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 (GaSx) 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.
Core Nanoparticle Engineering for Narrower and More Intense Band-Edge Emission from AgInS2/GaSx Core/Shell Quantum Dots