  Searching for materials with single atom-thin as well as planar structure,
like graphene and borophene, is one of the most attractive themes in two
dimensional materials. Herein, using density functional theory calculations, we
have proposed a series of single layer planar penta-transition metal phosphide
and arsenide, i.e. TM$\mathrm{_2}$X$\mathrm{_4}$ (TM= Ni, Pd and Pt; X=P, As).
According to the calculated phonon dispersion relation and elastic constants,
as well as ab initio molecular dynamics simulation results, monolayers of
planar penta-TM$\mathrm{_2}$X$\mathrm{_4}$ are dynamically, mechanically, and
thermally stable. In addition, the band structures calculated with the screened
HSE06 hybrid functional including spin-orbit coupling show that these
monolayers are direct-gap semiconductors with sizeable band gaps ranging from
0.14 eV to 0.69 eV. Besides, the optical properties in these monolayers are
further investigated, where strong in-plane optical absorption with wide
spectral range has been revealed. Our results indicate that planar
penta-TM$\mathrm{_2}$X$\mathrm{_4}$ monolayers are interesting narrow gap
semiconductors with excellent optical properties, and may find potential
applications in photoelectronics.
