Interface state and energy level alignment of F4-TCNQ sandwiched between a pentacene film and the ethylene-terminated Si(1 0 0) surface

Abstract

The electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules sandwiched between a pentacene (Pn) film and the ethylene-terminated Si(100) surface were investigated using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS), and their effects on the morphology and the energy level alignment of the Pn film were investigated by atomic force microscopy (AFM) and polarization dependent ultraviolet photoelectron spectroscopy (UPS), respectively. The XPS and NEXAFS spectra show that F4-TCNQ molecules are involved in charge transfer with the surface, and they adsorb with tilt or random orientation. They consist of both neutral and anionic C≡N groups, and the neutral groups are still observed after Pn deposition, indicating the absence of charge transfer between F4-TCNQ and the Pn overlayer. We find that the Pn molecules take a standing-upright orientation on the ethylene/Si(100) surface. F4-TCNQ deposition on the ethylene/Si(100) surface induces some disorder in the Pn overlayer at the Pn thickness lower than 0.60nm. F4-TCNQ deposition leads to an increase in the vacuum level by 1eV, and thus a decrease in the hole injection barrier by 0.6–0.8eV. Thus, the decrease is mainly attributed to the charge transfer between the F4-TCNQ and the ethylene/Si(100) surface.