bluuebunny/crossref_metadata_2025_split huggingface dataset:
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].
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.
In situ
UHVEM
observation of atomic ordering in magnetic nanoparticles using a direct detection camera
Authors: Sato Kazuhisa , Yasuda Hidehiro 
Published: December 2016

Recent demands for ultrahigh density magnetic storage technology require the development of recording media with higher magnetocrystalline anisotropy energy in order to ensure thermal stability of magnetization as well as ultrahigh recording density. For such a purpose, equiatomic CoPt alloy nanoparticles (NPs) are one of the candidate materials [1]. The hard magnetic property of this alloy is attributed to the tetragonal L1
0
‐type ordered structure; the anisotropy energy is dependent on the degree of order of the ordered phase. Therefore, formation of the L1
0
ordered phase is the key issue for practical applications. In this study, we hence intend to observe atomic ordering of CoPt NPs by ultra‐high voltage electron microscopy (UHVEM) equipped with a direct detection camera.

Thin films of disordered CoPt alloy NPs were synthesized by co‐deposition of Co and Pt targets using rf‐magnetron sputtering onto NaCl(001) substrates kept at 620 K. After the deposition of Co and Pt, surface of the NPs were coated by an amorphous carbon (a‐C) thin film. The specimen films were removed from the NaCl substrate by immersing the substrate in distilled water, and then floating films were mounted onto microgrids for electron transparency. Structure and composition of the NPs were characterized using a JEOL JEM‐ARM200F TEM (200 kV). Electron irradiation experiments and the simultaneous in situ heating observation were carried out using a JEOL JEM‐1000EES UHVEM (1 MV) equipped with a Gatan K2‐IS electron direct detection CMOS camera newly installed at the Research Center for UHVEM, Osaka University [2].
Figure 1 shows a selected area electron diffraction (SAED) pattern and a TEM image of as‐sputtered CoPt alloy NPs with disordered fcc structure. As seen, (100)‐oriented CoPt alloy NPs with sizes of ~15 nm are dispersed. Average alloy composition was Co‐45at%Pt as determined by EDX analysis (Fig. 1(c)).

Figure 2 shows two successive TEM images of a CoPt NP acquired in situ with a frame rate of 1/400 s. The observation was made at 583 K under electron irradiation at 1 MV with a dose rate of 1.5 × 10
25
e/m
2
s. In Fig. 2(a), disordered phase with the fcc structure is seen. After 2.5 ms, the ordered fringes suddenly appeared as shown in the middle of the NP surrounded by the square (Fig. 2(b)). The atomic ordering can also be confirmed by appearance of weak 001 superlattice reflection in the attached Fast Fourier Transform (FFT) pattern. The observed kinetic ordering temperature of 583 K for binary CoPt NPs is lower than that reported in the previous study (> 800 K) [3]. The low temperature atomic ordering can be attributed to the enhancement of atom migration by high‐energy electron irradiation [4].
3‐D reconstruction of surface topography inSEMby means of energy filteredSEandBSE
Authors: Kupreenko Stepan , Rau Eduard , Tatarintsev Andrey , Zaytsev Sergey 
Published: December 2016

The reconstruction of surface topography based on the detection of backscattered or secondary electrons in SEM has found increasing interest during long time [1‐5].For more accurate reconstruction one should know angular dependecies of secondary electron emission (SE) coefficient δ and backscattered electron (BSE) coefficient η, which, in general, are quite well investigated and defined [6]. But for increasing sensitivity in determination of local surface slopes it is offered to detect BSE and SE not integrally, as in previous works, but differentially by energy, i. e. by value of signal detected in narrow energy range of emitted electrons. As it was shown in our experiments, sensitivity increases in times in such formulation of experiment. Experiments were carried out in LEO‐1455VP (Zeiss) equipped with a two‐channel toroidal electron spectrometer [7]. We measured energy spectra of SE and BSE for various angles of incidence α (Fig. 1). SE spectra are slightly deformed because of applying negative bias Vs=‐33 V to stage (electrons leave surface nonisotropically). Comparison of integral and differential coefficients is depicted on Fig. 2. Values of integral coefficients δ and η were calculated by formulas from [8] (by substituting R to Rcosα [9]) and [10] respectively. Values of integral coefficient at fixed angle of detection θ were obtained by simple integration of measured spectra. LineScans of Ti‐ball, obtained as difference signal between two symetrically mounted detectors at fixed energy of electrons, are depicted on Fig. 3.Obtained results allow us to conclude that secondary electrons are best suited for proplem of surface relief reconstruction than backscattered electrons.This work was funded thanks to the support of RFBR (grant 15‐02‐01557)
Environmental tomography of liquid latex suspensions in
STEM
Authors: Xiao Juan , Roiban Lucian , Foray‐Thevenin Genevieve , Masenelli‐Varlot Karine 
Published: December 2016

ESEM (Environmental Scanning Electron Microscopy) allows the observation of liquids under specific conditions of pressure and temperature. Moreover, when working in the transmission mode,
i.e
. in STEM (Scanning Transmission Electron Microscopy), nano‐objects can be analyzed inside a liquid. Those acquired images are quite similar to those obtained in TEM (Transmission Electron Microscopy) using a closed cell [1]. By implementing a Peltier stage in the tomography device which was developed for the characterization of the 3D structure of non‐conductive and low‐contrast materials [2], the acquisition of image series of wet samples (wet‐STEM tomography) [3]. Moreover,
in situ
evaporation of water can be performed to study the materials evolution from the wet to the dry state.

In our experiment, SBA‐PMMA, a copolymer derived from styrene and metacrylic acid esters in aqueous solution, is chosen as the sample. A 3% PMMA shell, which plays the role of steric surfactant, has already included in the SBA particles. Some other surfactants can be introduced to further stabilize the latex for some special applications [4].

We first present measurements of the sample temperature using different experimental conditions, to show that the sample can be preserved in the dry state. Then, we show series of tilted images that are acquired during
in situ
hydration / dehydration experiments. Reconstructed volumes are obtained using the image processing softwares
etomo
and
Fiji
(see Figure 1), and the resolution is calculated using Fourier Shell Correlation (FSC). The influence of irradiation damage on the reconstructed volumes are discussed.

Afterwards, the volumes are segmented (see Figure 2) and quantified to follow the evolution of the microstructural parameters during water evaporation. This shows the potentialities of wet‐STEM tomography for the characterization of suspensions from the wet to the dry state.
Quantitative compositional characterisation of fuel‐cell catalysts using EDX ionisation cross sections
Authors: MacArthur Katherine , Slater Thomas , Haigh Sarah , Ozkaya Dogan , Heggen Marc , Nellist Peter , Lozano‐Perez Sergio 
Published: December 2016

Hydrogen fuel‐cells are a ‘zero emission’ technology because they only release H
2
O, making them a very attractive source of clean electric power. Unfortunately, fuel‐cells such as the polymer exchange membrane (PEM) fuel‐cell are heavily reliant on a platinum catalyst at both the anode and cathode in order for the reaction to proceed at the low operating temperatures. It is largely the cost of this platinum metal which limits their more wide‐scale manufacture and use.

In light of this, the search is on for more active catalysts with lower platinum content. A great deal of success has been seen in the research of bimetallic alloy catalysts where the expensive platinum metal is combined with a much less expensive metal such as nickel or cobalt. Such catalysts not only provide a reduction in the amount of platinum used but also an increase in the activity, which may be due to the compressive strain that the alloying element introduces.
In order to develop these catalysts further and make them a viable alternative to platinum, we need to understand what is happening at the nanometre and even sub‐nanometre scales. For this we need to be able to characterise our catalysts at high resolution, understanding both the composition and structure. A new method for quantitative energy dispersive x‐ray (EDX) [1] analysis in the scanning transmission electron microscope (STEM) has been developed with this in mind.
In the same way that the scattering cross section, σ, can be calculated from ADF image intensity and for ionisation edges in EELS, it is possible to calculate an EDX partial cross section using an approach that demonstrates similarities with the ζ‐factor method. Rather than the ratio approach provided by the traditional k‐factor method, this is a direct measurement which yields the number of atoms of each element after quantification, as such thickness can easily be extracted as well. This quantification method was applied to PtCo alloy nanoparticles that have been acid‐leached to provide platinum enrichment (or rather cobalt depletion) at the particle surface [2]. It is possible to quantify the levels of cobalt depletion in the first few atomic layers of the particle, showing that the leaching produces a localised surface depletion that can only be determined by this high resolution EDX quantification.