Time-Dependent Tunneling Spectroscopy for Studying Surface Diffusion Confined in Nanostructures

By confining a diffusion atom in a nanometer region using surface potential heterogeneity, we have successfully employed a time-dependent tunneling spectroscopy to quantitatively study its random motion. A hopping rate in the range of 1--10⁴ Hz, ~3 orders of magnitude faster than those accessible by the existing diffusion methods based on scanning tunneling microscopy, was demonstrated for single Cu atoms diffusing in the faulted half unit cell of Si(111)-(7×7). Our technique is potentially useful to detect fast diffusion processes such as H quantum diffusion at atomic scale.