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Register a new userOrganometallic Hexahapto Functionalization of Single Layer Graphene as a Route to High Mobility Graphene Devices
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Organometallic hexahapto chromium metal complexation of single layer graphene, which involves constructive rehybridization of the graphene pi-system with the vacant chromium d orbital, leads to field effect devices which retain a high degree of the mobility with enhanced on-off ratio. This hexahapto mode of bonding between metal and graphene is quite distinct from the modification in electronic structure induced by conventional covalent sigma-bond formation with creation of sp3 carbon centers in graphene lattice and this chemistry is reversible.
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Chemical functionalization is a promising route to band gap engineering of graphene. We chemically grafted nitrophenyl groups onto exfoliated single-layer graphene sheets in the form of substrate-supported or free-standing films. Our transport measurements demonstrate that non-suspended functionalized graphene behaves as a granular metal, with variable range hopping transport and a mobility gap ~ 0.1 eV at low temperature. For suspended graphene that allows functionalization on both surfaces, we demonstrate tuning of its electronic properties from a granular metal to a gapped semiconductor, in which charge transport occurs via thermal activation over a gap ~ 80 meV. This non-invasive and scalable functionalization technique paves the way for CMOS-compatible band gap engineering of graphene electronic devices.
The thermoelectric response of high mobility single layer epitaxial graphene on silicon carbide substrates as a function of temperature and magnetic field have been investigated. For the temperature dependence of the thermopower, a strong deviation from the Mott relation has been observed even when the carrier density is high, which reflects the importance of the screening effect. In the quantum Hall regime, the amplitude of the thermopower peaks is lower than a quantum value predicted by theories, despite the high mobility of the sample. A systematic reduction of the amplitude with decreasing temperature suggests that the suppression of the thermopower is intrinsic to Dirac electrons in graphene.
The quantum Hall effect, with a Berrys phase of $pi$ is demonstrated here on a single graphene layer grown on the C-face of 4H silicon carbide. The mobility is $sim$ 20,000 cm$^2$/V$cdot$s at 4 K and ~15,000 cm$^2$/V$cdot$s at 300 K despite contamination and substrate steps. This is comparable to the best exfoliated graphene flakes on SiO$_2$ and an order of magnitude larger than Si-face epitaxial graphene monolayers. These and other properties indicate that C-face epitaxial graphene is a viable platform for graphene-based electronics.
We investigate the spin-to-charge conversion emerging from a mesoscopic device connected to multiple terminals. We obtain analytical expressions to the characteristic coefficient of spin-to-charge conversion which are applied in two kinds of ballistic chaotic quantum dots at low temperature. We perform analytical diagrammatic calculations in the universal regime for two-dimensional electron gas and single-layer graphene with strong spin-orbit interaction in the universal regime. Furthermore, our analytical results are confirmed by numerical simulations. Finally, we connect our analytical finds to recent experimental measures giving a conceptual explanation about the apparent discrepancies between them.
The unique capabilities of capacitance measurements in bilayer graphene enable probing of layer-specific properties that are normally out of reach in transport measurements. Furthermore, capacitance measurements in the top-gate and penetration field geometries are sensitive to different physical quantities: the penetration field capacitance probes the two layers equally, whereas the top gate capacitance preferentially samples the near layer, resulting in the near-layer capacitance enhancement effect observed in recent top-gate capacitance measurements. We present a detailed theoretical description of this effect and show that capacitance can be used to determine the equilibrium layer polarization, a potentially useful tool in the study of broken symmetry states in graphene.
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