Atomistic modelling of electron beam induced structural transformations in deposited metal clusters

Structural transformations in gold clusters deposited on a graphite substrate induced by the focused electron beam of a scanning transmission electron microscope are investigated using the classical molecular dynamics (MD) approach. The particular case study concerns Au₃₀₉ clusters softly deposited on few-layer graphite and exposed to a 300 keV electron beam. Two mechanisms of energy transfer to the cluster during the irradiation are considered: (i) through the relaxation of collective electronic excitations and (ii) through the momentum transfer by the energetic primary electrons. A relativistic MD approach implemented in the MBN Explorer software package is used to simulate the collisions of energetic primary electrons with cluster atoms and to evaluate the amount of energy transferred to the cluster for different collision geometries. Characteristic times for the occurrence of these energy deposition events are estimated for realistic experimental irradiation conditions. The MD simulations of the cluster dynamics after irradiation show that the cluster temperature decreases rapidly during the first few tens of picoseconds, and the cluster cools down to a temperature close to its initial temperature within several hundred picoseconds. This time period is comparable to the characteristic time between two successive energy transfer events induced by plasmon excitations in the deposited cluster. A large number of successive energy transfer events (on the order of ∼10³-10⁴) during irradiation can cumulatively lead to substantial heating of the deposited cluster and induce its structural transformations.