Cu segregation in Au-Cu nanoparticles exposed to hydrogen atmospheric pressure: how is fcc symmetry maintained?

In a recent work [A. Nassereddine et al., Small 2021, 17, 2104571] we reported the atomic-scale structure and dynamics of sub-4 nm sized Au nanoparticles (NPs) supported on titania in H2 at atmospheric pressure obtained by using aberration-corrected environmental transmission electron microscopy (ETEM), density functional theory (DFT) optimizations and ab initio molecular dynamic (AIMD) simulations. Our results showed unstable Au NPs losing their face-centred cubic (fcc) symmetry (from fcc to non-fcc symmetries) and revealed the drastic effect of hydrogen adsorption. In this work, we use the same approach to study the dynamics of equiatomic Au-Cu NPs in the same range of size and the results show an enhanced structural stability upon alloying by Cu. In spite of the morphology evolution from facetted to rounded shapes, the observed Au-Cu NPs are found to keep their fcc symmetry under atmospheric hydrogen pressure. AIMD simulation evidences a Cu segregation process from the sub-surface toward the upper surface layer, and a reversed segregation of Au atoms from the surface towards the sub-surface sites. The analysis of the chemical ordering in the core shows a tendency to a local chemical ordering where Au-Cu hetero-atomic bindings are favoured. The segregating Cu seems to play a major role in reducing the fluxionality of Au-Cu NPs in H2 and thus, maintaining their fcc symmetry.

A multiscale in situ high temperature high resolution transmission electron microscopy study of ThO2 sintering.

Two-grain model systems formed by ThO2 nanospheres have been used to experimentally study for the first time the initial stage of sintering from room temperature to 1050 °C using high temperature high resolution transmission electron microscopy. In each grain, oriented attachment drove the reorganization and growth of the crystallites up to 300 °C to form a pseudo single crystal. Crystallite size kept growing up to 950 °C. At this temperature, a fast transformation probably corresponding to the elimination of stacking faults or dislocation walls led to the formation of single-crystals. The contact formed at room temperature between the two grains was stabilized during heat treatment by a slight reorientation of the crystallographic planes (T≈ 400 °C), leading the neck to be formed by numerous boundaries between the crystallites. At higher temperatures, the neck evolved and stabilized in the form of a plane of crystallographic orientation mismatch between the grains, which corresponds to the usual definition of the grain boundary. The growth of the neck by the addition of atomic columns was further observed in real time and quantified. At T = 950 °C, the evolution of the microscopic sintering parameter λ was obtained from HT-HRTEM images and indicated that the neck formation mostly proceeded through volume diffusion.

Nanoscale temperature measurement during temperature controlled in situ TEM using Al plasmon nanothermometry.

Over recent years, the advent of microelectromechanical system (MEMS)-type microheaters has pushed the limits of temperature controlled in situ transmission electron microscopy (TEM). In particular, by enabling the observation of the structure of materials in their application environments, temperature controlled TEM provides unprecedented insights into the link between the properties of materials and their structure in real-world problems, a clear knowledge of which is necessary for rational development of functional materials with new or improved properties. While temperature is the key parameter in such experiments, accessing the precise temperature of the sample at the nanoscale during observations still remains challenging. In the present work, we have applied aluminium plasmon nanothermometry technique that monitors the temperature dependence of the volume plasmon of Al nanospheres using electron energy loss spectroscopy for in situ local temperature determination over MEMS-type microheaters. With access to local temperatures between room temperature to 550 ∘C, we have assessed the spatial and temporal stabilities of these microheaters when they operate at different setpoint temperatures both under vacuum and in the presence of a static H2 gas environment. Temperature comparisons performed under the two environments show discrepancies between local and setpoint temperatures.

Nanoalloying bulk-immiscible iridium and palladium inhibits hydride formation and promotes catalytic performances.

The hydrogen sorption properties of oxide-supported Ir-Pd nanoalloys have been determined for the first time, and correlated with their catalytic behavior. The addition of Ir to Pd suppresses hydride formation and leads to improved catalytic performances with respect to pure metals in the preferential oxidation of CO in H2 excess (PROX).

Combining moiré patterns and high resolution transmission electron microscopy for in-plane thin films thickness determination.

This paper reports the coupling of HRTEM and moiré pattern observations, allowing the determination of the thickness ratio of two superimposed crystals. Pseudo-lattice fringes are observed using identical TEM experimental conditions as for observing moiré patterns. The pseudo-lattice spacing is first calculated in the dynamical theory framework in two beam conditions. This approach shows a linear behavior of the spacing as a function of the thickness ratio of the two crystals. The roles of sample crystallographic orientation and sample thickness on the thickness ratio determination are discussed from multi-beam simulations. Finally, the method is applied on a bimetallic CuAg core-shell nanoparticle of a known structure. It is demonstrated that for this particle, the thickness ratio of Cu and Ag can be determined with an error that results in a precision less than 0.75 nm on the Cu and Ag thicknesses. The advantages of the technique are the use of an in-plane sample configuration and a single HRTEM image.

[Parent satisfaction with the Loire Infant Follow-up Team (LIFT) premature and at-risk infant network in the Pays-de-la-Loire area (France)]. / Satisfaction des parents dans le réseau de suivi des prématurés et des enfants à risques « Grandir ensemble ¼ des Pays-de-la-Loire.

BACKGROUND: The Loire Infant Follow-up Team (LIFT) is a network for caring for premature infants whose gestational age is 34 WA or less and at-risk neonates in the Pays-de-la-Loire area in France. The network aims to screen for clinical anomalies early and to propose adapted care. Trained physicians follow the included children in a standardized manner at 3, 6, 9, 12, and 18 months and 2 years, with a specific examination by psychologists at 2 years. The aim of the study was to assess the satisfaction of the parents of the children followed. METHODS: To evaluate parent satisfaction, a questionnaire from the Consumer Satisfaction Survey (CSS) in its French version was sent to parents whose infants were 2 years old, stratifying on the presence of an anomaly. The questioner had 39 items, with 8 specific items on the network and 31 from the CSS. The questionnaire was mailed twice in September 2006. RESULTS: Out of 300 questionnaires mailed, 269 were returned (rate 89.7 %). The questionnaire was assessed using principal component analysis with 2 dimensions for the 30 items common to all children, one of which covered empathy with physicians and the other with the consulting psychologists at 2 years. The validity was good (Cronbach coefficient, 0.91). The answers to overall questions such as "We are satisfied with the care in the network" scored 16.1±0.7/20, with 90 % "totally agree" or "moderately agree" responses. The "The care is perfect" scored 14.6±0.7/20 with 78 % agreeing with the statement. The total score for 30 general questions was 14.6±3.1 (median, 14.9). The total score was lower for infants with anomalies: 13.7±3.3 versus 14.9±2.9 (P<0.01). The answers with a low score (<10) were given by 22 parents (8.2 %). There was no significant relation between the total score or the satisfaction score and neonatal events. CONCLUSION: A postal survey is helpful to know the views of parents on the follow-up of their infants. This good level of satisfaction seems to stem from the parents feeling they belong to the network, the quality of the relationships with personnel, and the doctors' empathy, as well as the number of contacts between parents and the network coordinator.

Ostwald ripening in nanoalloys: when thermodynamics drives a size-dependent particle composition.

Ostwald ripening has been broadly studied because it plays a determinant role in the evolution of cluster size during both chemical and physical synthesis of nanoparticles. This thermoactivated process causes large particles to grow, drawing material from the smaller particles, which shrink. However, this phenomenon becomes more complex when considering the coarsening of metallic alloy clusters. The present experimental and theoretical investigations show that the relative composition of CoPt nanoparticles can be strongly modified during high temperature annealing and displays a size-dependent behavior. This compositional change originates from the higher evaporation rate of Co atoms from the nanoparticles. More importantly, this effect is expected in all alloy clusters containing species with different mobilities.

Size and shape effects on the order-disorder phase transition in CoPt nanoparticles.

Chemically ordered bimetallic nanoparticles are promising candidates for magnetic-storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. Here, the effects of size and morphology on the order-disorder phase transition temperature of CoPt nanoparticles (T(C)(NP)) have been investigated experimentally, using transmission electron microscopy, and theoretically, with canonical Monte Carlo simulations. For 2.4-3-nm particles, T(C)(NP) is found to be 325-175 degrees C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. Furthermore, we establish that T(C)(NP) is also sensitive to the shape of the nanoparticles, because only one dimension of the particle (that is, in-plane size or thickness) smaller than 3 nm is sufficient to induce a considerable depression of T(C)(NP). This work emphasizes the necessity of taking into account the three-dimensional morphology of nano-objects to understand and control their structural properties.

Comparing electron tomography and HRTEM slicing methods as tools to measure the thickness of nanoparticles.

Nanoparticles' morphology is a key parameter in the understanding of their thermodynamical, optical, magnetic and catalytic properties. In general, nanoparticles, observed in transmission electron microscopy (TEM), are viewed in projection so that the determination of their thickness (along the projection direction) with respect to their projected lateral size is highly questionable. To date, the widely used methods to measure nanoparticles thickness in a transmission electron microscope are to use cross-section images or focal series in high-resolution transmission electron microscopy imaging (HRTEM "slicing"). In this paper, we compare the focal series method with the electron tomography method to show that both techniques yield similar particle thickness in a range of size from 1 to 5 nm, but the electron tomography method provides better statistics since more particles can be analyzed at one time. For this purpose, we have compared, on the same samples, the nanoparticles thickness measurements obtained from focal series with the ones determined from cross-section profiles of tomograms (tomogram slicing) perpendicular to the plane of the substrate supporting the nanoparticles. The methodology is finally applied to the comparison of CoPt nanoparticles annealed ex situ at two different temperatures to illustrate the accuracy of the techniques in detecting small particle thickness changes.

STEM nanodiffraction technique for structural analysis of CoPt nanoparticles.

Studying the structure of nanoparticles as a function of their size requires a correlation between the image and the diffraction pattern of single nanoparticles. Nanobeam diffraction technique is generally used but requires long and tedious TEM investigations, particularly when nanoparticles are randomly oriented on an amorphous substrate. We bring a new development to this structural study by controlling the nanoprobe of the Bright and Dark Field STEM (BF/DF STEM) modes of the TEM. The particularity of our experiment is to make the STEM nanoprobe parallel (probe size 1 nm and convergence angle <1 mrad) using a fine tuning of the focal lengths of the microscope illumination lenses. The accurate control of the beam position offered by this technique allowed us to obtain diffraction patterns of many single nanoparticles selected in the digital STEM image. By means of this technique, we demonstrate size effects on the order-disorder transition temperature in CoPt nanoparticles when their size is smaller than 3 nm.

Enhanced fluorescence cell imaging with metal-coated slides.

Fluorescence labeling is the prevailing imaging technique in cell biology research. When they involve statistical investigations on a large number of cells, experimental studies require both low magnification to get a reliable statistical population and high contrast to achieve accurate diagnosis on the nature of the cells' perturbation. Because microscope objectives of low magnification generally yield low collection efficiency, such studies are limited by the fluorescence signal weakness. To overcome this technological bottleneck, we proposed a new method based on metal-coated substrates that enhance the fluorescence process and improve collection efficiency in epifluorescence observation and that can be directly used with a common microscope setup. We developed a model based on the dipole approximation with the aim of simulating the optical behavior of a fluorophore on such a substrate and revealing the different mechanisms responsible for fluorescence enhancement. The presence of a reflective surface modifies both excitation and emission processes and additionally reshapes fluorescence emission lobes. From both theoretical and experimental results, we found the fluorescence signal emitted by a molecular cyanine 3 dye layer to be amplified by a factor approximately 30 when fluorophores are separated by a proper distance from the substrate. We then adapted our model to the case of homogeneously stained micrometer-sized objects and demonstrated mean signal amplification by a factor approximately 4. Finally, we applied our method to fluorescence imaging of dog kidney cells and verified experimentally the simulated results.

Ion Beam "Photography": Decoupling Nucleation and Growth of Metal Clusters in Glass.

We demonstrate that room temperature MeV ion irradiation of a glass containing copper oxide initiates nucleation of pure Cu clusters via the inelastic "electronic" component of the ion energy loss, when the latter is above a threshold value. The clusters grow under subsequent thermal annealing, following Lifshitz-Slyozov-Wagner kinetics. The decoupling of nucleation and growth is analogous to that occurring in the photographic process. It allows total control over the cluster density, average size, and size distribution.