Role of coverage and vacancy defect in adsorption and desorption of benzene on Si(001)-2×n surface

We investigated the adsorption and desorption characteristics of benzene molecules on Si(001)-2×n surfaces using a variable-low temperature scanning tunneling microscopy. When benzene was adsorbed on a Si(001)-2×n surface at a low coverage,five distinct adsorption configurations were found: tight-binding (TB),standard-butterfly (SB),twistedbridge,diagonal-bridge,and pedestal. The TB and SB configurations were the most dominant ones and could be reversibly interconverted,diffused,and desorbed by applying an electric field between the tip and the surface. The population ratios of the TB and SB configurations were affected by the benzene coverage: at high coverage,the population ratio of SB increased over that of TB,which was favored at low coverage. The desorption yield decreased with increasing benzene coverage and/or density of vacancy defect. These results suggest that the interaction between the benzene molecules is important at a high coverage,and that the vacancy defects modify the adsorption and desorption energies of the benzene molecules on Si(001) surface.