Boosting quantum efficiency and suppressing self-absorption in CdS quantum dots through interface engineering

Applications of photoluminescence (PL) from semiconductor quantum dots (QDs) have faced the dichotomy of excitonic emission being susceptible to self-absorption and shallow defects reducing quantum yield (QY) catastrophically, and doped emissions sacrificing the tunability of the emission wavelength via a quantum size effect, making it extremely challenging, if not impossible, to optimize all desirable properties simultaneously. Here we report a strategy that simultaneously optimizes all desirable PL properties in CdS QDs by leveraging interface engineering through the growth of two crystallographic phases, namely wurtzite and zinc blende phases, within individual QDs. These engineered interfaces result in sub-bandgap emissions via ultrafast energy transfer (∼780 fs) from band-edge states to interface states protected from surface defects, enhancing stability and prolonging the PL lifetime. These sub-bandgap emissions involving the interface states show a high Stokes shift, significantly reducing self-absorption while achieving near-ideal quantum efficiencies (> 90%); we also achieved extensive emission tunability by controlling the QD size without sacrificing efficiency. Theoretical calculations confirm that the interface states act as planar antennas for an efficient energy transfer from the bandgap states, while the extended nature of these states imparts tunability via quantum confinement effects, underpinning remarkable optical performance. This interface-engineered approach offers a powerful strategy to overcome limitations in QD-based optoelectronic applications.