Core Nanoparticle Engineering for Narrower and More Intense Band-Edge Emission from AgInS₂/GaS_x Core/Shell Quantum...

Highly luminescent silver indium sulfide (AgInS₂) nanoparticles were synthesized by dropwise injection of a sulfur precursor solution into a cationic metal precursor solution. The two-step reaction including the formation of silver sulfide (Ag₂S) nanoparticles as an intermediate and their conversion to AgInS₂ nanoparticles, occurred during the dropwise injection. The crystal structure of the AgInS₂ nanoparticles differed according to the temperature of the metal precursor solution. Specifically, the tetragonal crystal phase was obtained at 140 °C, and the orthorhombic crystal phase was obtained at 180 °C. Furthermore, when the AgInS₂ nanoparticles were coated with a gallium sulfide (GaS_x) shell, the nanoparticles with both crystal phases emitted a spectrally narrow luminescence, which originated from the band-edge transition of AgInS₂. Tetragonal AgInS₂ exhibited narrower band-edge emission (full width at half maximum, FWHM = 32.2 nm) and higher photoluminescence (PL) quantum yield (QY) (49.2%) than those of the orthorhombic AgInS₂ nanoparticles (FWHM = 37.8 nm, QY = 33.3%). Additional surface passivation by alkylphosphine resulted in higher PL QY (72.3%) with a narrow spectral shape.