Photo-and Electroluminescence from Nitrogen-Doped and Nitrogen-Sulfur Codoped Graphene Quantum Dots

As opposed to inorganic counterparts, organic quantum dots often exhibit lower fluorescence efficiencies and are complex to synthesize. Here we develop nitrogen-doped (N-GQDs) and nitrogen-sulfur codoped (NS-GQDs) graphene quantum dots exhibiting high-yield visible and near-IR emission that are synthesized via a single-step microwave-assisted hydrothermal technique with a single glucosamine-HCl starting material (thiourea precursor used for NS-GQDs). As-synthesized N-GQDs and NS-GQDs are well-dispersed (average sizes of 5.50 and 3.90 nm) with high crystallinity and pronounced G-band. Formed by the bottom-up assembly of glucosamine, they contain amine linkage and a variety of oxygen-containing functional groups assessed by Fourier-transform infrared spectroscopy with approximate to 2% sulfur for NS-GQDs. The synthetic procedure allows varying their size and the bandgap. Unlike other graphene-based quantum dots, these GQDs exhibit bright, stable fluorescence both in the visible and near-IR with high quantum yields of up to 60%. Excitation-dependent visible fluorescence is attributed to size-dependent bandgaps, with near-IR emission potentially arising from the emissive defect states/their arrangements. Advantageous properties of these GQDs are utilized to develop exciton recombination layer for organic light-emitting devices exhibiting both photoluminescence and electroluminescence in the visible. Produced by ecofriendly one-step scalable synthesis brightly-emissive N-GQDs and NS-GQDs become a promising material for novel organic optoelectronics.