Photoexcited hole dynamics in TiO2 nanocrystalline films characterized using a lens-free heterodyne detection transient grating technique

A lens-free heterodyne detection transient grating (LF-HD-TG) technique has been applied to study the ultrafast carrier dynamics of anatase and rutile TiO2 films made from TiO2 nanoparticles. The decay of photoexcited hole concentrations in TiO2 films can be directly detected using the technique, since the effective mass of holes in TiO2 is much smaller than that of electrons. We find that the relaxation of holes in anatase occurs via first-order kinetics as fast as 1 ps, which is attributed to a hole trapping process. However, the lifetimes of holes in rutile are found to be longer than hundreds of ps.