First-principles investigation of in-plane anisotropies in XYTe₄ monolayers with X = Hf, Zr, Ti and Y = Si, Ge

In-plane anisotropic materials can introduce additional degrees of freedom while tuning their physical properties, which expand the range of opportunities for designing novel semiconductor devices and exploring distinct applications. In this work, we investigate the in-plane anisotropic electronic, elastic, transport and piezoelectric properties in a family of isostructural telluride XYTe₄ (X = Hf, Zr and Ti, Y = Si and Ge) monolayers based on first-principles calculations. Six types of structures are verified to harbor direct bandgaps at the Γ point ranging between 0.98 and 1.36 eV. The orientation-dependent in-plane elastic stiffness of XYTe₄ reveals the anisotropic and ultrasoft nature. Superior dielectric constants and giant switching effects are found in TiGeTe₄ monolayers because of giant in-plane anisotropy. Strikingly, the piezoelectric coefficients of XSiTe₄ differ by an order of magnitude along the two main directions. The strong in-plane anisotropic elastic properties of XYTe₄ monolayers together with outstanding piezoelectric responses show that these structures can compete with that of transition metal dichalcogenides for applications in the field of flexible electronic devices.