Ultrafast spectral migration of photoluminescence in graphene oxide.

We use subpicosecond time-resolved photoluminescence measurements to study the nature of photoluminescence in graphene oxide and reduced graphene oxide. Our data indicate that, in contrast to prior suggestions, the photoluminescence spectra of graphene oxide and reduced graphene oxide are inhomogeneously broadened. We observe substantial energy redistribution and relaxation among the emitting states within the first few picoseconds, leading to a progressive red shift of the emission spectrum. Blue shifts that arise in time-integrated spectra upon photothermal reduction are easily understood within this dynamical context without invoking a modified distribution of dipole-coupled states. Rather, reduction increases the nonradiative electron-hole recombination rate and curtails the red-shifting process, which is consistent with an increase in quenching through the introduction of larger and/or more numerous sp(2) clusters. Polarization memory measurements show energetic signatures of electron-hole correlations, established on a subpicosecond time scale and developing little thereafter.