Frame-by-frame observations of structure fluctuations in single mass-selected Au clusters using aberration-corrected electron microscopy

The multi-dimensional potential energy surface (PES) of a nanoparticle, such as a bare cluster of metal atoms, controls both the structure and dynamic behaviour of the particle. These properties are the subject of numerous theoretical simulations. However, quantitative experimental measurements of critical PES parameters are needed to regulate the models employed in the theoretical work. Experimental measurements of parameters are currently few in number, while model parameters taken from bulk systems may not be suitable for nanosystems. Here we describe a new measurement methodology, in which the isomer structures of a single deposited nanocluster are obtained frame-by-frame in an aberration-corrected scanning transmission electron microscope (ac-STEM) in high angle annular dark field (HAADF) mode. Several gold clusters containing 309 ± 15 atoms were analysed individually after deposition from a mass-selected cluster source onto an amorphous carbon film. The main isomers identified are icosahedral (Ih), decahedral (Dh) and face-centred-cubic (fcc) (the bulk structure), alongside many amorphous (glassy) structures. The results, which are broadly consistent with static ac-STEM measurements of an ensemble of such clusters, open the way to dynamic measurements of many different nanoparticles of diverse sizes, shapes and compositions

Growth pathways of Cu shells on Au and AuCu seeds: interdiffusion, shape transformations, strained shells and patchy surfaces

The growth of AuCu nanoparticles obtained by depositing Cu atoms on starting seeds of pure Au and on mixed AuCu seeds is studied by molecular dynamics simulations. Depending on the shape of the seed, its composition and the growth temperature, different growth pathways are observed, in which several types of structural transformations take place. The final growth structures comprise Au@Cu core@shell arrangements as well as Janus-like structures with patchy surfaces. The results of the growth simulations are rationalized in terms of the activation of different diffusion processes, both on the surface and inside the growing clusters. These diffusion processes regulate structural transitions between different motifs and the occurrence of dewetting phenomena. The simulation results show that depositon of Cu atoms on pure Au or mixed AuCu seed can be an effective tool for producing clusters with uncommon surface atom arrangements of potential interest for catalysis.AuCu nanoparticles with uncommon surface atom arrangements are produced by depositing Cu atoms on preformed seeds using computer simulations

Growth mechanisms from tetrahedral seeds to multiply twinned Au nanoparticles revealed by atomistic simulations

International audienceAtomic level simulations supported by density-functional theory calculations identify the key mechanisms of the twinning process in gold tetrahedral nanoparticles, which is shown to originate from the growth kinetics of the pure, ligand-free metal

Growth of size-matched nanoalloys – a comparison of AuAg and PtPd

The gas-phase growth of AuAg and PtPd clusters up to sizes ~3 nm is simulated by Molecular Dynamics. Both systems are characterized by a very small size mismatch and by a tendency of the less cohesive element to segregate at the nanoparticle surface. The aim of this work is to figure out the differences in the behavior between these two bimetallic systems at the atomic level. For each system, three simulation types are performed, in which either one species or both species are deposited on preformed bimetallic seeds. Our results show that core@shell and intermixed chemical ordering arrangements can be obtained, in agreement with the available experimental data. In the case of core@shell arrangement, the purity of the surface layer is perfect for Ag-rich and Pd-rich nanoparticles, whereas in Au-rich and Pt-rich ones, some tendency to surface migration of minority atoms (Ag or Pd) is observed. This tendency is somewhat stronger for Ag than for Pd. The analysis of the internal arrangement of the nanoparticles indicates that in the growth process the mobility of Pd and Ag minority atoms is stronger than that of Au and Pt minority atoms

Two-Steps Versus One-Step Solidification Pathways of Binary Metallic Nanodroplets

International audienceThe solidification of AgCo, AgNi, and AgCu nanodroplets is studied by molecular dynamics simulations in the size range of 2−8 nm. All these systems tend to phase separate in the bulk solid with surface segregation of Ag. Despite these similarities, the simulations reveal clear differences in the solidification pathways. AgCo and AgNi already separate in the liquid phase, and they solidify in configurations close to equilibrium. They can show a two-step solidification process in which Co-/Ni-rich parts solidify at higher temperatures than the Ag-rich part. AgCu does not separate in the liquid and solidifies in one step, thereby remaining in a kinetically trapped state down to room temperature. The solidification mechanisms and the size dependence of the solidification temperatures are analyzed, finding qualitatively different behaviors in AgCo/AgNi compared to AgCu. These differences are rationalized by an analytical model