Optical Transitions in Silver Indium Selenide Nanocrystals: Implications for Light-Emitting and Light-Imaging Applications

Among the alternatives for the successful family of Cd- and Pb-chalcogenide nanocrystals (NCs), I–III–VI NCs appear to be highly promising as they combine low toxicity with up-and-coming light-emitting and light-imaging applications. Much of the focus has been placed on the prototype Cu–In–S NC system, while Ag-based NCs are significantly less developed. Herein we report spectroscopic studies of robust Ag3In5Se9 NCs fabricated via an amide-promoted synthesis, using NC size as the structural varying parameter. Temperature-dependent photoluminescence, combined with Gaussian line-shape analysis, reveals the presence of three radiative channels in colloids and films of the Ag3In5Se9 NCs. On the basis of their energetics, dynamics, and size- and temperature-dependent behavior, we assign the radiative channels to optical transitions from the conduction-band minimum to (i) in-gap states of self-trapped holes, (ii) valence-band maximum, and (iii) lower-energy valence-band states. Furthermore, by combining pump–probe experiments with tight-binding band-structure calculations whose parameters were derived from first principles, we reveal the presence of two main interband absorption transitions associated with NC electronic states. The dynamics of such transitions are influenced by shallow surface states that act as sites in which fast exciton trapping and much slower detrapping occurs, whereas the trapping and detrapping rates are gauged by confinement and temperature.