Fermi energy dependence of ultrafast photoluminescence from graphene

The application of graphene in new light-emitting devices has been extensively studied since the demonstration of the ultrafast luminescence from single-layer graphene. The control of luminescence using doping techniques is crucial for these applications. In particular, for the application of graphene in flexible and wearable devices, electrochemical doping is a promising approach, and its influence on luminescence properties of the resulting material needs to be examined. In this study, we demonstrate the effect of the electrochemical doping of graphene using an ion gel on the photoluminescence (PL) of graphene at the emission energy $ℏω$ of 0.9 eV. The Fermi energy $EF$ of graphene was controlled from $+40$ to $−560meV$⁠, and femtosecond PL was observed. The PL intensity was maximum when $EF$ was $−440meV$ $(|EF|≈ℏω/2)$⁠. This trend of the PL intensity is due to (i) an increase in the PL emission rate owing to the doping-induced empty states in the valence band acting as the final states of the radiative relaxation of hot electrons and (ii) an increase in the non-radiative relaxation rate owing to the acceleration of carrier–carrier scattering by the doping-induced increase in the density of states around the $EF$⁠.