Strong Vertical Piezoelectricity and Broad-pH-Value Photocatalyst in Ferroelastic Y₂Se₂BrF Monolayer

With the development of miniaturized devices, there is an increasing demand for 2D multifunctional materials. Six ferroelastic semiconductors, Y₂Se₂XX' (X, X' = I, Br, Cl, or F; X ≠ X') monolayers, are theoretically predicted here. Their in-plane anisotropic band structure, elastic and piezoelectric properties can be switched by ferroelastic strain. Moderate energy barriers can prevent the undesired ferroelastic switching that minor interferences produce. These monolayers exhibit high carrier mobilities (up to 10⁴ cm² V^-1 s^-1) with strong in-plane anisotropy. Furthermore, their wide bandgaps and high potential differences make them broad-pH-value and high-performance photocatalysts at pH value of 0-14. Strikingly, Y₂Se₂BrF possesses outstanding d₃₃ (d₃₃ = -405.97 pm/V), greatly outperforming CuInP₂S₆ by 4.26 times. Overall, the nano Y₂Se₂BrF is a hopeful candidate for multifunctional devices to generate a direct current and achieve solar-free photocatalysis. This work provides a new paradigm for the design of multifunctional energy materials.