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The dynamics of charge carrier trapping and recombination are measured as a function of ZnO cluster diameter by ultrafast pump–probe absorption spectroscopy. A finite spherical potential well model which shows good agreement with previous experimental work is employed to predict ZnO cluster diameters from absorption onsets. The rate of electron trapping is measured for clusters of 3.2 and 6.2 nm, and is found to increase with increasing cluster size. This increase in trapping rate for increasing cluster size is not consistent with either a diffusional or quantum mechanical picture of electron trapping. A mechanism for electron trapping involving trap-to-trap hopping is discussed whereby the number density of optically accessible deep traps must increase with increasing cluster size. Differences in the dynamics and in the ratio of interior to exterior atoms on the cluster are correlated and discussed. The time-resolved absorption data of the subsequent electron–hole recombination shows the appearance of an early time signal which increases as the cluster size grows. The early time species decays away within the first 50 ps to a diameter-independent plateau value via second-order recombination, and is assigned to electrons trapped in the interior of the cluster. The electron–hole recombination is found to occur faster and to a greater extent in the largest nanoclusters.

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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 electrons 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) × 10-10 cm3/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.