Research on graphene was originally motivated by its peculiar electrical transport properties and the promise of future applications in nanoelectronics. [1,2] Graphene sheets, owing to their exceptional thermal and mechanical properties and high electrical conductivity, are also of great interest to serve as new nanoscale building blocks to create unique macroscopic materials. [3][4][5] Recent studies have shown that graphene sheets can be prepared in large quantity through chemical conversion from graphite, [6][7][8][9][10][11][12] which has facilitated the fabrication of graphene-based electronic devices [13,14] and has also made it possible to create new bulk materials comprising graphene sheets for a broader range of applications. [3][4][5]15,16] For example, it has been recently demonstrated that chemically modified graphene sheets can be dispersed throughout a polymeric [3] or inorganic matrix [15] to make electrically conducting composites with a percolation threshold as low as 0.1 vol %. Thin graphene films have also been used as potential transparent electrodes for solar cells. [16] More recently, Ruoff and co-workers have demonstrated that graphene oxide (GO) sheets dispersed in water can be assembled into a well-ordered structure under a directional flow, yielding ultrastrong GO paper.[4] GO paper is superior to many other paperlike materials in stiffness and strength, however, a lack of electrical conductivity limits its use. Although GO paper can be rendered conductive by thermal annealing, the structure and mechanical properties seriously deteriorate after this treatment (see below for further discussion). We have recently demonstrated that aqueous dispersions of graphene sheets can be readily produced without the need for polymeric or surfactant stabilizers. [12] This has enabled us to prepare electrically conductive graphene paper directly, using the same strategy that has been used to make carbon nanotube buckypaper and GO papers. [4,17,18] Here, we demonstrate that the resulting graphene paper displays a remarkable combination of thermal, mechanical, and electrical properties, whilst preliminary cytotoxicity tests suggest biocompatibility, making this new material attractive for many potential applications. Graphene dispersions were prepared by controlled reduction of GO dispersions with hydrazine using the procedure reported in one of our recent publications.[12] Graphene paper was then fabricated by filtration of a measured amount of graphene dispersion through an Anodisc membrane filter, followed by air drying and peeling from the filter. The samples of graphene paper were annealed at different temperatures before being cooled to room temperature for various measurements (see the Experimental section for details).We observe that as-prepared graphene paper displays a shiny metallic luster on both sides (Fig. 1A). Scanning electron microscopy (SEM) analysis reveals that the surface of the graphene paper is quite smooth (Fig. 1B) and the fracture edges of the papers exhibit a layered structure ...

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