First-principles prediction of high carrier mobility for β-phase MX₂N₄ (M = Mo, W; X = Si, Ge) monolayers

The recently synthesized monolayer MoSi₂N₄ (Science 2020, 369, 367) exhibits exceptional environmental stability, a moderate band gap, and excellent mechanical properties, presenting exciting opportunities for the exploration of two-dimensional (2D) MX₂Z₄ materials. However, the low carrier mobility of α-phase MoSi₂N₄ significantly limits its potential applications in field-effect transistor (FET) devices. In this study, we systematically investigate the structural stability, elastic properties, and carrier mobility of a novel family of β-phase MX₂N₄ (M = Mo, W; X = Si, Ge) monolayers through first-principles calculations. Our findings reveal that these β-phase MX₂N₄ monolayers demonstrate remarkable dynamic, thermal, and mechanical stability. Specifically, we identify the MoSi₂N₄, MoGe₂N₄, WSi₂N₄, and WGe₂N₄ monolayers as semiconductors with band gaps of 2.70 eV, 1.57 eV, 3.12 eV, and 1.93 eV, respectively, as calculated using the HSE06 functional. Moreover, the MX₂N₄ monolayers exhibit significant elastic anisotropy, characterized by high ideal tensile strengths and a critical tensile strain exceeding 25%. Notably, the WGe₂N₄ monolayer displays exceptional anisotropic in-plane charge transport, achieving mobility levels of up to 10⁴ cm²V^- 1S^- 1, surpassing those of the α-phase MX₂N₄ monolayers. These novel ternary monolayer structures have the potential to broaden the 2D MX₂Z₄ material family and emerge as promising candidates for applications in field-effect transistors.