Near-Unity Biexciton Emission Quantum Yield in CdS/CdSe/CdS Quantum Shells

Non-radiative Auger recombination is the primary multiexciton loss mechanism in colloidal nanocrystals and an impediment for prospective optoelectronic applications. Recent developments of new core/shell nanocrystal quantum dots (NQDs) with suppressed Auger recombination rates has opened the possibility for studying multicarrier states using single particle time-resolved photoluminescence (PL) spectroscopy. We recently developed a new class of semiconductor NQDs, CdS/CdSe/CdS “nanoshells” where excitons are localized in a large spherical CdSe shell, rather than in a small volume of the core of a conventional “core/shell” NQD. Here, we demonstrate that a quantum shell geometry strongly inhibits Auger processes, resulting in extraordinary improvements to biexciton (BX) quantum yield (QY) and multiexciton (trions and BX) lifetimes. From time-correlated PL of multistate blinking trajectories, the lifetimes and QY of neutral and charged excitons (trions) in individual dots are experimentally measured and a non-statistical scaling model is employed to calculate multiexciton QYs, radiative lifetimes and Auger decay rates. These results are corroborated by second-order correlation (“antibunching”) statistics of BX QY to confirm the validity of the scaling model. By varying the quantum well volume in CdS_bulk/CdSe/CdS quantum shells (CdS core diameter ranging from 4.5 to 8 nm), we demonstrate that larger-core nanoparticles exhibit the strongest suppression of Auger decay, corresponding to BX QY of nearly 100%. A combination of ultralong (>15 ns) BX emission lifetimes and strong exciton-exciton repulsion in large-core samples allowed demonstrating low-threshold amplified spontaneous emission (ASE) and large modal gain values for BX transitions. The observed combination of long BX lifetimes and large quantum yields makes the quantum shell geometry a promising candidate for light-emitting and lasing applications.