Reaction Time-Controlled Synthesis of Multicolor Carbon Dots for White Light-Emitting Diodes

Multicolor carbon dots (MCDs) have been receiving great attention because of their controllable fluorescence, which can be applied as novel optical sources. However, the preparation of MCDs is complex and the corresponding mechanism is still unclear. Here, using o-phenylenediamine and l-tryptophan as precursors, a facile one-step synthesis of MCDs (green, yellow, orange) is proposed through a reaction time engineering strategy. The structural and optical analyses combined with theoretical calculations are performed to disclose the synthetic and fluorescence mechanisms of MCDs. The result shows that increasing the reaction time improves the graphitization degree and surface states, making the emission wavelength red-shifted. In addition, the white-emissive CDs (w-CDs) are easily obtained using the same precursors. The structure and optical properties of w-CDs further confirm the importance of the graphitization degree and surface states in controlling the emissive color of CDs. Based on the optical properties, the MCDs are explored for water detection in organic solvents and multiple anticounterfeiting/information encryption. Moreover, multicolor light-emitting devices (LEDs) are successfully fabricated based on the MCD/PVP composition coated with 365 nm chips. In particular, high-performance white light-emitting devices (WLEDs) directly from w-CDs are obtained with Commission on Illumination (CIE) coordinates of (0.32 and 0.35) and a CRI of 87.2, similar to the standard w-LEDs.