Elastic Properties of Chemically Derived Single Graphene Sheets

Abstract

The elastic modulus of freely suspended graphene monolayers, obtained via chemical reduction of graphene oxide, was determined through tip-induced deformation experiments. Despite their defect content, the single sheets exhibit an extraordinary stiffness (E ) 0.25 TPa) approaching that of pristine graphene, as well as a high flexibility which enables them to bend easily in their elastic regime. Built-in tensions are found to be significantly lower compared to mechanically exfoliated graphene. The high resilience of the sheets is demonstrated by their unaltered electrical conductivity after multiple deformations. The electrical conductivity of the sheets scales inversely with the elastic modulus, pointing toward a 2-fold role of the oxygen bridges, that is, to impart a bond reinforcement while at the same time impeding the charge transport.Recent experiments have revealed the thermodynamic stability of graphene under ambient conditions, 1-3 which strongly revived the interest in the electrical and mechanical properties of this fascinating carbon nanostructure. Two major methods have been established for the fabrication of graphene monolayers, namely, mechanical exfoliation of graphite 2 and vacuum graphitization of silicon carbide. 3,4 More recently, chemical reduction of graphene oxide (GO) has been reported as an alternative, solution-based route, for obtaining graphenelike sheets which offers the advantages of being cheap and up-scalable. [5][6][7][8] Even though GO is a good insulator, deoxygenation has been shown to substantially enhance its electrical conductivity, albeit the obtained values of ∼1 S/cm remain 2-3 orders of magnitude below that of pristine graphene. 6,7 Due to the limited efficiency of the reduction process, the obtained sheets still contain residual oxygenated functional groups of the starting material. Microscopic studies of the reduced GO also indicate the coexistence of graphitic regions with defect clusters 6,9 in agreement with proposed models. [9][10][11] In addition to its interesting electrical characteristics, this 2D material is expected to have also unique mechanical properties. Recent studies have witnessed its suitability for the fabrication of composites 12,13 and paperlike materials 14 with excellent mechanical properties. However, in order to promote its application in nanotechnology, the mechanical characterization of single sheets is of upmost relevance.Here we present a study of the mechanical and electrical properties of suspended, chemically reduced GO single sheets. Graphite oxide prepared via Hummers method 15 was dispersed in water, deposited onto a Si/SiO 2 substrate, and subsequently reduced by hydrogen plasma treatment. The obtained samples exhibited a high yield of monolayers with lateral dimensions of 0.1-5 µm. Preselected layers were then contacted by standard e-beam lithography with Ti/Au electrodes. In order to freely suspend the layers (Figure 1), the samples were wet etched by buffered hydrofluoric acid, followed by critical point drying to prevent ...

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