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2009 CARBON ALLOTROPESElectric field doping of few-layer grapheneThe electronic properties of few-layer graphene have beeninvestigated at low temperature and high magnetic field.Few-layer graphene systems consist of a few tens of planesof carbon atoms stacked upon each others and exhibit acomplex electronic band structure. Regarding their transportproperties, they are natural candidates to study thecross over from 3D graphite to 2D graphene. So far, manystudies have focused on graphene, where a linear dispersionrelation at the K and K ′ points of the first Brillouin zoneleads to transport properties governed by massless quasiparticles.Such charge carrier dynamic makes graphenea unique 2D system and is responsible, among other, tothe anomalous quantum Hall effect. On the other hand,graphite supports the presence of different groups of chargecarriers (electron-like and hole-like quasi-particles), amongwhich massless Dirac fermions have already been reported[Luk’yanchuck et al. Phys. Rev. Lett., 93, 166402 (2004)].The experimental investigation of few-layer graphene hasbeen undertaken in an attempt to determine the relative contributionsof the different types of charge carriers to electronictransport, in a system where a small number of carbonlayers makes its electronic properties half-way betweenthose of graphite and graphene.+70 V shows a monotonously increasing function with nohint of resistance maximum. We therefore infer that thesample is intrinsically p-doped and that the charge neutralitypoint is out of the experimental range. Contaminantsand/or defects are responsible for shifting the charge neutralitypoint away from V g = 0 V.Figure 24: 3D plot of the Hall resistance R xy as a function ofmagnetic field B and gate voltage V gFigure 23: Oscillatory part of R xx (B,V g ) after subtracting asmooth background function. For clarity, the curves are shiftedvertically by 1.5 kΩThe few-layer graphene sample is obtained by exfoliationof bulk graphite and deposited onto a heavily dopedSi/SiO 2 substrate with 300 nm oxide thickness. Fourelectrodes have been fabricated on the sample using photolithographyso that the two-probe longitudinal and Hallresistance could be simultaneously measured during a pulseof the magnetic field. A gate voltage is applied thoughthe SiO 2 dielectric in order to continuously tune the chargecarrier density. As compared to the simple plane capacitormodel usually accepted for graphene, the few-layergraphene system displays reduced gate efficiency, mostprobably due to large screening effects. The gate voltagedependence of the resistance in the range −70 < V g