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Publisher Rating 7
Nanoscience
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Nanoscale Fabrication of Graphene by Hydrogen-Plasma Etching

Takamoto Yokosawa Masahiro Kamada Taisuke Ochi Yuki Koga
Acknolowdgement NA

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Graphene, Hydrogen-plasma etching, Atomic force microscopy, Raman spectroscopy

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Abstract
Graphene is attracting vast interest due to its superior electronic and mechanical properties. However, the structure and electronic properties of its edge are often neglected, although they are important for nanoscale devices because the edge ratio becomes larger by decreasing the device size. In this study, we suggest a way to fabricate graphene with atomically aligned zigzag edges by applying the hydrogen-plasma etching (HPE) technique. By patterning graphene prior to HPE, it succeeded to shape graphene in desired structure. Both atomic force microscopy and Raman spectroscopy con rm that the graphene shaped by this technique preserves its honeycomb structure even on the edge, which is aligned with the zigzag structure. Although the mechanism of the anisotropic etching by hydrogen-plasma has not been clari ed yet, the sample position dependence of the etching rate suggests that the hydrogen radicals are responsible for the anisotropic etching.

Introduction
Graphene is a hopeful material for future electronic devices[1]. The electronic and mechanical properties of graphene itself, namely the bulk properties, had been studied extensively since its discovery[2{4]. However, the experimental study of its edge state is rather limited, even though the understanding and controlling of the edge property is important for nanometer scale devices since the edge portion becomes larger for smaller devices. It is especially the case for graphene because there are two types of edge structures, zigzag and armchair, with totally different electronic properties. The electronic property of the armchair edge is roughly the same as that of bulk graphene since the bipartite symmetry is preserved. On the other hand, the sublattice symmetry is broken along the zigzag edge and, therefore, the zigzag edge can possess a characteristic electronic state named a zigzag edge state[5]. A at band appears at the Fermi energy (EF), and results in an electronic state strongly localized on the edge. Such zigzag edge states had rst been con rmed around edges on graphite surfaces by scanning tunneling microscopy and spectroscopy (STM/S)[6{8], in which a single peak appears at around EF in the electronic local density of state (LDOS) only around the edge. Interestingly, such a zigzag edge state is expected to be spin polarized[5]. Spins can be polarized ferromagnetically along an edge, while antiferromagnetically between the edges for a zigzag graphene nanoribbon (z-GNR). How- ever, the experimental study for such a state is limited due to the diculty to fabricate a zigzag edge in ideal shape. The zigzag edge should not only be atomically precise but also be terminated by only one hydrogen (H) atom to preserve the sp2 bonding. Graphene is often micro-fabricated by either oxygen- plasma etching (OPE)[9, 10] or cutting by scanning probes[11, 12]. But one cannot expect atomically aligned edges by such a top-down technique. Moreover, it is not clear how the edges are terminated. On the other hand, one can obtain GNRs with atomically precise and sp2 bonded edges by polymerization of benzene-based molecules[13, 14]. However, the size of a GNR is limited by such a bottom-up technique. Rather recently, it is found that the ideal zigzag edge can be fabricated by etching graphene and graphite sur- face using H-plasma[15{18], in which monatomic deep hexagonal nanopits surrounded by zigzag edges are cre- ated. The LDOS on the edge shows a sharp peak at EF and the suppression of the LDOS next to the peak sug- gests that the edge shows zigzag structure with atomic precision[19]. High-resolution electron energy loss spectroscopy (HREELS) suggests that the edge is sp2 bonded and terminated by only one H[20]. It also shows that the termination is robust in ambient condition. Moreover, STS studies show that the single peak in LDOS around an isolated zigzag edge changes to a double peak on edges of a z-GNR, which strongly suggest the appearance of the spin polarized state[21]. Here, although the detailed mechanism of the anisotropic etching have not been clari ed yet, the etching process can be understood in two steps, i.e., the H-plasma rst creates defects and then en-larges these defects into nanopits. Therefore, if the position of the defect as a nucleation center of nanopit can be controlled, one can expect to shape a graphene in desired structure with zigzag edges. Although such technique has been suggested from a research group[15, 22], it is not well established. In this paper, we set up a H-plasma etching (HPE) system and established a way to shape a graphene by HPE. The study of the etching behavior of our setup suggests that not only the transverse distance from the plasma glow but also a radial position inside the cylindrical chamber makes the etching results di erent. The sudden change of the anisotropic etching at temperature between 400 °C and 500 °C shown in Reference[18] was re- produced with our setup. By preparing nucleation center for hexagonal nanopit using CHF3-plasma, we succeeded to shape a graphene into a desired structure by applying HPE. The atomic force microscopy (AFM) and Raman spectroscopy suggest that the graphene is at to the very edge of the device, and the edge is zigzag structure.

Conclusion

The T and radial position dependences of our HPE setup were shown rest in this paper. The T dependence shows a sudden jump in Dmax at between 400 °C and 500 °C similarly to the case in Reference[18]. It may provide some hint to understanding the etching mechanism. In addition, it is also found that the effect of the anisotropic etching (defect creation) is stronger (weaker) at the bottom of the chamber than at the centre. This fact suggests that the radial distribution of the plasma component, such as H radicals and H ions, make the difference in the etching. Considering that the H ions are stronger radial distribution than H radicals, the anisotropic etching can be attributed to the H radicals, while the defect creation can be to the H ions.