Radiative dynamics of AgIn_(1−x)Ga_xS₂ quantum dots

We present and analyze the static spectroscopy and radiative dynamics of very high quality, well characterized silver indium gallium sulfide quantum dots. We find that the static spectroscopy and radiative dynamics are very different from those in II-VI and III-V quantum dots. The absorption spectrum is broad and featureless, but sharp, high quantum yield (90%) band edge photoluminescence is observed. Despite the spectrally sharp photoluminescence, the photoluminescence decays are long and nonexponential, having components ranging from 40 to 150 ns. The spectroscopy and radiative kinetics are understood in the context of the radial composition profile of each element, as determined by energy dispersive x-ray spectroscopy line profiles. The radially dependent compositions show that the core is largely Ag₂S with the fraction of gallium increasing with radial distance. We suggest that random spatial fluctuations in the local silver concentration localize holes at the most silver-rich regions of the particle. The lowest energy transition is nominally parity forbidden and the parity selection rule is relaxed in the random alloy crystal environment. The sites at which holes localize have varying degrees of local crystal asymmetry and varying magnitudes of internal electric fields. Both types of perturbations can break the local symmetry and thereby relax the parity selection rule, giving rise to inhomogeneity in the radiative rates. We also consider the possibility that the nonexponential photoluminescence decay kinetics can be explained by a delayed emission model but consider this to be less likely than the inhomogeneous radiative rate model.