First-principles prediction of β-phase SnA₂N₄ (A = Si; Ge) monolayers: outstanding mechanical anisotropy and high...

The recently synthesized monolayer MoSi₂N₄ (Y.-L. Hong, Z. Liu, L. Wang, T. Zhou, W. Ma, C. Xu, S. Feng, L. Chen, M.-L. Chen and D.-M. Sun, Chemical vapor deposition of layered two-dimensional MoSi₂N₄ materials, Science, 2020, 369(6504), 670-674.) boasts extraordinary environmental stability and superior comprehensive performance, offering exciting opportunities for the exploration of two-dimensional MX₂Z₄ materials. However, the low carrier mobility of α-MoSi₂N₄ significantly limits its practical applications in field-effect transistor (FET) devices. In this study, first-principles calculations were utilized to systematically investigate the structural stability, photoelectronic properties, tensile mechanical behavior, and carrier mobility of a novel family of β-SnA₂N₄ (A = Si, Ge) monolayers. Our findings reveal that these β-SnA₂N₄ monolayers demonstrate remarkable dynamic and thermal stability. Specifically, calculations based on the HSE06 functional reveal that the SnSi₂N₄ and SnGe₂N₄ monolayers are semiconductors with band gaps of 3.36 eV and 2.13 eV, respectively. Additionally, the SnA₂N₄ monolayers exhibit distinct mechanical anisotropy, characterized by high ideal tensile strengths and critical tensile strains exceeding 27%, indicating outstanding ductility. Importantly, the SnA₂N₄ monolayers display exceptional anisotropic in-plane charge transport, achieving electron mobility levels of up to 10³ cm² V^-1 s^-1, surpassing those of the α-phase MA₂N₄ (M = Mo, W; A = Si, Ge) monolayers. These novel ternary monolayer structures are expected to enrich the 2D MA₂Z₄ material family and emerge as promising candidates for FET applications.