Measurements of surface diffusion depend on the state of the system whether the state was equilibrium versus non-equilibrium. Equilibrium experiments carried out in 2-d overlayers measure the collective diffusion coefficient Dc and could test theoretical predictions in two-dimensional statistical mechanics. Growth experiments typically carried out at low temperatures and/or high flux rates probe systems under non-equilibrium conditions where novel diffusion mechanisms could potentially exist. The use of STM to study both types of measurements was discussed. Dc could be measured from the autocorrelation of time-dependent tunneling current fluctuations generated by atom motion in and out of the tunneling area. Controlled experiments as function of temperature, coverage and tip–surface separation confirmed that the signal was diffusive. For growth experiments the unusually uniform height island (for Pb/Si(111) In/Si(111)) has revealed a novel and intriguing type of diffusion at low temperatures that accounts for the high degree of the self organization. By monitoring the evolution of the stable islands out of a mixture of stable and unstable islands the unusual role of the wetting layer surrounding the growing islands was revealed.

Surface Diffusion Experiments with STM: Equilibrium Correlations and Non-Equilibrium Low Temperature Growth. M.C.Tringides, M.Hupalo: Journal of Physics - Condensed Matter, 2010, 22[26], 264002