Single molecule diffusion at step edges

We have tracked the motion of single Rhodamine 123 molecules in tetrakis-(2-ethylhexoxy)-silane close to a microscopic surface step on freshly cleaved mica sheets. Introducing a local directional measure of the mobility of single molecules we are able to unravel the anisotropy of single molecule diffusion close to the surface step. Direct comparison of the experimental results with theoretical calculations reveals a strong effective trapping potential on the order of a few 100 meV. This potential confines molecular motion into a thin layer close to the step surface, in which the mobility of molecules is slowed down by one order of magnitude as compared to the flat surface.