Although the changes in melting behaviour on the nanoscale have long attracted the interest of researchers, the mechanism by which nanoparticles melt remains an open problem. We report the direct observation, at atomic resolution, of surface melting in individual size-selected Au clusters (2–5 nm diameter) supported on carbon films, using an in situ heating stage in the aberration corrected scanning transmission electron microscope. At elevated temperatures the Au nanoparticles are found to form a solid core-liquid shell structure. The cluster surface melting temperatures, show evidence of size-dependent melting point suppression. The cluster core melting temperatures are significantly greater than predicted by existing models of free clusters. To explore the effect of the interaction between the clusters and the carbon substrate, we employ a very large-scale ab initio simulation approach to investigate the influence of the support. Theoretical results for surface and core melting points are in good agreement with experiment.

The application of Mg as a potential hydrogen storage material has been hindered due to the slow absorption rate of hydrogen in Mg films. Herein, we explore the hydrogenation process theoretically using DFT calculations, and compare the energy barriers for hydrogen diffusion through Pd nanoparticle/Mg film interfaces and their variations, i.e. Pd(H)/Mg(O).Decomposing the mechanism into basic steps, we show that Pd undergoes a strain-induced crystallographic phase transformation near the interface, and indicate that hydrogen saturation of Pd nanoparticles enhances their efficiency as nanoportals. Using energetic arguments, we explain why hydrogen diffusion is practically prohibited through native Mg oxide, and seriously suppressed through existing hydride domains. We experimentally investigate hydrogen flux through the nanoportals in Pd nanoparticle-decorated Mg films by PCT hydrogenation measurements, and demonstrate the effect of the surface oxide layer and film thickness on hydrogenation kinetics. We theoretically calculate an r~t 1/3 relationship for the radial growth of hemispherical hydride domains, and confirm this relationship by AFM. The diffusion constant of hydrogen in Mg films is estimated as D H film ≈8×10 -18 m 2 s -1 , based on TEM characterization. Our unique nanoportal configuration allows direct measurement of hydride domain sizes, thus forming a model system for the experimental investigation of the hydrogenation process in any material.

The simulation and analysis of the thermal stability of nanoparticles, a stepping stone towards their application in technological devices, require fast and accurate force fields, in conjunction with effective characterisation methods. In this work, we develop efficient, transferable, and interpretable machine learning force fields for gold nanoparticles based on data gathered from Density Functional Theory calculations. We use them to investigate the thermodynamic stability of gold nanoparticles of different sizes (1 to 6 nm), containing up to 6266 atoms, concerning a solid-liquid phase change through molecular dynamics simulations. We predict nanoparticle melting temperatures in good agreement with available experimental data. Furthermore, we characterize the solid-liquid phase change mechanism employing an unsupervised learning scheme to categorize local atomic environments. We thus provide a data-driven definition of liquid atomic arrangements in the inner and surface regions of a nanoparticle and employ it to show that melting initiates at the outer layers.

Cu 3 N with a cubic crystal structure has been prepared from Cu on fused SiO 2 under a flow of NH 3 :O 2 between 400 and 600 °C. All Cu 3 N layers exhibited distinct maxima in differential transmission at ∼500, 550, and 630, 670 nm with the same spectral structure and shape on a ps timescale as shown by ultrafast pump-probe spectroscopy. We show that the maxima at 630 (≡1.97 eV) and 670 nm (≡1.85 eV) correspond to the M and R direct energy band gaps of Cu 3 N, in excellent agreement with density functional theory calculations of the electronic band structure. These findings are corroborated further by the fact that Cu 3 N as-deposited by reactive sputtering under 100% N 2 at 25 °C and 10 −2 mbar did not exhibit a fine spectral structure due to a smeared density of states, poor crystallinity, and a high density of defects, but annealing under NH 3 :H 2 at 300 °C revealed a similar spectral structure to Cu 3 N obtained from Cu under NH 3 :O 2 . In contrast to the above, we suggest that the peaks at 500 (≡2.48 eV) and 550 nm (≡2.25 eV) might correspond to the M and R direct gaps of certain regions of Cu 3 N under strain that changes the lattice constant and band gap. We discuss the charge carrier generation and recombination mechanisms in terms of Cu interstitials and vacancies that are known to be energetically located near the band edges, thus allowing the observation of the direct energy band gaps in this defect tolerant semiconductor.

The structure of heterogeneous nanocatalysts supported on metal oxide materials and their morphological changes during oxidation/reduction processes play a crucial role in determining the resulting catalytic activity. Herein, we study the thermal oxidation mechanism of Pd nanoparticles supported on CuO nanowires by combining in situ environmental transmission electron microscopy (TEM), ex situ experiments, and ab initio density functional theory (DFT) calculations. High-resolution TEM imaging assisted by geometric phase analysis enabled the analysis of partially oxidized, fully oxidized, and distinct onion-like Pd nanoparticles with subsurface dislocations. Furthermore, preferential crystalline orientations between PdO nanoparticles and the CuO nanowire support have been found. Hence, the CuO–Pd interface is crucial for the thermal oxidation of Pd nanoparticles, as corroborated by electron energy loss spectroscopy and DFT calculations. The latter revealed a considerably lower energy barrier for penetration of oxygen into the Pd lattice at the CuO–Pd interface, promoting nanoparticle oxidation. The obtained results are compared with those of literature reports on different material systems, and potential implications for catalysis and chemoresistive sensing applications are discussed.

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Neuromorphic computing systems may be the future of computing and cluster-based networks are a promising architecture for the realization of these systems. The creation and dissolution of synapses between the clusters are of great importance for their function. In this work, we model the thermal breakage of a gold nanofilament located between two gold nanoparticles via molecular dynamics simulations to study on the mechanisms of neuromorphic nanoparticle-based devices. We employ simulations of Au nanowires of different lengths (20–80 Å), widths (4–8 Å) and shapes connecting two Au1415 nanoparticles (NPs) and monitor the evolution of the system via a detailed structural identification analysis. We found that atoms of the nanofilament gradually aggregate towards the clusters, causing the middle of wire to gradually thin and then break. Most of the system remains crystalline during this process but the center is molten. The terminal NPs increase the melting point of the NWs by fixing the middle wire and act as recrystallization areas. We report a strong dependence on the width of the NWs, but also their length and structure. These results may serve as guidelines for the realization of cluster-based neuromorphic computing systems.

GaAs/AlGaAs core-shell nanowires were grown on Si(111) by Ga-assisted molecular beam epitaxy via the vapor-liquid-solid mechanism. High-resolution and scanning transmission electron microscopy observations showed that nanowires were predominantly zinc-blende single crystals of hexagonal shape, grown along the [111] direction. GaAs core nanowires emerged from the Si surface and subsequently, the nanowire growth front advanced by a continuous sequence of (111) rotational twins, while the AlGaAs shell lattice was perfectly aligned with the core lattice. Occasionally, single or multiple stacking faults induced wurtzite structure nanowire pockets. The AlGaAs shell occupied at least half of the nanowire's projected diameter, while the average Al content of the shell, estimated by energy dispersive X-ray analysis, was x = 0.35. Furthermore, molecular dynamics simulations of hexagonal cross-section nanowire slices, under a new parametrization of the Tersoff interatomic potential for AlAs, showed increased atom relaxation at the hexagon vertices of the shell. This, in conjunction with the compressively strained Al 0.35 Ga 0.65 As shell close to the GaAs core, can trigger a kinetic surface mechanism that could drive Al adatoms to accumulate at the relaxed sites of the shell, namely along the diagonals of the shell's hexagon. Moreover, the absence of long-range stresses in the GaAs/Al 0.35 Ga 0.65 As coreshell system may account for a highly stable heterostructure. The latter was consolidated by temperaturedependent photoluminescence spectroscopy.