgen , Bali Rantej
Published: December 2016

The alloy Fe
60
Al
40
is described by a paramagnetic B2 structure in its ordered phase, which transforms into a ferromagnetic A2 structure by chemical disordering that can be induced locally by ion irradiation [1,2]. This mechanism allows writing arbitrary magnetic nanostructures on paramagnetic thin films e.g. by means of a focused ion beam available in novel scanning ion microscopes. However, reproducible fabrication of nanoscale magnets requires knowledge about the depth and lateral distribution of the induced magnetization in dependence on irradiation parameters.

Off‐Axis Electron Holography provides suitable insights by revealing the local distribution of the projected magnetic flux density with nanometer resolution [3]. By means of the coherent superposition of an electron wave passing through the object with one passing through vacuum, interference fringes can be formed at the detector plane encoding the amplitude and phase of the electron wave. The phase of an electron wave shifted by electric and magnetic fields of the object permits direct field mapping at the nanometer scale.

In cross‐sectional samples of irradiated thin films, we studied the effect of the kinetic ion energy ranging from 5‐30 keV on the depth distribution of the induced magnetization [4]. In agreement with irradiation damage simulations [2], we found a magnetized film adjacent to the ion entrance surface growing in depth with increasing kinetic ion energy. We conclude that a homogeneous magnetization depth distribution in a 40 nm thick film requires a kinetic Ne
+
ion energy of at least 20 keV. The resolution of the ion beam nano‐pattering is mainly limited by the effect of lateral ion scattering blurring the magnetization distribution at the pattern edges. To study this effect, we fabricated 500 nm wide magnetized stripes separated by non‐ferromagnetic (i.e. non‐irradiated) spacers (Fig. 1) using a focused Ne
+
ion beam (2 nm probe size) at 25 keV in a helium ion microscope [5]. The flux distribution at the stripe facets is an indicator for the effect of lateral scattering but is difficult to directly interpret in terms of magnetization because of the superposition with stray fields. Therefore, we applied a magneto‐static model for the field distribution around the nanoscale magnet as a function of the magnetization blurring, which returns a width of lateral scattering of about 20 nm fitting best to experimental results [4].
Structure and topology of chemical vapour deposited graphene by scanning electron diffraction
Authors: Johnstone Duncan N. , Leary Rowan K. , Eggeman Alexander S. , Hodge Stephen , Sassi Ugo , Fazio Domenico De , Ferarri Andrea C. , Midgley Paul A.
Published: December 2016

Structural and topological features of graphene have been investigated widely in the (scanning) transmission electron microscope and include: grain structure [1], layer number and mis‐stacking [2], dislocations [3] and out of plane buckling [4, 5]. Here we explore new insights offered by scanning electron diffraction (SED), including quantitative analysis of crystal orientation and local strain. SED involves scanning the electron beam across a specimen and recording a diffraction pattern at each point. This provides a four‐dimensional (4d) dataset combining real and reciprocal space information with nanoscale spatial resolution [6]. SED can be performed over areas of a few square micrometres, and the rich 4d data can be analysed using a number of versatile schemes, as described below. This automated analysis enables numerous regions to be considered, an example of which is shown (Fig.1) from a graphene sample grown by chemical vapour deposition on copper [7].

‘Diffraction images’ can be formed by plotting the intensity of a particular sub‐set of pixels in each diffraction pattern as a function of probe position to elucidate any variations in the diffraction condition. In Fig.1a, integration windows are selected around a particular set of first‐order ((1,0)‐type) and second‐order ((1,1)‐type) reflections to yield the ‘virtual’ dark field images in Fig.1b. These images reveal the local grain structure, in this case a grain in the lower right area of the map. They also show light/dark fringes associated with a small island (arrowed) as well as a fold (starred). This contrast can be attributed to deviation from perfect stacking between the island and the underlying graphene grain, or between layers in the fold. The contrast is understood in terms of variations in the interference condition for electrons scattered from atoms in each of the layers as their relative position varies spatially [5]. The most notable benefits of SED lie in further computational analysis. Orientation images can be produced by matching each diffraction pattern to a library of simulated patterns to automatically map the grain structure and determine the local orientation. All grains are then revealed, and the disorientation across grain boundaries can be determined (Fig. 1c). Strain and small orientation variations are also of considerable importance, and can be mapped with SED by comparing each pattern to an unstrained reference. Fig.1d shows up to 3% strain around the fold, as well as the rotation associated with the ~2º small angle grain boundary. Our approach can thus provide a comprehensive 'crystal cartography' of layered materials, paving the way for thorough understanding and exploitation of their unique structure and topology.
Hexagonal patterning of 1‐nm
G
d nano‐fibers based on dislocation templates in
M
g‐
G
d alloys
Authors: Zhu Guo‐zhen , Li Yangxin
Published: December 2016

Mg alloys, the lightest structural metal, generally suffer from low strength and poor deformability, and therefore severely restrict their widespread applications in automotive, aircraft, and aerospace industries. [1] The underlying reason for such mechanical behaviors is the anisotropic response inherent in the hexagonal close‐packed (hcp) lattice of Mg. [2‐3] Therefore, the key of advancing their applications is reducing the anisotropic behavior of different deformation modes, through regulating the relative activities of (easy) basal slips, (hard) non‐basal slips, and twinning.

Herein, we demonstrate the new microstructure, the self‐assembled hexagonal 1‐nm Gd nano‐fibers pattern within binary Mg‐Gd alloys. [4] As shown in Fig.1, such hexagonal patterns are typically a few hundred nanometers in width and a few microns in length. These patterns are associated with dislocation templates. Such patterns include Gd‐segregated dislocations with approximately 1 nm in diameter, which have a c‐rod shape, and these in turn become effective inhibitors for basal slips since they have to cut these nano‐fibers. On the other hand, non‐basal slips suffer much less effect because the glide of non‐basal dislocations has much less chances of cutting these Gd‐segregations. Thus, these patterns can strengthen Mg alloys mainly through pinning basal dislocations; more importantly, tune the relative activities of basal and non‐basal slips, and thus improve the deformability of Mg alloys. It is also worth mentioning that such patterning structure can be synthesized through a generally economical hot extrusion approach.

In summary, 1‐nm Gd nano‐fibers, with c‐rod shape, are self‐assembled into hexagonal patterns in Mg matrix, which can tune the relative activities of deformation modes and thus improve mechanical properties of Mg alloys. Our results open up a new path of engineering advanced Mg alloys though manipulating their microstructure.
Graphite‐to‐diamond (13C) direct transition in a diamond anvil high‐pressure cell
Authors: Tyukalova Elizaveta , Kulnitskiy Boris , Perezhogin Igor , Kirichenko Alexey , Blank Vladimir
Published: December 2016

As the hardest material in nature, diamond is of great importance and interest for scientific studies. However, formation of a diamond is complicated process and requires extreme conditions. Bundy and Kasper (1967) for the first time synthesized a new form of carbon—hexagonal diamond – under conditions of static pressure exceeding about 13 GPa and temperature greater than about 1000°C [1]. At room temperature the crystal structure of graphite is stable up to pressure 15 GPa and loses some of the graphite features at higher pressure, forming metastable graphitic or amorphous phases [2]. Transition of polycrystalline graphite to diamond occurs after hydrostatic pressure treatment near 70 GPa [3]. The development of solid‐state phase transitions, including those at the stage of nucleation and development of a new phase practically always is connected with the relaxation of elastic stress [4], and in case of graphite‐diamond transformation the latter can play main role.

The goal of the present work is the formation of diamond from graphite in direct phase transition in a diamond anvil high‐pressure cell, where the relaxation of elastic stress can be realized by means of plastic deformation of the sample. The experiment was performed at room temperature without a catalyst.
13
C was subjected to the shear deformation under pressure of 25 GPa. The structure studies of the obtained material were made by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). In order to prevent the confusion of the diamond obtained in the experiment with one of the diamond anvils we used graphite composed of
13
C carbon isotope atoms as a precursor. The diamond anvils consisted of conventional
12
C diamond. Before TEM examination of each sample a Raman spectroscopy was used to verify that it contains only
13
C (diamond) and no
12
C. TEM and EELS were carried out using JEOL JEM‐2010 high‐resolution transmission electron microscope.

TEM analysis has shown that the samples obtained in the series of our experiments contain several phases of carbon simultaneously. After the high pressure treatment in shear diamond anvil cell (SDAC) there were observed some fragments of the sample, which contained both hexagonal and rhombohedral graphite (significant amounts of the last one), and also diamond and lonsdaleite. Fig. 1 shows the fragment, where the rhombohedral graphite presents.

Fig. 2a shows the diamond structure fragment with {111}‐planes composing 70º. Interplanar distances are 0.206 nm. Fig. 2b shows the EELS‐spectrum which can be unambiguously attributed to a diamond.

Thus, it was shown that
13
C‐graphite directly transforms into
13
C‐diamond (at least particularly) without a catalyst at room temperature after treatment in SDAC under pressure of 25 GPa.
Analysis of nanoscale band gap fluctuations in
Cu(In,Ga)Se2
solar cells by
VEELS
Authors: Keller Debora , Buecheler Stephan , Reinhard Patrick , Pianezzi Fabian , Rossell Marta D. , Pohl Darius , Surrey Alexander , Rellinghaus Bernd , Hage Fredrik , Ramasse Quentin , Erni Rolf , Tiwari Ayodhya N.
Published: December 2016

Thin film solar cells based on CuInGaSe
2
(CIGS) absorber layers have demonstrated conversion efficiencies of more than 22% and they manifest a promising potential for the development of highly efficient, flexible and low‐cost solar cells [1]. However, even though such high efficiencies are reached, still many fundamental properties that govern the performance are not yet clearly understood. The interfacial regions of the thin film layer stack and also the bulk of the polycrystalline CIGS absorber layer contain a high variety of structural and chemical inhomogeneities which consequently affect the electronic properties of the photovoltaic device in various ways. In order to push the achievable efficiency in a targeted way further towards the theoretical limit, a better understanding of local electronic loss mechanisms related to compositional inhomogeneities is needed. Tools that allow for characterizing the electronic properties in CIGS in the nanometer range are therefore sought.

Valence electron energy loss spectroscopy (VEELS) provides access to electro‐optical properties down to the nanometer scale which makes it a very promising technique for local electronic characterization of CIGS solar cells. Besides others, information about the band structure and in particular about the local band gap energy may be extracted from VEEL spectra. However, various artifacts may influence the measured VEEL spectra which complicates the data interpretation and requires careful consideration of the results. In case of CIGS, the average band gap energy is rather small, i.e. 1.1‐1.4 eV for typically used CIGS compositions, which pushes the required energy resolution towards the achievable limits of state‐of‐the‐art electron sources [2].

Within this work, the applicability of VEELS for probing local band gap fluctuations in CIGS is comprehensively assessed. Based on the results obtained on different microscopes we discuss the achievable precision and accuracy in correlation with the performance of the respective instrument as shown in Figure 1. This analysis suggests that a monochromator is required to reliably identify the small band gap variations in CIGS. Finally, we use a microscope equipped with a second generation monochromator to study the electronic properties in CIGS solar cells at specific sites such as grain boundaries and interfaces.
EELS
analysis of the interaction betweeen frozen acqueous samples and incident electrons in
TEM
Authors: Maigne Alan , Wolf Matthias
Published: December 2016

Radiation damage is a major limiting factor in obtaining high resolution images in cryo‐electron microscopy. Quantifying and understanding those radiation damages are considerable challenges. While it seems commonly admitted that the major part of the damages comes from radiolysis[1], local charging within the ice or the sample has been mentioned several time as a source of damage[2].

Due to the isolator behavior of frozen H
2
O, samples undergo electrostatic charging. While it has never been directly measured, this charging effect is often observable in cryo‐electron microscopy in the form of beam deflection or Coulomb‐explosion (as shown in Figure 1).

Aranova et al. showed that Electron Energy Loss Spectroscopy (EELS) could be used to measure the production of H
2
and O
2
associated to “bubbles” often observed in cryo‐EM when damages start to occurs [3]. A very interesting finding in their report is that no bubbles were observed in pure frozen water (without sample in it). One possible explanation is that radicals recombine more quickly in H
2
O than in frozen hydrated specimens in a Transmission Electron Microsocope.

During this presentation, we will study with mono‐chromated EELS the effects of the incident electron beam on frozen H
2
O as well as on frozen aqueous samples. Both the radiolysis and the electrostatic aspects of the beam/matter interaction can be measured with EELS using Kramer‐Kronig formula (through dielectric formalism) and Valence EELS (to look at chemical structure changes and bubbles production). We will show that depending on the type of electron illumination (STEM or TEM), the nature and intensity of those damages can vary. We will, then, look at several possible solutions to reduce such damages.
Transmission Mode in theSEM: Direct measurement of charge load up, secondary electron yield and backscattering coefficient in dependence on the energy of the incident electrons
Authors: Jaud Bianca , Bernhard Jörg , Kaiser Ute , Golla‐Schindler Ute
Published: December 2016

One essential precondition for a successful characterization of material by using electron microscopy is a sufficient electronic conductivity. Insulating or non sufficient conductive specimens becomes negatively or positively charged beside the balance points E1,2,3where the same amount of backscattered and secondary electrons leave the sample than primary electrons remain in the sample [1]. This charging effects are additionally an important factor for microscopy accessories like phase, monochromator plates etc [2]. We have implemented a modified transmission unit [3,4] in a SEM/FIB NVISION 40 SEM, which allows to measure the adsorbed and transmitted current simultaneously with the BioLogic SP‐200 Potentiostat and the specimen current monitor of the SEM respectively. Four different free supporting films were investigated: formvar (C5H8O2), leaf gold, amorphous carbon and amorphous PCS (Pd77.5Cu6.0Si16.5) alloy. Fig. 1 shows the studied formvar film at a) 3 kV and b) 20 kV without and with charging effects. The measurement of the transmitted current delivers the electron range and the application of the formula of Fitting [5], the film thickness of the investigated samples. Fig 1 c) show the normalized transmitted currents It/Ip(It= transmitted current, Ip=probe current) in dependence on the accelerating voltage for two objective aperture sizes of 30 and 120 µm. The electron range is given as the point where the first time transmitted electrons can be detected. The results show the expected independence of the electron range on the aperture size and therefore on the beam current, except for formvar where the differences are caused by charging effects. The obtained thicknesses for gold (157±16 nm) and , Formvar (209±21nm ) agree well with Monte Carlo simulations and those obtained for amorphous carbon (56±6 nm ) and amorphous PCS (20±2 nm ) with the expected film thicknesses of 50 nm and 20 nm respectively [2]. With the knowledge of sample thickness, scan speed and adsorbed current, the implemented charge per volume N can be calculated. Fig 1 d) present N in dependence on the energy of the incident electrons exemplary for formvar and gold with the 30 µm aperture. The obtained E3points are 2,0±0,3 keV and 4,3±0.3 keV. In a second step the adsorbed and transmitted current was measured with and without an additional potential of 50 eV on the sample for two different working distances. With the experimentally obtained currents,this enables to calculate the secondary electron yieldδ and the backscattered coefficientη in dependence on the energy of the incident electrons. The results are shown for the amorphous carbon film in Fig.1 e) and f).