Unconventional low-temperature luminescence kinetics in micro- and nanopowders of the anatase phase of titanium dioxide

It is shown that the low-temperature (5 K) decay kinetics of the known luminescence band of titanium dioxide (anatase) in the range of 2.0–2.5 eV has a power-law character. For microcrystals, this behavior is observed in a wide range of delay times from $\sim$20 ns to 1 ms. The instantaneous luminescence decay time at the end of this range is as long as $\sim$100 $\mu$s. A simple model is proposed to reconstruct the statistics of the lifetime distribution of emitting states using the measured power-law decay curves. Within this model, the observed power-law decay kinetics is associated with radiative recombination mechanisms where a donor-bound electron recombines with an acceptor-bound hole. In crystals with sizes of $\sim$10 nm, power-law luminescence decay, characteristic of this mechanism, is also observed; however, the luminescence kinetics changes with time, which is explained by the nonradiative recombination of electron-hole pairs via surface states.