How different are the surfaces of semiconductor Ag₂Se quantum dots with various sizes?

The surface of nanocrystals plays a dominant role in many of their physical and chemical properties. However, controllability and tunability of nanocrystal surfaces remain unsolved. Herein, we report that the surface chemistry of nanocrystals, such as near-infrared Ag₂Se quantum dots (QDs), is size-dependent and composition-tunable. The Ag₂Se QDs tend to form a stable metal complex on the surface to minimize the surface energy, and therefore the surface chemistry can be varied with particle size. Meanwhile, changes in surface inorganic composition lead to reorganization of the surface ligands, and the surface chemistry also varies with composition. Therefore, the surface chemistry of Ag₂Se QDs, responsible for the photoluminescence (PL) quantum yield and photostability, can be tuned by changing their size or composition. Accordingly, we demonstrate that the PL intensity of the Ag₂Se QDs can be tuned reversely by adjusting the degree of surface Ag⁺ enrichment via light irradiation or the addition of AgNO₃. This work provides insight into the control of QD surface for desired PL properties.