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

Abstract

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.

Framework flexibility of ZIF-8 under liquid intrusion: discovering time-dependent mechanical response and structural relaxation

“…Although it has been reported that the crystal size of ZIF-8 may influence the water intrusion pressure, 33 it remains to be established whether ZIF-8's framework flexibility and relaxation behavior are also size dependent subject to liquid intrusion. It is envisaged that our findings not only will open up the field of rate-dependent MOF mechanics, [39][40][41] but also lead to new MOF-based nanofluidic innovations. 22,42 For example, highrate liquid intrusion performance is critical for applications like impact mitigators and shock absorbers afforded by nanoporous materials.…”

Self‐Assembled, Fluorine‐Rich Porous Organic Polymers: A Class of Mechanically Stiff and Hydrophobic Materials

“…The densities of FPOP-100 (1 = 1440.03 AE 46.29 kg m À3 ) and FPOP-101 (1 = 1451.61 AE 20.67 kg m À3 )a re higher than that of ZIF-8 ( Figure 6), which has nearly the same E but higher H (H{110} of ZIF-8:5 31 AE 28 MPa). [26,33] This higher hardnesso f ZIF-8 indicatesi ts ability to withstand higher load when the projected area of residual impression stays the same for both ZIF-8 and FPOPs. In other words, under the same indentation load, FPOPs suffer more permanent deformation,w hich could be plastic deformation of the network until eventual bond breakage.…”

“…Densities ρ of FPOP s were measured by using a Mettler Toledo instrument on the basis of the Archimedes principle. The densities of FPOP‐100 ( ρ =1440.03±46.29 kg m −3 ) and FPOP‐101 ( ρ =1451.61±20.67 kg m −3 ) are higher than that of ZIF‐8 (Figure ), which has nearly the same E but higher H ( H {110} of ZIF‐8: 531±28 MPa) . This higher hardness of ZIF‐8 indicates its ability to withstand higher load when the projected area of residual impression stays the same for both ZIF‐8 and FPOPs.…”

Transformation of hard pollen into soft matter

“…Intact pollen particles were chosen for extracting the Young's modulus of the exine layer and broken pollen particles, which exposed the inner layer, were chosen to determine the Young's modulus of the intine layer. In order to avoid the substrate effect, we followed the 10% depth rule where the indentation depth should be less than 1/10th of the layer thickness 38 . The average thickness of the exine layer was~0.6 μm for defatted pollen particles and 0 h KOH-treated pollen specimens, and became thinner with increasing KOH treatment time (~0.5 μm).…”