  Phosphorene, an emerging elemental two-dimensional (2D) direct band gap
semiconductor with fascinating structural and electronic properties
distinctively different from other 2D materials such as graphene and MoS2, is
promising for novel nanoelectronic and optoelectronic applications. Phonons, as
one of the most important collective excitations, are at the heart for the
device performance, as their interactions with electrons and photons govern the
carrier mobility and light-emitting efficiency of the material. Here, through a
detailed first-principles study, it is demonstrated that monolayer phosphorene
exhibits a giant phononic anisotropy, and remarkably, this anisotropy is
squarely opposite to its electronic counterpart and can be tuned effectively by
strain engineering. By sampling the whole Brillouin zone for the mono-layer
phosphorene, several "hidden" directions are found, along which small-momentum
phonons are "frozen" with strain and possess the smallest degree of
anharmonicity. Unexpectedly, these "hidden" directions are intrinsically
different from those usually studied armchair and zigzag directions. Light is
also shed on the anisotropy of interlayer coupling of few-layer phosphorene by
examining the rigid-layer vibrations. These highly anisotropic and
strain-tunable characteristics of phosphorene offer new possibilities for its
applications in thermal management, thermoelectronics, nanoelectronics and
phononics.
