Tunable photoluminescence of graphene quantum dots synthesized via laser ablation in ethanol

Abstract

GQDs were synthesized via eco-friendly pulsed laser ablation in the liquid (PLAL) method, employing ethanol as the solvent and carbon black as the carbon source. The photoluminescence (PL) behavior of GQDs was systematically analyzed using emission-excitation matrices across varying concentrations and pH conditions. Increasing GQD concentrations resulted in redshifts of maximum PL excitation bands, transitioning from 235 nm to 272 nm and subsequently to 365 nm. The corresponding PL emission shifted from 310 nm to 410 nm and finally to 495 nm. These findings reveal aggregation-induced quenching (AIQ) in the blue region and aggregation-induced emission (AIE) at longer wavelengths, emphasizing the role of aggregation in modulating GQD PL properties. The photoluminescence of GQDs was also strongly influenced by pH. At acidic pH, dual emission maxima were observed at 375 nm and 450 nm, linked to higher aggregation states due to the protonation of edge groups. Conversely, at pH values above 7.0, the GQDs exhibited a single emission peak at 470 nm, reflecting controlled aggregation dominated by core group contributions. Neutral pH yielded a redshifted emission maximum at 460 nm, attributed to oxygenated groups in the core structure. This work elucidates the crucial influence of aggregation and pH on GQD photoluminescence, offering essential insights into their mechanisms when synthesized without acidic agents. Adopting a sustainable synthesis approach reveals the tunable luminescent properties of high-purity, undoped GQDs, advancing their potential for environmentally friendly applications and supporting the design of graphene-based nanomaterials for targeted functionalities.