Local field effects on electron transport in nanostructured TiO2 revealed by terahertz spectroscopy

We study electron mobilities in nanoporous and single-crystal titanium dioxide with terahertz time domain spectroscopy. This ultrafast technique allows the determination of the electron mobility after carrier thermalization with the lattice but before equilibration with defect trapping states. The mobilities reported here for single-crystal rutile (1 cm²/(V s)) and porous TiO₂ (10^-2 cm²/(V s)) therefore represent upper limits for electron transport at room temperature for defect-free materials. The large difference in mobility between bulk and porous samples is explained using Maxwell-Garnett effective medium theory. These results demonstrate that electron mobility is strongly dependent on the material morphology in nanostructured polar materials due to local field effects and cannot be used as a direct measure of the diffusion coefficient.