Capturing transiently charged states at the C60/TiO2(110) interface by time-resolved soft X-ray photoelectron spectroscopy

Abstract

Time-resolved soft X-ray photoelectron spectroscopy is utilized to determine an energy level alignment and the photoexcited carrier dynamics at a C60/TiO2(110) interface. The interface electronic structure is characterized by a type II junction, which favors an injection of photoexcited electrons from C60 to TiO2. Ultraviolet (UV) laser pulse irradiation induces transient shifts of both C 1s and Ti 2p core levels towards the higher binding energies. These energy shifts are caused by a laser-induced charge transfer between the C60 layer and the TiO2(110) surface. Upon UV absorption, valence electrons of C60 are promoted to unoccupied levels, followed by a resonant transfer to TiO2, leaving C60 in a cationized state. On the TiO2(110) side, the electrons are injected into the conduction band to raise the carrier density so that downward bending of the TiO2 band is induced. The UV-excited states of C60 and TiO2 have sufficiently longer lifetime than the lifetime of the electron–hole pairs in solid C60. The C60/TiO2(110) interface is, thus, proved to be efficient for separating the electron–hole pairs generated within the C60 layer.