Ultra-small aqueous glutathione-capped Ag-In-Se quantum dots: luminescence and vibrational properties.

We introduce a direct aqueous synthesis of luminescent 2-3 nm Ag-In-Se (AISe) quantum dots (QDs) capped by glutathione (GSH) complexes, where sodium selenosulfate NaSeSO is used as a stable Se precursor. A series of size-selected AISe QDs with distinctly different positions of absorption and PL bands can be separated from the original QD ensembles by using anti-solvent-induced size-selective precipitation. The AISe-GSH QDs emit broadband PL with the band maximum varying from 1.65 eV (750 nm) to 1.90 eV (650 nm) depending on the average QD size and composition. The PL quantum yield varies strongly with basic synthesis parameters (ratios of constituents, Zn addition, duration of thermal treatment, ) reaching 4% for "core" AISe and 12% for "core/shell" AISe/ZnS QDs. The shape and position of PL bands is interpreted in terms of the model of radiative recombination of a self-trapped exciton. The AISe-GSH QDs reveal phonon Raman spectra characteristic for small and Ag-deficient tetragonal Ag-In-Se QDs. The ability of ultra-small AISe QDs to support such "bulk-like" vibrations can be used for future deeper insights into structural and optical properties of this relatively new sort of QDs.