Vapor-deposited gold film formation on highly oriented pyrolitic graphite. A transition from pseudo-two-dimensional branched island growth to continuous film formation

Abstract

Growth of vapor-deposited gold islands on highly oriented pyrolitic graphite (HOPG) using evaporation rates in the range 0.01–0.1 nm/s has been investigated with ex situ scanning tunneling microscopy. Equivalent mean gold thickness and substrate temperature were varied from 1 to 20 ML and 333 to 358 K, respectively. Gold ad-islands several layers high grow both atop large HOPG terraces and along HOPG steps. Images show split and stable tip branched islands, depending on individual ad-island height and equivalent mean gold thickness. Initially, the diffusion of gold atoms towards island edges occurs via the HOPG surface, whereas soon thereafter it includes diffusion via gold terraces and steps. These two very dissimilar possibilities are due to both misfit strain variation with local gold-deposit thickness, and large differences in sticking probabilities of gold on HOPG and gold. Addition of gold to islands more than 20 layers high produces island-top flattening, branch coarsening with pronounced facetting, and tip stabilization, as a result of diffusion of gold from island tops to valleys and interbranch spaces. Extended flat areas present monoatomic-high triangular gold terraces. The transition from split to stable-tip branched islands is associated with a surface diffusion barrier which decreases with island height as a result of reduced lattice mismatch. Seemingly, early stages of branched gold island formation can be assimilated to a deposition, diffusion, and aggregation-type model that nucleates surface atom at island or terrace edges. Once flat gold terraces set in, the growth process can be described by an Edwards–Wilkinson-type model. Coalescence of gold ad-islands more than 20 layers high explains the well-known continuous films obtained in metal coaters under comparable growth conditions.