  Black phosphorus has been revisited recently as a new two-dimensional
material showing potential applications in electronics and optoelectronics.
Here we report the anisotropic in-plane thermal conductivity of suspended
few-layer black phosphorus measured by micro-Raman spectroscopy. The armchair
and zigzag thermal conductivities are ~20 and ~40 W m$^{-1}$ K$^{-1}$ for black
phosphorus films thicker than 15 nm, respectively, and decrease to ~10 and ~20
W m$^{-1}$ K$^{-1}$ as the film thickness is reduced, exhibiting significant
anisotropy. The thermal conductivity anisotropic ratio is found to be ~2 for
thick black phosphorus films and drops to ~1.5 for the thinnest 9.5-nm-thick
film. Theoretical modeling reveals that the observed anisotropy is primarily
related to the anisotropic phonon dispersion, whereas the intrinsic phonon
scattering rates are found to be similar along the armchair and zigzag
directions. Surface scattering in the black phosphorus films is shown to
strongly suppress the contribution of long-mean-free-path acoustic phonons.
