Self-Organization of Quasi-Free-Standing Monolayer Graphene Nanoribbon Networks


Yuya Murata, Makoto Takamura, Hiroyuki Kageshima, and Hiroki Hibino
Materials Science Laboratory

  Graphene, a two-dimensional (2D) crystalline sheet of carbon, has generated considerable attention owing to its ultrathin geometry and high carrier mobility, with potential application in high-performance low-power electronics. It has been shown that the band gap of graphene can be controlled when it is tailored into a ribbon shape with a width of up to several nanometers [1]. However, methods such as lithography do not satisfy the requirements for the ideal nanoribbons in terms of production efficiency, uniformity of crystal orientation, and edge structure. On the other hand, it has been reported that a buffer layer on a SiC(0001) surface is decoupled from the substrate by H intercalation and it turns into graphene (quasi-free-standing monolayer graphene, QFMLG) [2]. Although the buffer layer is a 2D sheet of carbon like graphene, it is electrically insulating due to covalent bonds with the substrate. It is desirable to desorb H atoms partially and create insulating regions in QFMLG for fabrication of graphene nanostructures. We investigated H desorption from QFMLG and found a self-organization of QFMLG nanoribbon networks during the H desorption process.
   We performed in-situ STM observations of the H desorption process on QFMLG. Above 630°C, H-desorbed regions expand from SiC step edges (Fig. 1). Each H-desorbed region increases in size and is split into several patches by QFMLG nanoribbons. Finally, H-desorbed patches separated by the network of QFMLG nanoribbons cover the entire surface. The average width and length of the QFMLG naoribbons are a few nanometers and ~10 nm, respectively. The nanoribbons run along the SiC[112(-)0] directions, with uniform armchair edges. The nanoribbon shows the graphene~ surface structure, indicating electron scattering at the boundary between the QFMLG and H-desorbed regions (Fig. 2) [3].

[1] M. Y. Han et al., Phys. Rev. Lett. 98 (2007) 206805.
[2] C. Riedl et al., Phys. Rev. Lett. 103 (2009) 246804.
[3] K. Sakai., Phys. Rev. B 81 (2010) 235417.

Fig. 1. Series of time-lapse STM images at 590°C, with elapsed time of (a) 0, (b) 10, and (c) 64 min (182 nm ~ 250 nm), and at 630°C, with (d) 0, (e) 37, and (f) 69 min (273 nm ~ 341 nm).
Fig. 2. QFMLG nanoribbon networks (a) 200 nm ~ 200 nm, (b) 30 nm ~ 30 nm, (c) 10 nm ~ 10 nm, (d) 2.5 nm ~ 2.5 nm. The rhombus shows a graphene~ unit cell.