Optical properties of nitrogen-doped graphene thin films probed by spectroscopic ellipsometry

Nitrogen-doped graphene thin films were prepared by either chemical vapor deposition (CVD) or electrochemical exfoliation (ECE). Their optical properties were determined in the spectral region of 0.73-6.42 eV and at temperatures between 200 and 350 K by spectroscopic ellipsometry. The parameters of the dispersive structures were derived by numerical fitting of the experimental data to the stacked layer model. The optical absorption spectrum of the CVD-grown thin films is characterized by an asymmetric Fano resonance in the ultraviolet frequency region. In contrast, the line shape of the ECE-grown thin films displays less asymmetric. The excitonic resonance of the nitrogen-doped thin films is overall blue shifted by ∼ 0.2-0.3 eV compared with that of undoped analog. We interpret these results in terms of the exothermic nature of triazine molecule adsorption due to binding to graphene's surface via electron rich nitrogen.