Diminishing Surface Defects in Silver Indium Sulfide Quantum Dots Enables Bandgap‐Excitonic Characteristics

Abstract I‐III‐VI quantum dots are eco‐friendly alternatives for next‐generation light‐emitting and harvesting devices. The study investigates the optical properties of Ag‐In‐S‐based quantum dots, focusing on excitonic transitions and sub‐bandgap defects. Uniquely synthesized AgInS 2 quantum dots emit via free excitons but dominate with broad, red‐shifted emission from energy‐distributed donor–acceptor pair defects. Photoluminescence excitation spectroscopy unveils charge carrier relaxation pathways with varying efficiencies within the defect states. Gallium sulfide coating forms a core/shell structure, passivating defects and transforming the inhomogeneous sub‐bandgap energy landscape into a more homogeneous one. A narrow free exciton emission and sharp absorption onset with two exciton signatures, X1 and X2, become prominent. Theoretical calculations reveal the band structure, showing that these transitions result from the excitation between two distinct valence band maxima and the conduction band minimum. Femtosecond transient transmission spectroscopy shows a defect trapping time of 590 fs in core quantum dots, while exciton filling prevails in core/shell, with formation times of 560 fs (X1) and 320 fs (X2). This study provides new insights into the optical properties of Ag‐In‐S‐based quantum dots by demonstrating how the computed band structure explains the experimentally observed excitonic features, with ultrafast spectroscopy capturing the exciton formation and trap‐state filling dynamics.