  We ascertain the anisotropic thermal conductivity of alumina passivated black
phosphorus (BP), a reactive 2D nanomaterial with strong in-plane anisotropy. We
measure the room temperature thermal conductivity by time-domain
thermoreflectance for the three crystalline axes of exfoliated BP. The thermal
conductivity along the zigzag direction (86 +/- 8 W/(m*K)) is ~2.5 times higher
than that of the armchair direction (34 +/- 4 W/(m*K)).
