Enhancement Mechanism of the Photoluminescence Quantum Yield in Highly Efficient ZnS–AgIn₅S₈ Quantum Dots with Core/Shell Structures

Abstract

The optical properties of ZnS–AgIn5S8 quantum dots (QDs) with core/shell structures are examined to clarify the enhancement mechanism of the photoluminescence (PL) quantum yield (QY). Two types of QDs are synthesized by varying the concentration of zinc precursors, with alloyed-core (ZnS–AgIn5S8, ZAIS), inner-shell (ZnIn2S4, ZIS), and outer-shell (ZnS) structures, such as ZAIS/ZIS/ZnS and ZAIS/ZnS. Upon alloying/shelling processes from the preformed AgIn5S8 QDs, the evolution of the band gap energy indicates the formation of the solid solution of ZAIS. Due to the difference in the degree of alloying between ZAIS/ZIS/ZnS and ZAIS/ZnS QDs, the blue shift of PL, Stokes shift, and QY are different. The alloying/shelling processes improve the QY of the intrinsic defect states more effectively than the QY of the surface defect states, while the time-resolved studies suggest that the enhanced radiative rate of the intrinsic states is responsible for the improvement of the QY, in addition to the reduced nonradiative rate. In ZAIS/ZIS/ZnS QDs, the QY increases to 85%, which is attributed to the existence of the ZIS layer, as well as the reduced nonradiative states and the enhanced radiative states by the alloying/shelling processes. The ZIS layer mitigates the lattice strains and provides the appropriate levels of the electronic structures in the QDs, which further reduces the nonradiative rate and enhances the radiative rate, respectively, leading to the unprecedentedly high PL QY of ZAIS/ZIS/ZnS QDs.

FRET-Based Analysis of AgInS2/ZnAgInS/ZnS Quantum Dot Recombination Dynamics

“…Water-soluble negatively-charged AIS/ZAIS/ZnS quantum dots, which we used throughout the work, were prepared in three steps, as was described in [ 22 , 69 ]: (1) synthesis of AIS cores in the organic solvent; (2) two-step shell growth; (3) ligand exchange with L-cysteine. According to the literature, ZnS shell growth over the ternary QD cores may lead to partial alloying of the original particles [ 12 , 70 ]. Therefore, we describe the inner ZnS shell in terms of the ZAIS alloy layer, while the additional outer ZnS shell eliminates surface defects and ensures localization of the electron-hole pair in the QD core.…”

“…To improve the luminescent properties of the QDs, a wide-bandgap material shell, most commonly ZnS, can be grown to localize the exciton in the QD core. It has been shown, that the growth of the shell reduces the overall contribution of surface defects to the ternary QD luminescence, and consequently the contribution of the shorter lifetime component to the total decay curve [ 10 , 65 , 70 ]. Moreover, the small lattice mismatch between AIS and ZnS crystal structure favors the interdiffusion of their components, what leads to formation of a 2–3 monolayer ZnAgInS (ZAIS) alloy with the variable composition at the junction with the core [ 14 , 70 ].…”

“…In our case, zinc primarily replaces silver when embedded into the AIS lattice, so the Ag:In ratio decreases. Despite the AIS and ZnS structures are close, they still differ in lattice parameters by several percent [ 70 ]. As a result, structural stresses are created at the interface, that can also lead to new recombination channels and variations in the decay times.…”

“…All three lifetime components increase with the wavelength. The shortest lifetime ( ), which is usually attributed to the surface defect-related emission [ 10 , 12 , 65 , 70 ], has the smallest contribution of 5–10%. Since our QDs were covered with the double-shell, it is reasonable that the surface defect-related component did not contribute significantly to the overall emission.…”

“…As a result, the contribution (i.e., the relative number of photons emitted) of the alloy-related recombination channel is higher than of the core-related. Moreover, alloys with higher zinc content emit light at higher energies than the pure AIS [ 70 ], so the relative contribution of decreases towards the end of the spectrum, while increases ( Figure 4 b, cyan and dark blue trend lines, respectively). Notably, both the lifetimes and the relative contributions of our AIS/ZAIS/ZnS QDs are essentially similar to that of quaternary ZnAgInS QDs, according to the publication by Chevallier et al [ 12 ].…”