Rational design of 2D Janus P3m1 M2N3 (M = Cu, Zr, and Hf) and their surface-functionalized derivatives: ferromagnetic, piezoelectric, and photocatalytic properties

Abstract

In this study, first-principles calculations were employed to rationally design two-dimensional (2D) Janus transition metal nitrides of P3m1 M₂N₃ phases, where M is a d-block element (Sc–Zn, Y–Cd, Hf–Hg). Among the 29 examined 2D M₂N₃, three 2D phases, namely P3m1 Cu₂N₃, Zr₂N₃, and Hf₂N₃, exhibit excellent thermodynamic, dynamic, mechanical, and thermal stabilities. These novel Janus 2D materials exhibit ferromagnetic metallic and half-metallic behavior. The related 2D Janus surface-functionalized derivatives, Cu₂N₃H, Cu₂N₃F, Cu₂N₃Cl, Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F, are all dynamically stable. The 2D Janus P3m1 phases of Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F, all with M in the +IV oxidation state, act as semiconductors in the visible region, with energy band gaps of 2.26–2.70 eV at the HSE06 level of theory. On the other hand, the 2D Janus P3m1 Cu₂N₃X phases (where X = H, F, and Cl) are ferromagnetic half-metals. Additionally, it has been unveiled that there are high hole mobilities (∼6 × 10³ cm² V⁻¹ s⁻¹) derived from the moderate deformation potential and effective mass in the 2D Janus P3m1 Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F phases. Uniaxial strain engineering has demonstrated the outstanding in-plane piezoelectric properties of 2D Janus P3m1 Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F with high d₁₁ values (∼99.91 pm V⁻¹). Furthermore, the desirable band-edge alignments and high anisotropic carrier mobilities of 2D Janus P3m1 Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F phases indicate their potential as visible light-driven photocatalysts for water splitting reactions on different facets. These properties render 2D Janus P3m1 Zr₂N₃H, Hf₂N₃H, and Hf₂N₃F phases promising for use in optoelectronics, piezoelectric sensing, and photocatalysis applications.