Excited state analysis of absorption processes in metal decorated graphene nanoribbons

Transition density matrix (1TDM) based excited state analysis has been reported using the time dependent density functional theory (TDDFT) results. The structures considered in this work are from our previous study, which are single metal atom (X = Ni, Fe, Ti, Co+, Al+ and Cu+) doped graphene C29H14-X (metal decorated graphene nanoribbons, MGNRs) and pure graphene (C30H14). Transition dipole moments, oscillator strengths, exciton sizes, electron-hole correlation plots and natural transition orbital analysis have been calculated and discussed. Charge transfer during excitation is studied using the electron-hole correlation plots. The charge transfer occurs from the metal to the rest of graphene. The same is confirmed from natural transition orbital (NTO) analysis where the electron density moves away from the metal, in MGNRs. Exciton sizes (in angstrom) are determined and give a clearer indication than NTOs regarding the electron delocalization during excitations, which is found in the ascending order: Al+ > Fe > Cu+ > Co+ > Ni > Ti. Furthermore, the dominant transitions (S-0 -> S-n) were analyzed with the help of transition dipole moments (mu(0n)) and oscillator strengths (f). And, mu(0n) is found more useful than f in determining the maximum absorption wavelength.