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.

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.

Attachment of iron oxide nanoparticles to carbon nanofibers studied by in-situ liquid phase transmission electron microscopy.

By using liquid phase transmission electron microscopy (LP-TEM), the dynamics of iron oxide nanoparticle (Fe-NP) attachment to carbon nanofibers (CNFs) and oxygen functionalized CNFs (CNF-Ox) were studied in-situ. The beam effect on the stability of the sample in various liquids was examined, and it was found that toluene provided the highest stability and resolution to image both CNF supports and Fe-NPs. Flowing particles dispersed in toluene through the liquid cell allowed direct monitoring of the attachment process at ambient temperature. Using CNF-Ox as a support led to a large extent and irreversible attachment of iron nanoparticle compared to a lower extent and reversible attachment of Fe-NPs to pristine CNF, indicating the influence of surface functionalization on colloidal particle attachment. The results were confirmed by lab-scale experiments as well as experiments performed with the electron beam switched off, verifying the notion that beam effects did not affect the attachment. This study revealed previously unknown phenomena in colloidal particle - support interactions and demonstrates the power of LP-TEM technique for studying such nanoscale processes.

Luminescence properties of ZnGa2O4:Cr3+,Bi3+ nanophosphors for thermometry applications.

Chromium(iii) and bismuth(iii) co-doped ZnGa2O4 nanoparticles are synthesized by a hydrothermal method assisted by microwave heating. The obtained nanoparticles, with a diameter smaller than 10 nm, present good luminescence emission in the deep red range centered at 695 nm after coating with a silica layer and calcination at 1000 °C during 2 h. Persistent luminescence and photoluminescence properties are investigated at several temperatures. Bandwidth and luminescence intensity ratio of persistent emission do not present enough change with temperature to obtain a competitive nanothermometer with high sensitivity. Nevertheless, persistent luminescence decay curves present a significant shape change since the trap levels involved in the deexcitation mechanism are unfilled with increase of temperature. Even if the sensitivity reaches 1.7% °C-1 at 190 °C, the repeatability is not optimal. Furthermore, photoluminescent lifetime in the millisecond range extracted from the photoluminescence decay profiles drastically decreases with temperature increase. This variation is attributed to the thermal equilibrium between two thermally coupled chromium(iii) levels (2E and 4T2) that have very different deexcitation lifetimes. For ZnGa2O4:Cr3+ 0.5%,Bi3+ 0.5%, the temperature sensitivity reaches 1.93% °C-1 at 200 °C. Therefore, this kind of nanoparticle is a very promising thermal sensor for temperature determination at the nanoscale.

Recording low and high spatial frequencies in exit wave reconstructions.

Aberration corrected Transmission Electron Microscope (TEM) images can currently resolve information at significantly better than 0.1 nm. Aberration corrected imaging conditions seek to optimize the transfer of high-resolution information but in doing so they prevent the transfer of low spatial frequency information. To recover low spatial frequency information, aberration corrected images must be acquired at a large defocus which compromises high spatial frequency information transfer. In this paper we present two a posteriori solutions to this problem in which the information bandwidth in an exit wave reconstruction is increased. In the first we reconstruct the electron exit wavefunction from two focal series datasets, with different, uniform focal steps, experimentally demonstrating that the width of the transfer interval can be extended from 0.2 nm⁻¹ (∼5 nm) to better than 10 nm⁻¹ (0.1 nm). In the second we outline the use of a focal series recorded with a non-uniform focal step to recover a wider range of spatial frequencies without the need for a large number of images. Using simulated data we show that using this non-uniform focal step the spatial frequency interval for a five image data set may be increased to between 0.25 nm⁻¹ (4 nm) and 8.3 nm⁻¹ (0.12 nm) compared to between 0.74 nm⁻¹ (1.4 nm) and 8.3 nm⁻¹ (0.12 nm) for the standard focal series geometry.

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.

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.