▎ 摘　　要

Graphene quantum dots (GQDs) are nanometer-sized pieces of a graphene sheet possessing a number of fascinating electrical and optical properties. In this work, GQDs were synthesized by a top-down method using lignin as a carbon source due to its high carbon content and high abundance in the nature. GQDs were prepared via an intermediate step, by the synthesis of graphene-encapsulated metal nanoparticles (GEMNs). First the GEMNs were synthesized from lignin and three different transition metal chlorides (Fe, Co, Ni), and then metal nanoparticles were removed, and graphene capsules thus obtained were then decomposed to GQDs by using an alkaline hydrothermal treatment. GQDs have a diameter around 20-25 nm and exhibit a yellowish emission under UV light. Characterization results obtained by using the IR, Raman and UV-vis spectroscopies showed that the GQDs obtained from the three types of GEMNs have a graphene structure with various oxygen-containing functional groups. A comprehensive study on the luminescence properties of the GQDs revealed that the three kinds of GQDs show an excitation-wavelength-dependent photoluminescence. The life times are in the range of 5.2-5.5 ns and the quantum yield values vary within the limits of 11.7-12.4%.