Graphene as a subnanometre trans-electrode membrane

Isolated, atomically thin conducting membranes of graphite, called graphene, have recently been the subject of intense research with the hope that practical applications in fields ranging from electronics to energy science will emerge1. Here, we show that when immersed in ionic solution, a layer of graphene becomes a new electrochemical structure we call a trans-electrode. The transelectrode's unique properties are the consequence of the atomic scale proximity of its two opposing liquid-solid interfaces together with graphene's well known in-plane conductivity. We show that several trans-electrode properties are revealed by ionic conductance measurements on a CVD grown graphene membrane that separates two aqueous ionic solutions. Although our membranes are only one to two atomic layers2,3 thick, we find they are remarkable ionic insulators with a very small stable conductance that depends on the ion species in solution. Electrical measurements on graphene membranes in which a single nanopore has been drilled show that the membrane's effective insulating thickness is less than one nanometer. This small effective thickness makes graphene an ideal substrate for very high-resolution, high throughput nanopore-based single molecule detectors. The sensitivity of graphene's in-plane electronic conductivity to its immediate surface environment, as influenced by trans-electrode potential, will offer new insights into atomic surface processes and sensor development opportunities. We measured the trans-electrode ionic conductance of a 0.5 × 0.5 mm, CVD grown, sheet of graphene mounted across the surface of a 200 × 200 nm aperture in a 250 nm thick, freestanding, insulating SiNx layer on a Si substrate chip (Fig. 1). Micro-Raman spectroscopy scans of the G, G' peaks from the graphene showed it to consist of a mixture of 1 and 2 atomic layer regions of graphene3,4 with ~10μ m domains. The chip-mounted membrane was inserted in a fluidic cell so that it separated two compartments, each subsequently filled with ionic solutions electrically contacted with Ag/AgCl electrodes. The small diameters of the PDMS seals in the fluidic cell precluded ionic solution from leaking around the edges of the graphene. Users may view, print, copy, download and text and datamine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature.com/authors/editorial_policies/license.html#terms *Corresponding Authors Correspondence and requests for materials should be addressed to J.A.G. ([email protected]). *S.G. ([email protected]).. Author Contributions Graphene samples were grown by J.K and A.R. Experiments and calculations were performed by S.G. Other activities, including data interpretation, conclusions, and manuscript writing, were carried out collaboratively at Harvard by S.G., B. H., D. B. and J. A. G. NIH Public Access Author Manuscript Nature. Author manuscript; available in PMC 2011 March 9. Published in final edited form as: Nature. 2010 September 9; 467(7312): 190–193. doi:10.1038/nature09379. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript With 100 mV bias applied between the two Ag/AgCl electrodes, current measurements in a variety of chloride electrolytes show that the graphene membrane's trans-conductance is far below the nS level (Table I). The highest conductances are observed for solutions with the largest atomic size cations, Cs and Rb, correlated with a minimal hydration shell that mediates their interaction with the graphene5,6. We attribute this conductance to ion transport through defect structures in the free-standing graphene. Contributions from electrochemical currents to and from the graphene can be ruled out (Methods). The observed conductances for different cations falls much faster than the solution conductivities on going from CsCl to LiCl (Table 1), suggesting an influence of graphene-cation interactions. Nevertheless we cannot completely rule out ionic transport through graphene that is in contact with the chip surface. Small asymmetries and nonlinearities in the I–V curves were observed in the data for Table 1 and elsewhere (e.g., Fig. 2), reflecting asymmetrical properties of the graphene surfaces associated with its CVD growth3 or transfer to the chip. E-beam drilling a single nanometer scale pore7 in the graphene trans-electrode membrane increases its ionic conductivity by orders of magnitude (Fig. 2). Experiments with known nanopore diameters and solution conductivities allow one to deduce graphene's effective insulating thickness. The ionic conductance G of a pore of diameter d in an infinitely thin insulating membrane is given by8