  We perform molecular dynamics (MD) simulations with phonon spectral analysis
aiming at understanding the two dimensional (2D) thermal transport in suspended
and supported graphene. Within the framework of equilibrium MD simulations, we
perform spectral energy density (SED) analysis to obtain the lifetime of
individual phonon modes. The per-mode contribution to thermal conductivity is
then calculated to obtain the lattice thermal conductivity in the temperature
range 300-650 K. In contrast to prior studies, our results suggest that the
contribution from out-of-plane acoustic (or ZA) branch to thermal conductivity
is around 25-30% in suspended single-layer graphene (SLG) at room temperature.
The thermal conductivity is found to reduce when SLG is put on amorphous SiO2
substrate. Such reduction is attributed to the strengthened scattering in all
phonon modes in the presence of the substrate. Among them, ZA modes are mostly
affected with their contribution to thermal conductivity reduced to around 15%.
As a result, thermal transport is dominated by in-plane acoustic phonon modes
in supported SLG.
