Ambient Degradation Anisotropy and Mechanism of van der Waals Ferroelectric NbOI2

The spontaneous centrosymmetry-breaking and robust room-temperature ferroelectricity in niobium oxide dihalides spurs a flurry of explorations into its promising second-order nonlinear optical properties, and promises potential applications in nonvolatile electro-optical and optoelectronic devices. However, the ambient stability of the niobium oxide dihalides remains questionable, which overshadows their future development. In this work, the chemical degradation of NbOI2 is comprehensively investigated using combined chemical and optical microscopies in conjunction with spectroscopies. We unveil the highly anisotropic degradation kinetics of NbOI2 driven by the hydrolysis process of the unstable dangling iodine bonds dominantly on the (010) facet and progressing along the c axis. Knowing its degradation mechanism, the NbOI2 flake can then be stabilized by the hexagonal boron nitride encapsulation, which isolates the air moisture. These findings provide direct insights into the ambient instability of NbOI2, and they deliver possible solutions to circumvent this issue, which are essential for its practical integration in photonic and electronic devices.