Homo- and hetero-epitaxial deposition of Au by surface limited redox replacement of Pb underpotentially deposited layer in one-cell configuration

Abstract

This work reports on homo- and hetero-epitaxial Au deposition realized by surface limited redox replacement (SLRR) of Pb underpotentially deposited (UPD) layer by [AuCl4]− ions in one-cell configuration. An over-stoichiometric amount of deposited Au hinted at by the electrochemical quartz-crystal micro balance (EQCM) kinetic experiments and validated by separate pulse experiments has been attributed to reduction in each SLRR event of a pre-adsorbed layer of [AuCl4]− ions. This phenomenology, known as “spontaneous deposition”, along with strict control over the Pb UPD coverage enables a fine tunable epitaxial Au deposition in the sub-monolayer to monolayer range. After as little as one replacement event on Pt (111) the deposit displays a Pb UPD cyclic voltammetry (CV) curve as would be observed on an Au (111) substrate. High quality epitaxial Au has been grown for up to 40 SLRR events as evidenced by CV and ex situ scanning tunneling microscopy (STM). However, growth of thicker films yields a significant surface roughness evolution of 2.5 times. The thicker deposit features a dendritic structure when viewed via scanning electron microscopy. X-ray photoelectron spectroscopy shows less than 0.1at% Pb in the Au deposit and rules out defect induced roughening. STM results suggest that the surface roughness is likely due to formation of Au islands that grow rapidly in vertical direction and serve eventually as seeds of the dendritic growth transition. Despite its complexity, SLRR of Pb by Au proves to be a viable method for quick and cost effective growth of continuous homo- or hetero-epitaxial Au ultra-thin films.