(413b) 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.