Unveiling strain-dependent structure-property relationships in Janus WSeTe bilayers for next-generation optoelectronic applications: A first-principles outlook

The future of nanoscale electronic materials is increasingly focused on two-dimensional (2D) Janus bilayers. Using first-principles density functional theory (DFT), we investigated the effect of biaxial strain (ranging from –8% to +8%) on the structural, electronic, optical, and dielectric properties of Janus WSeTe bilayers, with and without spin-orbit coupling (SOC) effect. Among the preferred AA′ and AB stacking configurations with Te-Se-Te-Se ordering, binding energy calculations, phonon dispersion curves, and Born-Huang criteria confirm energetic, dynamical, and mechanical stability, yielding a Young’s modulus of 154 N/m and a Poisson’s ratio of 0.28. At equilibrium, the bilayers exhibit an indirect semiconducting bandgap of 0.74 eV within the PBE framework. Compressive strain progressively narrows this bandgap (down to 0.40 eV), indicating an impending semiconductor-to-metal transition under higher compression. Conversely, applying + 8% tensile strain successfully drives an indirect-to-direct bandgap transition. Incorporating SOC reduces the equilibrium bandgap to 0.60 eV due to relativistic effects, while preserving the overall strain-response trends. The optical characteristics strongly correlate with these electronic modifications. The real part of the dielectric function reveals marked polarization anisotropy, achieving a static dielectric constant of 17 along the in-plane (XX=YY) direction at zero photon energy. Negative dielectric values across specific energy ranges indicate enhanced optical reflectivity driven by interband transitions. Notably, dominant peaks in the imaginary dielectric function occur near the direct transition energy at the K-point. Under tensile strain, these spectral features shift toward lower photon energies (redshift) with enhanced absorption across the infrared-to-ultraviolet spectrum, whereas compressive strain induces a blueshift.