Electrical and spectroscopic characterizations of ultra-large reduced graphene oxide monolayers

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 H ₂SO₄ 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)//(G), is well correlated to the effective hole mobility values in reduced GO sheets.