Mechanisms of radiation-induced structural transformations in deposited gold clusters

TL;DR

This paper investigates how electronic excitations induced by electron beams cause structural transformations in gold clusters, combining theoretical analysis and molecular dynamics simulations to explain experimental observations.

Contribution

It demonstrates that plasmon excitations triggered by secondary electrons can drive structural changes in deposited gold clusters, a novel insight into beam-induced transformations.

Findings

Plasmon excitations can induce structural transformations in gold clusters.

Transformations from icosahedral to fcc structures are driven by vibrational excitations.

Theoretical and simulation results align with experimental observations.

Abstract

Physical mechanisms of structural transformations in deposited metallic clusters exposed to an electron beam of a transmission electron microscope (TEM) are studied theoretically and computationally. Recent TEM experiments with size-selected Au$_{923}$ clusters softly deposited on a carbon substrate showed that the clusters undergo structural transformations from icosahedron to decahedron and face-center cubic (fcc) structures upon exposure to a 200-keV electron beam. In this paper, we demonstrate that the relaxation of collective electronic (plasmon) excitations formed in deposited metal clusters can induce the experimentally observed structural transformations. Such excitations in the clusters are formed mainly due to the interaction with low-energy secondary electrons emitted from a substrate. The characteristic occurrence times for plasmon-induced energy relaxation events are several orders of magnitude shorter than those for the momentum transfer events by energetic primary electrons to atoms of the cluster. The theoretical analysis is complemented by molecular dynamics simulations, which show that an icosahedral Au$_{923}$ cluster softly deposited on graphite is transformed into an fcc-like structure due to the vibrational excitation of the cluster.