A molecular monolayer adsorbed at the interface between the basal plane of graphite and an organic solution of didodecylbenzene has been observed in situ by scanning tunneling microscopy at a resolution of 0.2 nm. The monolayer forms a 2D polycrystal with crystallite sizes in the range of a few nanometers. The predominant domain boundary is described by a glide plane. Fast image recording allowed, for the first time, direct observation of the molecular dynamics on the time scale of 100 ms. In particular, the motion of domain boundaries could be associated with diffusing ‘‘free volume.’’

• Received 30 November 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.2096