Novel stable Janus HfNXH (X = S, Se, Te) monolayers: insights into piezoelectric, electronic, and transport behaviors.

In this work, we propose a novel class of two-dimensional Janus HfNXH (X = S, Se, Te) monolayers and systematically investigate their stability, electronic structure, piezoelectricity, and carrier mobility using first-principles calculations. Our analysis reveals that HfNSH and HfNSeH are indirect-bandgap semiconductors, whereas HfNTeH exhibits metallic behavior. The intrinsic out-of-plane structural asymmetry in these materials induces a substantial vacuum level offset and pronounced piezoelectric responses. Most notably, HfNTeH demonstrates an exceptionally large in-plane piezoelectric coefficient (|d 11| = 20.41 pm V-1) alongside a large out-of-plane response (d 31 = 0.25 pm V-1). Furthermore, the inclusion of spin-orbit coupling yields a substantial spin splitting of up to 0.26 eV, highlighting their strong potential for spintronic applications. We find that the intrinsic carrier mobilities of Janus HfNXH monolayers are relatively low, and that acoustic deformation potential scattering is the primary mechanism limiting carrier transport. Interestingly, the mobility limited by polar optical phonon scattering exhibits a strong dependence on carrier concentration. These findings provide critical insights into the complex interplay between structural asymmetry, electronic band topology, and scattering mechanisms in functionalized Janus materials.