Diffusion of small two-dimensional Cu Islands on Cu(111)

TL;DR

This study investigates the diffusion behavior of small two-dimensional copper islands on Cu(111) surfaces using a novel self-learning kinetic Monte Carlo method, revealing how different microscopic processes influence diffusion barriers and island mobility.

Contribution

It introduces a self-learning Kinetic Monte Carlo approach that automatically builds a database of diffusion processes, including those involving hcp sites, to better understand island diffusion mechanisms.

Findings

Diffusion barriers increase almost linearly with island size when concerted motion is included.

Presence of hcp site processes significantly affects diffusion behavior.

Diffusion coefficients do not show clear size-dependent scaling at various temperatures.

Abstract

Diffusion of small two dimensional Cu islands (containing up to 10 atoms) on Cu(111) has been studied using the newly developed self-learning Kinetic Monte Carlo SLKMC method. It is based on a database of diffusion processes and their energetics accumulated automatically during the implementation of the SLKMC code. Results obtained from simulations in which atoms hop from one fcc hollow site to another are compared with those obtained from a parallel set of simulations in which the database is supplemented by processes revealed in complementary molecular dynamics simulations at 500 K. They include processes involving the hcp (stacking-fault) sites, which facilitate concerted motion of the islands. A significant difference in the scaling of the effective diffusion barriers with island size is observed between the two cases. In particular, the presence of concerted island motion leads to an almost linear increase in the effective diffusion barrier with size, while its absence accounts for strong size-dependent oscillations and anomalous behavior for trimers and heptamers while the diffusion coefficients (calculated at 300 K, 500 K, and 700 K) do not display any characteristic scaling with size. We also identify and discuss in detail the key microscopic processes responsible for the diffusion and examine the frequencies of their occurrence, as a function of island size and substrate temperature.