Unveiling the Structural Stability and Optoelectronic Properties of Pristine and Janus Monolayers

The versatile and tunable elemental composition of the monolayer-based two-dimensional (2D) MA₂Z₄ family (M is an early transition metal, A is Si or Ge, and Z is N, P, or As) has garnered significant attention in the optoelectronic applications. In this work, we have designed novel phosphorus and arsenic based tungsten digermanium monolayers (WGe₂P₄ and WGe₂As₄), and their Janus phase (WGe₂As₂P₂), respectively. We explored both the 2H and 1T phases for all three structures, finding that the 1T phase is energetically unfavorable and dynamically unstable based on total energy and phonon calculations. In contrast, the 2H phase shows excellent ground state, mechanical, and dynamical stabilities. To precisely predict the electronic properties of these monolayer systems, we employed Perdew-Burke-Ernzerhof (PBE) and Heyd-Scuseria-Ernzerhof (HSE06) functionals were employed. The HSE06 functional notably increased the bandgap values, enhancing WGe₂P₄ from 0.45 to 0.71 eV, WGe₂As₄ from 0.41 to 0.66 eV, and the Janus structure WGe₂As₂P₂ from 0.37 to 0.71 eV. In addition, the optical response of these narrow bandgap monolayers indicated absorption peaks in the range of 0.4-0.7 eV, making these materials suitable for infrared (IR) laser technology. Furthermore, the electronic and optical properties can be effectively tuned for these structures using biaxial strain engineering. These findings provide an understanding of the fundamental properties of WGe₂X₄ (X = P, As) and their Janus structure WGe₂As₂P₂, leading to advances in optoelectronic devices including infrared detectors, terahertz (THz) devices, and many other photonic applications, which are essential in medical diagnostics, telecommunications, and environmental monitoring.