Enhancing Dielectric Screening for Auger Suppression in CdSe/CdS Quantum Dots by Epitaxial Growth of ZnS Shell

Abstract

Auger recombination is the main nonradiative process in multicarrier states of high-quality quantum dots (QDs). For the most-studied CdSe/CdS core/shell QDs, we effectively reduce the biexciton Auger rate by enhancing dielectric screening of band-edge carriers via epitaxial growth of additional ZnS shells. Super volume scaling of negative-trion Auger lifetime for CdSe/CdS core/shell QDs is achieved with the outermost ZnS shells. The volume of CdSe/CdS/ZnS QDs can be less than half that of CdSe/CdS QDs with the same negative-trion Auger lifetime. Auger suppression by the ZnS shells is more pronounced for QDs with wave functions of band-edge carriers spreading close to the inorganic–organic interface, such as CdSe/CdS QDs with small cores. A maximum drop of biexciton Auger rate of ∼50% and a maximum enhancement of biexciton emission quantum yield of 75% are achieved. Auger engineering by dielectric screening opens up new opportunities to improve the emission properties of multicarrier states in QDs.

Optical properties of fully inorganic core/gradient-shell CdSe/CdZnS nanocrystals at the ensemble and single-nanocrystal levels

“…As these ratios are close to the factor of 2 expected for the negatively charged exciton [61][62][63], we tentatively assign τ 2 to the emission of X 1− , with a QY comparable to that of X 0 for both QD species, compatible with the measured emission quantum yields (QY) of 70 % (OA) and 50 % (halide). Large NCs have in fact been shown to have preferred emission through the X 1− channel [55][56][57], consistent with the dominant contribution (60 % of detected photons) of this component in our measured decay curves. The linear power-dependence with saturation of the number of detected photons attributable to τ 2 , Fig.…”

“…4 c and g, again suggesting single-exciton emission. As it has been surmised previously that the emission lifetime of the positively charged exciton X 1+ is shorter than that of X 1− (our τ 2 candidate) in CdSe/CdS [62], CdSeS/ZnS NCs [40] and CdSe/CdS/ZnS [57], we assign τ 3 to the emission of X 1+ . Finally, the number of photons detected from the fastest component (τ 4 < 0.5 ns) are the only ones whose power dependence is a convex function, see Fig.…”

“…Surprisingly, the lifetimes found for OA-and halide-capped QDs were very similar and did not change much for each species going from solution to thin films. This invariability of the lifetimes makes it seem unlikely that coupling exists for the halide-capped NCs, contrary to what one might have expected for QDs whose small ligands allow for tight packing in a thin-film sample [54]; given that QD dynamics usually depend on their surface states [55][56][57], a possible explanation for the observed lifetime robustness is a large potential barrier that protects the exciton from environmental influence, leading to a confinement closer to type I than quasi-type II. The relative weights of the decay components, on the other hand, were affected by the OA-tohalide ligand exchange: While component τ 2 remained dominant and accounted for about 60 % of detected signal photons from both QD species, the weight of the longest time τ 1 was reduced in the halide-capped NCs with a concomitant increase (about four-fold) of the strength of the two fastest components τ 3 and τ 4 .…”

Band Alignment Engineering of Semiconductor Nanocrystal Heterostructures Toward Emerging Applications

“…In previous studies, tuning the rates of Auger recombination could be mostly achieved by changing the size of the core and shell, but the huge volumes and reduced spectral tunability would limit applications in some aspects. Peng et al [ 47 ] reported an additional layer of ZnS shell with a large bandgap upon CdSe/Cds core/shell NC, the outermost shell can simultaneously restrict both conduction band minimum and valance band maximum wave functions away from the interface and enhance dielectric screening for Auger suppression without altering delocalization of the wave functions by changing the size of NCs (the band alignment is shown in Figure a). They synthesized CdSe/4CdS/2ZnS core/shell/shell QDs, which exhibited sharp absorption peak features and narrow emission peak, and also possessed near unity PL QY (92%) and monoexponential PL decay dynamics at a single‐exciton regime.…”

“…Reproduced with permission. [ 47 ] Copyright 2021, American Chemical Society. d) Schematic illustration of the band alignment of CdS/ZnSe/ZnS heterostructures.…”