Origin of high photoluminescence yield and high SERS sensitivity of nitrogen-doped graphene quantum dots

Herein, we elucidate the origin of high photoluminescence quantum yield (PL QY) and high surface-enhanced Raman scattering (SERS) sensitivity of in-situ nitrogen-doped graphene quantum dots (N-GQDs). We show that the doping of heteroatoms in GQDs facilitates the excited-state charge transfer and improved recombination in N-GQDs yielding a PL QY of ∼34%. Our study reveals that growth of GQDs in dimethyformamide solvent enables the reduction of the nonradiative sites in N-GQDs and the rearrangement of the charge distribution in the graphitic plane. Further, we demonstrate N-GQDs as an efficient SERS substrate with an enhancement factor of 3.2 × 10³ with 10^-4 M RhB target, which is ∼7-fold higher than the previous reports. The comparative studies of the SERS for undoped and N- and S-doped GQDs allow us to assess the contribution of the Förster resonance energy transfer (FRET) and chemical enhancement (CM) factors. Further, by controlling the functional groups of N-GQDs with vacuum annealing, and with different laser excitations, we isolate the contributions of CM and FRET, for the first time. The optimized N-GQDs exhibits a SERS detection limit of 10^-10 M for RhB. Finally, N-GQDs combined with RhB is utilized to demonstrate a white light emitter with a CIE coordinate (0.30, 0.34).