Luminescence enhancement mechanisms of AgInS2/ZnS core/shell nanoparticles fabricated by suppressing quaternary alloying

Abstract

Nanoparticles of chalcopyrite semiconductors with I–III–VI2 compositions have attracted attention as potential non-toxic alternatives to highly luminescent Cd–chalcogenide semiconductor nanoparticles. Numerous studies have reported that chalcopyrite nanoparticles capped with ZnS shells exhibit enhanced luminescence efficiencies; however, neither the actual core/shell structure nor the origin of the enhanced emission has been explained. Moreover, luminescence enhancement in I–III–VI2 nanoparticles alloyed with Zn (e.g. Zn–Ag–In–S) complicates our understanding of the origin of enhanced luminescence efficiencies in core/shell nanoparticles. In this study, we found the optimal heating condition for fabricating ZnS shells around AgInS2 nanoparticles by suppressing the alloying of the core and shell components. For AgInS2 nanoparticles with an average diameter of 3.4 nm, maximum luminescence enhancement was achieved when ZnS shells were formed at 393 K. Analyses of the carrier recombination mechanisms indicate that the luminescence improvement was due to both passivation of the non-radiative recombination centres on the nanoparticle surfaces by the ZnS shells and the formation of additional radiative recombination centres induced by a slight inclusion of Zn2+ ions in the core/shell interface region. When ZnS coatings were applied at higher temperatures, AgInS2–ZnS alloy nanoparticles formed because of the penetration and diffusion of Zn2+ ions into the nanoparticle bodies followed by the heat-accelerated cation exchange reaction between the Zn2+, Ag+ and In3+ ions. The boundary fabrication temperature between the formation of core/shell nanoparticles and that of alloy nanoparticles was estimated to be 420–430 K.