  We investigated thermal conductivity of free-standing reduced graphene oxide
films subjected to a high-temperature treatment of up to 1000 C. It was found
that the high-temperature annealing dramatically increased the in-plane thermal
conductivity, K, of the films from 3 W/mK to 61 W/mK at room temperature. The
cross-plane thermal conductivity, Kc, revealed an interesting opposite trend of
decreasing to a very small value of 0.09 W/mK in the reduced graphene oxide
films annealed at 1000 C. The obtained films demonstrated an exceptionally
strong anisotropy of the thermal conductivity, K/Kc ~ 675, which is
substantially larger even than in the high-quality graphite. The electrical
resistivity of the annealed films reduced to 1 - 19 Ohms/sq. The observed
modifications of the in-plane and cross-plane thermal conductivity components
resulting in an unusual K/Kc anisotropy were explained theoretically. The
theoretical analysis suggests that K can reach as high as ~500 W/mK with the
increase in the sp2 domain size and further reduction of the oxygen content.
The strongly anisotropic heat conduction properties of these films can be
useful for applications in thermal management.
