Highly efficient multicolor carbon quantum dots synthesized via a hydrothermal method and their applications in epoxy composites.

To avoid the extensive use of toxic solvents and reduce energy consumption, an environmentally friendly method was employed to synthesize high-yield full-color carbon quantum dots (CDs) using p-phenylenediamine (PPD) and suitable anhydrides (maleic anhydride, glutaric anhydride and itaconic anhydride) as raw materials under mild hydrothermal conditions (180 °C/4 h). The emission wavelengths of the resulting CDs covered the visible light region range from 441 to 641 nm. The relative photoluminescence quantum yields for the blue light-emitting carbon dots, green light-emitting carbon dots and red light-emitting carbon dots were 60%, 66.5% and 23.2%, respectively. The different optical features of these CDs are attributed to various graphitization degrees and surface functional groups. The resulting CDs had good thermal properties with initial thermal decomposition temperatures in the range of 184-203 °C, and also displayed significant pH-dependent fluorescence relationships. Moreover, the resulting CDs displayed stable fluorescence performance after being heated at 100 °C for 1 h and stored at room temperature for 30 days. After the CDs were introduced into epoxy resins (EPs), the resulting EP composites displayed good fluorescence performance. Even after heat treatment at 100 °C for 1 h and after 365 nm UV radiation for 10 h, the fluorescence intensities of the EP composites could be maintained at high levels, more than 91%. Light-emitting diodes fabricated by coating the CDs-epoxy composite on top of UV-chips could emit bright monochromatic light under 365 nm UV light and showed excellent UV resistance performance.