STS Observations of the Spin-Polarized Edge State in Zigzag Graphene Nanoribbons

A peculiar electronic state in graphene nanoribbons (GNRs) which is localized only at the zigzag
type edge, the so called “edge state”, was proposed in 1996 by M. Fujita and his coworkers [1]. In
2005−2006, the existence of the edge state was verified by scanning tunneling microscopy and
spectroscopy (STM/S) measurements at zigzag step edges on graphite surfaces by two groups
including ourselves [2,3]. In very narrow GNRs of widths less than ∼20 nm with zigzag edges on
both sides (zGNRs), the edge atoms are predicted to possess high spin polarizations (10−20%) even
under weak electron interactions [1], and the edge magnetic moments on opposite sites will align
ferromagnetically (FM) or antiferromagnetically (AF) depending on the ribbon width [4]. Moreover,
the edge state in FM-zGNRs will be gapless (metallic), whereas that in the AF-zGNRs will have a
band gap of the order of 100−400 meV (semiconducting) again depending on the width [4]. Several
experimental groups have reported band gap features, which could be caused by the magnetic edge
state, in zGNRs prepared by various ways [5]. However, the results are not necessarily consistent
each other quantitatively, and the detailed atomic structures of the edges are not clear.
In this talk, I will show preliminary results of our new low temperature STM/S measurements on
the edge magnetism of zGNR samples of 6−20 nm widths which are synthesized in between two
adjacent hexagonal nanopits anisotropically etched by hydrogen plasma on graphite surfaces [6]. The
6 nm wide ribbon clearly shows two peaks separated by 50−60 meV in the local density of states on
the two opposite edges (gap-like structures), and the separation decreases with increasing width.
Details of the sample preparation by anisotropic H-plasma etching and the STM/S measurements will
be discussed.
This work was carried out as a collaboration with Tomohiro Matsui and André E. B. Amend.
* hiroshi@phys.s.u-tokyo.ac.jp
[1] M. Fujita et al., J. Phys. Soc. Jpn. 65, 1920 (1996); K. Nakada et al., Phys. Rev. B 54, 17954
(1996).
[2] Y. Niimi et al., Appl. Surf. Sci. 241, 43 (2005); Y. Niimi et al., Phys. Rev. B 73, 085421 (2006).
[3] Y. Kobayashi et al., Phys. Rev. B 71, 193406 (2005).
[4] Young-Woo Son et al., Phys. Rev. Lett. 97, 216803 (2006).
[5] C. Tao et al., Nature Physics 7, 616 (2011); M. Ziatdinov et al., Phys. Rev. B 87, 115427 (2013);
Y.Y. Li et al., Nature Comm. 5, 4311 (2014); G. Z. Magda et al., Nature 514, 608 (2014).
[6] T. Matsui et al., to appear; A. E. B. Amend et al., e-J. Surf. Sci. and Nanotech., 16, 72 (2018).