Carrier relaxation dynamics and photoluminescence efficiency in semiconductor quantum dots

Summary form only given. Due to the discrete structure of energy levels and an enhanced surface-to-volume ratio, energy relaxation and recombination dynamics in colloidal quantum dots (QDs) are significantly different from those in bulk materials. Energy relaxation leads to a fast establishment of quasi-equilibrium populations of electron and hole quantized states (sub-ps time scale). Depopulation of these states occurs via a variety of radiative and nonradiative mechanisms. The competition between these mechanisms determines the efficiency of the QD phololuminescence (PL). The most important processes competing with radiative recombination are carrier trapping at defect states and Auger recombination. At low pump intensities (less than one excitation per dot on average), the role of Auger effects is negligible and nonradiative carrier losses are dominated by surface trapping. We study the effect of surface/interface states on carrier dynamics and PL efficiency for strongly confined CdSe QDs (radius R=1-4 nm) with a range of surface properties. Our data indicate that significant changes in the PL quantum yield observed in samples with different surface passivations are primarily due to changes in hole relaxation.