First-principles simulations of scanning tunneling microscopy images exhibiting anomalous dot patterns on armchair-edged graphene nanoribbons

To interpret our recent scanning tunneling microscopy (STM) experiment of nanographene sheets, in which the existence of rectangularlike lattices is confirmed, we conducted first-principles calculations of both simple and wrinkled armchair-edged graphene nanoribbons (AGNRs). We focused on the origin of this unique lattice and simulated STM images in the occupied band for the above-mentioned ribbons with different widths. Combined with the band structures of AGNRs, we investigated the features of electron distribution in detail under various magnitudes of applied sample biases (−1 to −0.05 V). In contrast to the well-known hexagonal structure of graphene at a large bias of −1 V, a rectangularlike lattice emerges in the STM images of AGNRs with specific widths when they are generated solely by electronic states occupying the valence band maximum. Furthermore, we find that a wrinkle in graphene parallel to the edges in AGNR significantly affects the STM images of a neighboring flat graphene region, even when its height is only several angstroms. By comparing the partial density of states of the wrinkle with flat graphene, we discussed the role of a wrinkle in functionalizing graphene sheets.