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 similar to 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. (C) 2014 Elsevier B.V. All rights reserved.