Ultrafast charge carrier dynamics of SnO2 nanoclusters:: A refined-interpretation of the electron-hole kinetics in metal oxides

Femtosecond experiments on 15 nm diameter SnO2 nanoclusters measure the elementary charge carrier reactions of electron trapping and electron-hole recombination. From the early time transient absorption data, an electron-trapping time of 200 +/- 20 fs is determined. In addition, an experimental scheme to determine the effect of electron thermalization on the relaxation of photoexcited elections is presented. Excess excitation energy above the conduction band increases the decay time to 500 +/- 50 fs indicating that thermalization plays an important role in the electron-trapping kinetics. The dynamics of charge carrier recombination are investigated by an ultraviolet pump intensity study. A second-order rate constant of (1.0 +/- 0.3) x 10(-10) cm(3)/s is found to fit all of the decays. The early time decay kinetics in metal oxide nanoclusters do not agree with a recently proposed fractal kinetic study but are consistent with trapped electron/free hole recombination. The assignment of the early time transient absorption at 620 nm to trapped electrons is supported by comparing the transient absorption kinetics to ground state recovery results in both SnO2 and TiO2 nanoclusters.