Energy level alignment and two-dimensional structure of pentacene on Au(111) surfaces

Abstract

X-ray photoemission, ultraviolet photoemission spectroscopy (UPS), and scanning tunneling microscopy (STM) have been used to determine the energy level alignment and the molecular ordering of monolayer and submonolayer pentacene films on Au(111) in ultrahigh vacuum. Pentacene evaporated onto the van der Waals surface of SnS2 was used as a noninteracting substrate for comparison. A large interface dipole was measured for pentacene on Au(111) (0.95 eV) whereas pentacene on SnS2 showed a relatively small interface dipole (0.26 eV). The different interface dipoles are related to the different orientations of the pentacene molecules due to different pentacene substrate interaction energies. Differences in the UPS spectra also support changing molecular orientations of the two substrates. STM images of pentacene on Au(111) revealed that the molecules lay flat on the substrate and are oriented parallel to each other, forming striped structures that are commensurate with the Au(111) lattice. The pentacene coverage influences the packing of the striped structures that can form a variety of unit cells. Three related unit cells with pentacene molecules tilted [(2×2√7), (2×√31), and (2×√39)] or perpendicular (2×3√3) to the row direction were identified on Au(111).