Multicolor carbon dots with concentration-tunable fluorescence and solvent-affected aggregation states for white light-emitting diodes

Multicolor emissive carbon dots (M-CDs) have tremendous potential applications in manifold fields of bioimaging, biomedicine and light-emitting devices. Until now, it is still difficult to produce fluorescence tunable CDs with high quantum yield across the entire visible spectra. In this work, a type of M-CDs with concentration-tunable fluorescence and solvent-affected aggregation states was synthesized by solvothermal treatment of citric acid (CA) and 1-(2-pyridylazo)-2-naphthol (PAN) and the formation mechanism was monitored by different reaction time and raw material ratio. The fluorescence spectra of M-CDs in organic solvents can range from 350 to 750 nm by adjusting the concentration. M-CDs possess different aggregation states in water and organic solvents, accompanied by different fluorescence emission, which is attributed to the different surface states of various component CDs in M-CDs. Moreover, the obtained products can be uniformly dispersed into polymethylmethacrylate (PMMA) solutions as well as epoxy resins to fabricate transparent CDs/PMMA films and CDs/epoxy composites, which can effectively prevent the aggregation and produce multicolor and white light-emitting diodes (WLED). In addition, the prepared WLED with Commission Internationale de L'Eclairage (CIE) of (0.29, 0.31) by using M-CDs/epoxy resin as packages, demonstrating the M-CDs exhibit potential applications for light-emitting devices.