The optical absorption of triangular nanographenes with well-defined edge structure, that is, zigzag-and armchair-type edges, is studied using the tight-binding method within the Hückel approximation. The absorption spectra of zigzag triangular nanographenes exhibit rich peak structures originating from the excitations associated with the edge states, while those of armchair ones do not. The main feature in the absorption spectra is reproduced by time-dependent density functional theory calculations within a real-time scheme. By varying the size of the nanographenes, we elucidate the role of the edge states in the optical absorption, which becomes conspicuous when the graphene size is on a nanometer scale. Our finding in this paper provides a way to capture the edge-state signature by optical experiments with visible light.
Photoabsorption spectra of graphitic ribbons (GR) and triangular graphitic flakes (TGF) are investigated by time-dependent density-functional theory calculations within a real-time scheme. Major peaks in the low-energy region of the spectra are attributed to the π-π * electronic transitions. The peak of the strength function appears at 3.0 eV owing to π-π * transitions via edge states for a TGF with hydrogen (H) termination and at 6.0 eV owing to π-π * transitions at the Γ point for GR with H termination. Small structures in the strength function emerge at 0 ∼ 2 eV for both graphitic structures after extraction of H atoms. The new structures are found to originate from the electronic transitions between dangling-bond states. Thus, the present study shows that the electronic states unique to nanoscale graphitic structures can be captured in the optical absorption spectra.
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