bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of spark plasma sintering on structure and phase stability in half‐Heusler thermoelectric alloys
Authors: Tofan Raluca , Echevarria‐Bonet Cristina , Berland Kristian , Schrade Matthias , Sørby Magnus , Hauback Bjørn Christian , Persson Clas , Prytz Øystein , Hansen Vidar , Gunnæs Anette Eleonora
Published: December 2016

Thermoelectric (TE) materials have the capability to directly convert a temperature difference into an electric voltage across the material and vice versa. Half‐Heusler (HH) compounds are a promising group of TE materials. They are ternary intermetallic compounds, with the general formula XYZ – X and Y transition metals (e.g., X=Ti, Zr, Hf and Y=Ni, Co) and Z a metal or metalloid (e.g., Sn, Sb). The electronic structure and charge carrier concentration can be manipulated by atomic substitution on each crystallographic site to enhance the TE properties. However, further increase of efficiency of the alloys is limited by the relatively high thermal conductivity. Nanostructuring is reported to reduce the thermal conductivity as a result of an enhancement of the phonon scattering, thus increasing the efficiency of the TE materials [1].

Spark plasma sintering (SPS) is reported to give rapid densification of powders into bulk specimens and retain nanostructures (grain boundaries) from the starting powders. In the current study, n‐type XNiSn alloys with different (Ti, Zr, Hf) compositions on the X‐site were prepared by ball‐milling of arc‐melted and thermally annealed ingots, followed by SPS for 10 minutes, with temperatures between 850 and 1100 °C and applied pressures between 65 and 80 MPa. The SPS prepared alloys were investigated with a combination of X‐ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM) techniques, supported by density functional theory (DFT) thermodynamic calculations.

In all SPS samples, graphite is present in the surface regions. At the interface between the graphite and the XNiSn alloys, carbides are present as illustrated in figure 1 for the HfNiSn alloy – sintered at 1100 °C and 80 MPa. In agreement with previous reports on HH studies, single phase HH does not form in any of the alloy systems [2]. Secondary phases are distributed along compositional HH boundaries as illustrated in figure 2 from the TiNiSn alloy, sintered at 900 °C and 80 MPa. Phase separation of HH phases, with different composition X
i
and HH lattice parameter, is generally evident by splitting of the HH reflections in the XRD diffractograms. In the case of the TiNiSn alloy, such splitting of the HH reflection is not observed; however, we find compositional variations of the HH phase from the nominal composition – consistent with the variations in the shades of grey seen in the HH regions in figure 2. In addition to the HH and secondary phases seen in the figure, grains of full‐Heusler TiNi
2
Sn exist in the alloy.
In situ cyclic telescoping of multi‐walled carbon nanotubes
Authors: Moore Katherine , Cretu Ovidiu , Golberg Dmitri
Published: December 2016

In this work, we perform an in‐depth study of the cyclic telescoping of multi‐walled carbon nanotubes (MWCNTs) inside a transmission electron microscope. The nanotubes are observed in real time, while simultaneously measuring their electrical properties. To date, examples of in situ TEM nanotube telescoping have remained few in number due to the highly specialized equipment and technical expertise required.

Experiments were conducted by first ‘sharpening’ a MWCNT by approaching it with a piezo‐controlled tungsten tip and applying a voltage pulse. This results in controlled and localized peeling of the outer walls, giving rise to a telescope structure. The exposed core is then contacted with the tungsten tip and reproducibly pulled out then re‐inserted, while measuring conductance behavior at each movement step. Figure 1 shows an example of in situ telescoping of a MWCNT in real time.

Results demonstrate that an electrical current can be maintained for multiple in‐and‐out cycles (up to 11 cycles have been tested to date), and for telescoping distances of up to 650 nm. This is exemplified in Figure 2. The systems display complex conductance behavior, with of some MWCNT telescopes demonstrating improved conduction (and current) as a function of the number of cycles, while for most these properties diminish.

Control experiments have been conducted to investigate the effect of the electron beam on the behavior of the MWCNTs, and have indicated that changes in conductance behavior are not strongly influenced by the imaging process under normal beam conditions.

One potential application for these structures is use as flexible electrical contacts, which allow for relative motion of the two electrodes, while maintaining electrical conduction.

The authors would like to acknowledge funding as provided by the National Institute for Materials Science (NIMS), through the International Center for Materials Nanoarchitectonics (MANA).
Correlative Raman spectroscopy and scanning electron microscopy of thermosetting carbon nanotube composite microstructures
Authors: Fisher Sandra , Shaffer Milo
Published: December 2016

In recent years carbon nanotubes (CNT) have attracted significant research into their processing, properties and applications due to their extraordinary mechanical [1], electrical [2] and thermal properties [3]. Incorporating CNTs into polymer matrices to produce composite materials is one strategy to harness the potential of these materials. The development of a powder based processing route for thermosetting nanocomposites allows the manufacture of materials with high loading fractions (up to 20wt%) of CNTs [4]. Typically, nanocomposites with randomly dispersed CNTs show a decline in strength and plateau in elastic modulus beyond a few volume percent CNTs, as well as severe embrittlement [5, 6]. However, in recent work, the highest strength and modulus of the powder‐based composites increased up to the highest loading [4]. Differential interference contrast (DIC) optical reflective microscopy of these nanocomposites have revealed a texture with domains on the length scale of the original powder particles, suggesting the migration of resin out of the nanocomposite particles to fill voids at particle interfaces. Raman spectroscopy combined with scanning electron microscopy is a powerful tool to identify the presence of epoxy rich regions and variations in CNT density. The correlated chemical and morphological analysis provides insight into the unique microstructure of the nanocomposite not possible by elemental analysis methods, such as EDS. Using correlated Raman and SEM techniques, the relationship between CNT loading on the “grain size” is quantified and calibrated resulting in an enhanced understanding of how the microstructure affects the macro mechanical properties of the nanocomposite.
Direct determination of calibration factors for quantitative
DPC
measurements
Authors: Schwarzhuber Felix , Wild Johannes , Zweck Josef
Published: December 2016

Differential phase contrast microscopy (DPC) is a measurement technique which is utilized in a scanning transmission electron microscope (STEM) equipped with a special direction sensitive detector [1,2]. It is based on the deflection of the electron beam by an angle α due to either Lorentz or Coulomb force, when the electron probe is scanned over an area with intrinsic magnetic or electric fields. This deflection causes a shift of the diffraction disk (DD) in the detector plane of the microscope, which is proportional to strength and direction of the field within the specimen. By measuring this shift with a direction sensitive segmented ring detector one obtains information about the intrinsic fields.

In this work we present an approach to determine calibration factors relating the qualitative DPC signals to absolute magnetic and electric field strengths or deflection angles just by measuring the size of the DD on the DPC detector. For this we derived a formula for the calibration factor by considering geometrical properties of the annular DPC detector and the DD. With this we can for example describe the absolute electric field strength E(x,y) at a certain specimen position (x,y) by:

E(x,y)[V/m] = ( S
DPC
(x,y) / S
sum
(x,y) ) · ( κ
el
[V] / t(x,y)[m] ) (1)

S
DPC
(x,y) being the DPC signal normalized with the sum signal S
sum
(x,y) of all four detector segments, the specimen thickness t(x,y) and the calibration factor κ
el
for electric fields. The latter can be described by:

κ
el
= [ (R
2
‐ r
2
) / (R · C) ] · [ (m
rel
· v
rel
2
) / e ] (2)

with R and r being the radii of the DD in the detector plane and the detector hole (see fig. 1) and C the used camera length of the microscope. The second term describes the energy of the accelerated electrons with their relativistic mass m
rel
, velocity v
rel
and elemental charge e.

Equation 2 shows us two important things. Firstly that the calibration factor is highly dependent on the radius R of the DD (see fig. 2 and 3). This means that even a small change of the DD radius due to beam broadening caused by the specimen can lead to significant different field values when they are quantified with a calibration factor determined for the same set of microscope parameters but with a slightly smaller disk radius.

The other one is, that it is possible to calculate κ
el
just by the measurement of R, because all the other variables in eq. 2 are well known parameters of the DPC measurement. The disk size itself can be easily determined e.g. with a CCD camera. This is a convenient way to calibrate a DPC system for quantitative measurements. Further it is possible to obtain individual calibration factors for each DPC measurement performed. The latter allows to minimize the error of the quantification due to disk broadening.

In addition to the theoretical approach we will also present our first experimental results showing the validity of the statements made above.
Control of Polarity, Structure and Growth Direction in
S
n‐Seeded
G
a
S
b Nanowires
Authors: Zamani Reza R. , Ghalamestani Sepideh Gorji , Niu Jie , Sköld Niklas , Dick Kimberly A.
Published: December 2016

Among III‐V semiconductor materials GaSb is highly interesting for several device applications such as optoelectronics.
1
The epitaxial growth of GaSb nanowires has mainly been done using Au as the seed material which demonstrated several limitations like direct nucleation and crystal structure tuning.
2
In this work we have investigated the epitaxial growth of Sn‐seeded GaSb nanowires directly nucleated on GaSb (111)A substrates with controlled Ga‐ and Sb‐polarities, and study their structural and physical properties.

Nanowires are grown by metal organic vapor phase epitaxy (MOVPE) in a standard low pressure (100 mbar) horizontal MOVPE reactor (Aixtron 200/4). Sn‐seed particles are formed in‐situ by using Tetraethyltin (TESn) precursor at 530°C. Following the particle formation, the reactor temperature was changed to the nanowire growth temperature in the range of 490‐570°C. Trimethylgallium (TMGa) and trimethylantimony (TMSb) are used as precursors for nanowire growth.

The polarity of the nanowires is confirmed by aberration‐corrected scanning transmission electron microscopy (STEM) as can be seen in Figure 1. It is shown that there are differences in the structural properties (i.e. growth direction, composition of the seed particle, and crystal purity) of Ga‐ and Sb‐polar nanowires; and their growth mechanism is studied. In addition, the formation of inclined twins which only occurs in the Sb‐polar nanowires is explained by using simulation methods. Also high‐angle annular dark‐field (HAADF‐) STEM image simulation is employed for realization of subtle features in the experimental images, as shown in Figure 2. In this Figure, presence of twin boundaries which is not perpendicular to the zone axis is proven. Finally, photoluminescence response of the Sn‐seeded GaSb nanowires is compared with their Au‐seeded counterpart, suggesting incorporation of Sn atoms from the seed particle into the nanowires.
3
Comparison of different
EM
methods for observation of lipidic structures in the cell nucleus
Authors: Schrenkova Jana , Sobol Margarita , Philimonenko Vlada , Hozak Pavel
Published: December 2016

Permanent progress in electron microscopy (EM) techniques enables us to reveal the finest ultrastructural details of the cell nucleus. Our recent work is focused on the previously unrecognized nucleoplasmic structures composed of phosphatidylinositol 4,5‐bisphosphate, so called PIP2 islets. The super‐resolution light microscopy enabled us to localize PIP2 islets inside the cell nucleus and reveal their colocalization with several components of Pol II transcription machinery and chromatin. However, only EM provides sufficient resolution to reveal the PIP2 islets ultrastructure and composition via immunostaining.

The usage of appropriate method for EM sample preparation is a crucial step in the whole procedure as PIP2 islets are lipidic structures which are predisposed to extraction or translocations. Here we compare different EM sample preparation techniques which we have optimized for the best ultrastructural preservation and antigenicity retention of PIP2 islets. We have analyzed several chemical fixation and cryoimmobilization approaches followed by various embedding media including both acrylic and epoxy resins. Moreover, different immunolabeling techniques were used such as pre‐embedding, on‐section labeling, and Tokuyasu method. With all these approaches we were able to observe roundish PIP2 islets (40‐100 nm) recognized by anti‐PIP2 antibodies. Preliminary results demonstrate that PIP2 islets are stable nuclear structures resistant to aldehyde fixation and extraction during preparation and cutting procedures. Based on the upcoming statistical analysis, the best approach for immunological studies of PIP2 islets will be selected.
T
i
O
2 nanotubes array decorated by Ag and modified in reduction atmosphere for photo‐catalytic application using visible light
Authors: Gajovic Andreja , Plodinec Milivoj , Grčić Ivana , Willinger Marc
Published: December 2016

TiO
2
nanostructures with promising physical and chemical properties, such as a high specific surface area, a high photo‐activity and environmental stability, but also the low costs of synthesis inspire numerous studies over the past three decades. Titania nanostructures can be used for photo‐catalysis, in solar cells (DSSC) and sensors, still it can absorb light and behave as photo‐catalytically active only under irradiation with UV light since TiO
2
has a relatively large band‐gap (3.2 eV for anatase and 3 eV for rutile phase). So, one of the key parameters necessary to develop enhanced photo‐catalytic properties in the visible and near‐IR regions is to optimize the band‐gap of TiO
2
materials towards the bandgap of monocrystal silicon. In our study we use Ag nanoparticles and hydrogenation to manipulate with the bandgap, photo‐absorption of TiO
2
nanotubes with the aim to enhance photo‐catalytic properties.

TiO
2
nanotubes array (TiNTA) was synthesized by anodization of titanium foils as broadly used method for production of nanotubes arrays with different morphology and dimensions of nanotubes. In case of decorated nanotubes, the silver particles were obtained by the photo‐reduction of AgNO
3
under UV light. Pure and silver‐decorated nanotubes were additionally heat treated in hydrogen. For the structural characterization we used conventional and analytical transmission electron microscopy (TEM) including high angle annular dark field imaging (HAADF), scanning electron microscopy (SEM) techniques, X‐ray diffraction (XRD) and Raman spectroscopy. Using UV–ViS‐NIR spectroscopy we studied bandgaps and the photo‐catalytic activity (on a model organic compound ‐ wastewater) for all the samples before and after hydrogenation.

We found that decoration of TiNTA with Ag broadened the light absorption also in visible range due to the surface plasmon resonance of Ag nanoparticles. Ag nanoparticles with different shapes and diameters are attached on TiO
2
array surface (Fig. 1) as well as inside the nanotubes (Fig. 2). Therefore, the frequency of their plasmonic oscillations covers a wide range, permitting absorbance over a visible and UV part of solar spectrum. Moreover, the hydrogenation at high temperature was shown as essential to considerably increase the absorption of light in the visible and near‐IR regions and enhance photo‐catalytic activity. This is a consequence of the synergetic effects between the silver and the defects at the surface of TiO
2
nanoparticles (the most probably oxygen vacancies) that increase the efficiency of the formation of electron–hole pairs and the charge transfer to the surface of the nanoparticles.
Characterisation of theMedipix3detector for electron imaging
Authors: Mir Jamil , Mir Jamil , Clough Robert , MacInnes Ruaraidh , Gough Christopher , Plackett Richard , Sawada Hide , MacLaren Ian , Maneuski Dima , O'Shea Val , McGrouther Damien , Kirkland Angus
Published: December 2016

Hybrid pixel sensors, originally developed for particle physics, incorporate advanced analogue processing and digital conversion circuitry at the individual pixel level. Medipix3 [1] is an example of such a sensor and we have investigated its performance as an imaging detector for transmission electron microscopy (TEM). Measurements were performed with electron beam energies in the range, 60–200 keV on a JEOLARM200cF TEM/STEM [2] utilising a 256x256 pixel Medipix3 detector with 300 µm thick Si sensor layer.

In order to characterise the Modulation Transfer Function (MTF) and the Detective Quantum Efficiency (DQE) performance, 32 repeated datasets were acquired containing images of free space and a knife‐edge for known beam current conditions at each energy across the full range of relevant Medipix3 energy threshold values. Data was acquired in Single Pixel Mode (SPM) and in Charge Summing Mode (CSM) [3], where, in the latter mode, effects from charge spreading in individual electron events are corrected for on the detector. We have also measured DQE(0) using the methods described in [4].

At high lower threshold (THL) DAC values the MTF for this counting detector in single pixel mode is better than the theoretical maximum due to the reduction in the effective pixel size [4] as shown in Figure 1. However, the DQE at such high THL DAC values in single pixel mode is significantly reduced as seen in Figure 2, seeing as many real electron events are now not counted as the charge is deposited in more than one pixel and therefore falls below the threshold for detection. Consequently, there is a balance to be made between optimizing DQE and MTF, depending on the exact requirements in the given application.

As is shown in Figure 3, the use of the CSM allows the achievement of a much higher MTF whilst retaining high DQE by using a lower threshold DAC value. This therefore offers additional benefits over the more conventional SPM, thus allowing very high efficiency imaging whilst preserving maximal detail in the images, which is particularly beneficial for minimizing the required electron dose to the sample required to produce interpretable data, with obvious applications in beam sensitive materials.
Structural characterisation of nanomaterials incorporated into bioengineered samples: An electron microscopy approach
Authors: Hobbs Christopher , Metel Sonia , Nicolosi Valeria
Published: December 2016

This study analyses the properties of nanohydroxyapatite (nHA) and layered double hydroxide (LDH) materials that can be used as non‐viral gene delivery vectors. The former is a biocompatible, non‐toxic calcium phosphate mineral which makes it a suitable candidate as a delivery platform
1
. On the other hand, LDHs are composed of brucite‐like layers with a charge compensating anionic interlayer
2
.

(Scanning) Transmission electron microscopy ((S)TEM) and Energy Dispersive X‐ray Spectroscopy (EDX) methods were utilised to characterize LDH and nHA nanomaterials and their respective interactions with plasmid DNA structures (pDNA). The current work also characterizes the ultrastructure of mesenchymal stem cells (MSCs), as well as investigating the uptake and localisation of the LDH gene delivery vectors at the nanoscale.

Fig. 1(a) and Fig. 1(b) present the TEM images of the nHA and LDH particles respectively. The nHA particles are found to have a spherical structure of diameter ~50nm, indicating that nHA is a good candidate for cellular transfection. LDHs present a well‐defined hexagonal platelet‐like structure of ~100nm, providing promising results towards their application as a gene delivery vector.

The interaction of nHA and LDH particles with pDNA was also investigated using STEM. Morphological variations are clearly evident in both the pDNA‐nHA and pDNA‐LDH composites. An annular‐like structure is presented in the case of pDNA‐nHA, whereas the DNA structures absorb to the surface of the LDH particles, displayed in Fig. 2(a) and Fig. 2(b) respectively.

The application of STEM to MSCs highlight intracellular features such as membranes and nuclei, as shown in Fig. 3. The complimentary EDX techniques portray evidence of the LDH gene delivery vectors within the cell due to a presence of an Mg, Al and P peak, hence confirming uptake and localisation at the molecular level. The enhanced resolution provided by electron microscopy techniques allows for the study and understanding of cellular biological processes, such as that of pDNA‐nanoparticle delivery.
Shape transformations during the growth of gold nanostructures
Authors: Ahmad Nabeel , Ricolleau Christian , bouar Yann Le , Alloyeau Damien
Published: December 2016

Liquid cell transmission electron microscopy (LCTEM) has rapidly emerged as a potent tool for understanding the dynamical processes taking place at solid / liquid interfaces. Imaging colloidal solutions with the high temporal and spatial resolutions of TEM enables understanding the growth mechanisms that control the final size and morphology of nanoparticles. Nevertheless, conclusive LCTEM experiments require understanding the effects of electron‐irradiation on the nanoscale phenomena under study. Radiolytic syntheses driven by the electron beam were performed in this work to study the effects of the dose history (including the instantaneous dose rate and the cumulative dose) and the solvent nature on the shape of gold nanoparticles. The straightforward control over the concentration of reducing agents (radiolytically‐produced hydrated or solvated electrons) provides mechanistic insights on the growth of highly desired nanocrystal shapes for plasmonic applications.

Liquid cell experiments were carried out on an aberration corrected JEOL ARM 200F operated at 200KV, by using a commercial liquid‐cell holder provided by Protochips Inc. 1mM HAuCl
4
in water or methanol was analyzed in a 150 nm‐spacer liquid cell. Growth experiments were conducted under two extreme regimes of dose rate (over 150 electrons/Å
2
s and below 1 electrons/Å
2
s) in both TEM and STEM modes.

Under high dose rate we observed the growth of dendritic nanostructures (Fig. 1a). By comparing LCTEM observations with an extended diffusion‐limited aggregation model (Fig. 1b), we explicitly reveal the molecular and atomic diffusion processes that impact the shape of these dendritic nanostructures.[1] Besides the well‐established link between the dose rate and the growth speed of the nanostructures,[2,3] we have demonstrated that the cumulative dose in the irradiated area can also induce drastic transitions in the growth mode of the nanostructures. For instance, high dose rate observation severely affects the concentration of precursors in and around the irradiated area, resulting in the formation of anisotropic tree‐like structures over spherical nanoparticles (Fig. 1a).

Under low dose rate, reaction‐limited growth leads to the formation of highly facetted nanoclusters. The growth is then dominated by thermodynamic effects, because the lower adsorption rate of gold atoms provides enough time for the clusters to reach an equilibrium shape that depend on intrinsic and extrinsic parameters.[3] We show that crystal defects (intrinsic parameter) or the nature of the solvent that modulates the surface energies of crystal facets (extrinsic parameters) can both drive shape transformations during the growth of the nanoparticles. Remarkably, we reveal the formation mechanisms of highly symmetric 2D and 3D nanostars enclosed by high‐index facets. . These
in situ
studies could help in designing new seed‐mediated methods or capping strategies to fabricate metallic nanostars.
Nano‐scale strain measurements from high‐precision ADF STEM
Authors: Jones Lewys , Varambhia Aakash , Wenner Sigurd , Nord Magnus , Ninive Per Harald , Løvvik Ole Martin , Holmestad Randi , Nellist Peter
Published: December 2016

High‐resolution annular dark‐field imaging in the scanning transmission electron microscope (ADF STEM) offers a powerful and readily interpretable mode for materials analysis at the atomic scale. However, like all serial (scanned) imaging techniques subtle disturbances in the instrument's surroundings can lead to deleterious artefacts in the recorded data. Recent developments in non‐rigid registration of multi‐frame data allows a route to mitigate these scanning errors [1] and obtain accurate strain information in the STEM [2].

Here we make use of these new multi‐frame techniques, but go further and use experiment design to optimally fractionate an allotted total electron‐budget to achieve maximum precision. For the conditions used this occurs at around 20‐25 ADF frames. Using these optimised conditions we record data from two specimens as proof‐of principle examples; a rod‐like AlMgSi precipitate in an Al matrix and a Pt
3
Co dealloyed nanoparticle.

In the case of the Al precipitate, Figure 1, the wide field‐of‐view allows geometric phase analysis (GPA) to be used to analyse the strain field [3]. At a 0.6nm resolution we achieve a strain precision of 0.3%. The accuracy of these results were confirmed through comparison with DFT simulation.

For the Pt
3
Co nanoparticle GPA is no longer appropriate owing to its limited size and the need for atomic resolution detail. Here a real‐space approach was used where atomic‐column positions were compared to sites defined by a pair of base vectors. The offsets from these are shown in Figure 2. Here we see the last monolayer and a half exhibit an expanded lattice parameter consistent with a platinum enriched outer shell.
In situ
TEM
measurements of mechanical properties of individual spherical
BN
nanoparticles of different morphologies
Authors: Firestein Konstantin , Steinman Alexander , Sukhorukova Irina , Kovalskii Andrey , Matveev Andrei , Golberg Dmitri , Shtansky Dmitry
Published: December 2016

Boron nitride (BN) nanostructures exhibit excellent mechanical properties. For example, the
in situ
tensile tests on individual BN nanotubes (BNNTs), which were conducted
in situ
in a transmission electron microscope (TEM) column, demonstrated the strength and Young's modulus of ~33 GPa and ~1.3 TPa, respectively [1].Such superb mechanical properties of BN nanostructures make them very attractive materials as a reinforcement phase in lightweight composites. Besides nanotubes, nano‐BN can be obtained in the form of spherical nanoparticles (BNNPs) with different morphologies – solid or hollow, with smooth or rough surfaces. High electrical and chemical resistance, thermal stability and biocompatibility of BNNPs were reported but their mechanical properties have not been detailed yet.

BNNPs in the present study were synthesized by CVD technique. By changing technological parameters of CVD process particles of various morphologies were obtained. For each type of individual BNNPs
in situ
compression tests were accomplished in a TEM column. As‐synthesized BNNPs were dispersed in acetone and the suspension was placed onto a Si wafer. The truncated cone‐like shape indenter with a diameter of top circle of 1 μm was used. Elastic moduli were determined for all tested particles. The obtained results (Fig. 1) show that the high strength enables the hollow spherical BNNPs to withstand a considerable compressive deformation before failure (up to 50% of the hollow particle diameter). The structures also demonstrated a high percentage of elastic recovery.
Structural characterization of the protein
P2
, the aphid transmission factor of the cauliflower mosaic virus
Authors: Lecorre Francois , Him Joséphine Lai Kee
Published: December 2016

Cauliflower mosaic virus (CaMV) is transmitted from plant to plant through an interaction with aphid vectors, according to a non‐circulant transmission (1). Until recently, it was admitted that CaMV transmission was not specific. However, our colleagues have recently identified a proteic receptor at the extreme tip of the aphid maxillary stylets involved in interaction with the viral protein P2, the transmission factor of CaMV, suggesting a specific and complex interaction process (2).

As a consequence, it is now believed that CaMV transmission involves an aphid receptor and two viral proteins, P2 and P3. P2 binds both to the aphid receptor and to P3, which is tightly associated with the CaMV viral particle, forming the transmissible viral complex (Figure 1). We have previously resolved the P3 structure by X‐ray crystallography (3) and proposed a model for the CaMV:P3 complex based on single particle cryo‐electron microscopy (cryo‐EM) studies (4). We are now trying to characterize P2, the viral helper‐component, which acts as a bridge between the aphid receptor and the viral complex CaMV:P3, thus playing a major role in plant‐vector transmission. Structure predictions indicate that P2 is composed by a N‐terminal globular domain, followed by two coiled‐coil domains. Due to the presence of these coiled‐coil domains which seem to confer the protein a strong tendency to aggregate, it is extremely difficult to work and manipulate the native P2 protein. Hence, we have adopted a hybrid strategy combining X‐ray crystallography and cryo‐EM studies to investigate the structural organization of P2. In this context, we are trying to crystallize various domains of the P2 protein over‐expressed in bacteria. Additionally, we want to obtain the tertiary structure of P2 by using electron microscopy. Indeed, as His‐tagged P2 protein can form paracrystals when expressed in
sf9
cells (Figure 2), we use cryo‐electron tomography and sub‐tomogram averaging methods to get first structural data on P2 and interactions between coiled‐coil domains. Moreover, a P2 mutant (157m) was shown to decorate microtubules in
sf9
cells (Figure 3). We defined conditions of purification and used these decorated microtubules to engage a single particle analysis in cryo‐EM and image processing.

This poster presents the first structural results in the organization of the P2 protein and its multiple interactions.
In‐Situ Formation of Crystallographically‐Oriented Semiconductor Nanowire Arrays via Selective Vaporization
Authors: Huang Xing , Yu Yongqiang , Jones Travis , Fan Hua , Wang Lei , Xia Jing , Wang Zhujun , Meng Xiangmin , Willinger Marc
Published: December 2016

Mass production of high‐quality semiconductor nanowire arrays (SNAs) with precisely controlled orientation and structure is essential for their application in nanoelectronics and optoelectronics. Here, we present a single‐step approach for large‐scale fabrication of [0001]‐oriented SNAs
via
direct heating of their bulk components in a H
2
/Ar atmosphere. Real‐time imaging during
in‐situ
SEM experiments clearly demonstrates that the SNAs form through a selective vaporization process with respect to the crystallography of wurtzite crystals. We propose that the selective vaporization relies on the low symmetry of the wurtzite structure with large differences between surface energies of low‐index planes. Further, we demonstrate that the approach can be extended to zinc‐blende type structures through a cation exchange process. Finally, the high‐quality of the SNAs is demonstrated by fabrication of photodetectors that present outstanding optoelectronic performances. We believe that our method represents a transformative new fabrication platform for large‐scale fabrication of orientated SNAs with novel functionalities.
Gap measurements via low‐loss
EELS
on atomically thin
M
ox
W
(1‐x)
S
2 nanoflakes
Authors: Pelaez Fernandez Mario , Suenaga Kazu , Arenal Raul
Published: December 2016

The properties of alloyed materials are a fundamental issue in Materials Science. For years, layered semiconductors of the TX
2
type (T=Mo, W; X=S, Se, Te) have been the subject of a varied range of studies due to their interesting electric, optical, catalytic and structural properties. In this sense, Mo
x
W
(1‐x)
S
2
alloys have been recently reported [1,2]; but most of their properties haven't been delved into yet. In this contribution, we focus on the local optoelectronic properties of such atomically thin (up to 6‐8 layers) of Mo
x
W
(1‐x)
S
2
. These properties have been probed by low‐loss EELS measurements [3] and we have examined the bandgap behavior for different alloying degrees of such nanomaterials as well as a function of the number of layers.

These works have been carried out using a FEI Titan Cs probe‐corrected microscope equipped with a monochromator (working at 80 KV and with an energy resolution of ~180 meV).

Figure 1 displays three HRSTEM‐HAADF micrographs of three different monolayers of MoS
2,
Mo
0.5
W
0.5
S
2
and WS
2
samples, respectively. The alloying effects of such materials can be easily distinguished from the HAADF image corresponding to the Mo
0.5
W
0.5
S
2
sample (Fig. 1(a) second image from the left). The areas of reduced number of layers have been selected via optical and low‐magnification TEM images and identified for the low‐loss measurements, see Fig. 1(b).

Low‐loss EEL spectra, using the spectrum‐line mode, have been recorded in regions where different stacks of a few layers can be noticed, see Fig. 2. In each of these stacks, easily recognisable for being the flat regions in the HAADF intensity profile (Figure 2(b)), several spectra have been integrated over a window of 10 to 12 nm. After zero loss peak (ZLP) extraction, the different spectra are fitted to obtain the band‐gap value in each of these zones. In parallel, the thickness of these areas has been estimated, using the standard procedure [4]. Finally, these results show a relation between the band‐gap of the material and the number of layers for every composition (Figure 2(c)).

All these results will be deeply discussed in the framework of previous experimental (photoluminescence) and theoretical (DFT calculations) works carried out in these material [1,2]. In conclusion, the present studies improve our knowledge of the optoelectronic properties of atomically thin layered alloys of dichalcogenides and provides further insight into the potential applications of these materials.
Structure of
C
o
A
g nanoparticles embedded in a matrix
Authors: Loiselet Ophelliam , Tournus Florent g , March Katia
Published: December 2016

Since CoAg nanoparticles are composed of two immiscible metals, their theoretical structure is predicted to be Co@Ag due to the weaker surface energy of Ag [1]. In the literature these particles are synthesized by chemistry techniques [2‐3], the structure obtained is Co(core)Ag(shell) and has been confirmed by the optical response in UV‐Vis spectroscopy [2]. The localized surface plasmon resonance (LSPR) of silver shell and the magnetic anisotropy of cobalt core open the access to magneto‐plasmonic studies. That is why we have decided to elaborate those particles in a different approach using physical method under ultra high vacuum (UHV).

We have elaborated by low energy cluster beam deposition (LECBD [4]) technique Co
50
Ag
50
nanoparticles of different diameter (approximatively 1 to 10 nm) in UHV which are mass selected by a quadrupole ion deflector. These clusters are embedded in different matrices (metal oxides, amorphous carbon) to protect them from oxidization. The clusters stoichiometry has been checked by EDX measurements.

The structure of Co
50
Ag
50
clusters in carbon matrix is polycrystalline fcc. On high resolution transmission electron microscopy (HRTEM) images we can distinguish two different phases between the top and the bottom of the particle (figure 1.a) by using the intensity as a discrimination factor. Then we have determined by Fourier transforms analysis that the silver is in fcc structure and we measured approximatively the same lattice parameter than the silver bulk. Nevertheless we could not identify expected bcc, hcpor fcc [5]crystallinecobalt structure. High angle annular dark field electron microscopy (HAADF) measurements provided us informations about the contrast between cobalt and silver (figure 1.b), which depends of their respective atomic number. Inhomogeneous Janus‐like structures have been identified, with cobalt assumed to be in dark and silver in bright (figure 1.b), this segregation is partially in agreement with theoretical predictions [1] which expect a core/shell segregated structure.

In Al
2
O
3
matrix another phenomena occurs, an amorphous shell appears on each particle. As we can see on TEM image (figure 1.c and 1.d), we can distinguish a core in the particles composed of a dark part and a brighter one and an amorphous bright shell, assumed to be a reaction with the matrix. SQUID magnetic measurements confirmed us that the clusters are still ferromagnetic, an antiferromagnetic behaviour is expected for CoO, which means that the Co cannot be completely oxidized. As in carbon matrix, we can distinguish an inhomogeneous structure and an intensity contrast between the topand the bottom of the particle (figure 1.d). Then the silver is also in fcc structure and the lattice parameter obtained by Fourier analysis measurements matches with the bulk one. Moreover the same problem encountered in carbon matrix appears when we try to identify the Co crystalline structure.
Exploring and Tailoring Structural Properties of the Two‐Dimensional Family
of
MX
enes
Authors: Persson Per
Published: December 2016

MXenes constitute a rather recent addition to two‐dimensional materials, which exhibit a large range of tunable properties [1]. These materials can be synthesized in both bulk form as well as thin film structures. Although many of their properties remain unexplored they exhibit outstanding properties for applications in electrochemical energy storage devices, EESDs [2].

MXenes originate from a large family of naturally nanolaminated materials known as MAX phases, where
M
is a transition metal,
A
a group A element and
X
is either C or N, following the general formula M
n+1
AX
n
., see e.g. the schematic in fig. 1a. Upon chemical etching, the A element leaves the MAX phase and results in single sheets, with surfaces functionalized, T
x
, by O, OH and F to give the MXene formula M
n+1
X
n
T
x
[3]. Bulk made MXene hence exhibit an extremely large surface area per volume, and are consequently attractive for energy storage. The material is easily intercalated and delaminated using standard methods.

To date, shy of 20 different 2D structures in this family has been synthesized and the wide range of choice of e.g. M elements enable a formidable playground for materials engineering. Among the more interesting structures synthesized to date, are those which are achieved while alloying different M elements to achieve (M
1
,M
2
)
n+1
X
n,
see e.g. [4]. Through theoretical predictions and subsequent materials synthesis, we have been able to tune the material to exhibit
out of‐plane
ordering as well as
in‐plane
ordering, by means of repetitively alternating M elements. Additionally, by choosing the M elements carefully, we can incorporate those which are loosely bound and prone to chemical etching. This further enable a reduced MXene structure exhibiting
in‐plane vacancy ordering
. The process is shown in fig. 1, where a MAX phase of 2M elements is schematically etched to produce the reduced, in‐plane vacancy ordered structure as seen in cross‐section (fig 1b) and plan‐view (fig 1c). The corresponding synthesized structure is shown by HRSTEM methods in fig 2. These new MXenes additionally enable further tuning by vacancy ripening. Upon irradiating and heating the sample, the vacancies ripen to produce 2D sheets with a mesoporous structure, which exhibits a significant application in filtering and for enhancing ion mobilities in EESD applications.

This presentation will highlight the research front in this new family of materials, and present the opportunities given through its vast tailoring ability. Electron microscopy, and in particular aberration corrected STEM in combination with EELS and EDS has proven critical in the exploration of these new materials.
In situ electro‐chemical liquid
TEM
experiments to study
LiFe0
.
5Mn0
.
5PO4
Nanoplatelets
Authors: Dachraoui Walid , Karakulina Olesia , Hadermann Joke , Demortière Arnaud , Dachraoui Walid
Published: December 2016

Liquid cell electron microscopy is a developing technique that allows us to apply the powerful capabilities of the electron microscope to image and analyze materials immersed in liquid. We are thus able to examine liquid based processes in materials science and physics that are traditionally inaccessible with conventional TEM. The liquid/bias sealed cell (Protochips Poseidon 510) consists of two designed silicon echips having Si
3
N
4
windows and microelectrodes fabricated by lithography technique (
figure 1
). Electrochemical echips separate the liquid from the microscope vacuum also confining it into a layer that is thin enough for imaging with transmitted electron. The importance of liquid cell TEM in electrochemistry is that liquid cell experiments enable direct imaging of key phenomena during battery operation and relate the structural and compositional changes
[1]
to electrochemical signature
[2,3]
.

In this work, we carried out
in situ
liquid TEM studies of LiFe
0.5
Mn
0.5
PO
4
(LMFP) nanoplatelets synthesized with a colloidal procedure
[4]
, while are very promising nano‐objects for high‐rate batteries
.
The synthesized nanoplatelets LMFP were studied using advanced TEM: high angle annular dark field STEM (HAADF‐STEM), EDX‐STEM mapping, and EELS‐STEM mapping to find crystal structure and atomic distributions. The figure 2 shows high‐resolution HAADF‐STEM image of single LMFP nanocrystal oriented along [010] direction in which Li/(Fe, Mn) anti‐site defects is observed (see plot profile). EFTEM and EELS mapping show the coexistence and homogenous distribution of Fe, Mn, P elements in single nanoplatelets. For
in situ
liquid experiments, this cathode material was deposited onto glassy carbon (on silicon nitride membrane) of the top echip used to encapsulate conventional liquid electrolyte LP30 (LiFP
6
/EC/DMC). A gap of 500 nm between both echips and a flow mode introduction were used for the liquid medium. The assembly was followed with TEM/STEM imaging and spectroscopy (EELS, EDX) during electrochemical cycling so as to monitor in real time the delithiation process of LMFP single nanocrystals.

We succeed to follow by TEM imaging in real time the structural and chemical changes taking place on positive electrode materials during the electrochemical cycling. Figure 3 shows electrochemical cyclic voltammetry (25mV/s) corresponding to LFMP in LP30 measured in three‐contact mode with silver as reference electrode. Using EELS, it was possible to follow the transition between LFMP and FMP during extraction and insertion of lithium from LiFe
0.5
Mn
0.5
PO
4
showed by the appearance a peak at 5 eV (in figure 3), also confirmed by EFTEM. The structural changes due to the lithiation/delithiation processes during electrochemical cycling were imaged by high‐resolution TEM showing local structural distortion.
Surface effects in nano‐cobalt ferrites
Authors: Saidani Mohamed , Belkacem Wajdi , Patout Loïc , Charai Ahmed , Mliki Najeh
Published: December 2016

Magnetic nanoparticles have been the issue of continuous and growing interest, from both fundamental and technological points of view, in the last decades. Their unique physical objects with remarkable magnetic properties differ significantly from their parent massive materials. These properties are due to finite size effects of the magnetic core, related to the reduced number of spins cooperatively linked within the particle, and to surface and interface effects, related to the lack of coordination for the surface ions, inducing broken exchange bonds that can result in frustration and spin disorder. A description of ferrite magnetic properties requires a through characterization of the crystallographic structure down to atomic scale. Indeed, synthesis processes significantly alter the chemistry of the involved compounds, and then their physical properties.

In this work, experimental investigations on cobalt ferrite nanoparticles CoxFe3‐xO4 (1 < x < 1.8) are reported, providing a comprehensive description of different surface and interface effects. Surface and exchange anisotropy in ferrite nanoparticles are investigated by means of several experimental techniques such as X ray diffraction, Transmission Electron Microscopy and magnetization measurements. A strong correlation between the structural and the magnetic properties has been revealed. It is shown that the proposed synthesis technique leads to the formation of a spinel nanoparticles well dispersed. Many of them are faceted having cub‐octahedral polygonal growth forms with exclusively the {1 1 1} and {1 0 0} type faces. Rather than core‐shell structures, a number of ordered defects were observed.
Nanotomy and
CLEM
techniques shed new light on biomedical processes
Authors: Giepmans Ben
Published: December 2016

A spectrum of dyes and probes now enable to localize molecules of interest within living cells by fluorescence microscopy. With electron microscopy (EM), cellular ultrastructure has been revealed. Bridging these two modalities, correlated light microscopy and EM (CLEM) opens new avenues [1].

The first focus will be on recent developed labeling strategies for molecules that allow CLEM (Fig.1). These include particles and substrates to highlight endogenous proteins that are targeted using affinity, but also genetically‐encoded probes [2], and traditional stains for light microscopy that aid in EM‐analysis of samples. Probes that can only be detected in a single modality, and require image overlay, as well as combinatorial probes that can be visualized both at LM and EM levels will be discussed.

In addition, new approaches for large scale EM (“nanotomy”), either TEM‐based [3] or S(T)EM‐based [4,5], to visualize macromolecules and organelles in the context of organized cell systems and tissues will be highlighted. Matching the areas of acquisition in CLEM and EM will not only increase understanding of the molecules in the context, but also is a straight forward manner to combine the LM and EM image data. While these new developments aid to better understand the contribution of molecules, organelles and cells in tissue‐function, the amount of data is huge and quantification and sharing data need new solutions. We will show how millions of particles can be recognized and quantified using Fiji. Moreover, our open‐access data‐sharing (Fig.2) via
nanotomy.org
will be highlighted to easily access the ultrastructure in a variety of research projects, including blistering diseases that affect skin and mucosa in men and Type 1 diabetes.

Covering a variety of probes and approaches for image overlay will help to enable (new) users to broadly implement CLEM and/or nanotomy to better understand how molecules (mal)function in biology [6].