Single-atom lithium adsorption-induced electronic and optical modulation in armchair graphene nanoribbons: a PBE and screened-hybrid DFT study.

CONTEXT: Lithium adsorption can alter the electronic and optical response of graphene nanoribbons, but earlier studies have generally treated adsorption stability, electronic structure, and optical spectra separately. The present analysis links adsorption-site energetics and charge transfer to the width-dependent electronic and optical response of hydrogen-terminated armchair graphene nanoribbons. RESULTS: The hollow site was lowest in energy, with adsorption energies of - 1.32 and - 1.25 eV and Li-to-ribbon charge transfers of 0.86 and 0.84 electron for widths 6 and 8, respectively. PBE band gaps of 0.78 and 0.11 eV increased to 1.23 and 0.24 eV under the screened-hybrid benchmark, whereas Li adsorption produced bands crossing the Fermi level under both treatments. The first longitudinal absorption peak shifted from 1.70 to 0.90 eV for width 6 (red shift) but from 0.30 to 1.10 eV for width 8 (blue shift). The previously reported static longitudinal refractive index of 21.02 was identified as a transcription error and corrected to 2.10. CONCLUSIONS: Within the independent-particle approximation, Li adsorption produces a robust donor-induced metallic response, while the direction and magnitude of the optical shift remain width-family dependent. These results support further experimental and many-body validation rather than direct device-level claims. METHODS: Full-potential DFT calculations were performed with WIEN2k using PBE-GGA, explicit structural relaxation, site-resolved adsorption energies, charge-density-difference and electron-localization analyses, band structures, orbital-projected density of states, and polarization-resolved optical functions. Widths 5-10 and a concentration series were evaluated, and representative width-6 and width-8 structures were benchmarked with the WIEN2k screened-hybrid implementation.