Electron–Phonon Scattering Dominated Intrinsic Mobility in Janus <i>M</i> BNS ₂ ( <i>M</i> = Ti, Zr, Hf) Monolayers

We present a comprehensive first-principles investigation of a series of proposed two-dimensional Janus MBNS2 (M = Ti, Zr, and Hf) monolayers using density functional theory. The study systematically explores their structural stability, mechanical properties, Raman spectra, piezoelectric behavior, electronic band structures, and carrier transport characteristics. The results indicate that all proposed materials are semiconductors with dynamically and mechanically stable crystal structures. The calculated band gaps of the Janus MBNS2 monolayers range from 1.41 to 1.71 eV. Notably, the Janus HfBNS2 monolayer exhibits a large Zeeman spin-splitting of 225.2 meV due to strong spin–orbit coupling, highlighting its potential for spintronic applications. Carrier mobility in these materials is found to be intrinsically low, primarily limited by electron–phonon interactions. Detailed scattering analysis reveals that acoustic deformation potential scattering is the dominant mechanism limiting charge transport, while other scattering mechanisms contribute less significantly. The proposed Janus MBNS2 monolayers also display considerable mechanical robustness, with in-plane Young’s moduli reaching up to 206.29 N/m. Moreover, the vertically asymmetric crystal structures of these compounds give rise to both in-plane and out-of-plane piezoelectric responses, enhancing their potential for use in piezotronic and electromechanical devices. These findings not only extend the family of functional 2D Janus semiconductors but also provide valuable insights into the fundamental properties, particularly transport limitations arising from electron–phonon interactions, offering guidance for future materials engineering and optimization.