Luminescence Mechanisms of Quaternary Zn-Ag-In-S Nanocrystals: ZnS:Ag, In or AgInS2:Zn?

Abstract

Highly emissive Zn-Ag-In-S nanocrystals have attracted attention as derivatives of I-III-VI2-type nanocrystals without the use of toxic elements. The wide tunability of their luminescence wavelengths is attributed to the controllable bandgap of the solid solution between ZnS and AgInS2. However, enhancement of the photoluminescence quantum yield (PL-QY) depending on the chemical composition has not been elucidated. Here, the luminescence mechanisms of Zn-Ag-In-S nanocrystals were studied from the perspective of ZnS doped with Ag and In, although previous research has proposed a hypothesis that Zn is a radiative recombination centre in the AgInS2 host. The Zn-Ag-In-S nanocrystals were synthesized by systematically varying the Zn, Ag, and In contents. The nanocrystals exhibit a structure in which a part of the Zn in the cubic ZnS is substituted with Ag and In. Luminescence was ascribed to a donor-acceptor pair (DAP) recombination between electrons trapped in In donors and holes trapped in Ag acceptors. The composition-dependent enhancement of PL-QYs was attributed to an increase in donor and acceptor concentrations. The DAP characteristics were maintained over a wide range of Ag and In contents because of the localized character of the band edge states dominated by Ag and In orbitals, as suggested formerly by simulation.