Nanomechanical Properties and Failure Mechanisms of Two-Dimensional Metal-Organic Framework Nanosheets

Nanoscale mechanical properties measurement of porous nanosheets presents many challenges. Herein we show atomic force microscope (AFM) nanoindentation to probe the nanoscale mechanical properties of a 2‑D metal‑organic framework (MOF) nanosheet material, termed CuBDC [copper 1,4‑benzenedicarboxylate]. The sample thickness was ranging from ~10 nm (tens of monolayers) up to ~400 nm (stack of multilayers). In terms of its elastic‑plastic properties, the Young’s modulus (E ~ 22.9 GPa) and yield strength (𝜎_Y ~ 448 MPa) have been determined in the through-thickness direction. Moreover, we have characterized the failure mechanisms of the CuBDC nanosheets, where three failure mechanisms have been identified: interfacial sliding, fracture of framework, and delamination of multilayered nanosheets. Threshold forces and corresponding indentation depths corresponding to the failure modes have been 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 be pertinent to the study of other 2‑D hybrid nanosheets and van der Waals solids.