Enhanced photoluminescence of pyrrolic-nitrogen enriched graphene quantum dots

Abstract

Graphene quantum dots (GQDs) have attracted tremendous attention due to their unique optical and optoelectronic properties. Doping GQDs with nitrogen atoms can effectively tune their intrinsic properties and exploit new applications in many fields. In this paper, we report a facile yet effective strategy for enhancing the photoluminescent (PL) intensity of nitrogen-doped GQDs (N-GQDs) with enriched pyrrolic-N content using urea as a dopant. The morphology, structure, components, and optical properties of the N-GQDs were characterized systematically. The average diameter of N-GQDs was about 7.5 nm, which was larger than undoped GQDs (∼3 nm). Both GQDs and N-GQDs exhibited excitation-independent PL behavior and the PL quantum yields were improved from 9% to 24% after N-doping. Moreover, the PL lifetime decay of GQDs (1.74 ns) and N-GQDs (7.40 ns) can be fitted to a single exponential function very well, indicating that GQDs/or N-GQDs have one single PL origin. Considering the results of Fourier transform infrared spectra and X-ray photoelectron spectra, –COOH groups and pyrrolic-N rings might be feature chromophores of GQDs and N-GQDs, respectively.