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Graphene

Graphene is a monatomic sheet of carbon atoms in which the atoms are densely packed in a honeycomb lattice. The charge carriers show unique linear dispersion relation, and behave as massless Dirac fermions. As a result, Graphene shows unusual electronic properties such as anomalous integer quantum-Hall-effect and weak anti-localization, etc. It is also attracting considerable attention due to its possible applications in many emerging fields such as graphene-based electronic devices.
We are recently studying following interesting topics related Graphene.

  1. Zigzag edge states in Graphene and Graphite
  2. Bandgap opening in Graphene

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Zigzag edge state in Graphene and Graphite

There are two types of edges in graphene, i.e. zigzag and armchair structures.
It is theoretically predicted in 1996 that electrons are strongly localized along the zigzag edge, while it is not the case for armchair edge. It is because the bipartitle symmetry is broken at the zigzag edge. We have confirmed such a localized state, named zigzag edge state, experimentally by using STM/S at monatomic sted edges of graphite. [1][2]
Moreover, it is also expected that the spin degeneracy could be lifted under an electron-electron interaction, and ferromagnetically spin polarized state appears along zigzag edge. The ferromagnetic edge state is considered to stabilize in a nano-ribbon between two zigzag edges (zigzag nanoribbon) through anti-ferromagnetic coupling between edges.

However, such spin polarized edge states had not been observed, yet. This is because of the difficulty to obtain atomically controlled edge structure and chemical conditions.
To overcome such difficulties and to obtain hydrogen terminated zigzag edges, we tried hydrogen-plasma etching of graphite surfaces. By exposing graphite to hydrogen-plasma under high temperatures, hyxagonal pits with monatomic depth are found to be created. The size and the density of the pit can be controlled by tuning plasma density, temperature and time duration of the process. Moreover, and most importantly, the edges of the pit are aligned to zigzag direction and hydrogen terminated. Therefore, one can obtain zigzag nanoribbon in betweeen two hexagonal pits, where the spin poralized zigzag edge state can be expected to observe by STM/S measurments.

  1. Y. Niimi, T. Matsui, H. Kambara, and Hiroshi Fukuyama: "Scanning tunneling microscopy and spectroscopy of the electronic local density of states of graphite surfaces near monatomic step edges", Physical Review B 73, 085421 (2006).
  2. Y. Niimi, T. Matsui, H. Kambara, K. Tagami, M. Tsukada and Hiroshi Fukuyama: "Scanning tunneling microscopy and spectroscopy of graphite edges", Applied Surface Science 241, 43-48 (2005).

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Bandgap opening in Graphene

Graphene is hopeful material for future device applications. However, graphene itself is not proper enough to apply to electronic devices because of its linear energy dispersion. One cannot obtain good on/off ratio without energy gap in the band structure. Therefore, it is one of the important subjects to study, how to induce a band gap in graphene.
So far, many possibilities are proposed and, among them, we are focuing on a mechanism to break the chiral symmetry of graphene by decorating with atoms/molecules. For example, it is theoretically expected that the band gap can be induced when atoms are adsorbed on graphene to form (√3x√3)R30° structure.
To verify this possibility, we use Kr atom as an adsorbate, because Kr atom is confirmed to form (√3x√3)R30° structure on the surface of graphite by our STM measurement.

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