Composition effects on exciton recombination dynamics of blue-emitting alloyed Cd1−xZnxS/ZnS quantum dots

Abstract

The effects of composition on the intrinsic and trapping excitonic recombination in core/shell Cd_1−xZn_xS/ZnS quantum dots (QDs) with different x (x = 0.2, 0.3, 0.4, 0.5) are investigated using steady-state photoluminescence (PL), time-resolved PL, and transient absorption techniques. The PL emission peak blue-shifts from 460 to 410 nm with x increasing from 0.2 to 0.5, which means that the bandgap can be modulated by the cation ratio. The exciton binding energy, as determined by the temperature dependent PL intensity, increases from 62 to 90 meV with x increasing from 0.2 to 0.5 because of the lattice strain and dielectric polarization in the QDs. The PL dynamics is composed of a short-lived band-edge excitonic state and a long-lived shallow trapping state and therefore leads to the observation of biexponential decay kinetics. The average PL lifetimes decrease with increasing temperature from 80 to 220 K and then increase until 350 K, which is ascribed to the splitted exciton fine structure and thermal PL quenching. The multi-exciton Auger recombination dynamics mechanism is illustrated by sub-picosecond time-resolved transient absorption spectra considering the effects of composition and pump fluence. A positive absorption band is observed under high pump fluence and is assigned to the composition-related trapping state at the core/shell interface. We reveal that Cd_1−xZn_xS/ZnS QDs are a class of potential materials for application to blue-emitting displays, LEDs, and lasing devices, and the performances are tunable by controlling the cation ratio.