Size-Dependent Mechanical Properties of a Metal–OrganicFramework: Increase in Flexibility of ZIF‑8 by Crystal Downsizing

Size engineering is an emerging strategy to modulate the mechanical properties of crystalline materials. Herein, micro- and nanodimensional single crystals of the prototypical metal–organic framework (MOF) ZIF-8 are generated using solvothermal and solution methods, respectively. Atomic force microscopy-based nanoindentation technique was used to measure the Young’s modulus values of micro- and nanodimensional individual ZIF-8 crystals. We demonstrate that crystal downsizing to nanoscale dimensions results in a 40% reduction in crystal stiffness. The change is attributed to a greater contribution of surface effects to the physical properties of nanocrystalline ZIF-8. The observed change in the mechanical properties may be used to explain reported size-dependent changes in gas adsorption of ZIF-8, thought to be a result of differences in framework flexibility at the nanoscale. Our work provides an important example on how downsizing of crystalline metal–organic materials can give rise to specific and tunab...