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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
The photoluminescence mechanism of the N-GQDs: (A) the excitation spectrum obtained under the monitor emission wavelength at 447 nm and the excitation in the range of 200–430 nm; (B) the fluorescence lifetime of the N-GQDs; and (C) the schematic energy level of the N-GQDs.