(color online) (a) Overall and decomposed phonon density of states in (b) x direction, (c) y direction, and (d) z direction for phosphorene and grapheme respectively. Two phenomena are observed. First, the PDOS of graphene is anisotropic, while that of phosphorene is isotropic. Second, the active phonon frequency in phosphorene is severely limited compared to that of graphene. Reproduced from Ref. [101] with permission from the Royal Society of Chemistry.