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We discuss transport through double gated single and few layer graphene devices. This kind of device configuration has been used to investigate the modulation of the energy band structure through the application of an external perpendicular electric field, a unique property of few layer graphene systems. Here we discuss technological details that are important for the fabrication of top gated structures, based on electron-gun evaporation of SiO$_2$. We perform a statistical study that demonstrates how --contrary to expectations-- the breakdown field of electron-gun evaporated thin SiO$_2$ films is comparable to that of thermally grown oxide layers. We find that a high breakdown field can be achieved in evaporated SiO$_2$ only if the oxide deposition is directly followed by the metallization of the top electrodes, without exposure to air of the SiO$_2$ layer.
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We induce surface carrier densities up to $sim7cdot 10^{14}$cm$^{-2}$ in few-layer graphene devices by electric double layer gating with a polymeric electrolyte. In 3-, 4- and 5-layer graphene below 20-30K we observe a logarithmic upturn of resistance that we attribute to weak localization in the diffusive regime. By studying this effect as a function of carrier density and with ab-initio calculations we derive the dependence of transport, intervalley and phase coherence scattering lifetimes on total carrier density. We find that electron-electron scattering in the Nyquist regime is the main source of dephasing at temperatures lower than 30K in the $sim10^{13}$cm$^{-2}$ to $sim7 cdot 10^{14}$cm$^{-2}$ range of carrier densities. With the increase of gate voltage, transport elastic scattering is dominated by the competing effects due to the increase in both carrier density and charged scattering centers at the surface. We also tune our devices into a crossover regime between weak and strong localization, indicating that simultaneous tunability of both carrier and defect density at the surface of electric double layer gated materials is possible.
We report a systematic study of the contact resistance present at the interface between a metal (Ti) and graphene layers of different, known thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate that the contact resistance is quantitatively the same for single-, bi-, and tri-layer graphene ($sim800 pm 200 Omega mu m$), and is in all cases independent of gate voltage and temperature. We argue that the observed behavior is due to charge transfer from the metal, causing the Fermi level in the graphene region under the contacts to shift far away from the charge neutrality point.
Series connection of four quantum Hall effect (QHE) devices based on epitaxial graphene films was studied for realization of a quantum resistance standard with an up-scaled value. The tested devices showed quantum Hall plateaux RH,2 at filling factor i = 2 starting from relatively low magnetic field (between 4 T and 5 T) when temperature was 1.5 K. Precision measurements of quantized Hall resistance of four QHE devices connected by triple series connections and external bonding wires were done at B = 7 T and T = 1.5 K using a commercial precision resistance bridge with 50 microA current through the QHE device. The results showed that the deviation of the quantized Hall resistance of the series connection of four graphene-based QHE devices from the expected value of 4*RH,2 = 2h/e^2 was smaller than the relative standard uncertainty of the measurement (< 1*10^-7) limited by the used resistance bridge.
Graphene-based heterostructures display a variety of phenomena that are strongly tunable by electrostatic local gates. Monolayer graphene (MLG) exhibits tunable surface plasmon polaritons, as revealed by scanning nano-infrared experiments. In bilayer graphene (BLG), an electronic gap is induced by a perpendicular displacement field. Gapped BLG is predicted to display unusual effects such as plasmon amplification and domain wall plasmons with significantly larger lifetime than MLG. Furthermore, a variety of correlated electronic phases highly sensitive to displacement fields have been observed in twisted graphene structures. However, applying perpendicular displacement fields in nano-infrared experiments has only recently become possible (Ref. 1). In this work, we fully characterize two approaches to realizing nano-optics compatible top-gates: bilayer $text{MoS}_2$ and MLG. We perform nano-infrared imaging on both types of structures and evaluate their strengths and weaknesses. Our work paves the way for comprehensive near-field experiments of correlated phenomena and plasmonic effects in graphene-based heterostructures.
We analyze the response of bilayer graphene to an external transverse electric field using a variational method. A previous attempt to do so in a recent paper by Falkovsky [Phys. Rev. B 80, 113413 (2009)] is shown to be flawed. Our calculation reaffirms the original results obtained by one of us [E. McCann, Phys. Rev. B 74, 161403(R) (2006)] by a different method. Finally, we generalize these original results to describe a dual-gated bilayer graphene device.
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