  Ab-initio calculations based on density functional theory (DFT) have been
performed to study the optical properties of pure graphene and have been
compared to that of individual boron (B), nitrogen (N) and BN co-doped graphene
sheet. The effect of doping has been investigated by varying the concentrations
of dopants from 3.125 % (one atom of the dopant in 32 host atoms) to 6.25 %
(six dopant atoms in 50 host atoms) for individual B and N doping and from 37.5
% (one B/N pair in 32 host atoms) to 18.75 % for BN co-doping. Positions of the
dopants have also been varied for the same concentration of substitution
doping. The dielectric matrix has been calculated within the random phase
approximation (RPA) using VASP (Vienna ab-initio Simulation Package) code. The
dielectric function, absorption spectrum and energy loss-function of single
layer graphene sheet have been calculated for light polarization parallel and
perpendicular to the plane of graphene sheet and compared with doping graphene.
The calculated dielectric functions and energy-loss spectra are in reasonable
agreement with the available theoretical and experimental results for pure
graphene. It has been found that individual B and N doping does not
significantly affect the imaginary dielectric function and hence the absorption
spectra. However, significant red shift in absorption towards visible range of
the radiation at high doping is found to occur for the B/N co-doping. The
results can be used to tailor the optical properties of graphene in visible
region.
