Scanning tunnelling microscopy was used to study the clean (001) surface, using sample bias voltages of about -0.4V. Under this bias, many dimer vacancies were found to be highlighted by a bright feature on the neighboring dimers. In the case of other defects, this situation was reversed and the area around the defect became dark at low voltages. In both cases, this contrast disappeared at bias voltages of about -0.8V. First-principles calculations of charge density, as a function of energy, were used to simulate scanning tunnelling microscopic images of proposed structures for the single dimer vacancy. These images exhibited a significant bias-voltage dependence, and the low-bias voltage images differed markedly between the modelled structures. By this means, an identification of the re-bonded structure with the bright defect, and of the non-rebonded structure with the dark defect, was made.

J.H.G.Owen, D.R.Bowler, C.M.Goringe, K.Miki, G.A.D.Briggs: Surface Science, 1995, 341, L1042-7