S. J. Stranick, M. M. Kamna, and P. S. Weiss*
Surface Science 338, 41 (1995).
We have used ultrahigh vacuum scanning tunneling microscopy to study the structure and dynamics of benzene on Cu{111} at low coverage and low temperature. We find that at 77K adsorbed benzene molecules preferentially form two-dimensional (2D) solid structures along Cu substrate step edges while benzene molecules on terraces move freely across the surface as a 2D molecular gas. At the interface between the 2D solid and 2D gas phases we observe lateral diffusion, 2D adsorption, and 2D desorption. The motion of the benzene is detected by observing the partial and transient occupation of preferred adsorption sites at the interface. The outward growth of the 2D benzene solid is assisted by the modulations in the local density of states due to Cu{111} surface state electrons scattered from step edges and from the tightly bound benzene molecules. These modulations in the local density of states strongly influence the binding and dynamics of the benzene molecules and are also imaged using the scanning tunneling microscope. The relative strengths of the interactions at various step and terrace sites due to nanometer-scale variations in the surface electronic structure are discussed and the resulting benzene overlayer structures described.