In-plane anisotropic mechanical properties of two-dimensional NbOI₂

Two-dimensional niobium oxide diiodide (NbOI₂) has recently attracted extensive attention due to its highly anisotropic band structures and rich physical characteristics in electronics and optoelectronics. Nevertheless, mechanical properties of NbOI₂ have not been systematically investigated, which are critical parameters for applications. Here, we determine the directional dependence of Young's modulus of thin NbOI₂ flakes by using an atomic force microscopy-based nanoindentation technique. We find that Young's moduli along two perpendicular in-plane crystalline axis, the c-axis and the a-axis, were 97.27 ± 2.12 and 51.51 ± 8.21 GPa, respectively. The anisotropic ratio is up to 1.89, which is a high anisotropy value in two-dimensional materials reported so far.