Longitudinal Piezoelectricity and Polarization‐Insensitive Oxidation in Janus vdWs Nb3SeI7

Abstract As a newly discovered Janus van der Waals (vdW) material, semiconducting Nb 3 SeI 7 offers several notable advantages, including spontaneous out‐of‐plane polarization, facile exfoliation to the monolayer limit, and significant out‐of‐plane emission dipole in second harmonic generation. These properties make it a promising candidate for piezoelectric and piezophototronic applications in highly efficient energy conversion. However, Nb 3 SeI 7 is prone to oxidation when exposed to oxygen, which can severely limit the exploration and utilization of these intriguing physical properties. Therefore, understanding the oxidation mechanism of pristine Nb 3 SeI 7 and its correlation with electrical polarization—an area that remains largely unexplored—is highly significant. In this study, the out‐of‐plane piezoelectricity of Nb 3 SeI 7 is experimentally demonstrated, with a piezoelectric coefficient (| d 33 eff |) of 0.76 nm V −1 . Furthermore, by combining near ambient‐pressure X‐ray photoelectron spectroscopy (NAP‐XPS), Time‐of‐Flight secondary ion mass spectrometry (ToF‐SIMS), and Density functional theory (DFT) calculations, it is revealed that the oxidation of Nb 3 SeI 7 is self‐limiting and independent of its electrical polarization, owing to the similar defect formation energies of Se and I atoms. This self‐limiting and polarization‐insensitive oxidation provides valuable insights into the stabilization mechanisms and expands the potential applications of out‐of‐plane piezoelectricity and other intriguing physical properties in Janus vdW Nb 3 SeI 7 .