Vibrational behaviors, piezoelectricity, spin-splitting, and carrier mobility in Janus HWSZ (Z = N, P, As) monolayers

Janus two-dimensional (2D) materials provide a versatile platform for symmetry-driven physics. However, the influence of surface functionalization on their intrinsic properties remains an open question. In this Letter, we propose and systematically investigate a class of Janus materials formed by the one-side hydrogenation of the Janus WSZ (Z = N, P, As) or HWSZ monolayers using first-principles density functional theory. Asymmetric hydrogenation acts as an additional symmetry-breaking mechanism, markedly reshaping the band structure and strengthening spin-orbit coupling. This transition yields a robust spin splitting up to 0.46 eV, highlighting the role of surface functionalization in engineering giant spin-dependent characteristics. Furthermore, the intrinsic vertical dipole induces a significant out-of-plane piezoelectric response, with the d ₃₁ coefficient reaching 0.31 pm V^-1. In particular, we evaluate the electron mobility by explicitly incorporating multiple phonon scattering mechanisms to identify the fundamental transport limits.