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 synthesize isotopic 12C/13C random mixes and isotope superlattices (SLs) with periods ranging from 46 to 225 nm by chemical vapor deposition. Raman optothermal conductivity measurements of these SL structures show an approximately 50% reduction in thermal conductivity compared to pristine 12C graphene. This average reduction is similar to the random isotope mix. The reduction of the thermal conductivity in the SL is well described by a model of pristine graphene and an additional quasi-one-dimensional periodic interfacial thermal resistance of (2.5 ± 0.5) × 10-11 m2 K W-1 for the 12C/13C boundary. This is consistent with a large anisotropic thermal conductivity in the SL, where the thermal conductivity depends on the orientation of the 12C/13C boundary.

We report the Raman spectroscopy of C12/C13 graphene isotope superlattices (SLs) synthesized by chemical vapor deposition. At large periods the Raman spectrum corresponds to the sum of the bulk C12 and C13 contributions. However, at small periods we observe the formation of mixed C12/C13 modes for Raman processes that involve two phonons, which results in the tripling of the 2D and 2D′ Raman peaks. This tripling can be well understood in the framework of real-space Raman spectroscopy, where the two emitted phonons stem from different regions of the SL. The intensity of the mixed peak increases as the SL half-period approaches the mean free path of the photoexcited electron-hole pairs. By varying the SL period between 6 and 225 nm we have a direct measure of the photoexcited electron mean free path, which is found to be 18 nm for suspended graphene and 7 nm for graphene on SiO2 substrates.

We developed a method of precise isotope labeling to visualize the continuous growth of graphene by chemical vapour deposition (CVD). This method allows us to observe, as a function of time, the growth of graphene monocrystals at a resolution of a few seconds. This technique is used to extract the anisotropic growth rates, as well as investigate the formation of dendrites, and the dependence of growth rate on adsorption area of methane on copper. This technique also gives precise start times for individual graphene nucleation sites. We obtain a physical picture of the growth dynamics of graphene and its dependence on various parameters. Finally, our method is relevant to other CVD grown materials.