Abstract
Optical gain in ultrasmall semiconductor nanocrystals requires that some of the nanoparticles in the ensemble be excited with multiple electron−hole pairs (multiexcitons). A significant complication arising from this multiexciton nature of optical amplification is the ultrafast gain decay induced by nonradiative Auger recombination. Here, we develop a simple model for analyzing optical gain in the nanocrystals in the presence of exciton−exciton (X−X) interactions. This analysis indicates that if the X−X interaction is repulsive and its energy is large compared to the ensemble line width of the emitting transition, optical gain can occur in the single-exciton regime without involvement of multiexcitons. We further analyze theoretically and experimentally X−X interactions in type-II heteronanocrystals of CdS (core)/ZnSe (shell) and ZnTe (core)/CdSe (shell) and show that they can produce giant repulsion energies of more than 100 meV resulting from a significant local charge density generated as a result of s...
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