Surface state modulation of red emitting carbon dots for white light-emitting diodes

Controlling surface states using different functional groups is an effective and facile way to modulate the fluorescence of carbon dots (CDs), but the underlying mechanisms are still unclear and urgent to solve. In this work, we synthesized red emitting CDs and achieved multiple emission states (red, green and blue emission) by tailoring the surface states with different amino groups. In particular, the luminescence mechanism and the role of surface states were studied in detail. It is found that the multiple emission states are related to the speciation of nitrogen (pyridinic N, pyrrolic N, graphitic N and amino N) on the surface of CDs, of which the fluorophore formed by the deformation of p-phenylenediamine contributes to the red emission, pyridinic N is responsible for the green emission state, and pyrrolic N enhances the blue emission. A possible energy level diagram was proposed to disclose the electron transition process and relative levels induced by different surface groups on CDs. In addition, the modulated CDs with multiple emission states can be used as single light converters to fabricate a white light-emitting diode, which has white light color coordinates of (0.31, 0.32), a high color rendering index (CRI) of 85, a luminous efficiency of 8.8 lm W−1 and good stability.