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.

Extra-High Mechanical and Phononic Anisotropy in Black Phosphorus Blisters.

Strain is an effective strategy to modulate the electrical, optical, and optoelectronic properties of 2D materials. Conventional circular blisters could generate a biaxial stretching of 2D membranes with notable strain gradients along the hoop direction. However, such a deformation mode cannot be utilized to investigate mechanical responses of in-plane anisotropic 2D materials, for example, black phosphorus (BP), due to its crystallographic orientation dependence. Here, a novel rectangular-shaped bulge device is developed to uniaxially stretch the membrane, and further provide a promising platform to detect orientation-dependent mechanical and optical properties of anisotropic 2D materials. Impressively, the derived anisotropic ratio of Young's modulus of BP flakes is much higher than the values obtained via the nanoindentation method. The extra-high strain-dependent phononic anisotropy in Raman modes along different crystalline orientations is also observed. The designed rectangular budge device expands the uniaxial deformation methods available, allowing to explore the mechanical, and strain-dependent physical properties of other anisotropic 2D materials more broadly.