K2Be2P2 monolayer: a predicted strain-tunable two-dimensional topological insulator exhibiting multifunctional properties

Abstract

Exploring new two-dimensional (2D) materials with unique electronic and topological properties is key to developing next-generation electronic devices. We predict, using first-principles density functional theory calculations, a novel two-dimensional (2D) material, the K₂Be₂P₂ monolayer with a square Bravais lattice, and investigate its structural, electronic, mechanical, and optical properties. Our comprehensive stability analyses, encompassing phonon dispersion, cohesive energy (−3.1 eV per atom), formation energy (−2.46 eV per atom), elastic constants, and ab initio molecular dynamics, confirm that K₂Be₂P₂ is dynamically, thermodynamically, and mechanically stable, suggesting its experimental realizability. While initial PBE–GGA calculations suggest a near-zero bandgap, more accurate HSE06 hybrid functional calculations reveal that pristine K₂Be₂P₂ is a direct-bandgap semiconductor with a gap of 165 meV at the Γ point. Crucially, we demonstrate that the application of biaxial compressive strain induces a topological phase transition (TPT) from a trivial insulator to a topological insulator. This TPT, occurring at approximately −2% strain, is characterized by bandgap closure and reopening, accompanied by p–p type band inversion near the Fermi level. The topological nature of the strained phase is unambiguously confirmed by the topological invariant () and the presence of topologically protected edge states, calculated using a semi-infinite Green's function approach. Furthermore, we find that K₂Be₂P₂ exhibits in-plane mechanical anisotropy, with a relatively low Young's modulus (68.59 N m⁻¹), suggesting potential for flexible electronics applications. The optical properties, characterized by the frequency-dependent dielectric function, reveal strong absorption in the visible and near-infrared regions, with a pronounced anisotropy dependent on light polarization, and an exceptionally low work function of 1.49 eV. Our findings position K₂Be₂P₂ as a promising candidate for strain-engineered topological phase transitions in two-dimensional materials, showcasing the tunability of its electronic and topological properties for next-generation electronic and spintronic devices.