Trapping dynamics of electrons and holes in a nanocrystalline TiO2 film revealed by femtosecond visible/near-infrared transient absorption spectroscopy

The trapping dynamics of electrons and holes in TiO2 nanocrystlline films excited by ultraviolet laser pulses (266-nm wavelength) were studied with femtosecond visible/near-infrared transient absorption spectroscopy. UV irradiations of the TiO2 film generated hot carriers in the conduction and valence bands. The formation rate of deeply trapped holes was estimated to be 200 +/- 50 fs. The rate was limited by intraband relaxation (cooling) of hot holes. The spectral shift of the transient absorption indicated that trapped holes relax from shallow sites to deep ones. This relaxation was occurring more than 100 ps after photoexcitation.