  Thermal conductance of graphene nanoribbons (GNRs) with the width varying
from 0.5 to 35 nm is systematically investigated using nonequilibrium Green's
function method. Anisotropic thermal conductance is observed with the room
temperature thermal conductance of zigzag GNRs up to ~ 30% larger than that of
armchair GNRs. At room temperature, the anisotropy is found to disappear until
the width is larger than 100 nm. This intrinsic anisotropy originate from
different boundary condition at ribbon edges, and can be used to tune thermal
conductance, which have important implications for the applications of GNRs in
nanoelectronics and thermoelectricity.
