  Structural, electronic, ferroelectric, and optical properties of
two-dimensional (2D) BiN monolayer material with phosphorene-like structure are
studied in terms of the density functional theory and modern Berry phase
ferroelectric method. Both phonon spectra and molecular dynamics simulations
indicate that the BiN monolayer is a room-temperature stable 2D ferroelectric
with polarization as large as 580 pC/m. Further studies show that the
polarization in the BiN monolayer can be easily switched from [100] to [010]
direction over the bridging saddle phase by applying a tensile [010] stress of
2.54 N/m or compressive [100] stress of -1.18 N/m. This phase transition makes
its lattice constants vary in a large range compared to other non-ferroelectric
2D materials. Moreover, through applying uniaxial tensile stress parallel to
the polarization, one can fix the polarization and change the semiconductor
energy gap from direct to indirect one. The optical properties feature a very
strong anisotropy in reflectivity below the photon energy of 4 eV. All these
significant ferroelectric, electronic, and optical properties make us believe
that the 2D BiN monolayer can be used to make stretchable electronic devices
and optical applications.
