  In addition to its exotic electronic properties graphene exhibits unusually
high intrinsic thermal conductivity. The physics of phonons - the main heat
carriers in graphene - was shown to be substantially different in
two-dimensional (2D) crystals, such as graphene, than in three-dimensional (3D)
graphite. Here, we report our experimental study of the isotope effects on the
thermal properties of graphene. Isotopically modified graphene containing
various percentages of 13C were synthesized by chemical vapor deposition (CVD).
The regions of different isotopic composition were parts of the same graphene
sheet to ensure uniformity in material parameters. The thermal conductivity, K,
of isotopically pure 12C (0.01% 13C) graphene determined by the optothermal
Raman technique, was higher than 4000 W/mK at the measured temperature Tm~320
K, and more than a factor of two higher than the value of K in a graphene
sheets composed of a 50%-50% mixture of 12C and 13C. The experimental data
agree well with our molecular dynamics (MD) simulations, corrected for the
long-wavelength phonon contributions via the Klemens model. The experimental
results are expected to stimulate further studies aimed at better understanding
of thermal phenomena in 2D crystals.
