FEMTOSECOND STUDY OF THE SIZE-DEPENDENT CHARGE-CARRIER DYNAMICS IN ZNO NANOCLUSTER SOLUTIONS

The dynamics of charge carrier trapping and recombination are measured as a function of ZnO cluster diameter by ultrafast pump-probe absorpt ion 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 tra pping is measured for clusters of 3.2 and 6.2 nm, and is found to incr ease 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 elec tron trapping involving trap-to-trap hopping is discussed whereby the number density of optically accessible deep traps must increase with i ncreasing cluster size. Differences in the dynamics and in the ratio o f interior to exterior atoms on the cluster are correlated and discuss ed. The time-resolved absorption data of the subsequent electron-hole recombination shows the appearance of an early time signal which incre ases as the cluster size grows. The early time species decays away wit hin the first 50 ps to a diameter-independent plateau value via second -order recombination, and is assigned to electrons trapped in the inte rior of the cluster. The electron-hole recombination is found to occur faster and to a greater extent in the largest nanoclusters. (C) 1995 American Institute of Physics.