An STM investigation of surface diffusion on iodine modified Au(111)

Step edge fluctuations on Au(111) electrodes have been investigated using STM in electrolytes containing iodide, as a way of monitoring surface diffusion. The influence of the iodine adsorbate on the gold step edge mobility and diffusion of the Au surface is analysed. It was found that there is a sudden increase in step edge fluctuations upon iodide adsorption followed by a local minimum when an ordered iodine ad-layer is formed. The time correlation function of step edge fluctuations has been analysed using established methods to reveal the diffusion mechanism at different electrode potentials. As a result of this analysis it was found that the mechanism of diffusion in the disordered phase involves the movement of gold atoms (assisted by the 2D iodine gas on the terrace) from step edges out onto terraces. By contrast, evidence was found that diffusion is predominantly restricted to the step edge as soon as an ordered ad-layer is formed. Images show that this ad-layer packs closely to the step edge, thus largely restricting mobility of Au to the immediate vicinity of the step edge. In this case it is suggested that diffusion along the step edge involves a co-operative movement of gold and iodide.