Near-infrared-emitting colloidal Ag₂S quantum dots exhibiting upconversion luminescence

Ag₂S quantum dots (QDs) coated with thioglycolic acid (Ag₂S QDs-TGA) have been synthesized in an organic solvent via a stepwise addition of reagents. When excited by a 980 nm laser, the near-infrared-emitting colloidal Ag₂S QDs-TGA exhibit upconversion luminescence (UCL). The observed photoluminescence (PL) was attributed to the presence of ligand-modified Ag₂S on the QD surfaces. Hence, upon dilution of the solution, the PL intensity initially increased before subsequently decreasing, accompanied by a blue shift in the PL spectra. The PL phenomena can be attributed to the increase in the amount of ligand-modified Ag₂S on the QD surfaces upon dilution, which in turn affected the fluorescence resonance energy transfer (FRET) and re-emission of the surface energy level. The relations between the emission intensity of Ag₂S QDs-TGA and the excitation power are investigated, and the results confirm that the UCL in Ag₂S QDs-TGA can be ascribed to a two-photon-assisted absorption process via a real energy state.