Abstract
Gold nanoclusters have been the focus of numerous computational studies, but an atomistic understanding of their structural and dynamical properties at finite temperature is far from satisfactory. To address this deficiency, we investigate gold nanoclusters via ab initio molecular dynamics, in a range of sizes where a core–shell morphology is observed. We analyze their structure and dynamics using state-of-the-art techniques, including unsupervised machine-learning nonlinear dimensionality reduction (sketch-map) for describing the similarities and differences among the range of sampled configurations. In the examined temperature range between 300 and 600 K, we find that whereas the gold nanoclusters exhibit continuous structural rearrangement, they are not amorphous. Instead, they clearly show persistent motifs: a cationic core of 1–5 atoms is loosely bound to a shell which typically displays a substructure resulting from the competition between locally spherical versus planar fragments. Besides illuminating the properties of core–shell gold nanoclusters, the present study proposes a set of useful tools for understanding their nature in operando.
Highlights
Transition-metal nanoclusters have been the focus of a large number of experimental and computational studies in physics and chemistry because of their expected technological applications in diverse areas, including catalysis,[1] optics,[2] and biomedicine.[3]
Most of the reported ab initio studies have been dedicated to searching for their putative global minimum configurations in the gas phase at zero temperature.[14]
An empirical-potential molecular dynamics (MD) study of Au75, Au146, and Au457 showed that their melting temperatures were well below the melting temperature of bulk gold (1090 K, at standard pressure) and that structural solid-to-solid transitions were present well below their melting temperatures.[24]
Summary
Together with the bond lifetime analysis (Figure 3), this observation further strengthens the picture of typical gold cluster structures at 300 K and higher temperatures with weak coupling between core and shell and larger configurational freedom ( lower free energy) than relaxed structures. We showed that in the considered range of temperatures (between 300 and 600 K) the gold clusters are not amorphous Instead, they exhibit a dynamical core−shell structure, with a cationic core (single Au for Au25 and mainly tetrahedral Au4 for Au38 and Au40) loosely bound to an outer anionic flexible shell.
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