Planar penta-transition metal phosphide and arsenide as narrow-gap semiconductors with ultrahigh carrier mobility

Searching for single-atom thin materials in the planar structure, like graphene and borophene, is one of the most attractive themes in two-dimensional materials. Using density functional theory calculations, we have proposed a series of single-layer planar penta-transition metal phosphides and arsenides, i.e., TM₂X₄ (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-TM₂X₄ are dynamically, mechanically and thermally stable. In addition, screened HSE06 hybrid functional calculations including spin–orbit coupling show that these monolayers are direct band gap semiconductors, with band gaps ranging from 0.14 to 0.77 eV. Ultrahigh carrier mobilities up to 10⁴–10⁵ cm² V⁻¹ s⁻¹ both for electrons and holes have been confirmed, among the highest in 2D semiconductors. Our results indicate that planar penta-TM₂X₄ monolayers are interesting narrow-gap semiconductors with ultrahigh carrier mobility as well as excellent optical properties, and may find potential applications in nanoelectronics and photoelectronics.