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172 Graphene – Synthesis, Characterization, Properties and Applications relative to the substrate we induce a nonzero charge, this is equivalent to injecting a number of electrons in the upper half or holes in the lower half of the Dirac cones. This effect is known as the field effect (Figure 11). Fig. 11. (a) Energy dispersion relations of graphene. Inset shows the energy dispersion relations along the high symmetry axes near the Dirac point. (b) Conical low-energy spectrum E(k), indicating changes in the position of the Fermi energy EF with changing gate voltage Vg. Positive (negative) Vg induce electrons (holes) in concentrations n = Vg where the coefficient 7.2 1010 cm-2 V-1 for field-effect devices with a 300 nm SiO2 layer used as a dielectric. 3.1 Graphene for nanoelectronics Graphene exhibits a pronounced ambipolar electric field effect such that charge carriers can be tuned continuously between electrons to holes. Single layer graphene atop a thermally grown SiO2 layer on a highly doped Si substrate may serve as a prototype of a field effect transistor. Under this configuration SiO2 serves as an insulating layer, so a back-gate voltage can be applied to vary carrier concentration (figure 11b). Early graphene FET devices demonstrated by Novoselov exhibited dopant concentrations as high as 1013 cm–2 and achieved a mobility that could exceed 10,000 cm2 /Vs (Novoselov, Geim et al. 2004). This translates into ballistic transport on submicron scales. The room-temperature mobility is limited by impurities or corrugations of the graphene surface, which means that it can still be improved significantly up to the order of 105 cm2 /Vs (Bolotin, Sikes et al. 2008; Du, Skachko et al. 2008). Electrons in graphene behave like massless relativistic particles, which govern most of its electronic properties. One of the most important consequences of such unusual dispersion relation is the observation of half-integer Quantum Hall Effect and the absence of localization, which can be very important for graphene-based field effect transistors (Geim and Novoselov 2007). Mechanical exfoliation of highly ordered pyrolitic graphite (HOPG) or high purity graphite flakes can lead to obtain graphene crystals with very few defects, which in turn exhibit high mobility of the charge carriers. Figure 12 shows scanning electron microscopy (SEM) and atomic force microscopy (AFM) of the graphene- based device reported in the literature as having the highest electron mobility to date (Bolotin, Sikes et al. 2008). The graphene film was obtained by mechanical exfoliation of graphite on Si/SiO2 substrate in which the oxide layer underneath the graphene was etched in order to obtain a free-standing graphene flake connecting the metal electrodes.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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