MgXN 2 (X = Hf/Zr) Monolayers: Auxetic Semiconductor with Highly Anisotropic Optical/Mechanical Properties and Carrier Mobility.

Two-dimensional (2D) semiconducting materials with distinct anisotropic physical properties have attracted intense interests. Herein, we show theoretical predictions that MgXN 2 (X = Hf/Zr) monolayers are auxetic semiconductors with highly anisotropic electronic, optical, and mechanical properties. The density functional theory calculations coupled with a PSO algorithm (global-minimum search) suggest that both MgHfN 2 (MgZrN 2 ) monolayers exhibit orthorhombic symmetry ( Pmma ) and are direct-gap (indirect-gap) semiconductors with a bandgap of 2.43 eV (2.13 eV). Specifically, the MgHfN 2 monolayer exhibits highly anisotropic hole mobility as well as very high electron mobility (∼10 4 cm 2 V -1 s -1 ). G 0 W 0 +BSE calculations indicate that both monolayers bear notable optical anisotropy and relatively large exitonic binding energy (∼0.6 eV). In addition, both monolayers acquire remarkable mechanical anisotropy with a negative in-plane Poisson's ratio (∼-0.2) and high Young's modulus (∼260 N/m). The combination of highly anisotropic electronic, optical, and mechanical properties endows MgXN 2 monolayers as potentially useful parts in multifunctional nanoelectronic devices.