# Lattice Engineering Novel 2D Monolayer in Zinc Pnictides

## TL;DR

This paper explores new 2D monolayer structures of zinc pnictides and identifies the most stable and feasible design for potential experimental synthesis.

## Contribution

The study introduces a novel 2D-L1 monolayer structure of ZnX with superior stability and auxetic properties.

## Key findings

- The 2D-L1 monolayer is the ground-state structure in zinc pnictides, showing dynamic and thermal stability.
- ZnAs in the 2D-L1 structure exhibits a negative Poisson’s ratio, indicating auxetic behavior.
- Band gaps in the 2D-L1 monolayer are wider and vary in directness across ZnAs, ZnSb, and ZnBi.

## Abstract

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.