Ultra large and single-layer graphene oxide sheets (up to millimeter in lateral size) are obtained by a modified Hummers’ method, where we replace the first aggressive oxidation process with a short sonication step in H2SO4 solutions. The lateral size of obtained GO sheets can be adjusted by the sonication period: it decreases with the increasing sonication time. The thin-film electrodes made from ultra large reduced GO sheets exhibit lower sheet resistance compared with those from small-size reduced GO sheets. Moreover, the transistor devices made from these single-layer GO sheets after 800 °C thermal reduction exhibit the effective hole mobility ranged between 4 and 12 cm2/(V s). Raman spectroscopic results suggest that the enhancement in mobility at a higher-mobility regime is well explained by the graphitization of GO rather than the removal of functional groups. The ratio between the 2D and G peak areas, I(2D)/I(G), is well correlated to the effective hole mobility values in reduced GO sheets.

Solution-based processes involving the chemical oxidation of graphite and reduction of the obtained graphene oxide (GO) sheets have attracted much attention for preparing graphene films for printed electronics and biosensors. However, the low electrical conductivity of reduced GO is still hindering the development of electronic applications. This article presents that GO sheets reduced by high-temperature alcohol vapors exhibit highly graphitic structures and excellent electrical conductivity. The sheet resistance of thin transparent films is lowered to ∼15 kΩ/◻ (>96% transparency). Field-effect transistors produced from these reduced GO sheets exhibit high effective field-effect hole mobility up to 210 cm(2)/V x s. Raman spectroscopic studies reveal that the conductivity enhancement in the low mobility regime is attributed to the removal of chemical functional groups and the formation of six-fold rings. In the high mobility regime, the growth of the graphitic domain size becomes dominant for enhancing its electrical conductivity. The excellent electrical conductivity of the reduced GO sheets promises potential electronic applications.

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