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

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.