2 eV band gap tuning and optical properties of AgIn₅S₈ quantum dots

In this work, we demonstrate the colloidal bottom-up synthesis of spinel AgIn₅S₈ quantum dots (QDs) with tunable optical properties. The QD size, and consequently their band gap energy (E_g), is effectively controlled by reaction temperature and ultrasound (US) irradiation. Under combined conditions of 75 °C and US irradiation, ultrasmall QDs with an average size of 2.6 nm are obtained, exhibiting a wide band gap of 3.77 eV. In the absence of US, reactions conducted at 55 °C and 75 °C yield larger QDs (∼5 nm and 31 nm, respectively), with reduced band gaps of 3.09 eV and 2.18 eV. The elevated temperature (75 °C) suppresses sulfur-chain formation that otherwise limits growth at 55 °C, while acoustic cavitation induced by US enables narrowest size distribution. Annealing of as prepared QDs, at 200 °C for 2 h, promotes coalescence resulting in QDs with increased size of ∼34 nm, with a bulk like band gap of 1.73 eV for QDs prepared without US. In contrast, annealing of the QDs, prepared with US, results in polycrystalline QDs with average size of ∼21 nm. High-resolution transmission electron microscopy reveals a strong correlation between QD size, structural ordering and optical behavior. The as-prepared 2.6 nm QDs exhibit lower Urbach energy, attributed to their single-crystalline nature, unlike the less ordered QDs synthesized without US. Annealing improves structural ordering and reduces Urbach energy in QDs prepared at 75 °C, while stacking faults and grain boundaries in other QDs hinder such improvements. Photoluminescence measurements further confirm a strong relationship between QD structure, size, and emission characteristics. The synthesized AgIn₅S₈ QDs exhibit remarkable band gap tunability of up to 2 eV across the visible spectrum and sharp band-edge emission, underscoring their potential for applications in optoelectronic and biomedical devices. This work provides a robust and sustainable pathway to high-performance, non-toxic QDs, addressing a key bottleneck for their use in biocompatible and consumer electronics.