Core level photoemission studies of the interaction of pentacene with the Si(1 1 1) (7 × 7) surface

The chemical bonding interactions of molecular pentacene with the atomically clean Si(1 1 1) (7 × 7) surface were investigated by high resolution core level and valence photoemission spectroscopy. Thin films of pentacene were deposited from a thermal evaporator onto the atomically clean Si(1 1 1) surface in ultra-high vacuum at room temperature. Analysis of the Si 2p core level spectra reveal evidence of a strong chemical interaction between the first molecular layer and the surface with a chemically shifted component appearing on the lower binding energy side of the Si 2p core level. A 0.30 eV shift of the Si 2p core level towards the valence band maximum indicates that the bonding interaction between the molecules and the surface removes the bandgap states which pin the Fermi level midgap on the clean surface. Valence band spectra reveal that these interactions result in significant changes to the electronic structure of the chemically bonded molecular layer. Subsequent layers display the characteristic molecular orbital spectrum of the pentacene molecule. A reduction in the workfunction can be attribute to the formation of an interface dipole which reaches a maximum of 0.55 eV for a monolayer coverage.