(a) Low-magnification TEM image of NGQDs. Scale bar, 50 nm. (b) High-magnification TEM image of NGQDs. Scale bar, 2 nm. Inset shows a single NGQD containing zigzag edges as circled. The yellow line outlines the zigzag edge. Scale bar in inset, 1 nm. (c) Raman spectrum of NGQDs as compared with that of pristine GQDs. (d) High-resolution N 1s spectrum for NGQDs, deconvoluted into three sub-peaks representing pyridinic, pyrrolic and graphitic N. The value in parentheses is the corresponding N atomic concentration calculated based on N/(N+C). The inset is a schematic demonstrating zigzag edges and the N-bonding configuration with respect to pyridinic (black), pyrrolic (blue) and graphitic (pink) N. In comparison, the N 1s peak does not appear in GQDs sample.