Thermoelectric Properties of Thermally Reduced Graphene Oxide Observed by Tuning the Energy States

Reduced graphene oxide (rGO) possesses a similar electronic structure to graphene but can be synthesized on a larger scale. Hence, rGO is considered as an attractive alternative to graphene. Here we report the carrier transport properties of thermally reduced graphene oxide (TrGO) as a function of reduction temperature. The transfer curve of a field effect transistor fabricated with TrGO exhibited ambipolar properties, and the charge neutrality point of TrGO was shifted from negative to positive as the reduction temperature increased. Furthermore, as revealed in Arrhenius plots of the carrier densities and carrier mobilities, TrGO behaved as a metallic conductor at all reduction temperatures. To investigate the effect of reduction temperature on the thermoelectric properties of TrGO, the Seebeck coefficients of the fabricated TrGOs were calculated from the transfer curve using Mott's equation for metallic materials. All samples showed ambipolar carrier transport. At V-g = 0 V, the Seebeck coefficient switched sign from negative to positive as the reduction temperature became higher, indicating that electron and hole carrier transport dominates at higher and lower reduction temperature, respectively. The calculated Seebeck coefficients at zero gate bias were compared with the measured coefficients in TrGO bulk films. The thermoelectric properties of the measured and calculated coefficients showed similar trends with increasing reduction temperature, and the charged carrier transport (i.e., the energy states) of TrGO can be tuned by varying the reduction temperature without doping with impurities.