  Graphene has a high intrinsic thermal conductivity and a high electron
mobility. The thermal conductivity of graphene can be significantly reduced
when different carbon isotopes are mixed, which can enhance the performance of
thermoelectric devices. Here we compare the thermal conductivities of isotopic
c12/c13 random mixes with isotope superlattices with periods ranging from 46 to
225 nm. Raman Opto-Thermal conductivity measurements of these superlattice
structures show an approximately 50% reduction in thermal conductivity compared
to pristine c12 graphene. This average reduction is similar to the random
isotope mix. The reduction of the thermal conductivity in the superlattice is
well described by a model of pristine graphene and an additional quasi-one
dimensional periodic interfacial thermal resistance of (2.5\pm 0.5)\times
10^{-11} m^2 K/W for the c12/c13 boundary. This is consistent with a large
anisotropic thermal conductivity in the superlattice, where the thermal
conductivity depends on the orientation of the c12/c13 boundary.
