Fabrication and performance of graphene nanoelectromechanical systems
(Color online) (a) Synthesis of graphene by mechanical exfoliation. Graphite is thinned by repeatedly peeling layers apart with scotch tape, then rubbed against a layer of oxide on a silicon wafer. (b) An exceptionally large graphene flake obtained by exfoliation. Reproduced from Ref. 2. (c) Schematic for producing large-area graphene from graphene grown on copper foil, adopted from Ref. 38. Graphene is produced on a roll of copper foil and attached to a polymer support using light pressure between two rollers. Using additional rollers, the copper is dissolved and the graphene is transferred to the final substrate. (d) The results of a similar process on a silicon wafer, together with an optical image showing more than 95% monolayer coverage. The graphene can be stacked to form several-layer-thick sheets, and Raman spectra for various graphene thicknesses are shown. Reproduced from Ref. 38.
(Color online) Graphene resonators made by mechanical exfoliation. (a), (b) Schematic and SEM image of a graphene resonator made by exfoliating graphene over a trench, reproduced from Ref. 44. (c), (d) Schematic and AFM image of a graphene resonator made by exfoliating graphene over a well. The graphene sheet (c) bulges upward in response to pressure, and (d) self-tensions by adhering to the sidewalls of the well when not acted on by other forces. Reproduced from Ref. 48. (e)–(g) Schematic and SEM images of graphene resonators fabricated by mechanical exfoliation followed by lithographic processing. Reproduced from Ref. 49.
(Color online) Graphene resonators made using large-area graphene. (a) Epitaxial graphene resonators fabricated by shaping the graphene on SiC and undercutting it using a wet etch (Ref. 54). (b) Resonators made of reduced graphene oxide. Scale bar, 1 μm (Ref. 56). (c) Arrays of doubly clamped beam resonators (2 and 5 μm in length) made from CVD graphene (Ref. 59).
(Color online) Properties of the quality factor of monolayer graphene. Doubly clamped exfoliated graphene resonators (Ref. 49) (a) and a doubly clamped CVD graphene resonator (Ref. 59) (b) display similar temperature dependence of quality factor. (c) Drumhead resonators (inset top left) made of monolayer CVD graphene display size-dependent quality factors at room temperature. Inset bottom right, the highest published quality factor for a monolayer graphene resonator at room temperature, Q = 2400 ± 300. Reproduced from Ref. 61.
(Color online) Electrically contacted graphene resonators can sense mass, tension, and charge simultaneously. In a color plot of dI/df as a function of frequency f and gate voltage, the graphene resonance stands out as a U-shaped feature. The dependence of the resonant frequency on gate voltage can be fit to extract the density ρ and strain ɛ0 of the graphene sheet. Here, the deposition of pentacene on an as-fabricated exfoliated graphene resonator (a) causes the measured density and tension to increase (b). (c) Subsequent cleaning restores the density of the sheet to that of a pristine graphene resonator. (d) The addition of more pentacene increases the mass and the stress in the graphene. For each step, the charge can be studied by looking at the plots of conductance vs gate voltage. Figure adopted from Ref. 49.
(Color online) (a) Graphene is impermeable to gases, so that it can seal off a small volume of air. The pressure in a small volume enclosed by a graphene sheet can be measured by finding the resonance frequency of the graphene as a function of external pressure. Reproduced from Ref. 48. (b) A yeast cell is contained in a small volume of air beneath a graphene sheet. Because the graphene is transparent to electrons, it is possible to see the analyte inside the well, while the pressure inside the well can be determined via resonance measurements. (Barton et al., unpublished work).
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