The STM can be employed in an electrolytic environment containing some Ag⁺ to direct the growth of silver nanostructures. The purpose of this paper is to elucidate the mechanism by which silver nanostructures grow in this experiment. The mechanism which we propose is based on the results of three types of experiments: Ex-situ electrochemistry of HOPG and platinum, in-situ STM studies of silver nanostructure growth, and Monte Carlo simulations of silver transport in the STM configuration.

The prominent features of the proposed mechanism for silver nanostructure deposition are summarized schematically below:

A) A freshly prepared polymer-coated platinum ultramicroelectrode (UME) is employed as the tip in a scanning tunneling microscope (STM). This tip is immersed in a dilute (0.5 - 2.0 mM Ag⁺) aqueous silver plating solution in the STM and a tunneling junction is established with the graphite basal plane surface, B) Upon application of a +20 mM imaging bias (tip vs. sample), the tip potential is brought to within approx. 40 mV of the silver Nernst potential, and silver under-potential deposition occurs at the platinum UME resulting in the deposition of approximately one silver atomic layer, C) While the tip is tunneling at a defect-free region of the graphite basal plane surface, two bias pulses are applied in rapid succession - the first of these causes the formation of a shallow pit in the graphite surface having typical dimensions of 100Å in diameter and one graphite atomic layer in depth, D) The second bias pulse (having a smaller amplitude) causes the desorption of silver from the UPD layer at the tip. Desorbed silver ion is transported across the tip-sample gap (by diffusion and migration) and is reduced at the edges of the pit - silver metal nucleates at the high coordination sites present at the step edges of the pit, E) With continued deposition of silver for approx. 40 microsec., silver ion in the proximity of the pit is completely consumed and a silver particle is produced having typical dimensions of 100-500Å in diameter and 5-40Å in height. Immediately following the second pulse, the platinum surface of the UME is partially depleted of silver and regeneration of the UPD layer requires a minute or so.