Longitudinal Piezoelectricity and Polarization-Insensitive Oxidation in Janus vdWs Nb₃SeI₇

As a newly discovered Janus van der Waals (vdW) material, semiconducting Nb₃SeI₇ 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, Nb₃SeI₇ 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 Nb₃SeI₇ and its correlation with electrical polarization-an area that remains largely unexplored-is highly significant. In this study, the out-of-plane piezoelectricity of Nb₃SeI₇ is experimentally demonstrated, with a piezoelectric coefficient (|d₃₃ ^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 Nb₃SeI₇ 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 Nb₃SeI₇.