Nanomechanical behavior and interfacial deformation beyond the elastic limit in 2D metal-organic framework nanosheets.

Nanoscale mechanical property measurements of nanoporous nanosheets face many challenges. Herein we show atomic force microscope (AFM)-based nanoindentation to probe the nanoscale mechanical properties of a 2-D metal-organic framework (MOF) nanosheet material containing atomic-sized pores, termed CuBDC [copper 1,4-benzenedicarboxylate]. The sample thickness ranged from ∼10 nm (tens of monolayers) up to ∼400 nm (a stack of multilayers). In terms of its elastic-plastic properties, the Young's modulus (E ∼ 23 GPa) and yield strength (σ y ∼ 450 MPa) were determined in the through-thickness direction. Moreover, we characterized the failure mechanisms of the CuBDC nanosheets, where three failure mechanisms were identified: interfacial slippage, fracture of the framework, and delamination of multilayered nanosheets. Threshold forces and indentation depths corresponding to these failure modes were determined. To gain insights into the failure mechanisms, we employ finite-element models with cohesive elements to simulate the interfacial debonding of a stack of 2-D nanosheets during the indentation process. The nanomechanical AFM methodology elucidated here will pave the way for the study of other 2-D hybrid nanosheets and layered van der Waals solids.

Quantum-chemical simulation of the adsorption-induced reduction of strength of siloxane bonds.

Mechanical strength of silicate glasses is known to decrease markedly due to the adsorption of molecules from the environment, especially in aqueous alkali solutions. This effect, known as the adsorption-induced reduction of strength (AIRS), has not yet been fully understood. Here, the dependence on the chemical nature and electronic properties of adsorbates of the AIRS of siloxane bonds in silica was studied by means of quantum-chemical calculations at the wB97X-D3/def2-TZVP level of theory. A siloxane bond was modelled by H3Si-O-SiH3 and (HO)3Si-O-Si(OH)3 clusters, and the AIRS was simulated by a linear tensile deformation of the siloxane bond in the presence of the following adsorbates: OH-, Cl-, H2O, H+ and H3O+. Potential energy profiles and derivative force curves of the siloxane bond rupture were obtained. The varying effect of the adsorbates on the energy-force characteristics of the AIRS can be explained by changes in the bond lengths and electron occupancy. It is shown that the AIRS of the siloxane bonds increases with an increase in the nucleophilicity of the adsorbates, and correlates with an adsorbate-induced redistribution of electron density.