First-principles study of nitrogen-doped nanographene as an efficient charge transport and nonlinear optical material

Abstract

The prospective of nitrogen doped graphene (NDG) as useful nonlinear optical (NLO) and charge transport materials is explored using first principles methods. A dual methodological strategy is adopted by performing molecular level and bulk level calculations through first principles methods. The calculated molecular geometry and absorption wavelengths are found to be in reasonable agreement with experimental results of parent NDG compound 1 (di-peri-(tert-butylbenzo)-di-peri-(tri-methoxybenzo)-di-peri-(pyrimidino)-coronene). Similarly, first principles calculations of the solid-state crystal structure show that NDG compound 1 retains a value of 1.585 eV as its direct band gap, which is potentially useful for a direct bad gap semiconductor for optical device applications. The calculation of second- (β) and third-order (γ) nonlinear optical properties indicates that compound 1 possesses reasonably large values of β_tot (22.96 × 10⁻³⁰ esu) and 〈γ〉 (238.53 × 10⁻³⁶ esu) amplitudes, which may fall well within the figure of merit for second harmonic generation (SHG) and two-photon absorption (TPA) applications, etc. TD-DFT calculations along with the plots of the frontier molecular orbitals show an intramolecular charge transfer from the rest of the graphene fragment to the pyridine moieties (N-doped sites) accompanied by significant oscillator strengths for involved transitions in the absorption spectra. Additionally, several key optical parameters (dielectric and conductivity functions, refractive index and extinction coefficients) are calculated to assess the overall optical efficiency of parent NDG compound 1. The present investigation not only features the substantial potential of NDG compound 1 as an efficient optical material but also highlights the importance of the substitution of its terminal groups to enhance the first- and second hyperpolarizability amplitudes, as exemplified through the design of compound 3.