I seek to develop new ferroelectrics based on metal-organic frameworks with dipolar rotors. Ferroelectrics are targeted to be used as physically flexible memories and mechanical energy harvesters for biocompatible sensors and implantable monitoring devices.
As ferroelectrics can store and switch their polarity, they can be used as memories. Via the piezoelectric effect, they can harvest mechanical vibrations. The materials most compatible with flexible substrates, are soft matter materials. However, these so far don’t meet the requirements. Especially lacking is a combination of i) polarisation stability, ii) a sufficiently low energy barrier for polarisation switching and iii) fast switching. As energy harvesters, soft matter materials are hampered by low piezoelectric coefficients.
The main objective of this proposal is rational design of ferroelectrics by obtaining a fundamental understanding of the relation between structure and properties. I will achieve this uniquely by synthesizing polar rotors into 3D crystalline scaffolds that allow to alter the rotors’ nano-environement. I will achieve this via polar ligands in metal-organic frameworks (MOFs). The variability of MOFs allows to tune the nature of the hindrance towards rotation of the polar rotors. The tuneable flexibility allow to regulate the energy harvesting efficiency. Moreover, MOFs have already shown potential as biocompatible materials that can be integrated on physically flexible substrates.
The research consists of i) synthesis of polar rotor MOFs with targeted variations, ii) reliable characterisation and computational modelling of the electronic properties, iii) nanoscopic insight in the switching dynamics. The approach allows to understand how ferro- and piezoelectricity are related to the materials’ structure, and hence to develop materials with exceptional performance.
We identified a class of MOFs that are polar due to chains that show a pattern of short-long-short-long bond length, akin to the distribution of atoms that causes piezo- and ferroelectricity in inorganic ABO3. The piezoresponse is large, and ferroelectricity is likely. This class of materials might alleviate some problems associated with current materials for piezoelectric energy harvesting (brittle and/or a too low piezoelectric response).
We engineered metal-organic frameworks such that limited hindrance being neighbouring linkers took place, such that the linkers could rotate but not independently from one another. We uncovered that very intriguing behaviour could ensue in the case of hindered rotational linker dynamics: the hindered linkers have conformation in a A-B-A-B-A-B pattern, with A= planar, and B = non-planar. The dynamics are correlated, but in a surprising way, flipping of linkers in a planar (A) conformation makes the flipping of the nearest A (thus one but next neighbouring linker) more likely, but with a time delay of typically a few nanoseconds. This intricate and time-delayed correlated behaviour makes clear that concerted gear-like motion of rotors, a dream in the field, will require very careful engineering.
