Thermoelectric properties of CVD grown large area graphene

The thermoelectric power (TEP) of CVD (Chemical Vapor Deposition) grown large area graphene transferred onto a Si/SiO$_{2}$ substrate was measured by simply attaching two miniature thermocouples and a resistive heater. Availability of such large area graphene facilitates straight forward TEP measurement without the use of any microfabrication processes. All investigated graphene samples showed a positive TEP $\sim $ + 30 $\mu $V/K in ambient conditions and saturated at a negative value as low as $\sim $ -75 $\mu $V/K after vacuum-annealing at 500 K in a vacuum of $\sim $10$^{-7}$ Torr. The observed p-type behavior under ambient conditions is attributed to the oxygen doping, while the n-type behavior under degassed conditions is due to electron doping from SiO$_{2}$ surface states. It was observed that the sign of the TEP switched from negative to positive for the degassed graphene when exposed to acceptor gases. Conversely, the TEP of vacuum-annealed graphene exposed to the donor gases became even more negative than the TEP of vacuum-annealed sample.