Epitaxial graphene on SiC(0001) suffers from strong intrinsic n-type doping. We demonstrate that the excess negative charge can be fully compensated by non-covalently functionalizing graphene with the strong electron acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ). Charge neutrality can be reached in monolayer graphene as shown in electron dispersion spectra from angular resolved photoemission spectroscopy (ARPES). In bilayer graphene the band gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band gap. The reduction of the charge carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy. The structural and electronic characteristics of the graphene/F4-TCNQ charge transfer complex are investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The doping effect on graphene is preserved in air and is temperature resistant up to 200• C. Furthermore, graphene non-covalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultra-high vacuum but also by wet chemistry.

The intrinsic doping level of graphene prepared by mechanical exfoliation and standard lithography procedures on thermally oxidized silicon varies significantly and seems to depend strongly on processing details and the substrate morphology. Moreover, transport properties of such graphene devices suffer from hysteretic behavior under ambient conditions. The hysteresis presumably originates from dipolar adsorbates on the substrate or graphene surface. Here, we demonstrate that it is possible to reliably obtain low intrinsic doping levels and to strongly suppress hysteretic behavior even in ambient air by depositing graphene on top of a thin, hydrophobic self-assembled layer of hexamethyldisilazane (HMDS). The HMDS serves as a reproducible template that prevents the adsorption of dipolar substances. It may also screen the influence of substrate deficiencies.

Raman spectra were measured for mono-, bi-and trilayer graphene grown on SiC by solid state graphitization, whereby the number of layers was pre-assigned by angle-resolved ultraviolet photoemission spectroscopy. It was found that the only unambiguous fingerprint in Raman spectroscopy to identify the number of layers for graphene on SiC(0001) is the linewidth of the 2D (or D*) peak. The Raman spectra of epitaxial graphene show significant differences as compared to micromechanically cleaved graphene obtained from highly oriented pyrolytic graphite crystals. The G peak is found to be blue-shifted. The 2D peak does not exhibit any obvious shoulder structures but it is much broader and almost resembles a single-peak even for multilayers. Flakes of epitaxial graphene were transferred from SiC onto SiO 2 for further Raman studies. A comparison of the Raman data obtained for graphene on SiC with data for epitaxial graphene transferred to SiO 2 reveals that the G peak blue-shift is clearly due to the SiC substrate. The broadened 2D peak however stems from the graphene structure itself and not from the substrate. 2Graphene is the building block of graphite and carbon-based nanomaterials such as carbon nanotubes and fullerenes. Since the development of the micromechanical cleavage method to obtain thermodynamically stable mono-and few-layer graphene, our understanding of this purely twodimensional system has improved significantly. 1 , 2 , 3 , 4 Graphene exhibits unconventional electronic properties, 1,2,5,6 such as high and nearly equal mobilities at room temperature for both electron and hole conduction, which makes it a strong candidate for nanoelectronic circuit applications. 5,7 However, the micromechanical cleavage method is not suitable for obtaining large area graphene. For practical applications requiring large areas of graphene, full graphitization on SiC seems to be the more promising route. 8,9,10,11 Indeed, devices of epitaxial graphene on SiC have been prepared using conventional e-beam lithography and a patterning method based on O 2 plasma etching 8,9 although several problems still remain: the nature of the (6√3×6√3)R30° reconstruction at the interface between SiC and graphene is still under debate and the proper conditions for the production of large areas of homogeneous mono-, biand few-layer graphene are not well developed. 10,12Raman spectroscopy is known to be a powerful tool to determine the electronic properties of carbonbased materials and there have been several reports about Raman measurements on graphene layers which were micromechanically exfoliated from highly oriented pyrolytic graphite (HOPG) on SiO 2 substrates. 13,14,15,16,17 Recently, Ferrari et al. have demonstrated that the shape of the 2D Raman peak may serve as the fingerprint to distinguish mono-, bi-and few-layer graphene. The 2D peak stems from a double resonance electron-phonon scattering process. 14 For monolayer graphene the 2D peak can be fitted to a single Lorentzian, whereas the multiple bands in bilayers or few-layer gra...

Application of a strong perpendicular magnetic field B to a two-dimensional electron gas effectively quenches the kinetic energy of electrons and gives rise to flat energy bands called Landau levels (LLs) which contain a total of eB/h states, where e is the electron charge and h is Planck's constant. In graphene, each of these states has an additional fourfold degeneracy due to the spin and sublattice degrees of freedom, and the LLs possess an approximate SU(4) LLs. This occurs in graphene at filling factors Ȟ = neB/h = 4(N + 1/2) in the absence of interelectron interactions 7-9 , where n is the charge carrier density and N is the orbital index.Hence, the quantum Hall sequence is shifted by a half-integer, a distinctive signature that reflects the sublattice pseudospin of graphene.When disorder is low and at high magnetic field, Coulomb forces between electrons become important and many-body effects emerge. Recently, the fractional quantum Hall effect (FQHE) of Dirac fermions has attracted considerable attention [10][11][12][13][14][15][16][17][18][19][20][21][22][23] . In graphene, the low dielectric constant and unique band structure lead to fractional quantum Hall states with energy gaps that are larger than in GaAs at the same field, particularly in the N = 1 LL 11, 17, 18 .Moreover, the SU(4) symmetry of charge carriers in graphene could yield fractional quantumHall states without analogues in GaAs 12-14 . The FQHE was recently observed [24][25][26] in suspended graphene samples at Ȟ = 1/3 and 2/3, with an activation gap at Ȟ = 1/3 of approximately 2 meV at B = 14 T. Measurements of graphene on hexagonal boron nitride substrates 27 revealed further fractional quantum Hall states at all multiples of Ȟ = 1/3 up to 13/3, except at Ȟ = 5/3, but no conductance plateaus were observed at filling factors with higher denominators. It was suggested that the absence of a fractional quantum Hall state at Ȟ = 5/3 might result from lowlying excitations associated with SU(2) or SU(4) symmetry, but alternate scenarios associated with disorder could not be ruled out 27 .Here we report local electronic compressibility measurements of graphene performed using a scanning single-electron transistor (SET) 28, 29 . We observe a unique pattern of incompressible fractional quantum Hall states at filling factors with odd denominators as large as nine. Figure 1a shows a schematic of the measurement setup. By modulating the carrier density 4 and monitoring the resulting change in SET current, we measure both the local chemical potential µ and the local inverse electronic compressibility dµ/dn of the graphene flake.The inverse electronic compressibility as a function of carrier density and magnetic field is shown in Fig. 1b. At zero magnetic field, we observe an incompressible peak that arises from the vanishing density of states at the charge neutrality point in graphene. For B > 0, strong incompressible behavior occurs at Ȟ = 4(N + 1/2), confirming the monolayer nature of our sample. In addition to the expected single-particle quan...

The application of graphene in electronic devices requires large scale epitaxial growth. The presence of the substrate, however, usually reduces the charge carrier mobility considerably. We show that it is possible to decouple the partially sp 3 -hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp 2 -hybridized graphene layer with improved electronic properties.Electrons in graphene -sp 2 -bonded carbon atoms arranged in a honeycomb lattice -behave like massless Dirac particles and exhibit an extremely high carrier mobility [1]. So far, the only feasible route towards large scale production of graphene is epitaxial growth on a substrate. The presence of the substrate will, however, influence the electronic properties of the graphene layer. To preserve its unique properties it is desirable to decouple the graphene layer from the substrate. Here we present a new approach for the growth of highly decoupled epitaxial graphene on a silicon carbide substrate. By decoupling the strongly interacting, partially sp 3hybridized first graphitic layer (commonly referred to as zero layer (ZL) [2]) from the SiC(0001) substrate by gold intercalation, we obtain a completely sp 2 -hybridized graphene layer with improved electronic properties as confirmed by angleresolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM) and Raman spectroscopy.There are essentially two ways for large scale epitaxial growth of graphene on a substrate: by cracking organic molecules on catalytic metal surfaces [3][4][5][6][7] or by thermal graphitization of SiC [2,[8][9][10][11]. Unfortunately, the presence of the substrate alters the electronic properties of the graphene layer on the surface and reduces the carrier mobility. Even though it has been shown that the graphene layer can be decoupled from a metallic substrate [6,[12][13][14] the system remains unsuitable for device applications. This problem can be solved by decoupling the graphene layer from a semiconducting SiC substrate [15].On both the silicon and the carbon terminated face of a SiC substrate, graphene is commonly grown by thermal graphitization in ultra high vacuum (UHV). When annealing the substrate at elevated temperatures Si atoms leave the surface whereas the C atoms remain and form carbon layers. On SiC(0001), the so-called C-face, the weak graphene-tosubstrate interaction results in the growth of rotationally disordered multilayer graphene and a precise thickness control becomes difficult [16]. On the other hand, the rotational disorder decouples the graphene layers so that the transport properties resemble those of isolated graphene sheets with room temperature mobilities in excess of 200,000 cm 2 /Vs [17].On SiC(0001), i. e. the Si-face, the comparatively strong graphene-to-substrate interaction results in uniform, long-range ordered layer-by-layer growth. The first carbon layer (=ZL) grown on the Si-face is partially sp 3 -hybridized to the substrate, wh...