IISER Pune researchers turn insulating MOFs into semiconductors

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(From left) Dr. Nirmalya Ballav, Vikash Kumar and Barun Dhara of the Indian Institute of Science Education and Research team that achieved the feat.

After four years of intensive screening, researchers at Pune’s Indian Institute of Science Education and Research (IISER) have transformed insulating metal-organic frameworks (MOFs), which are generally used for gas storage and solvent separation, into semiconductor MOFs by incorporating polymers.

A team led by Dr. Nirmalya Ballav from the Department of Chemistry at IISER Pune has converted a cadmium-based MOF insulator into a semiconductor at room temperature through nanochemistry. The electrical conductivity increased nine-fold (a billion-fold increase) when chains of conducting polymers were introduced into the nanochannels of MOFs. The results were published recently in The Journal of Physical Chemistry Letters.

Initially, the pores of metal-organic frameworks are loaded with pyrrole monomers, which are not electrical conductors. The addition of iodine brings about an oxidation reaction and converts the monomers into polymers. Unlike monomers, polymers are electrically conducting in nature and this helps turn the metal-organic framework into a semiconductor.

The weak interaction between the MOF and the conducting polymer is the key behind the unusual increase in conductivity.“The size of the pyrrole monomer nearly matches the dimension of nanochannels of the metal-organic framework. So no branch polymer was formed but only a single-chain (linear) polymerisation took place,” says Dr. Ballav. “Branch polymers are generally less electrically conducting in nature than single-chain polymers.”

The amount of polymer loaded inside the one-dimensional MOF pores was only about 10 per cent. Though electrical conductivity may increase if more polymer is packed inside the pores, the restricted diffusion in the pore nanospace does not allow more polymers to be loaded.

The MOF continued to retain its fluorescence even after becoming electrically conducting. “If you bring about electrical conductivity in a fluorescent MOF the fluorescence is expected to vanish. But it was not so in our case,” he says. “It indicates the weak, non-covalent interaction between the conducting polymer and the MOF. The weak interaction was sufficient enough for the electrons to flow across the material and is the key behind the unusual increase in conductivity.”

“The unusual enhancement of electrical conductivity of the MOF was due to the presence of conducting polymer and the electronic interaction between the MOF and the polymer,” says Barun Dhara of IISER Pune and the first author of the paper.

“Conductivity and fluorescence is a rare combination that could provide a route towards multifunctonal MOFs suitable for optoelectronics, including solar cells and imaging devices,” says a news item in Chemistry World.

Though the researchers have been able to produce a nine-fold increase in the electrical conductivity of the MOF-nanocomposite, it is still far less than silicon. “But our work shows promise that organic materials can be used in electronic industry where silicon is primarily used. It will be an economic approach for the development of future electronic applications,” says Dr. Ballav.

By using a right combination of MOFs and conducting polymer, Dr. Ballav is confident of designing nanocomposites for specific purposes. The team is working on other monomers such as aniline and thiophene.

The hybrid nanocomposite can be used for fabricating electronic devices for gas sensing applications and for making electrochemical devices such as super capacitors, he says.

Published in The Hindu on November 1, 2016.

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