Investigations of graphene and its interactions with foreign atoms and molecules

Graphene is nowadays one of the most dynamic research topics in the field of nanoscience. It has potential applications in newly designed electronic devices with high charge mobility and ideal structural properties (lightness and flexibility). From the theoretical point of view, its specific structure represents an ideal background for testing and predicting novel electronic and structural effects which are not present in conventional materials.

Fig. 1 illustrates an atomic model of a honeycomb like network of a graphene sheet and the scanning tunneling microscopic (STM) image of a single layer graphene grown on silicon carbide.

Fig. 1a) An atomic model of a honeycomb like network of a graphene sheet. b) STM image recorded from the graphene layer at the size of 2x2 nm2

Research on the electronic properties of graphene has followed two parallel courses. One course involves the study of mechanically exfoliated graphene sheets. Thereby graphene flakes (typically micron size, Fig. 2(a)) are mechanically peeled from a bulk graphite crystal onto a supporting substrate. Once a single graphene sheet is successfully identified by optical microscopy, metal contacts are attached for transport studies. In the second research course graphene is directly grown on large area insulating or semiconducting substrates, as illustrated in Fig. 2 (b)-(c). Once grown, the films are lithographically patterned and metal contacts applied to make electronic devices. When considering the graphene/substrate interaction, it is remarkable that the mono- or multilayer graphene films grown on silicon carbide substrate (SiC) show electronic properties similar to an isolated graphene sheet. One may think that the substrate should influence the unusual properties of graphene. This is also one of the reasons why graphene grown on SiC has been the focus of research targeting a path towards graphene electronics. This is very fortuitous since SiC is a robust wide band gap semiconductor and has a superior range of properties from inert to bio-compatible and is excellently suited for high temperature and high power applications.

Fig. 2. (a) mechanical exfoliation graphene flake visualized by atomic force microscopy [2]. (b) LEEM image showing a graphene layer grown by high temperature annealing SiC wafer at 1310 °C in vacuum, field of view (FOV) 5 µm [3]. (c) graphene layer grown in inductively heated furnace at 2000 °C, FOV 50 µm. [1,4]

For a large scale integration of graphene-based nanoelectronics, the band engineering, electrical contacts, and a high-quality graphene sheet on a suitable substrate play an equally important role. The electronic band structure of a pure single graphene layer classifies it as gapless semiconductor (Fig. 3(a)). Some device applications require, however, a gap between the two bands as displayed in Fig. 2(b)-(d). One way to create such a gap is to grow more than one layer of graphene in a controllable way. Another possibility is to cut a narrow ribbon from the graphene sheet with a width of less than 100 nm, thereby confining the electrons and holes to a “quantum box” and splitting the energies of the two bands. Such cutting can modify further the properties of graphene because the dangling electron bonds at the ribbon edges are chemically active and can capture elements from the environment. However, recent findings suggested that the size of the gap can also be controlled by varying the amount of hydrogen on its surface. This may result in producing a tailored graphene gap without cutting graphene into ribbons.

Fig. 3. The π and π* bands near EF for 1-4 graphene layers, respectively [5]

However adding atomic hydrogen to graphene is not a simple task. Therefore we will find out how to saturate the dangling bonds in a simpler way or with simpler adsorbates. In addition, there is an urgent need for obtaining and classifying good metallic contacts on a graphene sheet. Especially for the epitaxial graphene sheet, the substrate may contribute to the contact and plays a role depending on the metals selected. Moreover, due to its intriguing electronic properties, it is also of interest for use in sensor applications in studies of adsorption phenomena ranging from atoms to bio-molecules.

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[2]. A.K. Geim and K. S. Novoselov, Nature Mat. 6, 183 (2007)
[3]. T. Ohta, F. El Gabaly, A. Bostwick, J. L. McChesney, K. V. Emtsev, A. K. Schmid, Th. Seyller, K. Horn, and E. Rotenberg, New J. Phys. 10, 023034 (2008)
[4]. C. Virojanadara, M. Syväjärvi,, R. Yakimova, L. I. Johansson, A. A. Zakharov, T. Balasubramanian, Phys. Rev. B 78, 245403 (2008)
[5]. T. Ohta, A. Bostwick, J. L. McChesney, Th. Seyller, K. Horn, and E. Rotenberg, Phys. Rev. Lett. 98, 206802 (2007)

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