Enhancement of graphene thermoelectric performance through defect engineering

We have achieved mobilities in excess of 200,000 cm^2/Vs at electron densities of ~2*10^11 cm^-2 by suspending single layer graphene. Suspension ~150 nm above a Si/SiO_2 gate electrode and electrical contacts to the graphene was achieved by a combination of electron beam lithography and etching. The specimens were cleaned in situ by employing current-induced heating, directly resulting in a significant improvement of electrical transport. Concomitant with large mobility enhancement, the widths of the characteristic Dirac peaks are reduced by a factor of 10 compared to traditional, non-suspended devices. This advance should allow for accessing the intrinsic transport properties of graphene.

The reported thermal conductivity (kappa) of suspended graphene, 3000 to 5000 watts per meter per kelvin, exceeds that of diamond and graphite. Thus, graphene can be useful in solving heat dissipation problems such as those in nanoelectronics. However, contact with a substrate could affect the thermal transport properties of graphene. Here, we show experimentally that kappa of monolayer graphene exfoliated on a silicon dioxide support is still as high as about 600 watts per meter per kelvin near room temperature, exceeding those of metals such as copper. It is lower than that of suspended graphene because of phonons leaking across the graphene-support interface and strong interface-scattering of flexural modes, which make a large contribution to kappa in suspended graphene according to a theoretical calculation.

Graphene monolayer has been grown by chemical vapor deposition on copper and then suspended over a hole. By measuring the laser heating and monitoring the Raman G peak, we obtain room-temperature thermal conductivity and interface conductance of (370 + 650/-320) W/m K and (28 + 16/-9.2) MW/m(2) K for the supported graphene. The thermal conductivity of the suspended graphene exceeds (2500 + 1100/-1050) W/m K near 350 K and becomes (1400 + 500/-480) W/m K at about 500 K.