Using Time-Resolved Thz Spectroscopy To Study Carrier Injection and Dynamics in TiO2 and SnO2 Nanomaterials

Charge injection from a variety of porphyrin-based dyes bound to TiO2 and SnO2 has been measured. The time scales and efficiencies are interpreted in terms of the identities (single vs. triplet) and energetic of excited electronic states. In addition, we have characterized the time-dependent conductivity after photoexcitation of dye-sensitized TiO2 nanotubes. It had been hoped that nanotubes would overcome low electron mobilities found in TiO2 nanoparticle films because the nanotubes can be many tens of microns long. However, recent macroscopic measurements found electron transport through nanotube and nanoparticle films to be comparable. Here we show that low electron mobility in polycrystalline TiO2 nanotubes is not due to scattering from grain boundaries but instead due to traps that manifest themselves in a single sharp resonance in the THz spectrum. The TiO2 nanotube spectra are fundamentally and qualitatively different than that for nanoparticles or the bulk material.