bluuebunny/crossref_metadata_2025 huggingface dataset:
Soft
X
‐ray tomography enhancement by focal series projections
Authors: Oton Joaquin , Pereiro Eva , Conesa Jose Javier , Sorzano Carlos Oscar S. , Marabini Roberto , Carrascosa José L. , Carazo Jose María
Published: December 2016

Soft X‐ray tomography (SXT) is one of the most recent structural biology techniques for quantitative three‐dimensional (3D) analysis of whole cells. When preserved in cryo‐conditions, whole cells can be imaged by Fresnel zone plate (FZP) lenses with spatial resolution in the few tens of nanometers. Information from these images can then be combined by tomographic techniques to generate 3D maps of the specimen. X‐ray photon‐energies in the so‐called
water window
(between 284 and 543 eV) are especially useful, since high contrast images of cryopreserved whole cells of ~10 microns can be obtained without use of staining agents. However, SXT is far from currently being delivering its promise to its full extend, mostly due to the intrinsic limited depth of field (DOF) of the microscope optical system and to the over simplicity of the standard reconstruction algorithms being used. Indeed, this limited DOF produces images where the contribution of each plane of the specimen is different, leading to three‐dimensional (3D) maps where certain parts of the specimen are blurred. We analyze the SXT image formation process in detail, providing a new mathematical theory allowing for the quantitative SXT inversion using focal series projections, images acquired at different defocus (Figure 1). Following these ideas, the SXT microscope at the Spanish synchrotron ALBA has been modified so as to automatically acquire the necessary additional information in a fast and reliable manner. Examples of the use of the new formulation on several biological systems using images acquired at ALBA are provided.

Here we apply our quantitative approach to SXT inversion to the study of an experimental biological system: a rotavirus infected cell tomogram (Figure 2).
Vibrational Spectroscopy in the Electron Microscope
Authors: Krivanek Ondrej , Aoki Toshihiro , Batson Philip , Crozier Peter , Dellby Niklas , Egerton Raymond , Lovejoy Tracy , Rez Peter
Published: December 2016

Vibrational spectroscopy in the scanning transmission electron microscope (STEM) was introduced two years ago [1, 2], and it has made much progress since. It has opened a new window on the world of materials, in which nothing is quite like it was before.

The main vibrational modes occur at energies of 0‐500 meV, and exploring them requires a monochromated STEM‐EELS system with an energy resolution

The energy of vibrational modes is given byΔE = ħ √(k/m), wherekis the force constant of the atomic bond andmthe effective mass of the vibrating nucleus. Strongly bonded light atoms give the highest vibrational energies, starting with hydrogen, an element that is nearly invisible in traditional electron microscopy. Fig. 1(a) shows a vibrational spectrum of Ca(OH)2[3], in which the peak at 452 meV is due to O‐H stretch, and Fig. 1(b) shows the particle from which the spectrum was recorded. Fig. 1(c) shows how the strength of the vibrational peak varied with the distance from the particle: the signal decayed only gradually outside the particle, and was still 50% strong 35 nm away.

Fig. 2 shows an EEL spectrum of guanine compared to an IR spectrum from the same specimen [4]. The agreement between the two types of spectra is very good. EELS has worse energy resolution (~10 meV), but much better spatial resolution than regular IR. As is typical of vibrational spectroscopies, the different peaks can be assigned to different types of bonds and vibration modes (see the inset in Fig. 1).

In order to minimize radiation damage, both the OH and guanine spectra were acquired in an “aloof” mode, with the electron beam parked just outside the sample [1, 3‐5]. Aloof spectroscopy makes it possible to select the maximum energy of the beam‐sample interaction, simply by adjusting the beam‐sample distance [4,5]. Its great import to vibrational EELS is that the vibrational signal can be excited even when the interaction energy is limited so that ionization damage of the sample cannot occur. It may even be possible to spatially map the vibrational features of a beam‐sensitive sample by “coarse step (leapfrog) scanning”: scanning with a discrete pixel increment of 10‐100 nm, so that even though the area that the beam traverses in each new position is essentially destroyed, large parts of the sample are not touched by the beam and remain in a pristine state [6].
Study of Nucleation of Microbubbles at a Solid/Water Interface with Optical Microscopy and Atomic Force Microscopy
Authors: Yang Chih‐Wen , Ko Hsien‐Chen , Chiang Ya‐Ling , Lu Yi‐Hsien , Hwang Ing‐Shouh
Published: December 2016

Microbubbles (MBs) or air bubbles have been explored for various applications in many fields of science and technology, such as water treatment, biomedical engineering and nanomaterials. The so‐called nanobubble was first proposed in 1994 and also confirmed their existence at the interfaces between water and hydrophobic solids [1]. In a previous study, microbubbles may evolve spontaneously on smooth modified hydrophobic Si surfaces following the decompression [2]. It was proposed that nanobubbles may be the gas micronuclei responsible for the microbubble formation. However, direct observation of microbubble nucleation sites is not yet reported. In this work, we investigated the formation process of microbubbles at a solid/water interface using pe‐degassed and N
2
‐saturated water. Optical microscopy was first used to observe the nucleation site of microbubbles and atomic force microscopy (AFM) was utilized to obtain high‐resolution images of the nucleation sites. In figures 1(a)‐(c), optical images show that the formation process of microbubbles on graphene‐coated mica surface in N
2
‐saturated water. Microbubbles tend to form on hydrophobic graphite or graphene‐coated areas but not on pure mica areas. Interestingly, there were certain sites on graphene‐coated areas that microbubbles prefer to form when we inject N
2
‐saturated water. We zoomed in an area with the preferential gas nucleation sites [figure 1(d)] and acquired high‐resolution AFM images [figure 1(e)] after injecting pre‐degassed water to make the microbubbles disappear. The preferential bubble nucleation sites appeared dark in optical images, but AFM images revealed that these sites were local protrusions in the height images. In addition, AFM images showed presence of nanobubbles on flat graphene‐covered areas outside the protrusions [figure 1(f)]. The relation between microbubble and nanobubble will be discussed.
Grain size determination in nanocrystalline materials using the
TKD
technique
Authors: Guitton Antoine , Guyon Julien , Zhang Yudong , Bouzy Emmanuel
Published: December 2016

Nanocrystalline (nc) materials,
i.e
. polycrystalline structures with grain sizes below 100 nm exhibit extraordinary properties strength. As a first assumption, such property derives from the short paradigm “smaller is stronger”. For grain sizes below 50 nm deformation mechanisms usually involve a quasi‐stationary balance between dislocation slip and grain boundary mediated mechanisms. But there is still an ongoing debate, which one of these mechanisms governs the deformation behavior of nc metals [1,2].

Therefore determination of grain size, analysis of the local texture and characterization of grain boundaries in nanocrystalline materials are crucial. Different techniques have been tested, such as automated crystal orientation mapping in Transmission Electron Microscopes (TEM) [3]. But they suffer a lack of accuracy due to the nanocrystalline nature of tested specimen. Grain overlapping, for instance, trends to observe smaller grains. By x‐ray diffraction, it is also possible to determine grain size, but measurements provide size of coherent domains only that we consider equal to grains.

In this framework, a new technique based of Transmission Kikuchi Diffraction (TKD) has been been recently introduced as a Scanning Electron Microscope (SEM) based method capable of giving orientation maps as the EBSD method but with a spatial resolution improved by up to one order of magnitude [4]. The technique requires a specimen thin enough to be transparent to the electron beam. In the current configuration, it uses hardware and software developed for the EBSD technique. We proposed a new configuration of the TKD where the detector is disposed horizontally on the axis of the microscope instead of being vertically positioned as in the conventional configuration (see Figure). This achieves better spatial resolution and angular resolution than the ones of the current TKD configuration [5]. Moreover, acquisition times are shorter than in the conventional technique, because the intensity of the forward scattered electrons is much higher than that of the large angle scattered electrons.

Firstly, technological challenges such as technique design will be presented. Secondly first results on electrodeposited nanocrystalline nickel will be discussed and they clearly show the full potentialities of this original characterization set‐up.
Plasmonic edge and breathing modes in aluminum nanotriangles
Authors: Campos Alfredo , Gerard Davy , Martin Jerome , Plain Jerome , Proust Julien , Arbouet Arnaud , Kociak Mathieu
Published: December 2016

The optical properties of metallic particles at nanometric scale have raised a great interest in scientific community due to the high promising technological applications such as optical communication and storage or quantum optics
1
. It is well known that optical properties of metallic nanoparticles are dominated by surface plasmons that are collective electron oscillations at a metal‐dielectric interface which can be exploited to manipulate light. In a metallic nanoparticle these oscillations are confined by the boundaries of the particles, resulting in discrete modes of oscillations (plasmon modes) which can be tuned by shaping the geometry of the nanoparticle. Recently, a detailed analysis of surface plasmons in flat structures
1
has allowed classifying plasmon modes in two groups. The first group corresponds to the so‐called edge modes which are localized at the periphery of the nanoparticle. These edge modes are well known in the literature and have been reported for several geometries, including nanorods and nanotriangles. On the other hand the second group of modes corresponds to the so‐called breathing modes (or cavity modes) which are localized on the center of the nanoparticles. Nowadays breathing modes have just been reported and explained for simple structures as disks
2
and squares
3
. More complex structures remain to be understood.

In this work we study aluminum nanotriangles (edge length size ranging from 125 to 622 nm) by electron energy loss spectroscopy (EELS) coupled with a transmission scanning electron microscope (STEM) in order to understand the complexity of plasmon modes in this kind of structures. Behind this geometry, a rich variety of edge and breathing modes are observed ranging from 1 to 5 eV (figure 1a). Thanks to the high spatial and energy resolution of STEM‐EELS technique, we were able to generate with high level of precision, plasmon maps for all modes (figure 1b). In order to understand the breathing modes in triangular nanoparticles, we propose an analytical model considering the interference of reflected waves at the boundaries of triangular cavities which allowed us to explain the symmetry of lobes shown in maps of figure 1b. Furthermore, plasmon modes dependence with nanotriangles size will be shown and interpreted based on our analytical model and rigorous theoretical simulations.
Gas sensing properties of cobalt ferrite nanooctahedra and nanocubes
Authors: Lopes‐Moriyama Andre‐Luis , Aritana Fernandes de Medeiros Indira , Madigou Veronique , Arab Madjid , Pereira de Souza Carlson , Giorgio Suzanne , Leroux Christine
Published: December 2016

The detection function of a sensing material is dependant of a high surface to volume ratio, but also to the exposed crystallographic facets. It should then be possible to tailor the reactivity and sensitivity of the sensing materials by controlling their shape and size, for a given composition [1]. We already showed that the composition of the cobalt ferrite Co
x
Fe
3‐x
O
4
influences their catalytic properties [2]. In order to understand and control the gas sensing properties as well as the catalytic properties, we synthesized cobalt ferrites as nanoparticles with various shapes and sizes. Nanoparticles with specific shapes allow to study the influence of the cristallographic facets, hence the cation distribution at the surface, in the gas interaction with the particles.

By solvothermal methods, we synthesised CoFe
2
O
4
nanooctahedra (Fig. 1) and nanocubes (Fig. 2). Conventional TEM coupled with EDS, high resolution TEM, environmental TEM, were carried out in order to understand the mechanisms involved in the growth of the grains and their reaction under gas. Octahedron‐like nanoparticles of CoFe
2
O
4
were submitted to H
2
‐O
2
cycles, at ambient temperature, under 1mbar gas pressure in an TEM 300 kV. The {100} facets extended which led to truncated octahedra and the {111} facets became more rounded under oxygen. The phenomenon was reversible and rounded particles under O
2
became facetted under H
2
(Figure 3). These CoFe
2
O
4
nanooctahedra exhibit a high sensibility to oxidative gases like NO
2
at low gas concentration, and the shape effect on sensibility was clearly demonstrated.The study of nanocubes under oxydo reduction cycles is under progress.
Optical and structural properties of facetted boron nitrides nanotubes
Authors: Pierret Aurélie , Schué Léonard , Fossard Frédéric , Barjon Julien , Ersen Ovidiu , Moldovan Simona , Ducastelle François , Loiseau Annick
Published: December 2016

Luminescent devices operating at sub‐250nm wavelength present a strong commercial interest. Applications such as antibacterial properties, high density optical storage or nanofabrication possibilies require reliable, portable and efficient devices. Actual Deep UltraViolet (DUV) sources are based on gaz active regions which are difficult to integrate in mobile devices, exhibit poor efficiencies and are harmful for environment. Thus, the development of solid state based DUV emitters is getting more and more relevant.

Hexagonal boron nitride is a wide band gap semiconductor (~ 6.5 eV), which meets a growing interest for DUV applications. In contrast to carbon nanotubes, Boron Nitride Nanotubes (BNNTs) are all semiconductors whatever their diameter and chirality and their luminescence emission occurs between 200 nm and 250 nm and is governed by strong excitonic effects. Until recently, the optical properties were poorly known due to both the scarcity of samples and suitable investigation tools. This situation has changed thanks to the development of dedicated photoluminescence (PL) and cathodoluminescence (CL) experiments running at 4K and adapted to the detection in the far UV range [1, 2, 3].

These previous studies on boron nitride nanotubes have mainly dealt with multi‐wall BNNTs with a large number of walls (20‐120 walls). These tubes luminesce between 226 and 234nm and this spectral range has been assigned, in hBN, to transitions involving defects. A critical point to further study the confinement effect on the excitonic transitions is therefore to elucidate the luminescence origin of these multiwalls. Furthermore it is important to investigate the luminescence of small diameter BNNTs (with a reduced number of walls), which actually appears to be very challenging.

Cathodoluminescence from a single BNNT with a large number of walls have been measured with a spatial resolution of about ten nanometers, thanks to an UV dedicated SEM system. Different areas along the tube were investigated, from which luminescence is detected at few wavelengths. From 224 to 228 nm, monochromatic cathodoluminescence images exhibit features, which can be linked to defects in the crystallographic structure, separately observed by Transmission Electron Microscopy (TEM) on the same tube.HRTEM observations and tomography experiments revealed that the BNNTs exhibit a peculiar shape. The section of the tube is polygonal with a number of facets between 6 and 9 (Fig 1). These facets forms an helix along the axis of the nanotube. An important consequence of this facetting is the formation of a large number of dislocations along the tube.

We will discuss the relations between these structural properties and the luminescence as shown on fig 2.
Near Band Edge excitation in
2D
materials by Transmission Electron Microscopy
Authors: Fossard Frédéric , Schué Léonard , Gaufrès Etienne , Andrieux Amandine , Ducastelle François , Loiseau Annick
Published: December 2016

In this work, we report on the characterization of near band edge excitation by electron energy loss spectroscopy (EELS). This technique is operated in a Transmission Electron Microscope and allows to rely the structure of a material obtained by HR‐TEM with its chemical and physical properties deduced from EELS. Indeed, when the energy transfered by a transmitted electron remains below 50 eV, it is possible to have access to the electronic structure of the material and more precisely to its dielectric function. [1] In other words, we are able to obtain informations such as, plasmon resonances, interband transitions and band gap measurements.

We used a Libra 200 equipped with an electrostatic monochromator operating at 80 kV. Thanks to the in‐column filter, energy loss signal is recorded on a CCD camera with a spectral resolution of 150 meV. The sharp cut‐off of the omega filter allows to probe the dielectric properties of semiconducting materials down to 1eV losses. We are able to determine bandgaps in several 2D materials and rely them to the number of layers. For instance, we can see the blue shift of the “optical absorption” from several MoS
2
layers (1.4 eV) down to a single layer (1.8 eV).

Recently, thanks to dedicated operating modes [2,3], we have been able to obtain additional informations on the plasmons and interband transitions over the Brillouin Zone in hexagonal Boron Nitride (hBN). Energy Filtered scattering patterns have been recorded in the TEM to have access to the symmetries of the dipole matrix elements involved in the observed transitions. Moreover, by dispersing the energy along specific crystallographic directions, we accessed to the related dispersion of plasmons and interband transitions with the so‐called ω‐q maps [2] as representated on fig 1. We show that, due to a strong electron‐hole interaction, the observed dispersion is related to the one of the exciton [4]. The experimental results are in good agreements with inelastic X‐ray scattering experiments [5] and calculations [6] as shown on fig 3.
Crystalline nanoplatelets of a
PCL
‐fructose block copolymer
Authors: Förster Beate , Ganda Sylvia , Stenzel Martina
Published: December 2016

Nanosheets have in recent years received considerable attention because of their unique electronic and mechanical properties. They can be prepared by exfoliation of layered materials, but can also be formed via self‐assembly which offers a much larger variability in size and chemical functionality. Self‐assembly of amphiphilic molecules and polymers though usually leads to the formation of spherical, cylindrical and vesicular structures. The generation of nanosheets requires one to adjust the molecular structure such that planar bilayers sheets are formed with large bending constant to prevent closure into vesicles.

We report the formation of nanosheets or nanoplatelets from an amphiphilic block copolymer consisting of a poly(caprolactone) (PCL) and a fructose‐functionalized poly(acrylate) (PFA) block (Scheme 1), prepared by RAFT‐polymerization, in water. The nanoplatelets have a core/shell‐structure, consisting of an inner core‐sheet of the water‐insoluble PCL‐blocks and an outer sheet of the water‐soluble PFA‐blocks. The nanoplatelets are stabilized by the crystallization of the poly(caprolactone) block providing a sufficiently large bending constant. We use transmission electron microscopy (TEM) with phosphotungstic acid (PTA) negative staining to determine the thickness and lateral dimension of the nanoplatelets (Fig. 1). Using selected area electron diffraction (SAED) we prove the crystallinity of the poly(caprolactone) core‐sheet (Fig. 2). The fructose‐functionalized nanoplatelets have a thickness of 8 nm and lateral dimensions of 50 – 200 nm and are potential vectors for targeted drug delivery, where the planar geometry allows for large‐area cooperative binding to cell surface receptors.
Characterization of intergranular corrosion defects in a 2024 T351 aluminium alloy
Authors: de Bonfils‐Lahovary Marie‐Laetitia , Laffont lydia , Blanc Christine
Published: December 2016

In the 2xxx series alloys, intergranular corrosion is generally related to the strong reactivity of copper‐rich intergranular precipitates leading to a copper enrichment of these particles. While the nature of the oxides formed inside the intergranular corrosion defects was assumed to strongly influence the intergranular corrosion propagation rate, it was not clearly identified due to the thickness of the oxide layer formed which required to use high resolution analytical techniques. The present work aims to characterize the intergranular corrosion defects formed for a 2024‐T351 aluminium alloy after a 24 hours continuous immersion in a 1 M NaCl solution and compares the results to literature data concerning the oxide layers formed on copper‐rich model alloys. An intergranular defect obtain after cyclic immersion (8 hours continuous immersion, 16 hours emersion) was also observed and characterised. In order to obtain a thin sample in a localized region, i.e. in an intergranular corrosion defect, a Focused Ion Beam (FIB) / Scanning Electron Microscope (SEM) FEI HELIOS 600i equipped with a field emission gun (FEG) was used. The thin sample preparation was done using conventional lift out procedure; it is summarized in Fig. 1. Location of interest (intergranular corrosion defect) was chosen (Fig. 1a) and a platinum coating was deposited using electron beam prior to using ion beam to protect the area beneath from being contaminated by the Gallium (Ga ions) (Fig. 1b). Using a large beam current for fast ion milling, two tranches were milled on either side of the Pt coating. The sample of size (10x10x7 µm
3
), so prepared, was then mounted on a TEM sample holder. A cross‐section view of the intergranular corrosion defect was therefore obtained (Fig. 1c). It was then polished using successive lower beam current. Finally, the sample was thinned to 100 nm or less using 1 keV ion beam to minimize the artefacts from sample preparation (Fig. 1d). A transparent section was obtained. Some intermetallic precipitates were visible inside and all around the intergranular corrosion defect.

Then, a combination of transmission electron microscopy (TEM) observations (Fig. 2) and electron energy loss spectroscopy (EELS) analyses was used to accurately characterize both the morphology and chemical composition of the intergranular corrosion defects. Results evidenced the dissolution of intergranular copper‐rich particles, the formation of a 10‐200 nm‐thin metallic copper‐rich layer at the oxide/metal interface and the incorporation of copper inside the amorphous alumina oxide film leading to the formation of structural defects of the oxide film.
Electron‐beam induced fluorescence superresolution with 100nm resolution in
CLEM
on labelled tissue sections
Authors: Voortman Lennard M. , Raja Aditi Srinivasa , Väkeväinen Aaro , de Boer Pascal , Giepmans Ben N.G. , Kruit Pieter , Hoogenboom Jacob P.
Published: December 2016

We present a novel optical superresolution (SR) technique using integrated correlative light and electron microscopy. Recent advances in SR techniques has revolutionized the field of optical microscopy by achieving image resolutions well below the diffraction limit, the fundamental resolution limit of traditional optical microscopy. Current SR methods involve stochastic techniques, beam‐shaping in combination with confocal scanning, external control over excited state relaxation pathways, and/or structured illumination [1]. Correlation of SR data with ultrastructural images obtained with electron microscopy (EM) has been demonstrated [2], but requirements for SR microscopy are often in conflict with those for EM. Moreover, the optical localization accuracy in the correlation image may be severely compromised compared to the SR resolution by the additional error introduced in aligning the separate SR and EM images. Here, we demonstrate a novel approach for correlative SR‐EM using a focused electron beam to locally modify the fluorescence signal of fluorophores, and detecting the change in fluorescence intensity with a wide‐field epi‐fluorescence microscope.

We use an integrated light‐electron microscope [3] that is used for correlative light and electron microscopy (CLEM) [2]. The integrated light microscope allows us to record the fluorescence signal while scanning the electron beam through the optical field of view. By correlating changes in the fluorescence decay with the instantaneous electron beam position and the other EM signals, we obtain a SR fluorescence image (Fig.1). This SR fluorescence image is in perfect registry with the simultaneously acquired EM image.

In first experiments on rat pancreas tissue, immuno‐labelled for insulin and guanine quadruplexes using different Alexa Fluor dyes, we have achieved a lateral resolution below 100nm (Fig. 2). We will discuss further implementation of our technique towards higher resolution, paving the way towards precise localization, within the EM ultrastructure, of bio‐molecules labelled with standard fluorescent dyes.
Study on the robustness of electron ptychography for phase imaging in the
STEM
using fast pixelated detectors
Authors: Martinez Gerardo T , Yang Hao , Jones Lewys , Simson Martin , Huth Martin , Soltau Heike , Strüder Lothar , Sagawa Ryuusuke , Kondo Yukihito , Nellist Peter D
Published: December 2016

The use of electron ptychography methods for phase imaging in the scanning transmission electron microscope (STEM) have gained renewed interest due to the recent developments of fast pixelated detectors. One of those being the pnCCD (S)TEM camera [1] developed by PNSensor and PNDetector, it allows the recording of a 4D‐dataset that consists of the full convergent beam electron diffraction pattern for each probe position that is scanned over the sample. This dataset contains all the scattering information generated by the electron sample interaction in the STEM experiment. The technique has allowed improved resolution beyond conventional limits [2] and phase contrast imaging at atomic resolution of different materials [3, 4]. In combination with other STEM techniques, it enables imaging of heavy and light elements of radiation‐sensitive materials at the atomic scale [4].

In this work, we present an analysis and discussion of two different phase reconstruction algorithms, such as the Single Side Band (SSB) [3, 5, 6] and the Wigner‐Distribution Deconvolution (WDD) [7] methods, in terms of their robustness to dynamical effects. The main difference of both methods is that the phase difference retrieved by the SSB method is still affected by the probe aberrations, meanwhile the WDD method allows to correct for this [4, 7]. However, in both cases a multiplicative interaction between the specimen function and the electron wave is assumed, which complicates the interpretation of the reconstructed wave when dynamical effects start to play a major role. Figures 1 and 2 show experimental results of retrieving the modulus (a) and the phase difference (b) of a gold nanoparticle measured with the pnCCD (S)TEM camera and using the discussed ptychography methods. It can be observed that by correcting the probe aberrations [3] while using the WDD method, the quality of the reconstructed image is dramatically improved. Moreover, a ring‐like shape of the reconstructed phase difference for some atomic columns is observed. This behavior is studied in detail by image simulations (c) and discussed in terms of how dynamical effects influence the reconstruction algorithm. Furthermore, we explore the comparison of this method with other phase imaging techniques.
Analytical electron tomographic investigations revealing self stabilization of core‐shell precipitates through opposing diffusion processes
Authors: Orthacker Angelina , Haberfehlner Georg , Taendl Johannes , Poletti Maria Cecilia , Sonderegger Bernhard , Kothleitner Gerald
Published: December 2016

For a thorough understanding of a material, investigations at the nanoscale are often essential. Analytical techniques like electron energy loss spectroscopy (EELS) and energy dispersive X‐ray spectroscopy (EDXS) in scanning transmission electron microscopy (STEM) can reveal important chemical information necessary for the development and improvement of high‐tech materials. The integrative character of the signal acquired through transmission, however, might hide important morphological details of the material, relevant for its properties. Those details can be revealed through electron tomography, where the data is acquired at different tilt angles and, after alignment, reconstructed to form a full 3D model of the material under investigation. The combination of both techniques, analytical STEM and tomography, gives full insight into structure and composition of a material [1].

In this study an industrially cast aluminum alloy, containing scandium (Sc) and zirconium (Zr) rich nano‐precipitates, was investigated at different stages of ageing. High resolution STEM and analytical EELS and EDX tomographic investigations were carried out. The resulting 3D elemental reconstructions delivered otherwise inaccessible information on the samples' chemistry and structure (figure 1). Additionally, EDX‐spectra of a long aged and electron‐beam re‐solidified (EBRS) sample were reconstructed channel by channel, resulting in a data cube, in which each voxel (volume pixel) contained an entire spectrum. Via a high angle annular dark field (HAADF) reconstruction and 3D masking, the voxels of the core and the different shell regions of a nano‐precipitate were extracted and summed to obtain pure spectra of those volumes, thus overcoming the intrinsic limitations arising from the integrative character of analytical STEM. Comparisons of these results with data from high resolution STEM imaging, HAADF tomography and data from literature have led to the conclusion ‐opposed to previous believes [2]‐ that a self‐limiting diffusion process takes place within the precipitates stabilizing their core‐shell structure (figure 2).
In‐situ observation of structural transition, ferromagnetic order breaking and magnetocrystalline anisotropy by
EMCD
in MnAs/GaAs(001)
Authors: Fu Xiaoxiao , Warot‐Fonrose Bénédicte , Arras Rémi , Demaille Dominique , Eddrief Mahmoud , Serin Virginie
Published: December 2016

EMCD (Energy‐Loss Magnetic Chiral Dichoism), an emerging technique based on energy‐loss spectroscopy (EELS) in a transmission electron microscopy (TEM) [1‐2], aims at measuring the element‐specific local magnetic moment of solids at a nanometer scale. The signal of interest that brings the magnetic information comes from two chiral positions in the electron diffraction pattern, as sketched in figure 1(a).

In this work, EMCD is carried out to epitaxial MnAs thin films grown on a GaAs(001) substrate. As illustrated in figure 1(b‐c), a breaking of the ferromagnetic order in MnAs thin film is locally studied
in‐situ,
together with the associated crystallographic transition from hexagonal α‐MnAs to quasi‐hexagonal β‐MnAs, by modifying the temperature of the crystal inside the electron microscope. To achieve quantitative information, applying the sum rules [3] to the dichroic signal of magnetic anisotropic materials is accurately discussed [4].

In addition, the orbital‐to‐spin moment ratio of ‐MnAs along the easy, hard, and intermediate magnetic axes is estimated by EMCD and compared to implemented density functional theory (DFT) calculations, as illustrated in table 1. The influence of the magnetocrystalline anisotropy is locally demonstrated [5]. This work in particular illustrates the feasibility of the EMCD technique for in‐situ experiments, and proves its potential to explore the anisotropy of magnetic materials.
Revealing the internal structure of microbial mats by
FIB
/
SEM
tomography
Authors: Zeitvogel Fabian , Obst Martin , Schroeppel Birgit , Burkhardt Claus J.
Published: December 2016

Microbial mats are macroscopically visible communities of microorganisms, which play an important role for the interaction of microorganisms with their environment [1]. Important examples are mats dominated by Fe(II)‐oxidizing bacteria, which are typically found in wet, Fe(II)‐rich environments such as lake sediments and mine tunnels [2,3].

The microscopic constituents of microbial mats can be analyzed using 2D microscopy approaches such as SEM. Other properties such as the microbial community and elemental composition are accessible using bulk analytical methods [e.g. 3]. However, little is known about the 3‐dimensional structure of these mats.

Samples of two microbial mats from an abandoned mine tunnel were analyzed by FIB/SEM tomography using a Zeiss Auriga CrossBeam instrument. The samples were embedded in resin. Metal ions and minerals within the sample were responsible for contrast generation. The voxel size was 30 x 30 x 30 nm³. The resulting 3D reconstructions show the original arrangement of the microscopic constituents in great detail (Fig. 1,2). The two mats were dominated by different types of Fe(II)‐oxidizing bacteria, which formed characteristic structures that were visible in the two respective volumes. A large part of both volumes appeared void.

The results of this analysis provide explanations for a series of macroscopically observed effects, such as the physical stability of the mats, high water content, efficient interaction with the feeding water, and the creation of a habitat for a variety of different microbial species. In addition, the results represent a valuable reference for the detection of microfossils in sedimentary rocks.
FeCrMg
composite and porous
FeCr
obtained by dealloying in metallic melt bath by Xray tomography and
SEM
Authors: Mokhtari Morgane , Maire Eric , Bourlot Christophe Le , Wada Takeshi , Kato Hidemi , Bonnin Anne , Duchet‐Rumeau Jannick
Published: December 2016

Nanoporous metals have attracted considerable attention for their excellent functional properties [Snyder, 2010]. The most promising technique used to prepare such nanoporous metals is dealloying in aqueous solution. Nanoporous noble metals including
Au
have been prepared from binary alloy precursors [Forty, 1979]. The less noble metals, unstable in aqueous solution, are oxidized immediately when they contact water at a given potential so this process is only possible for noble metals. Porous structures with less noble metals such as
Ti
or
Fe
are highly desired for various applications including energy‐harvesting devices [Sivula, 2010]. To overcome this limitation, a new dealloying method using a metallic melt instead of aqueous solution was developed [Wada, 2011]. Dealloying in the metallic melt is a selective dissolution phenomenon of a mono‐phase alloy solid precursor: one component (referred as soluble component) being soluble in the metallic melt while the other (referred as targeted component) is not. When the solid precursor contacts the metallic melt, only atoms of the soluble component dissolve into the melt inducing a spontaneously organized bi‐continuous structure (targeted+sacrificial phases), at a microstructure level. This sacrificial phase can finally be removed by chemical etching to obtain the final nanoporous materials. Because this is a water‐free process, it has enabled the preparation of nanoporous structures in less noble metals such as
Ti
,
Si
,
Fe
,
Nb, Co
and
Cr
.

In this study, nanoporous
FeCr
samples were prepared using
Ni
as the soluble component, in a metallic melt bath of
Mg
. To introduce structural and mechanical anisotropy, some samples were cold‐rolled before etching. The influence on the microstructure of the precursor composition, the dealloying conditions and the rolling process were investigated along the different steps by SEM‐EBSD and Xray tomography to correlate the process with the microsctructure. Xray tomography (cf. Fig. 2 and 4)enables us to characterize qualitatively and quantitatively the volume while SEM (cf. Fig. 1 and 3) enables us to analyze larger areas with higher resolution 2D images. To confirm the validity of Xray tomography results, SEM‐FIB analysis were also performed.
Calcination of pd nanocatalysts on alumina: ex‐situ analysis versus in‐situ environmental
TEM
Authors: Koneti Siddardha , Roiban Lucian , Epicier Thierry , Gay Anne‐Sophie , Avenier Priscilla , Cabiac Amandine , Dalmas Florent
Published: December 2016

Sustainable chemistry, reduction of pollution, greenhouse gases control, oil refinery, liquid and solid waste management, are essential societal topics related to what is called Environmental catalysis [1], a sector which is booming rapidly for the past 10 years. So, developing innovative catalysts is a very important aspect ahead of us. To develop more advanced catalysts, we have to understand the catalysts genesis in all the stages of preparation (impregnation, drying, calcination and reduction of the active phase). Nowadays Environmental Transmission Electron Microscopy (ETEM) enables dynamic studies of the catalysts down to the nanometer and even atomic scale., at high temperatures and under gaseous environment.

This contribution deals with the dynamic evolution of palladium nanoparticles (NP) supported on alumina during the catalyst preparation process. Here, the calcination step is studied in a FEI TITAN‐ETEM microscope operated at 300 kV. A Pd/delta alumina catalyst was investigated at different temperatures and under different gas pressures in order to follow the particles size evolution. Owing to the small size of the Pd NPs in the range of 1‐3 nm, we mainly used the STEM‐ADF imaging mode. Preliminary observations during heating under high vacuum up to 600°C show that particles as well as the supporting media remain stable and apparently not damaged by the electron beam under nominal and atomic resolution imaging STEM conditions (see Fig. 1). Some EELS (Electron Energy‐Loss Spectroscopy, Gatan Imaging Filter Tridiem Ers) analysis was performed in situ in order to ascertain the chemical nature of the observed particles. During this work it appeared essential to perform measurements systematically on the same areas at different temperatures. Fig. 2 is a typically illustration of the in situ evolution of palladium particles under oxygen partial pressure at different temperatures. From such micrographs, the NP size evolution was quantified and compared with post‐mortem TEM observations after ex‐situ experiments performed at same temperatures but atmospheric pressure in the course of the catalyst synthesis (Fig. 3).

This comparison shows that all measurements appear to be consistent except those performed in the bright field TEM imaging mode, where larger particles sizes are obtained, most probably due to irradiation effects which were further evidenced by a high‐induced mobility of NPs on their supporting media. This ETEM study brings direct in situ information on the transient stages that cannot be followed by post mortem experiments after ex‐situ treatments; in particular, we will also discuss about the crystallographic evolution of the Pd NPs during the calcination process [2,3].
EDX‐STEM phase mapping of semiconductor devices using multivariate statistical analysis tools
Authors: Saghi Zineb , Pimenta‐Barros Patricia , Goret Gael , Printemps Tony , Bernier Nicolas , Barraud Sylvain , Delaye Vincent
Published: December 2016

With the increasing miniaturization of electronic devices, high resolution structural and analytical characterization tools are necessary for the optimization of fabrication processes. Scanning transmission electron microscopy energy dispersive X‐ray (EDX‐STEM) spectroscopy is a well‐established technique that has recently gained momentum thanks to the introduction of high‐brightness electron sources and the Super‐X EDX system (4 SDD detectors), allowing fast EDX mapping with high collection efficiency. While the traditional EDX data analysis consists in extracting the elemental map of each element present in the sample [1], it is often the case that the aim of the analysis is to investigate the spatial distribution, shape and thickness of the different chemical phases present in the sample.

Multivariate statistical analysis tools, such as non‐negative matrix factorization (NMF) and independent component analysis (ICA), were shown to yield simplified interpretation of spectral datasets by rapid identification of phases (e.g. [2,3]). In this work, we applied NMF to the EDX analysis of Si/SiGe multilayers to validate the Sidewall Image Transfer (SIT) process developed for their patterning [4]. A FIB‐prepared lamella was characterized in an FEI Titan Themis operating at 200kV and equipped with a probe corrector and 4 SDD EDX detectors. An EDX‐STEM map was acquired with TIA, using a pixel size of 1nm and a dwell time of 20ms/pixel, and exported to hyperspy, a python‐based software for hyperspectral data processing [5].

Spectral unmixing using NMF led to the identification of five chemical phases in the sample: Si, SiGe, SiO2, TiN and C (see the component spectra in Figure 1 and the corresponding loadings in Figure 2(a‐e)). More specifically, NMF succeeded in: (1) separating the Si signal emanating from pure Si, SiGe and SiO2 layers; and (2) deconvoluting the C, N and O peaks. This greatly simplified the compositional analysis (Figure 2(f)), and allowed a more straightforward estimation of the thickness of the different layers, as shown in Figure 2(g).

NMF combined to EDX‐STEM tomography was recently applied to superalloy systems for aerospace applications [6,7]. We will show that this approach has also the potential to address materials characterization challenges currently facing the semiconductor industry, such as the chemical analysis of dopants and impurities [8].
In‐situ and cryogenic electron microscopic study of genesis and dynamics of cobalt nanoparticle formation
Authors: Su Hao , Bomans Paul , Friedrich Heiner , Sommerdijk Nico
Published: December 2016

Cobalt nanoparticles have a high potential as catalysts for the Fischer‐Tropsch synthesis, i.e. to convert hydrogen and carbon monoxide, which can be derived from a variety of renewable feedstocks, into industrially useful hydrocarbons. The cobalt nanoparticles are commonly created via a deposition precipitation (DP) process In the DP process, catalyst supports such as silica and titania are suspended in solutions of cobalt precursor salts, which upon increasing/decreasing the pH precipitated a cobalt intermediates e.g. cobalt hydroxides and cobalt carbonates on the liquid‐solid interface. At present how and where the precursor salts in solution are nucleated and how they crystallize as cobalt intermediates is still unknown. In this contribution the morphological and structural development of the cobalt intermediates with and without support materials during the DP process has been investigated using advanced cryo‐EM and
in situ
liquid cell TEM.

First, cryo‐TEM was employed to study Co nanoparticle formation during an decreasing pH induced by out‐gassing ammonia from the synthesis solution. At the early stage of the nucleation and growth, particles with a diameter of 1 to 2 nm were found. It is difficult to image these 1~ 2 nm particles through a 100 to 200 nm thick vitrified ice layer due to significant scattering of electrons in the embedding medium. Hence, we vitrified our sample on graphene oxide (GOx) supports which cover a normal TEM holy grid (figure 1a). Because of the very low background and high hydrophilicity of GOx, we generate ultra‐thin aqueous layers for high contrast cryo‐TEM imaging (figure 1b). In addition, the presence of GOx makes it possible to focus more accurately to acquire high contrast and resolution cryo images at only a few hundred nanometers of defocus.

Second, liquid cell TEM was employed to study in‐situ particle formation by exposing a solution of Co
2+
ions to the vapor ‐NH
3
and CO
2.
Here Co(NO
3
)
2
solution is flown through the liquid cell to fill the system, after which wet N
2
is then flown in to remove the Co(NO
3
)
2
solution in the tube (fig 2a, 2b). Because the chips are cleaned by O
2
plasma before mounting, the surface of two chips is highly hydrophilic. So that liquid between the two chips is not removed by the wet N
2
. This way a thin liquid layer containing Co
2+
ions is generated between two chips (fig 2b). Subsequently, a syringe containing (NH
4
)
2
CO
3
powder is connected to the other port and NH
3
and CO
2
vapor is released from the decomposition of solid (NH
4
)
2
CO
3
(figure 2c). In this way, we could generate a thin liquid layer with a thickness of 250 ~ 600 nm in the center in a repeatable manner (figure 3), making the edge of viewing area to be good position for (S)TEM imaging of particle nucleation and growth.
Revisiting Graphene Oxide Structure via Spatially‐Resolved Electron Energy Loss Spectroscopy
Authors: Tararan Anna , Zobelli Alberto , Benito Ana M , Maser Wolfgang K , Stéphan Odile
Published: December 2016

Graphene oxide (GO) is obtained by chemical oxidation and exfoliation of natural graphite. In the last decade, it has attracted a widespread interest for its mechanical strength, tunable optoelectrical properties, simple processability and its potential as precursor for a low‐cost and large‐scale production of graphene. Indeed, chemical and thermal treatments allow to almost completely remove the oxygen, yielding reduced graphene oxide (RGO). Nevertheless, after about 150 years, the atomic structure of GO and RGO is still greatly debated. At present, the most acknowledged model for GO considers a random functionalization of the carbon basal plane with epoxide and hydroxyl groups, forming graphitic and partially oxidized domains. However, no definitive evidence of this model has been reported due to the lack of chemical analysis at the proper scale. For these reasons, nanometrically spatially‐resolved spectroscopy of GO and RGO is highly suitable.

In this work we provide the first chemical characterization of GO and RGO thin flakes at the scale of few nanometres, thanks to core Electron Energy Loss Spectroscopy (EELS) in a STEM microscope. A major issue is represented by the extreme sensitivity of these materials to illumination and the use of this technique on GO and RGO has been so far very restricted. A new experimental set up combining a liquid nitrogen cooling system at the sample stage, a low accelerated electron beam (60 keV) and a liquid nitrogen cooled CCD camera with a low read‐out noise of three counts r.m.s. and a negligible dark count noise has allowed us to overcome this limitation. Optimal illumination conditions have been defined by monitoring the evolution of the sample under continuous illumination, defining a maximal electron dose before substantial chemical modification of the order of 10
3
e
‐
Å
‐2
and hence a 3 nm lower limit on the hyperspectral spatial resolution. Chemical maps of the atomic oxygen content of few layers GO and RGO show well separated domains on the scale of tens of nanometres. Overall, the oxygen amount has been observed to vary within 10‐50 at.% in GO and 5‐20 at.% in RGO. Energy‐Loss Near‐Edge Structures (ELNES) at the carbon K‐edge exhibit well‐defined features related to C‐O bonding, previously not reported. Moreover different oxidation levels in GO and RGO are characterized by specific ELNES profiles. The highly oxidized regions in GO (~50 oxygen at.%, i.e. 1:1 C/O ratio) correspond to a full functionalization of the carbon network. With the support of complementary DFT numerical calculations, we suggest a model for the highly oxidized regions consisting in a full functionalization with hydroxyls, forming a 2D‐sp
3
system.