Optical properties of pure graphene in various forms: a time dependent density functional theory study
Time dependent density functional theory (TDDFT) based calculations were performed for two basis sets 6-31G and 6-31G (d) on two sets of graphene samples: (a) graphene nano-ribbons (GNRs) of different geometries such as square, rectangle and triangle containing 48, 53, 60, 68, 70 and 77 atoms, and (b) six isomers of C46H18 GNR. UV-visible absorption spectra were obtained from the TDDFT calculations. Optical gaps, excitation wavelengths, oscillator strengths and dominant transitions of all the samples were calculated. The highest oscillator strengths were found for the rectangle shaped GNRs Rectangle60, Rectangle77 (f = 0.826 and 1.512) at the absorption wavelengths 416.5 and 439.2 nm. Amongst C46H18 isomeric GNRs, ISO4, ISO5 and ISO6 with high f (>0.6) were found to absorb more light in the visible region. Higher optical absorption has been obtained for all the isomeric GNRs in the range 500–900 nm making them suitable for the dye sensitized solar cell applications. The pi → pi* transitions were found to be the dominant transitions in the optical absorption of both the sets of GNRs.