Excited State Dynamics in Sulfur Quantum Dots: Influence of Size, Aggregation, and the Role of Oxidant Environment

Sulfur quantum dots (SQDs) are emerging as sustainable luminescent nanomaterials with distinctive optical properties and compatibility for optoelectronic, sensing, and photocatalytic applications. Despite their promise, the excited-state dynamics of SQDs remain poorly understood. In this work, SQDs were synthesized in oxidizing environments containing O2 and H2O2, using polyethylene glycol-400 (PEG) and ethylenediamine (EDA) as passivating agents, to systematically probe the influence of particle size and aggregation on ultrafast photophysical processes. Structural and optical characterization revealed that oxidant concentration governs particle size, surface chemistry, and emission behavior: low H2O2 concentrations promote aggregation, whereas higher concentrations yield smaller, more uniform SQDs with markedly enhanced fluorescence. Time-resolved fluorescence spectroscopy showed that increasing H2O2 concentration elevates the contribution of surface states, directly boosting quantum yield (QY). Ultrafast transient absorption spectroscopy further disentangled carrier dynamics, revealing a universal hot-exciton relaxation (∼3 ps) independent of particle size, an intermediate decay (40–70 ps) that vanishes at elevated H2O2 and is replaced by longer decays (200–250 ps), and a long-lived trap-state recombination pathway. By correlating photoluminescence (PL) and transient absorption (TA) results, this study establishes a mechanistic framework in which quantum confinement, surface oxidation, and aggregation jointly dictate SQDs’ luminescence. These insights provide a rational strategy for tailoring sulfur-based quantum dots with controlled excited-state dynamics and optimized optical performance for next-generation optoelectronic and energy applications.