Versatile photoluminescence from graphene and its derivatives

Graphene and its derivatives exhibit many interesting photoluminescence (PL) properties because of their unique electronic structures. In spite of the absence of the bandgap, graphene shows PL due to hot electrons. Graphene oxide (GO) fluorescence is different from that of a single organic fluorophore, for which the spectral properties and emission lifetime are independent of wavelength. Single-layered GO sheets are made of a large number of covalently connected independent fluorophores of varying sizes. These fluorophores are aromatic π-conjugated sp2-hybridized subsystems of carbon atoms surrounded by sp3 regions. The PL of GO is pH dependent because of the presence of many oxygen-containing groups in GO sheets. Reduced graphene oxide (rGO) PL is somewhat different from GO because the number and size of sp2 fragments are increased in rGO due to the elimination of the functional groups containing oxygen via reduction. Nanosized graphene/GO possesses a strong quantum confinement effect and hence emits intense excitation wavelength-dependent PL. Moreover, graphene quantum dots show upconversion PL due to anti-Stokes transition. The diverse PL properties including the effect of reduction, pH, and solvent have been reported in many recent studies. Here, the versatile PL features of graphene derivatives are reviewed to elucidate the mechanism of PL.