Measurement of the Internal Orbital Alignment of Oligothiophene-TiO2 Nanoparticle Hybrids

Abstract

X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and ultraviolet–visible spectroscopy (UV–vis) were used to investigate the electronic structure of the inner-molecule interface in oligothiophene-TiO2 organic molecule/nanoparticle hybrids. In these molecular structures the TiO2 nanoparticles were covalently bonded to the oligothiophene ligands. The discussed experiments utilized electrospray in-vacuum deposition, enabling the in situ characterization of these materials without the presence of significant ambient contamination. A comparison between the electronic structures of the individual molecular components of the hybrid with the assembled material enabled the determination of the orbital line-up at the internal interface of the hybrid molecules. Transmission electron microscopy (TEM) was used to investigate the morphology and size distribution of the nanoparticles. The results showed that the covalent bond causes an interface dipole at the junction between organic and inorganic parts of the hybrid. The dipole was determined to be 0.61 eV, and the hole injection barrier at the interface amounted to 0.73 eV. The electron injection barrier was estimated by taking into account the gap between highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO). This procedure suggested that there is only an insignificant barrier.