Zigzag edges in graphene and graphite defined with a cold hydrogen plasma

Abstract

Graphene has been considered to be an intriguing playground for novel physics, especially since its isolation ten years ago. For the investigation of the predicted properties of graphene, however, clean samples with crystallographically defined edges are of crucial importance. Here, we explore the interaction of graphite and graphene on SiO2 and hBN substrates with hydrogen ions and radicals in two different pressure regimes, searching for a reliable fabrication method for zigzag edged graphene nanoribbons. The samples are prepared by exfoliating graphite and depositing graphitic material on the substrate of interest. Subsequently the samples are exposed to the plasma at various gas pressures and sample-plasma distances. Exposing graphite flakes to a pure hydrogen plasma at pressures around 0.03 mbar, leads to the intercalations of hydrogen atoms in between the top graphite layers where the atoms recombine to hydrogen molecules. This process is reversible as the gas molecules can be released from within the substrate when the samples are heated to elevated temperatures. In this regime a partial hydrogenation of the graphite and graphene surfaces is measured and indicates the formation of graphane. Its band structure is expected to be gapped, opening the way for atomically thin devices employable in electronic industry. Increasing the gas pressure of the plasma to 0.4-1.7 mbar, the graphitic samples are etched by hydrogen radicals at intrinsic or predefined defects, evolving into hexagonally shaped holes, indicating an anisotropic etching process. The anisotropy of the etching process is, however, strongly dependent on the substrate and the amount of graphite layers exposed. The edges of the hexagons are expected to be of zigzag type and can be employed to fabricate graphene nanoribbons with well define edges. Zigzag graphene nanoribbons were predicted to have magnetic edge states and a band gap, opening the possibility of investigating spin filter devices in graphene structures. Stimulated by the unanswered question of strongly differing c-axis resistivities (rho_c) of natural and highly oriented pyrolytic graphite (HOPG), transport experiments, following older investigation are pursued. Reducing the sample height of natural graphitic flakes to micrometer sized samples, noticeably enlarges its rho_c, approaching it to the measured HOPG values. A recent theory unveils the discrepancy in rho_c of natural graphite and HOPG, linking the density of bulk disorder with the value of rho_c. The measurement result of the micrometer sized natural graphitic samples with increased rho_c, distinctly point to a strong influence of the bulk disorder in the graphite flakes on rho_c, confirming the recent theory put forward.