3100704 NGQD nitrogen-doped graphene quantum dots 18 3100701 N-GQD N-doped graphene quantum dot 5 3100703 NGQD nitrogen-doped GQD 2 3100702 N-GQD N-graphene quantum dot 1 3100706 N-GQD nitrogen doped GQD nanoparticles 1 3100707 N-GQD nitrogen incorporated graphene quantum dot 1 3100705 N-GQD ZnS@nitrogen graphene quantum dot 1