Real-space Raman spectroscopy of graphene isotope superlattices

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.