Near-infrared photoluminescence in the femtosecond time region in monolayer graphene on SiO₂

We investigate the dynamical properties of photoexcited carriers in a single monolayer of graphene at room temperature in air using femtosecond time-resolved luminescence spectroscopy. The luminescence kinetics are observed in the near-infrared region of 0.7-1.4 eV and analyzed based on the two-temperature model describing the cooling of thermalized carriers via the carrier-optical phonon interaction. The observed luminescence in the range 0.7-0.9 eV is well reproduced by the model. In the range 1.0-1.4 eV, however, the luminescence, which decays in ∼300 fs, cannot be reproduced by this model. These results indicate that the carrier system is not completely thermalized in ∼300 fs. We also show the importance of the carrier-doping effect induced by the substrate and surrounding environment in the carrier cooling dynamics and the predominance of optical phonons over acoustic phonons in the carrier-phonon interactions even at a temperature of ∼400 K.