Adsorption and Desorption of Stilbene from the Ag/Ge(111)-√3 Surface

Abstract

The adsorption and desorption of stilbene on Ag/Ge(111)-(√3 × √3)R30° (Ag/Ge(111)-√3) were investigated using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density functional theory (DFT). Both trans- and cis-stilbenes form a (2 × 1) overlayer structure on Ag/Ge(111)-√3 at a coverage of ∼1 ML. The STM images show parallel strips with three equivalent directions, indicating a self-ordered molecular structure. At a coverage of less than 1 ML, the TPD of cis-stilbene shows only one peak, attributed to submonolayer desorption. The TPD peaks are indistinguishable for desorption of trans-stilbene from the surface submonolayer and multilayer. This is due to the simultaneous desorption and/or thinning of adsorbed multilayers during the TPD process, as determined from the STM analysis of adsorbed trans-stilbene structures before and after annealing. The TPD traces fit the half-order kinetics for molecular desorption of stilbene from Ag/Ge(111)-√3 with desorption energies of 20.1 (cis-) and 21.3 kcal/mol (trans-), which are comparable with the calculated values using the DFT method. A plausible explanation for the stilbene desorption process on Ag/Ge(111)-√3 is proposed and discussed.