Anisotropic mechanical properties of α-MoO₃ nanosheets

The mechanical behaviors of 2D materials are fundamentally important for their potential applications in various fields. α-Molybdenum trioxide (α-MoO₃) crystals with unique electronic, optical, and electrochemical properties, have attracted extensive attention for their use in optoelectronic and energy conversion devices. From a mechanical viewpoint, however, there is limited information available on the mechanical properties of α-MoO₃. Here, we developed a capillary force-assisted peeling method to directly transfer α-MoO₃ nanosheets onto arbitrary substrates. Comparatively, we could effectively avoid surface contamination arising from the polymer-assisted transfer method. Furthermore, with the help of an in situ push-to-pull (PTP) device during SEM, we systematically investigated the tensile properties of α-MoO₃. The measured Young's modulus and fracture strengths along the c-axis (91.7 ± 13.7 GPa and 2.1 ± 0.9 GPa, respectively) are much higher than those along the a-axis (55.9 ± 8.6 GPa and 0.8 ± 0.3 GPa, respectively). The in-plane mechanical anisotropy ratio can reach ∼1.64. Both Young's modulus and the fracture strength of MoO₃ show apparent size dependence. Additionally, the multilayer α-MoO₃ nanosheets exhibited brittle fracture with interplanar sliding due to poor van der Waals interaction. Our study provides some key points regarding the mechanical properties and fracture behavior of layered α-MoO₃ nanosheets.