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Femtosecond visible-to-IR spectroscopy of TiO2 nanocrystalline films: dynamics of UV-generated charge carrier relaxation at different excitation wavelengths

The transient absorption of nanocrystalline TiO2 films in the visible-to-IR wavelength region was measured under UV excitation conditions at different wavelengths of 266 nm and 355 nm. Under weak 355 nm excitation, the generated charge carrier density could be reduced as low as the second-order electron-hole recombination process could be ignored as we reported previously (Y. Tamaki et al. Phys. Chem. Chem. Phys. 9, 1453-1460 (2007)). The result was compared with data obtained under 266 nm excitation, where the band-gap exaction was strong and efficient electron-hole recombination occurred due to the high charge carrier density. Taking into account the dynamics of the electrons and holes in the femtosecond to picosecond time range, such as ultrafast charge carrier trapping and slow deep trapping of electrons, intra-band relaxation in the conduction and the valence bands and intra-particle diffusion of electrons in the shallow trap levels were revealed.