Control of Crystal Growth in Local Electroless Gold Deposition by Pyridinium Based Surfactants

The formation and local deposition of well-shaped Au nanostructures on a nonconducting surface are described. Specifically, the local electroless deposition of Au in aqueous solutions in the presence of various n-alkylpyridinium surfactants is driven by electrochemically generating a flux of AuCl₄^- at a gold tip close to a 3-mercaptopropyltrimethoxysilane modified Si oxidized wafer. Two reducing agents, NaBH₄ and ascorbic acid, were used for the reduction of the gold ions. We studied the effect of the solution temperature, the potential applied to the gold tip and its distance from the surface, the reductant, and the nature of the alkylpyridinium on the structure of the gold deposit. The chloride salts of methylpyridinium, butylpyridinium, cetylpyridinium, 4-carbamoyl-1-cetylpyridinium, and 4-methyl-1-cetylpyridinium were added separately and showed remarkable effect on the shape of the structures that were formed. We find that short chain n-alkylpyridinium salts do not adsorb preferentially on the gold facets, whereas the longer chain n-alkylpyridinium ions cause the formation of well-faceted Au structures, such as cubes, hexagons, and even multipods. Moreover, comparison between local and bulk deposition revealed a significant difference in Au structures that were formed, presumably due to the different concentration profile of the AuCl₄^-.