Atomic diffusion inside a STM junction: simulations by kinetic Monte Carlo coupled to tunneling current calculations

The influence of a static scanning tunneling microscope (STM) tip on the diffusion of xenon atoms adsorbed on a Cu(1 1 0) stepped surface is studied. Semi-empirical potentials for the Xe-surface interaction and a N-body energy based method for the Xe-tip contribution are used to calculate the adsorption energy of adsorbates in the STM junction. First, we analyse the variation of this energy when the adatom is placed near a step edge and for different tip positions. When the tip is situated in the neighbourhood of the step edge, the Ehrlich-Schwoebel barrier experienced by the adatom is lowered. This opens a specific diffusion channel, allowing a possible crossing of the step edge. Second, through a kinetic Monte Carlo approach coupled to the elastic scattering quantum chemistry method, the noisy tunneling current created by the random motion of diffusing atoms in the vicinity of the tip can be analyzed. We show that, by counting the number of diffusion events, we can determine effective barriers related to the most dominant processes contributing to the diffusion at a particular temperature. We also demonstrate that the interaction mode of the tip (attractive or imaging) greatly modifies the diffusion processes.