Alloying Ag2S Quantum Dots with Gold: Controlling NIR-I Emission with Material Modification

Abstract

Herein, the preparation of tunable NIR-I emitting (700–900 nm) Au-alloyed Ag2S QDs is outlined, controlled through Au:Ag reaction stoichiometry. Increasing Au:Ag results in blue-shifted emission (λem = 1180–715 nm), accompanied with an increased measured bandgap (Eg = 1.6–2.3 eV). Au-alloyed Ag2S QDs exhibited enhanced PLQYs (φ = 26.7%) and prolonged lifetimes (τ = 4.57 µs) compared to their parent Ag2S QDs (φ = 5.9%, τ = 1.87 µs). Detailed structural characterization revealed that the series of NIR QDs possessed uniform size distributions with morphological changes due to the nucleation of Au on the QD surface. Cationic exchange of Ag+ for Au+ occurs, originating from these nucleation sites. Phase transformations in the crystal structure were evident, giving rise to the evolution of Ag3AuS2, AgAuS, and AgAu3 species. AgAuS QDs were transferred to aqueous media through ligand exchange with dihydrolipoic acid (DHLA), to produce colloidally stable QDs with an emission maximum in the far NIR-I region at 845 nm. NIR-I imaging phantoms were additionally measured to confirm their compatibility with commercial instrumentation. To our knowledge, this is the first report elucidating the relationship between structural evolution and optoelectronic properties of tunable NIR-I emitting Au-alloyed Ag2S QDs.