  The anharmonic behavior of phonons and intrinsic thermal conductivity
associated with the Umklapp scattering in monolayer MoS2 sheet are investigated
via first-principles calculations within the framework of density functional
perturbation theory. In contrast to the negative Gruneissen parameter occurring
in low frequency modes in graphene, positive Gruneissen parameter in the whole
Brillouin zone is demonstrated in monolayer MoS2 with much larger Gruneissen
parameter for acoustic modes than that for the optical modes, suggesting that
monolayer MoS2 sheet possesses a positive coefficient of thermal-expansion. The
calculated phonon lifetimes of the infrared active modes are 5.50 and 5.72 ps
for E'and A2'' respectively, in good agreement with experimental result
obtained by fitting the dielectric oscillators with the infrared reflectivity
spectrum. The lifetime of Raman A1' mode (38.36 ps) is about 7 times longer
than those of the infrared modes. The dominated phonon mean free path of
monolayer MoS2 is less than 20 nm, about 30-fold smaller than that of graphene.
Combined with the non-equilibrium Green's function calculations, the room
temperature thermal conductivity of monolayer MoS2 is found to be around 23.2
Wm-1K-1, two orders of magnitude lower than that of graphene.
