新学術領域「コンピューティクスによる物質デザイン：複合相関と非平衡ダイナミクス」
公募研究「A02：金属電極に架橋した単原子・単分子の電子・熱・スピン輸送ダイナミクスの解明」

Chemistry and electronic properties of defects in graphene
Shintaro Fujii, Maxim Ziatdinov, Toshiaki Enoki and Manabu Kiguchi
Dpt. of Chem., Tokyo Inst. of Tech., Tokyo, JapanTo understand better physical properties of nanomaterials on the atomic and molecular scale, it is
crucial to combine theory and experiment. Here we focused on the electronic properties of nanosized
two dimensional π-electron network of graphene. The local electronic properties of graphene are
crucially dependent on geometrical shape and chemistry of defect sites. According to current
understanding, while the zigzag edge of graphene supports the localized π-state (edge state) on its
boundary and gives rise to intra-valley scattering of extended electronic states, its armchair
counterpart does not possess the edge state and leads to inter-valley scattering of charge carrier. It istherefore appealing to tune electronic and magnetic properties of graphene-based nanostructures via
changing geometrical shape of defects in graphene. In a simple structural edge-defect model, eachedge site is passivated by a hydrogen atom to saturate the dangling bond and form
mono-hydrogenated graphene edges. We prepared atomically well-defined hydrogenated grapheneedges contained within nano-sized pits by atomic hydrogen etching of single-vacancies in thetopmost graphene layer of graphite. To simulate scanning tunneling microscopy (STM) data,
electronic calculations based on density functional theory (DFT) were performed. High-resolution
STM characterizations and DFT simulations demonstrate that mono-hydrogenated zigzag edges are
preferentially formed in the periphery of the nanoholes [Fig. (a)]. In addition to the conventionalmono-hydrogenated zigzag edge, we found a new type of zigzag edge, characterized bydisappearance of the edge state and profound interference pattern in a form of electronic superlattice[Fig. (b)]. To understand these two main features we have to take into account chemistry at zigzag
edge. Systematic DFT simulations show that the edge state can be removed when every third
zigzag-edge site is di-hydrogenated while two others remain mono-hydrogenated. To further explore
the effect of the edge chemistry on the electronic properties, oxidized graphene edges were preparedby electrochemical oxidation of graphitic surface. In sharp contrast to the conventional hydrogenatedgraphene edge [Fig. (a)] in previous reports, newly found oxidized edges (i.e., ketonated zigzagedge) with higher thermodynamic stability exhibited significantly modulated π-states near the Fermi
level, which is characterized by (i) spatially extended π-state distribution and (ii) energy-dispersive
character with quasi-band gap nature. This modulation can be understood by participation of theoxygen π electron into graphene π electron system and resultant change in the effective boundary
condition of the π electron network. Thesefindings open up a way for tuning electronic
properties of graphene edges not only bychanging the geometrical shape of edges but also
by controlling how chemical species are attachedto the graphene edge of a given geometry.
[1] M. Ziatdinov, S. Fujii, K. Kusakabe, M. Kiguchi, T. Mori, T. Enoki, Phys. Rev. B 87, 115427(2013).
[2] M. Ohtsuka, S. Fujii, M. Kiguchi and T. Enoki, ACS Nano 7, 6868-6874 (2013).