Electronic structure, chemical interactions and molecular orientations of 3,4,9,10-perylene-tetracarboxylic-dianhydride on TiO2(110)

The electronic structure, molecular orientations, and interfacial energy level alignment of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules on rutile TiO2(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 TiO2 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 TiO2 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/TiO2 interface could facilitate the interfacial charge delocalization, which is desirable for making PTCDA-based dye-sensitized solar cell (DSSC) devices