We use an atomic force microscope (AFM) to manipulate graphene films on a nanoscopic length scale. By means of local anodic oxidation with an AFM we are able to structure isolating trenches into single-layer and few-layer graphene flakes, opening the possi
which allows the formation of a water meniscus between the AFM-tip and the device surface; a schematic setup is shown in Fig. 3. By applying a positive voltage between the graphene sheet and the tip, the graphene can be locally oxidized below the tip following the concept of electrochemical oxidation. At the cathode the current-induced oxidation of carbon leads to the formation of a variety of carbon-based oxides and acids that will escape from the surface and a groove forms in the graphene sheet directly underneath the AFM tip.
Figure 4 shows the experimental realization of this principle: The (doped) silicon tip of the AFM is moved in contact mode slowly (vtip = 0.05 μm/s) across a contacted few-layer graphene flake with a constant voltage (Vox = 25 V) applied between the tip and the graphene sheet. During this process the environment is kept at a constant humidity of 55 % at a temperature of 27 °C. As the graphene flake is oxidized in half, the resistance measured across the flake drastically increases (Fig. 4c). Figure 4a shows an AFM micrograph of the resulting groove with a width of less than 30 nm, as can be seen in the cross section of Fig. 4b along the indicated line in Fig 4a. The remaining graphene on both sides of the groove stays intact, making them ideal for graphene in-plane gates [15, 32] in more complicated structures. The width of the oxidized grooves typically varies between 30 and 100 nm, mainly depending on the apex of the used AFM-tip, and thereby defines the limit on the resolution possible with this technique.
Although the principle of local anodic oxidation sound rather straightforward, it is important to remark that this technique only works if the line is stared at the edge of a graphene sheet. Oxidizing bulk graphite or starting the oxidation in the middle of a graphene sheet turned out to be practically impossible even with voltages up to 40 V. Most likely the carbon-carbon bonds in the center of a graphene sheet are too strong to be broken directly. In contrast, the edge-termination of graphene [33] by e.g. hydrogen atoms can substantially facilitate the initial
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