Ultrafast terahertz photoconductivity in nanocrystalline mesoporous TiO2 films

Terahertz time-resolved spectroscopy is used to investigate the transport of photoexcited electrons in nanocrystalline mesoporous TiO2 films prepared by the recently proposed “brick and mortar” technology [Szeifert et al.Chem. Mater. 21, 1260 (2009)] with a variable fraction of nanocrystalline titania “bricks” and amorphous titania “mortar.” Both long- and short-range conductivity is significantly enhanced upon calcination. After an ultrafast (subpicosecond) regime where the intrananograin conductivity dominates, the electron conductivity becomes limited by the interaction of electrons with the amorphous mortar. Comparison of the experimental results with Monte Carlo simulations of the electron motion allows us to determine the crystalline grain size after calcination and the yield of mobile photocarriers.