Facile High-Yield Synthesis of Ag-In-Ga-S Quaternary Quantum Dots and Coating with Gallium Sulfide Shells for Narrow Band-Edge Emission

Among cadmium-free quantum dots (QDs), ternary or quaternary chalcogenide semiconductor QDs composed of groups 11, 13, and 16 elements have extensively been studied. Herein, monodispersed quaternary nanoparticles of silver indium gallium sulfide (AgInxGa1-xS2) are synthesized by a new route that specifically produces the target product without solid byproducts. The difference in the reactivity between groups 11 and 13 metals with chalcogen is a common issue in synthesizing the groups 11, 13, and 16 ternary or quaternary semiconductor QDs. Instead of a common approach to suppress the reactivity of group 11 metals, this study utilizes their high reactivity; rapid injection of a Ag source into the solution containing a thiocarboxylate produced small Ag2S nanoparticles. These Ag2S nanoparticles are then exposed to indium-, gallium-, and sulfur-containing species in situ and then converted into quaternary nanoparticles comprising AgInxGa1-xS2. Due to the well-controlled reaction steps, the product yield based on Ag was increased to 60%, which significantly exceeds that obtained using our previous approach of heating all-mixed raw materials (5-15%). After coating with gallium sulfide (GaSy) shells, the core/shell QDs exhibited an intense, green-colored band-edge emission with a tunable peak wavelength between 499 and 543 nm. The excellent uniformity of the core nanoparticles in terms of size and composition demonstrated a quite narrow band-edge emission with the full width at half maximum of 31 nm, which is equal to the record-narrow values of green-emitting cadmium-free QDs. Additionally, a near-unity photoluminescence quantum yield was achieved after postsynthetic surface treatment by alkylphosphines, indicating the defect-free nature of the prepared AgInxGa1-xS2/GaSy core/shell QD system.