bluuebunny/crossref_metadata_2025 huggingface dataset:
Point defect driven ferromagnetism in
YBa2Cu3O7‐x
superconductor
Authors: Gazquez Jaume , Guzman Roger , Mishra Rohan , Bartolome Elena , Salafranca Juan , Magen Cesar , Varela Maria , Coll Mariona , Palau Anna , Valvidares S.M. , Gargiani Pierluigi , Pellegrin Eric , Herrero‐Martin Javier , Pennycook S.J. , Puig Teresa , Obradors Xavier
Published: December 2016

Although defects may be seen as detrimental, they play essential roles in many materials. In some complex oxides for instance, they present an opportunity to enhance particular properties, or even engineer new ones. An archetypal example is the high‐Tc superconductor YBa
2
Cu
3
O

7−
δ

(YBCO), where defects are indispensable to immobilize quantized vortices in the presence of magnetic fields. Therefore, determining the atomic structure of defects is critical to unravel and control their effect on the physical properties. In this talk, we will do precisely that with an unforeseen complex point‐defect that leads to an unexpected formation of ferromagnetic clusters embedded within a superconductor. The commonest YBCO defect comprises the doubling of Cu‐O chains between Ba‐O planes, a ubiquitous intergrowth regardless of the deposition technique. Here, we will unclothe the true nature of this defect using a combination of experiments and theory to provide a complete picture of the structure and chemistry of these intergrowths at the atomic‐scale and their effect on the electronic and magnetic properties of YBCO. First, we will show, by means of scanning transmission electron microscopy (STEM), how the system solves the local off‐stoichiometry induced by the extra Cu‐O chain, removing half of the Cu atoms in selected chains, and the distortions induced by the vacancies. Secondly, we will show using density functional theory (DFT) how the complex structure of these intergrowths affects the electronic properties, and yields a magnetization density that extends into the neighboring Cu‐O planes. Finally, we will present X‐ray magnetic circular dichroism (XMCD) spectroscopy results, which provide evidence of the theoretically predicted Cu magnetic moments and the presence of a dilute network of magnetic defects within the high‐Tc superconducting state, see Figure 1. See for more details “Emerging dilute ferromagnetism in high‐Tc superconductors driven by point defect clusters”, Adv. Sci. 2016, 1500295.
Time‐resolved imaging and analysis of single atom diffusion on graphene oxide
Authors: Furnival Tom , Leary Rowan , Tyo Eric C , Vajda Stefan , Thomas John Meurig , Bristowe Paul D , Midgley Paul A
Published: December 2016

Single atoms and small atomic clusters offer a range of novel, tunable properties for a number of applications such as selective catalysis [1]. Achieving precise control of the desired properties of these systems first requires an understanding of the interaction between the cluster and its support. Advances in aberration‐corrected scanning transmission electron microscopy (STEM) mean that atomic resolution imaging and characterisation is now achievable for many materials. Observing individual atoms and small clusters remains difficult, however, due to low signal‐to‐noise ratio and beam‐induced motions causing blurring during image acquisition. One route around these problems is to acquire rapid image sequences in an effort to reduce the electron dose and also to capture any dynamic behaviour of the atoms. Making use of the spatial and temporal correlations between frames, and using a novel processing method based on singular value thresholding [2], we have developed robust approaches to recover individual atomic positions, including STEM acquisition rates of 10 frames per second or better [3].

We have applied the approach to the study of catalytically‐important copper atoms on few‐layer graphene oxide (GO), where the presence of functional groups on GO may aid the control of deposited clusters by acting as preferential pinning sites. Processing and analysis of an annular dark‐field STEM image sequence reveals a range of behaviours, with some strongly‐pinned atoms and other more mobile atoms undertaking random walks on the surface (Figure 1b). Further investigation of the jump distances (Figure 1c) and mean‐squared displacements (Figure 1d) reveals that the diffusion of adatoms on GO is anomalous. We combine this information with ab‐initio DFT calculations to provide new insight into the formation and behaviour of small atom clusters under an electron beam, and the interactions between few‐atom catalysts and high surface area supports.
Dynamic oxidation and reduction of catalytic nickel nanoparticles using
E(S)TEM
Authors: Lloyd David , LaGrow Alec , Boyes Edward , Gai Pratibha
Published: December 2016

For many materials, nanoparticles show enhanced catalytic activity and selectivity compared to their bulk state [1]. While catalytic nanoparticles have many industrial, economic and environmental benefits, they also present new challenges, the most significant of which is to characterise the intermediate phases and structural changes to the active catalyst species under reaction conditions. TEM is uniquely suited to the study of solid‐state heterogeneous catalysis as it allows us to directly characterise the catalyst with regard to its nanostructure on the atomic scale. Complementary spectroscopy techniques such as EDS and EELS can be used in tandem with imaging to add chemical data to the structural observations. The in‐situ capabilities of the E(S)TEM allow for intermediate catalyst phases and transformations to be observed, some of which are only present under reaction conditions and would be lost using ex‐situ methodologies [2].

Here we present a study of the dynamic nature of Ni based catalysts under redox conditions relevant to industrial processes [3, 4]. The increased activity of finely divided Ni means that it is important to directly observe the Ni nanoparticles rather than interpreting models based on bulk or ex‐situ techniques. Many chemical processes that rely on Ni catalysts involve exposure to both oxidising and reducing gas in the reaction environment. Under reducing and oxidising conditions the structure of Ni nanoparticles is dynamic and involves transitions between single crystal Ni, core‐shell Ni/NiO and hollowed Kirkendall‐like structures [5, 6]. These changes are dependent on the material properties (size, shape and support interaction) and reaction parameters (temperature, gas type and pressure). Furthermore, these dynamic structure/shape changes influence the stability of the catalyst, which in turn is controlled by the reaction environment. As such, this area of research is crucial for understanding the conditions necessary for attaining and maintaining a particular catalytic species and designing the reaction routes required to reactivate spent Ni catalysts.

Environmental TEM/STEM has been used to probe the dynamic oxidation of Ni nanoparticles. The particle size, reaction time and temperature dependencies of the oxidation process have been investigated using both model and industrial Ni catalysts. Furthermore, we have applied the same in‐situ techniques to reveal the conditions needed for complete reformation/reactivation of the original Ni species.
TEM
analysis of deformation induced dynamic nanocrystallization in an amorphous
CoTi
alloy
Authors: Noisternig Stefan , Ebner Christian , Gammer Christoph , Rentenberger Christian , Gspan Christian , Karnthaler Hans‐Peter
Published: December 2016

Nano‐sized crystals in an amorphous matrix are considered to change the mechanical properties of an amorphous alloy. Therefore, it is of special interest to manipulate and control both the size and structure of nanocrystals. In many cases, nanocrystals are formed by special heat treatments. Here, we show that nanocrystals can emerge out of the amorphous phase during severe plastic deformation. This can be revealed by studying the composition and the atomic structure of the crystals using different transmission electron microscopy (TEM) methods.

In our work pure components are used to make a Co
3
Ti alloy. Homogenisation at 950°C for 100 hours leads to a L1
2
long range ordered single phase alloy. The samples were deformed by high pressure torsion (HPT) using 4 GPa pressure and 80 rotations. After deformation TEM imaging with a Philips CM200 yields both crystalline and amorphous regions present in the samples. The situation is similar to that of Zr3Al, a L12 alloy, that can be made amorphous by severe plastic deformation [1]. The striking result of the present study is that in the amorphous regions nanocrystals of about 2‐20 nm in size are embedded. Their average size is about 12 ± 0.5 nm and they exhibit a volume fraction of about 2 ± 1% (cf. Fig.1). From the analysis of bright field (BF) images taken from different sample sections it can be concluded that the nanocrystals have a spherical shape.

The chemical composition of the nanocrystals is analysed in a FEI Titan microscope by electron energy loss spectroscopy. The Ti atomic concentration for individual nanocrystals is 18 % higher than those of the surrounding amorphous matrix. This indicates that the nanocrystals are of the Laves phase Co
2.1
Ti
0.9
. Therefore we conclude that the nanocrystals are not retained crystalline material but rather formed during deformation by dynamic crystallisation.

For structural information high resolution transmission electron (HRTEM) images of the nanocrystals are acquired. The HRTEM images show lattice planes according to the Kagome layers of Laves phases (cf. Fig.2). Nevertheless, the analysis of the stacking sequence of the Kagome layers A, B and C does not reveal unambiguously the corresponding Laves phase due to a high density of faults. The structure can be described either by a faulted Co
2
Ti (stacking sequence ABC) or a faulted Co
2.1
Ti
0.9
(stacking sequence ABAC). In order to have reference images of an unfaulted Laves phase the as‐cast alloy containing the Co
2.1
Ti
0.9
Laves phase (C36) was studied. Therefore HRTEM images are acquired with a CM30 microscope. By using the Kikuchi patterns the sample was tilted to a [100] pole of Co
2.1
Ti
0.9
. Fig.3 shows the corresponding HRTEM image of the Kagome layers with an unfaulted ABAC stacking order.
In‐Situ
ESTEM
Observations of Asymmetric Oxidation and Reduction in Copper Nanoparticles
Authors: LaGrow Alec , Ward Michael , Lloyd David , Boyes Edward , Gai Pratibha
Published: December 2016

A fundamental understanding of the oxidation and corrosion mechanisms of metals is of critical importance to improving their performance in catalysis, and other industrial applications.
1
For applications in nanocatalysis a metals oxidation pathway and subsequent reduction can lead to the rearrangement of catalytically active surface facets
2
as well as deactivation through sintering and Ostwald ripening.
3
In particular we are studying copper which can readily oxidize at room temperature and has two native oxides, cuprous oxide (Cu
2
O) and cupric oxide (CuO). The oxidation of copper has been previously reported to be dependent on its crystallography
4
as well as the interaction between the copper and the substrate.
5

In this talk we will discuss the use of environmental scanning transmission electron microscopy (ESTEM)
6
to study the
in‐situ
oxidation of copper. Environmental STEM was carried out in a modified JEOL 2200 which allowed for the introduction of gases into the microscope and using a DENSsolutions holder to control the reaction temperature. The copper is studied in the form of nanoparticles of 2 – 50 nm in size. With high angle annular dark field (HAADF) STEM we use conditions that are ideal to track the oxidation front as it progresses across the copper nanoparticles by following the changes in Z‐contrast with time. In the case of copper, the oxidation occurred via the heterogeneous nucleation of the oxide phase (Cu
2
O) from the smallest point on the nanoparticle (Figure 1a and 1b). When the process is reversed, via reducing the particles with hydrogen, it was also observed that the reduction was initially nucleated from the smallest part of the nanoparticle and then spread across the particle. Preliminary analysis of the data suggests that once the oxidized or reduced phase is nucleated the reaction is mediated by the Cu/Cu
2
O interface.
Microstructure of glass ceramics synthesized from chromium waste
Authors: Varitis Savvas , Kavouras Panagiotis , Kaimakamis George , Pavlidou Eleni , Vourlias George , Karakostas Theodoros , Komninou Philomela
Published: December 2016

The microstructure of different glass ceramic materials, obtained by thermal processing of vitrified products synthesized from tannery waste, was investigated using electron microscopy (TEM) techniques. Preliminary structural characterization was conducted by X‐Ray Diffraction (XRD) while morphologies and compositions of the materials at the mesoscopic scale were attained using Scanning Electron Microscopy (SEM) and Energy Dispersive X‐ray Spectroscopy (EDXS).

A first series of materials were synthesized by sintering chromium rich ashes of tannery waste with low grade soda lime glass powder. Three different mixtures were prepared with proportions 30/70, 40/60 and 50/50 of Cr‐ash over glass powder and sintered at 600°C, 800°C, 1000°C and 1200°C [1]. Depending on the temperature, crystalline phases with chromium and non‐chromium content were solidified. At the low temperatures the resulting products were opaque ceramics with granular morphology and high porosity, as shown in Fig. 1. At the high temperatures they consisted of a vitreous matrix with dispersed crystalline phases having the morphology of typical glass‐ceramics as those pointed by the white arrow in Fig. 2.

A second series of materials were synthesized using chromium ash (10wt%‐20wt%) and SiO
2
, Na
2
O and CaO as vitrifying agents, in different relative proportions considering the low solubility of chromium inside the silicate melts [2]. Only the product with the lowest Cr‐ash proportion was X‐Ray amorphous with no indication of crystallites. HRTEM observations verified that the product retained the amorphous character even at the nanoscale since no nanostructured crystallites were detected. In all other vitrified products Eskolaite (Cr
2
O
3
) crystallites of hexagonal shape, shown in Figure 3, were grown in the melt. Devitrification of the as‐casted products resulted to various crystalline phases dispersed within the vitreous matrix, depending on the initial batch composition. Eskolaite crystallites were not affected by the thermal processing and Devitrite (Na
2
Ca
3
Si
6
O
16
), Combeite (Na
4
Ca
4
Si
6
O
18
) and Wollastonite (CaSiO
3
) crystallites were grown. Figure 4 illustrates a HRTEM image showing a Wollastonite (CaSiO
3
) crystallite in the devitrified product with 13wt% Cr‐ash, viewed along the [0 1 1] zone axis. This crystallite has the triclinic structure of 1A‐CaSiO
3
.
Quantitative evaluation of the (211)
B GaAs/InAs
quantum dot heterostructure
Authors: Kehagias Thomas , Florini Nikoletta , Kioseoglou Joseph , Dimitrakopulos George , Germanis Savvas , Katsidis Charalambos , Hatzopoulos Zacharias , Pelekanos Nikolaos
Published: December 2016

InAs QDs grown on high‐index GaAs(
h
11) surfaces seem to exhibit superior optical properties compared to the usual QD growth on GaAs(001), due to their prominent piezoelectric field, which can be functional in nano‐photonics and quantum computing. However, the morphology of the QDs, as well as their strain state and chemical composition, influence significantly the light emission and absorption, the lasing efficiency, and other optoelectronic properties of QD‐based devices. In order to shed light to these effects, we have explored the nanostructure, the strain properties, and the related chemical composition of buried InAs QDs grown on (211)B GaAs surface employing quantitative HRTEM techniques.

The InAs QD layer was grown by MBE under the Stranski‐Krastanow (S‐K) regime at 480°C with a growth rate of 0.9 ML/s for 2s, over a 20 nm GaAs layer grown at 620°C. The corresponding BEP was 8.5x10
‐7
mbar. The QD layer was then overgrown by a 30 nm‐thick GaAs cap layer, without growth interruption, by ramping up the temperature back to 620°C.

The QDs adopted an anisotropic pyramidal shape, elongated along the <111> direction and were delimited by the {110}, {100}, and {213} or {214} crystal facets [Fig. 1(a)], as clearly evidenced by the equivalent larger uncapped InAs QDs grown on (211)B GaAs surface under similar growth conditions [1]. Local strain measurements by the geometric phase analysis (GPA) method [2] showed that buried dots were pseudomorphically grown on GaAs without the presence of any interfacial or extended defects [Figs. 1(a) and 1(b)]. Assuming a plane stress state of the QDs, following the transformation of the elastic stiffness tensor in order to comply with growth along the [211] direction [3], we found a systematic increase of the local strain from the base area to their apex region [Figs. 1(c) and 1(d)]. Then, applying Vegard's law, we calculated the chemical composition of the QDs that was found to exhibit an indium composition gradient along the growth direction, obviously suggesting gallium segregation inside the dots (Fig. 2). Even though the gradual increase of indium concentration is a common trend for all QDs, various In‐content maxima (0.50 to 0.92) were measured at the apex area of different QDs. This variation can be attributed to the corrugated form of the (211) surface [4], resulting in local compositional fluctuations of the wetting layer at the nucleation sites of the QDs. Therefore, gallium segregation is already involved at the onset of the S‐K growth. Furthermore, photoluminescence (PL) and
μ
‐PL experiments, as well as simulations of the QDs' transition energies, showed variations in the emission energy of the QDs, which is in line with a graded In‐content along the growth direction instead of pure InAs, thus verifying the chemical composition profile of the QDs revealed by quantitative strain measurements.
Relationships between elaboration conditions, structural parameters and electrical properties in metal oxides nanometric periodic multilayers
Authors: Potin Valérie , Cacucci Arnaud , Martin Nicolas
Published: December 2016

Ti/TiOx and W/WOx multilayered thin films have been deposited by DC reactive sputtering using the reactive gas pulsing process (RGPP). It is implemented to produce regular alternations of metal‐oxide compounds at the nanometric scale [1, 2]. Structure and growth have been investigated by High Resolution Transmission Electron Microscopy (HRTEM), Scanning Transmission Electron Microscopy (STEM) Energy Dispersive X‐rays Spectroscopy (EDX) and Energy Electron Loss Spectroscopy (EELS). Regularity of titanium and tungsten based alternations, quality of interfaces as well as oxygen diffusion through the multilayered structure have been examined taking into account the reactivity of oxygen towards the metals. Electrical measurements have been also carried out with the van der Pauw method to determine resistivity changes with temperature.

CTEM has been performed to determine the thickness of the periodic metallic and oxide layers from 3 to 40 nm. HRTEM experiments have been carried out to study the atomic structure of the periodic layers. The study of HRTEM images has allowed determining a growth model with the following series: (rutile‐)TiO
2
, fcc‐TiO and α‐Ti for Titanium system. This result has been confirmed by SAED experiments. Chemical information was obtained from the core‐loss EELS and EDX spectra. Core‐loss study was particularly performed for TiOx samples to quantify the elemental composition from the Ti‐L
2,3
and O‐K edges (Fig. 1). The systematic presence of oxygen has been pointed out in the rich‐metal sub‐layer, corresponding to the TiO phase already pointed out by HRTEM. Concerning tungsten system, the layers appear to be mainly amorphous. Therefore, low‐loss EELS study was performed and has allowed determining a growth model with the following series: W/W
3
O/WO
3
/W
3
O (Fig. 2).

The knowledge of the structural parameters has allowed determining a first relation between the elaboration conditions (control of the pressure value) and the structural parameters. Electrical and structural results have also been related to propose a law linking the resistivity values ρ to the structural parameters as total thickness e
tot
, metal λ
met
and oxide λ
ox
layers thickness and metal elemental composition C
met
(Fig. 3).
Optically Coupled Plasmonic Nanopores Observed by Cathodoluminescence Scanning Transmission Electronmicroscopy
Authors: Sannomiya Takumi , Saito Hikaru , Juliane Junesch , Yamamoto Naoki
Published: December 2016

Control of the optical properties of nano‐plasmonic structures is essential for next‐generation optical circuits and high‐throughput biosensing platforms. Realization of such nano‐optical devices requires optical couplings of various nanostructured elements and field confinement at the nanoscale. In particular, symmetric coupling modes, also referred to as “dark modes”, have recently received considerable attention because these modes can confine light energy to small spaces. Although the coupling behavior of plasmonic nanoparticles has been relatively well‐studied, couplings of inverse structures, i.e., holes and pores, remain partially unexplored. Even for the most fundamental coupling system of two dipolar holes, comparison of the symmetric and anti‐symmetric coupling modes has not been performed. Here, we present a systematic study of the symmetric and anti‐symmetric coupling of nanopore pairs using cathodoluminescence by scanning transmission electron microscopy (CL‐STEM) and electromagnetic simulation.

The nanopore samples were fabricated by colloidal lithography and film transfer by wet etching of the sacrificial layer. To achieve very close separation of nanopore pairs and to obtain high spatial resolution in STEM, we chose ultra‐thin, free‐standing film structures. For the measurement of single and coupled pairs of nanopores, 135 nm nanopores in an AlN(8 nm)/Au(16 nm)/AlN(8 nm) trilayer membrane were used (Figure 1b). With this sandwich layer structure, it is possible to obtain very thin and stable metal layers, even at high temperatures.

For CL‐STEM measurement, 80 kV acceleration was used to avoid possible damage due to relatively high beam current. (5nA) Depending on the electron beam position it is possible to distinguish the symmetric and anti‐symmetric dipolar coupling modes.(Fig. 1) The observed symmetric coupling mode, approximated as a pair of facing dipoles, appeared at a lower energy than that of the anti‐symmetric coupling mode, indicating that the facing dipoles attract each other. The anti‐symmetric coupling mode splits into the inner‐ and outer‐edge localized modes as the coupling distance decreases. These coupling behaviors cannot be fully explained as simple inverses of coupled disks. Electromagnetic simulation by finite difference time domain (FDTD) also showed consistent coupling behaviors. Models of FDTD simulation showed that the inner‐ and outer‐edge anti‐symmetric modes become fully localized with minimal influence of the opposite edges as the coupling distance decreases. Symmetric and anti‐symmetric coupling modes are also observed in a short‐range ordered pore array (Fig. 2), where one pore supports multiple local resonance modes, depending on the distance to the neighboring pores.
Development of New Generation Cryo
TEM
Authors: Hosogi Naoki , Kaneko Takeshi , Ishikawa Isamu , Yuasa Syuuiti , Hiyama Kimitaka , Fujimoto Naoki , Chiyo Izuru , Kamoshita Akihito , Ohkura Yoshihiro
Published: December 2016

Cryo transmission electron microscopy (TEM) provides structural information of a specimen close to its natural state without any disturbance, due to the specimen preparation process, which exclude chemical reactions and physical stimulations. Recently, cryo TEM produces very exciting results of structural biology in combination with single particle analysis and electron tomography, since application field of the method expand to non crystalline samples or huge molecules.

We developed a new generation cryo TEM, which achieves high throughput and high usability. This microscope equips 200 kV field emission gun (FEG). Users can choose it from a Schottky‐type (TFEG) or a cold FEG (CFEG). Since the energy spread of the emitted electrons from the CFEG is about 50% of TFEG and the size of the virtual source is less than 10 nm, the electron beam has a high coherences. With such beam, cryo TEM image has high contrast due to its high spatial coherence and is less affected by chromatic aberration due to its high temporal coherence. In low dose imaging, where the image resolution is mainly determined with dose density for the image. In the low dose density, S/N of image mostly determined by a statistical noise of electrons, since dose density in cryo TEM is typically several tens of electrons for angstrome square. Namely, the resolution is determined with the competition between the statiscal noise and image contrast. It means CFEG has posibility to have higher resolution for cryo TEM works.

Since this microscope also has dedicated cryo stage, cryo TEM observation can be performed at low temperature < 100 K and with low grow ratio of ice contamination. In addition, this cryo stage is compatible for multi‐specimen auto‐loader, so users can exchange specimens automatically. It also has some automation functions, such as liquid nitrogen auto‐refill system and auto acquisition software (JEOL Automated Data Acquisition System: JADAS). These automation functions will help users to perform high throughput works. On another front, the electron gun chamber and the TEM column are evacuated with sputter ion pump and turbo‐molecular pump, because of this, a sample is kept in oil‐free environment.

In addition, this microscope is compatible with omega‐type energy filter and Zernike or hole‐free phase plate. The cryo specimens exhibit low contrast in TEM images even using large defocus phase contrast imaging. The Cryo TEM is more advantageous when it is combined with these techniques of contrast enhancement.
Charging kinetics features of different modifications of theAl2O3–dielectrics (sapphire, polycrystalline and ceramics) under electron‐beam irradiation
Authors: Rau Eduard , Tatarintsev Andrey , Khaidarov A , Kupreenko Stepan
Published: December 2016

The phenomenon of charging dielectrics via the electron‐beam irradiation has been studied very intensively. However, published in recent years the results of experimental and theoretical studies of dielectrics charged under electron irradiation gives contradictory description of this phenomenon [1–6]. These contradictions arise mainly due to the differences in the approaches and fragmentation of experimental studies and differences in used physical models of charging mechanisms.

In this report on the basis of extensive experimental data of simultaneous measurement of key charging parameters we are given a balanced assessment of the multiple factors influencing on the charging process. We present the new features of this process and given its interpretation.

Based on comprehensive methodology [7, 8], we measure simultaneously the main charging parameters of different modification Al2O3, such as surface potentialVS, electron‐emission currentIσ, displacement currentID, and accumulated chargeQversus time of electron irradiation. The experimental setup that allows simultaneously measure the basic charging parameters published in the [9].

The experimental results of complex research of the key charging parameters for sapphire and ceramics Al2O3are presented in Fig. 1, Fig. 2, Fig. 3 and Fig 4. The main conclusion of these investigations is that although charging processes of dielectric targets are self‐consistent and mutually dependent, the accumulation of the negative charge and, correspondingly, an increase in the charging potential, is the dominant (leading) effect, while the variation in the secondary electron emission coefficient, which depends on the surface potential, is a driven adjusting process. For this reason, a delay occurs in the time for attaining the equilibrium of two fundamental charging parameters, viz., the emission current (a short‐time process) and charging potential (a long‐time process). In other words, for all modifications Al2O3coefficient of secondary electron emissionσreaches an equilibrium value for used doses of electron irradiation significantly faster than the surface potential.

This work was funded thanks to the support of RFBR (grant 15‐02‐01557).
Quantitative measurement of doping and surface charge in a
ZnO
nanowire using in‐situ biasing and off‐axis electron holography
Authors: den Hertog Martien , Donatini Fabrice , McLeod Robert , Monroy Eva , Pernot Julien
Published: December 2016

Semiconducting nanowires (NWs) are widely studied because the properties that stem from their three‐dimensional, nanoscale nature open new opportunities for device design. In particular ZnO NWs are widely studied for their interesting piezoelectronic properties. Though NWs can be readily grown today with increased carrier concentration due to doping, the measurement of the doping concentration at the nm scale remains challenging.

We demonstrate that state‐of‐the‐art off‐axis electron holography in combination with electrical in‐situ biasing can be used to detect active dopants and surface charges quantitatively in ZnO nanowires. The outline of the contacted NW is described in Fig. 1. We have acquired series of holograms and averaged the phase to increase signal to noise but avoid blurring due to specimen drift. The 0V bias images were used to remove contrast not related to the varying electrostatic potential and to verify the nanowire was not electrically modified during the experiment. We analyzed the depletion width in the nanowire due to an applied reverse bias to a Schottky contact on the nanowire, using a fit to the data.

Comparison of the experimental data with 3D simulations that were similarly treated indicates an
n
‐type doping level of 1x10
18
at. cm
−3
and a negative surface charge around −2.5x10
12
charges cm
−2
. Fig. 2a shows the experimental vacuum corrected phase profiles converted to potential, and a fit to the data. The extracted depletion width is indicated with a cross. The inset shows the location of the phase profile in the NW core and two symmetrically defined phase profiles obtained in vacuum on either side of the NW. The average signal in vacuum was subtracted from the NW signal and the remaining phase signal was converted to potential using a thickness of 75 nm, much smaller than the 150 nm NW diameter. In Fig. 2b the experimental and simulated depletion length is compared for 3D simulations including varying doping and surface charge quantities. We expect that the real doping is between 1 and 2x10
18
at. cm
−3
by comparison of experiment and simulation. The surface charge results in a surface depletion to a depth of 36 nm. We found an active/undepleted core thickness of 70–75 nm, providing excellent agreement between the simulated thickness of the undepleted core and the active thickness observed in the experimental data.

Off‐axis electron holography thus offers unique capabilities for quantitative analysis of active dopant concentrations and surface charges in nanostructures with nanometer‐scale spatial resolution.
Studying synaptic architecture in the zebrafish Danio rerio
Authors: Helmprobst Frederik , Schuch Kristin , Schulze Katja , Berger Constantin , Lillesaar Christina , Dandekar Thomas , Stigloher Christian
Published: December 2016

A fixation technique, which allows a “near‐to‐native” state tissue preservation, such as high‐pressure freezing in combination with electron tomography is required for studying synapses systematically at high resolution in 3D. With these techniques we are studying the ultrastructural architecture of synapses in the zebrafish
Danio rerio
which can be high‐pressure frozen as intact animals at early stages. Currently we are focusing at a group of cytoskeletal proteins, the septins, which play important roles in synapse formation and synaptic function. We localize them with various microscopy techniques ranging from light to 3D electron microscopy in the nervous system. A limiting factor in 3D electron microscopy is the annotation of structures. We therefore developed an automatic vesicle annotation macro with Fiji, which helps us to automatically measure the size of synaptic vesicles and their density.
Interfaces and defects in directionally solidified oxide‐oxide eutectics
Authors: Londaitzbehere Laura , Lartigue‐Korinek Sylvie , Mazerolles Leo
Published: December 2016

Oxide‐oxide eutectic ceramic materials prepared by unidirectional solidification from the melt seem to be promising candidates for thermo‐mechanical applications at high temperatures such as for gas turbine parts applications. They are prepared from Al
2
O
3
‐ RE
2
O
3
‐ ZrO
2
(RE = Y, Er, Sm) systems and associate 2 or 3 phases among: Al
2
O
3
(corundum structure), RE
3
Al
5
O
12
(garnet structure), REAlO
3
(perovskite structure) and ZrO
2
(fluorite structure) phases. The microstructure consists of a three‐dimensional interpenetrated network of single‐crystal phases, free of grain boundary (fig. 1) and preferred orientation relationships occur between constituent phases
1
. Consequently, they exhibit a noticeable thermal stability of the microstructure and good mechanical properties
2,3
.

Compressive tests were carried out at 1450°C and 1550°C under 100 MPa and 200 MPa. Low strain rates obtained confirm good creep resistance at high temperature of these materials. Deformation mechanisms were studied by TEM with conventional two‐beam analysis. During creep tests, the specimens deform via micro‐twinning in alumina and dislocation activity (climb in most cases) in all phases (fig. 2).

The interface nature of directionally solidified eutectic ceramics is the origin of their good properties in comparison with sintered equivalent ceramics. Thus, the different types of interfaces were studied at atomic scale by HRTEM (fig. 3). In a first approach, the chosen observation direction axis was parallel to the growth direction. Investigations were made on as‐grown materials and after creep tests in order to:

‐ understand interface structure and defects at the atomic scale in as‐grown materials ;

‐ analyse modification of interfacial structure after creep deformation ;

‐ correlate microstructure and deformation mechanisms.
Rotavirus double and triple layered viral particles: correlative characterization using electron microscopy, disc centrifuge and capillary electrophoresis
Authors: Miseur Agnes , Coppens Astrid , Chapelle Céline , Hens Christine , Blain Philippe , Delpierre Ghislain , Deschuyteneer Michel
Published: December 2016

Rotavirus (Reoviridae) viral particles consist of capsids with 3 concentric protein layers surrounding the virus genetic material. The outer layer is composed of the VP4 and VP7 proteins, which independently elicit neutralizing antibodies and induce protective immunity.

Rotarix™, GSK live attenuated Rotavirus vaccine, was characterized by EM. The observations show that the vaccine contains both triple‐layered particles (TLPs) containing all 3 protein layers and double‐layered particles (DLPs) which lack the outer capsid proteins VP4 and VP7.

As DLPs are non‐infectious, it may be of importance to address the question of their abundance
vs
. TLPs in bulk preparations. Although such information can be obtained by EM, the method is relatively impractical for this purpose, particularly for routine analysis, as high number of particles should be observed and sized to get accurate relative abundances.

To determine the size and quantify the relative abundance of TLPs and DLPs on large populations of viral particles in bulk preparations, two size‐based, orthogonal analytical techniques were applied to detect these particles: disc centrifuge (DC) and capillary electrophoresis (CE).

To confirm the capacity of both methods to distinguish clearly the two types of capsids, TLPs and DLPs were partially purified using CsCL gradient. They were characterized by EM negative staining (Fig. 1) and cryo‐TEM (Fig. 2). Their apparent diameter difference of about 10 nm was well correlated in both techniques. The profiles obtained by DC and CE (Fig. 3) show the well‐resolved identification.

In addition, further confirmation was obtained by incubation of the preparations with EDTA, to remove the calcium required for VP7 protein stabilisation and thus to convert TLPs into DLPs [
1
]. The shift from TLPs to DLPs is clearly visible with CE (not shown), DC and also EM (Fig. 4).

Dynamic Light Scattering (DLS) was also evaluated and works fine with the partially purified samples of either TLPs or DLPs. The shift from TLPs to DLPs by this method is clearly observable as well (not shown). However, DLS does not allow distinguishing clearly the two populations of capsids in real samples with mixed TLPs and DLPs.

These orthogonal structural and sizing analysis methods can therefore be combined to assess and measure accurately DLPs/TLPs ratio. They thus provide a tool to understand the possible link of this ratio with the immunological potency of vaccine preparations.
Atomic structures of interfacial complexions between gold nanoparticles and nominally stable spinel‐substrate
Authors: Liu Fang , Xie Dong Yue , Fu Yong‐sheng , Zhu Guo‐zhen
Published: December 2016

The interfacial complexions, with distinct structures, are considered as quasi‐two‐dimensional phases, which undergo the structural and chemical changes associated with thermodynamic parameters [1‐2]. We recently discovered the formation of gold‐spinel interfacial complexions, with well‐defined atomic structures, and the related growth of nominally stable spinel lattice underneath the gold nanoparticles after annealing [3‐5]. As shown in Figure 1, the necking structure, which is tens of nanometers high, is detected under dewetted gold nanoparticles. Such necking structure has the same contrast with spinel substrate, maintains the spinel composition confirmed by the energy dispersive X‐ray spectroscopy, and keeps an ideal epitaxial relationship with the substrate. In associated with the substrate growth, an interfacial bilayer with distinct crystalline structure forms between gold nanoparticles and the substrate. Further studies reveal that the formation and migration of the interfacial complexions are related to the defects at the interfaces, such as, the intersections of gold twinned planes and the interface (see details in Figure 2). In spite of their importance in synthesizing nanostructures, the atomic structures of interfacial complexions have not been fully elucidated yet.

Herein, in this paper, we investigated the detailed atomic models of such interfacial complexions in combination of atomic‐resolution experimental images and first‐principle computations. Experimentally, we synthesized a series of Au‐MgAl
2
O
4
samples within different annealing profiles and proposed the initial atomic models based on atomic‐resolution scanning transmission electron microscopy (STEM) ‐ high‐angle annular dark‐field (HAADF) images in Figure 3. Experimental images with two orthogonal crystallographic directions were selected to provide the three‐dimensional structural information. A few possible atomic models, with different oxygen vacancies, were built through our
MATLAB
codes, and inputted into the density functional theory (DFT) computation. The relaxed atomic models, carried out with generalized gradient approximation (GGA) and Perdew‐Burker‐Ernzerhof (PBE) exchange‐correlation density functional [6], were applied to simulate the HAADF images [7] in order to further verify the validity of the structures. Our results provide a new clue to the field of interfacial complexions, particularly to the structural origins of the abnormal phenomenon of self‐assembled growth of oxide substrates underneath dewetted gold nanoparticles.
Valence state analysis on iron in minerals of Earth's lowermost mantle by electron energy loss spectroscopy
Authors: Dennenwaldt Teresa , Piet Hélène , Badro James , Gillet Philippe , Hébert Cécile
Published: December 2016

Determining the ratio of ferrous and ferric iron concentrations in lower mantle minerals is essential to understand the geophysical and geochemical properties of the deep Earth. The two iron‐bearing phases in the lower mantle are bridgmanite (Brg, (Mg,Fe)SiO
3
) and ferropericlase (Fp, (Mg,Fe)O) where the valence state and partitioning of iron have a strong impact on the physical and chemical properties [1,2]. Samples consisting of a Brg and Fp mineral assemblage have been previously synthesized from Al‐rich olivine composition [3]. Brg turns into an amorphous matrix after decompression to atmospheric pressure and Fp remains crystalline with grain sizes ranging from 50 – 400 nm. To understand the correlation between the distribution and oxidation states of iron, characterization methods with a high spatial resolution are required. Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is a useful method to investigate Fe
3+
/ ∑Fe in iron‐rich minerals [4]. Things to take care of during acquisition are e.g. beam damage of the material or electron‐beam‐induced oxidation of iron. EELS analysis of Brg and Fp phases synthesized at different temperatures and pressures prepared by focused ion beam (FIB) sectioning will be discussed.
Intracellular Organization in Cochlear Inner Hair Cells by
3D
Electron Microscopy
Authors: Fleck Roland , Vizay‐Barrena Gema , Bullen Anewn
Published: December 2016

The ways in which cell architecture is modelled to meet cell function is a poorly understood facet of cell biology. However, the compartmentalization of mutually exclusive reactions in different regions of cells by membrane‐enclosed organelles or by self‐assembling macromolecular complexes is a basic mechanism of life. Fluorescent tags can display proteins confined to compartments in living cells, but provides no glimpse of the underlying ultrastructure. The electron microscope has the resolution to display cellular ultrastructure. Transmission electron microscopy (TEM) tomography of thin slices (~100‐300nm depth) can reveal complex subcellular cellular ultrastructure. Serial Block Face Scanning Electron Microscopy (SBFSEM) can provide ultrastructural resolution throughout a much longer (μm to mm) sample depth, enabling quantitative analysis of ultrastructural features throughout the length of most complex cells.

We have studied the cytoarchitecture of a cell with highly specialised organisation, the cochlear inner hair cell (IHC), using multiple hierarchies of 3D electron microscopy analyses. We have shown that synaptic terminal distribution on the IHC surface correlates with cell shape, and the distribution of a highly organised network of membranes and mitochondria encompassing the infranuclear region of the cell. Structural linkages between organelles that underlie this organisation were identified by high resolution imaging. Together these techniques have the potential for clarifying functionally specialised cytoarchitecture of other cell types. Strategies employed to improve data quality will be discussed.
Accurate and fast electron microscopy simulations using the open source
MULTEM
program
Authors: Lobato Ivan , Van Aert Sandra , Verbeeck Johan
Published: December 2016

The most practical method for TEM image simulations is the multislice method, which is known to be an accurate numerical procedure for solving the quantum mechanical electron‐specimen interaction. Although most simulation codes treat the scattering process as purely elastic and coherent, inelastic scattering cannot be neglected and it has to be included in realistic simulations. Inelastic phonon scattering is often incorporated by using the frozen phonon model [1, 2] and the electronic excitations by using the density matrix approach [3, 4]. Nowadays, new computer technologies allow us to perform large TEM calculations with inclusion of accurate approximations of the electron‐specimen interaction in an acceptable amount of time.

A general overview of the MULTEM program along with a number of examples has been reported in [5]. In this work we present a new version of the open source MULTEM program, which adds key features including a graphical user interface, tapering truncation of the atomic potential, CPU multithreading, single/double precision calculations, STEM simulations using experimental detector sensitivities, ISTEM simulations, EFTEM simulations, STEM‐EELS simulations along with other improvements in the algorithms. A screenshot of the user interface is shown in Fig. 1. This figure shows the main available options of the program. In Fig. 2, a simulated HRTEM image and ED pattern of an isolated gold nanoparticle of 21127 atoms are shown. In these simulations, plane‐wave illumination is assumed with the following electron microscope setting: acceleration voltage (300 keV), spherical aberration (0.002 mm), defocus (19.8 Å), defocus spread (30 Å) and beam divergence angle (0.1 mrad). A numerical real space grid of 4096x4096 pixels has been used. The frozen atom simulation is performed by using the Einstein model with 200 configurations, slice thickness of 0.5Å and the three‐dimensional rms displacements of all the atoms are set to 0.085Å. For the HRTEM simulation, the spatial and temporal incoherences are included by applying the partially coherent microscope transfer function to each exit wave of the frozen atom. A multislice simulation of each frozen atom configuration only took 2.6 s on a Nvidia GeForce GTX TITAN GPU card.

The MULTEM's C++ classes, Matlab mex functions and the GUI are available in the GitHub repository
https://github.com/Ivanlh20/MULTEM
.
Combined EELS‐EDX analysis of nanoscale memristive NbOx and AlOx layers
Authors: Strobel Julian , Hansen Mirko , Haberfehlner Georg , Kothleitner Gerald , Ziegler Martin , Kohlstedt Hermann , Kienle Lorenz
Published: December 2016

Memristive devices – electronic components that can change their resistance depending on their history of operation – offer a new approach in various fields of digital computing. [1] This advancement in information technology is being pursued in order to satisfy the ever increasing need for computing power on the one hand and to enable the imitation of neuronal networks like the hippocampus on the other. The conductive properties in memristive devices are typically governed by atomistic effects like cation or anion movement: the change of resistance state is dependent on the nanoscopic layout. The mechanisms, however, behind a non‐binary, remanent, reversible and repeatable change of electrical resistance are not well understood. In this work we present the results of dedicated transmission electron microscopy (TEM) analysis involving highly precise electron energy loss spectroscopy (EELS) methods in a complex oxide multilayer stack of only about ten nanometer total thickness. The layer sequence Nb/Al/Al
2
O
3
/Nb
x
O
y
/Au (from bottom to top; Fig. 1 a) was deposited onto a Si/SiO
2
substrate. The memristive properties of such junctions were recently investigated. [2]

Combined energy dispersive X‐ray spectroscopy (EDX) and EELS experiments allow for analyses of both light and heavy metals as well as the reliable detection of oxygen. In contrast to the deposited layer sequence we observe an entire oxidation of the metallic Al and even a partial oxidation of the Nb bottom electrode. (Fig. 1 b) and Fig. 2) Furthermore, the results imply the abundant presence of oxygen vacancies apparent from the O K‐peak in the spectrum of the amorphous Al oxide layer which acts as a tunnel barrier. (Fig. 1 c) These puzzling results will be discussed in the framework of former studies on the Nb‐Al overlayer technique. [3]

Different conduction mechanisms are being discussed based on the findings and an outlook onto further research by electronic structure calculation is given. The results shed light on the fundamentals of tunnel barrier‐based memristive devices which are compatible to state‐of‐the‐art CMOS technology and were already built into integrated circuits.