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Constructing polar rotors in metal-organic frameworks for memories and energy harvesting

Periodic Reporting for period 2 - MemoMOFEnergy (Constructing polar rotors in metal-organic frameworks for memories and energy harvesting)

Reporting period: 2019-07-01 to 2020-12-31

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.
Series of metal-organic frameworks with potentially piezo-and ferroelectric behaviour have been synthesised. New synthesis metholdogies have been developed to grow large single crystals of these materials, which are paramount for reliable quantitative characterisation of their piezo-and ferroelectric properties. Some of these crystals showed unparalleled optical dichroism, meaning a change in colour upon change of polarisation of light. For a series of topologically related metal-organic we unravelled their piezoelectric behaviour computationally and experimentally. We identified a metal-organic framework with a piezoelectric coefficient around that of the currently best performing organic piezoelectric material (PVDF). We obtained insight into the role of framework flexibility and that of different metal cations in the piezoelectric response of metal-organic frameworks.
Metal-organic frameworks can have highly mobile linkers. For our goal of ferroelectric metal-organic frameworks, collective motion of these linkers is of paramount importance. Via a combination of experimental and computational work we could for the first time identify the emergence of cooperative linker motion in metal-organic frameworks.
Piezoelectricity of metal-organic frameworks has not been studied before. Based on our current results, the electronically favourable characteristics and tuneable nature of metal-organic frameworks, we expect that by the end of the project we will have identified MOFs that surpass the current piezoelectric materials for energy harvesting. Moreover, we expect to develop such understanding of collective linker dynamics in metal-organic frameworks such that we can develop polar frameworks with a favourable degree correlated motion such that we can achieve ferroelectric materials for which the electric field required for switching of polarity and the stability of the resultant polar phase should be very favourable.

With regard to methodology, we expect to have developed new methods for the characterisation of the shear component ofpiezoelectricity based on piezo-force microscopy, and to be able to detect fast ferroelectric switching via optical methods. We expect to develop computational methodology that allows for unravelling the correlated motions of MOF-linkers on larger length scales.
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