AFM Nanoindentation To Quantify Mechanical Properties of Nano- and Micron-Sized Crystals of a Metal-Organic Framework Material.

The mechanical properties of individual nanocrystals and small micron-sized single crystals of metal-organic frameworks (MOFs), hitherto, cannot be measured directly by employing the conventional instrumented nanoindentation approach. Here we propose the application of atomic force microscopy (AFM)-based nanoindentation technique, equipped with a calibrated diamond cube-corner indenter tip to quantify the Young's modulus, hardness, adhesion energy, and interfacial and fracture strengths of a zeolitic imidazolate framework (ZIF-8) porous material. We use ZIF-8 as a model MOF system to develop AFM nanoindentation leveraging the concept of unloading strain rate, enabling us to critically assess the practicality and technical limitations of AFM to achieve quantitative measurements of fine-scale MOF crystals. We demonstrate the advantages of using a high unloading strain rate (ε̇ > 60 s) to yield reliable force-displacement data in the few μN load range, corresponding to a shallow indentation depth of ∼10s nm. We found that the Young's moduli (∼3-4 GPa) determined by AFM nanoindentation of the nanocrystals (<500 nm) and micron-sized crystals (∼2 μm) are in agreement with magnitudes derived previously from other techniques, namely instrumented nanoindentation and Brillouin spectroscopy (however, these methods requiring large 100-μm sized crystals) and also in line with density functional theory predictions of an idealized ZIF-8 crystal.