Enhancement of biexciton generation via resonance energy transfer in nanoclusters of colloidal CdTe quantum dots for optical gain applications

Abstract

Colloidal CdTe quantum dots (QDs) of two different sizes (2.7 nm and 3.7 nm diameter) were self-assembled into mixed quantum dot donor-acceptor nanoclusters (QDANs) in aqueous solution. The ratio of small QDs (SQDs) to large QDs (LQDs) was 10:1, and the assembly was mediated by electrostatic attraction between oppositely-charged ligands (thioglycolic acid, TGA, and mercaptoethylamine, MA) on the QD surfaces. Under low excitation fluence (well below the threshold for biexciton formation), photoluminescence (PL) emission spectra and time-resolved decays in the QDANs revealed quenching in the SQDs and enhanced emission from the LQDs, providing evidence for resonance energy transfer (RET) from the wider band gap SQDs to the narrower band gap LQDs. The observed RET rate constant, determined from comparison of decays of free SQDs and those in the QDANs, was (3.4 ns)−1. Under these low-fluence conditions, the PL emission lifetimes of the LQDs in QDANs were the same as the free LQDs. Under higher excitation intensity, the decay of the LQDs in the QDAN showed a new, fast (sub-nanosecond), fluence-dependent component, suggesting formation of biexcitons and multiexcitons on the LQDs. The free LQDs required significantly (approximately 4-fold) higher excitation intensity than those in the QDANs to show similar evidence of biexciton formation through lifetime shortening. Since the QDANs contained a 10-fold excess of SQDs, the excitation energy that was initially distributed over the full population of QDs was funneled and concentrated into the small number of LQDs. These results support the idea that optical gain through biexciton formation can be enhanced by using RET, even when the rate constant for RET is much lower than that for biexciton decay.