Femtosecond study of the size-dependent charge carrier dynamics in ZnO nanocluster solutions

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Abstract
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.