Lattice Engineering Novel 2D Monolayer in Zinc Pnictides

In this work, the structural, electronic, and thermodynamic stabilities in the novel two-dimensional monolayer (2D-ML) structure of IIB-VA zinc pnictides, ZnX (X = As, Sb, Bi), have been systematically investigated via lattice engineering. We utilize the geometries of 3D bulk structures of ZnX in orthorhombic symmetry with space group Pbca(No.61) as parental material to model three different 2D monolayers of ZnX, denoted as 2D-(L1, L2, and L3). Their total relative energies and stabilities have been investigated and compared with the 2D monolayer geometries of tetragonal, hexagonal (planar honeycomb), and wurtzite (puckered honeycomb) symmetries. The spin-polarized density functional theory (DFT) with plane wave basis sets has been employed throughout the calculations, with the hybrid HSE06 functional to get an accurate description of thermodynamic stability and electronic band gap values consistent with experimental data. Our calculations suggest that the 2D-L1 monolayer with rectangular symmetry obtained from the lattice relaxation of quasi-layered rhomboid rings (Zn₂X₂) dramatically represents the ground-state monolayer in zinc pnictide compounds. While the 2D-ML in tetragonal geometry is energetically competitive in ZnSb or favorable in ZnBi, it shows slight dynamical instability, reinforcing that 2D-L1 is the only structure found to be dynamically stable at zero strain. The feasibility of the most stable 2D-L1 monolayer has been realized with its dynamical stability, as manifested by the absence of imaginary frequencies in phonon dispersion curves, together with mechanical and thermal stabilities via ab initio molecular dynamics (AIMD). The band gap becomes wider in the 2D-L1 monolayer compared to its bulk counterparts. The nature of the band gap is slightly indirect in 2D-L1 monolayer of ZnAs, whereas it is direct in that of ZnSb, and ZnBi. Notably, the indication of a negative Poisson's ratio in the most stable 2D-L1 monolayer in ZnAs signifies its auxetic property. Our theoretical findings provide a potential synthesis route for novel 2D-ML structures in ZnX, which have yet to be experimentally synthesized.