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ElelcetrcontircoTrnanicspoTrtrParonpesrtpieosorftFPewr-oLapyeerGrtriaephsenoefMaFteriawls-LayerGrapheneMaterials 149 conductive graphene channel, whereas the Fermi level of graphene is continuously driven from the valence to the conduction band by means of a gate voltage, see Fig. 6a. As the Fermi level is driven inside the conduction (valence) band, the conductivity increases with increasing the concentration of electrons (holes) induced by the gate voltage. At the touching point between the valence and conduction bands the Fermi level crosses the zero density of states point -i.e. the Dirac point- where the conductance reaches its minimum value. Indeed, despite the density of sates vanishes at the Dirac point the conductivity remains finite with a minimum value of ≈ 4e2/πh for the ballistic transport regime (Geim & Novoselov (2007); Katsnelson (2006); Novoselov & Geim (2005); Tworzydlo et al. (2006)). Theoretically a finite conductivity is expected for ballistic electrical transport in graphene at the neutrality point assisted by evanescent wave propagation. This evanescent wave propagation gives rise to a unique non-monotonous dependence of the Fano factor on the charge density in shot-noise (Danneau & Wu et al. (2008); Danneau & Craciun et al. (2008); Danneau et al. (2009); Tworzydlo et al. (2006)). The predicted minimum conductivity 4e2/πh has only been experimentally observed in devices with a short graphene channel. So far, most of the experimental studies on the minimum of conductivity have been conducted in supported graphene -e.g. graphene on SiO2 substrate. Ultrapure suspended graphene devices do not suffer of the presence of substrate-related disorder and are therefore the ideal candidate to address the physics governing electrical transport at the minimum of conductivity. The presence of disorder such as adatoms or molecules, ripples of the graphene sheet, atomic vacancies and topological defects is expected to affect the electronic properties of graphene such as the conductivity and charge carrier mobility. In particular, close to the Dirac point, charged impurities create electron/hole puddles which dominate the charge transport properties of graphene (Martin et al. (2008)). For small energies around the Dirac point recent theoretical advances also pointed out that strong short-range interaction caused by scattering off adatoms and molecules on the surface of graphene can actually be the dominant source of disorder limiting the charge carrier mobility. During the fabrication process of transistor devices, graphene is exposed to environmental conditions, it is therefore likely that for instance hydrocarbons are adsorbed on the surface of graphene. Whenever an hydrocarbon covalently bonds to the graphene, the 2pz delocalized electrons are localized into a σ-bond, i.e. a covalently bond hydrocarbon effectively act as a vacancy. The charge carrier scattering off the resonant states induced in the vicinity of the Dirac point together with scattering off charged impurities are likely to play a dominant role in graphene devices, and this is currently at the focus of both theoretical and experimental research (Ferreira et al. (2011); Peres (2010)). 3. Experimental observation of gate tuneable band structure in few-layer graphene One of the most remarkable physical property of graphene materials is the ability to reversibly tune the band structure of these systems simply by means of an external electric field (Craciun et al. (2011)). In standard semiconducting materials a precise value of the band-gap is engineered during the growth process, therefore the value of this energy gap cannot be reversibly controlled in situ in a device. Few-layer graphene is the only known class of materials to exhibit a gate tuneable band structure and this unprecedented property paves the way for devices with novel functionalities (Craciun et al. (2011)). Different experimental approaches have been implemented to address the band structure of graphene materials. Respectively, pioneering charge transport experiments (Castro et al. (2007); Craciun et al. (2009); Oostinga et al. (2008)) followed by photoemission spectroscopy (Ohta et al. (2006)) and infrared spectroscopy (Mak et al. (2009); Zhang & Li et al. (2008);PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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