Jj. Cavaleri et al., FEMTOSECOND STUDY OF THE SIZE-DEPENDENT CHARGE-CARRIER DYNAMICS IN ZNO NANOCLUSTER SOLUTIONS, The Journal of chemical physics, 103(13), 1995, pp. 5378-5386
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.