Electronic structure, chemical interactions and molecular orientations of 3,4,9,10-perylene-tetracarboxylic-dianhydride on TiO 2(110)

Abstract

The electronic structure, molecular orientations, and interfacial energy level alignment of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules on rutile TiO 2-(110) 1 × 1 surface have been investigated using synchrotron-based photoemission spectroscopy (PES) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The evolution of PES spectra as a function of PTCDA coverage is interpreted as possible chemical reactions that strongly couple the TiO 2 surface Ti and O atoms with PTCDA molecules. The emergence of an interfacial gap state observed at the lower binding energy side of the highest occupied molecular orbital (HOMO) of PTCDA corroborates the strong electronic coupling between PTCDA molecules and TiO 2 substrate. In addition, the molecular orientation of PTCDA is found to vary with coverage due to the strong interfacial interactions. It adopts a slightly tilting, disordered, and nearly lying-down geometry in the submonolayer, monolayer, and multilayer regimes, respectively. The strong electronic coupling and the relative energy alignment at the PTCDA/TiO 2 interface could facilitate the interfacial charge delocalization, which is desirable for making PTCDA-based dye-sensitized solar cell (DSSC) devices. © 2011 American Chemical Society.