Graphene research has attracted enormous attention since the discovery of the material in 2004 . Graphene can be viewed as a two-dimensional (2D) honeycomb lattice that is constructed with a single layer of carbon atoms. It not only provides a unique structure for fundamental research but is also an ideal material for future nanoelectronics and sensor applications [2,3]. Graphene has demonstrated fascinating chemical, mechanical, and electrical properties. For example, when exposed to different gases, graphene can be chemically doped to either p-type or n-type . A nanoindentation measurement of a freestanding monolayer graphene membrane using atomic force microscopy (AFM) shows that the material has an ultrahigh Young’s modulus of 1.0 TPa, which makes graphene one of the strongest materials ever measured . Depending on the number of layers in the sheet, the electrical properties of graphene alter accordingly, changing it from a semiconductor to a metal, or vice versa .
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