Sub-Picosecond Dynamics of Free Electrons in TiO2 Nanotubes

The sub-picosecond mobility and lifetime of free electrons injected into TiO2 nanotubes were probed with sub-picosecond temporal resolution using time resolved terahertz spectroscopy (TRTS). The photoconductivity spectra of the nanotubes revealed the free electrons to be very mobile, however most electrons appear to be trapped by a single kind of defect present at high concentration (> 1014 cm3) having a distinct resonance at 1.8 THz (7.5 meV). Synthesizing the nanotubes on the walls of a waveguide allows for probing the shallow defect structure of the nanotubes with even greater sensitivity than is possible in transmission though free space. Insights obtained by TRTS indicate the potential, and also reveal the challenges, of using TiO2 nanotubes and other semiconductor nanostructured materials as charge transport conduits in photovoltaic cells.