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Impermeable Atomic Membranes from Graphene Sheets



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Supplementary Information Impermeable Atomic Membranes from Graphene Sheets J Scott Bunch Scott S Verbridge Jonathan S Alden Arend M van der Zande Jeevak M Parpia Harold G Craighead Paul L McEuen Cornell Center for Materials Research Cornell University Ithaca NY 14853 Experimental Methods Graphene drumheads are fabricated by a combination of standard photolithography and mechanical exfoliation of graphene sheets First a series of squares with areas of 1 to 100 m2 are defined by photolithography on an oxidized silicon wafer with a silicon oxide thickness of 285 nm or 440 nm Reactive ion etching is then used to etch the squares to a depth of 250 nm to 3 m leaving a series of wells on the wafer Mechanical exfoliation of Kish graphite using Scotch tape is then used to deposit suspended graphene sheets over the wells To determine the elastic constants of graphene using equation 3 we extrapolate the deflection in Fig 1e inset to z 181 nm to account for a 40 minute sample load time assume an initial pressure difference across the membrane p 93 kPa and a negligible initial tension The latter two assumptions are verified using resonance measurements The actual deflection used in equation 3 is obtained by subtracting the extrapolated deflection z 181 nm from the initial deflection z0 23 3 nm at p 0 This initial deflection is determined from the AFM image in Fig 4a and AFM force distance curves Fig S1 1 Slack and Self Tensioning at p 0 Since the cantilever surface interaction is expected to be different for AFM measurements over the relatively pliable suspended and the rigid SiO2 supported graphene the depth of the membrane z0 at p 0 must be determined via force and amplitude calibrations of the cantilever over each surface Whittaker Minot et al 2006 A representative calibration measurement is shown in Fig S 1 Both the amplitude upper and deflection lower of the AFM tip is measured while approaching the surface Over the SiO2 supported surface the difference between the actual



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