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Metallic conduction at the interface of two electrically insulating materials

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Along with PhD student Pooja Sindhu and collaborators, Prof. Nirmalya Ballav's group from the Chemistry department at IISER Pune designed a thin film made of two materials, referred to as metal-organic frameworks (MOFs), and captured metallic conduction at the interface where the two MOFs meet. Considering that MOFs are usually electrically insulating by nature, this development opens up new possibilities in interfacial chemistry.

The interface between two materials has often been found to have unique and useful properties.  Interfaces of thin films of materials such as semiconductors or insulating transition metal oxides and organic molecules have been of particular interest. During the last two decades, various physical phenomena have been observed at the interfaces of electrically insulating materials and the chemistry of surfaces or interfaces has been of keen research interest for many.

In the current study, the team designed a hetero-structured thin film composed of two distinct copper-based MOFs, namely Cu-BPyDC (a band insulator) and Cu-TCNQ (a Mott-insulator).

Metallic conduction at the hetero-structured thin film made of two mal organic frameworks, namely, Cu(II)-BPyDC (a band insulator) and Cu(I)-TCNQ (a Mott insulator) (Image: Prof. Nirmalya Ballav)

The team observed conduction across the hetero-structured thin film, and found that the current-voltage profiles are consistent across micrometre scale height and centimetre scale width.

“The observation is intriguing and can have far reaching implications. Novel chemistry at the solid-liquid interface generating hetero-structured thin films has the capability to make an effective use of electrically insulating MOFs to potential device application possibilities” said Prof. Ballav.

Members from the research group of Prof. Nirmalya Ballav at IISER Pune: (Left to Right) Umashis Bhoi, M Kalyani, Sauvik Saha, Nirmalya Ballav, Pooja Sindhu, Rimpa Mandal, Nahid Hassan, and Ajay Shahu Ugale. Ballav and Sindhu are the authors from IISER Pune in the current paper. (Photo: Prof. Nirmalya Ballav)

The team attributed such unusual behaviour to the strong interlayer hybridization and charge-transfer, leading to an overlap of the conduction band and the valence band in the Cu-TCNQ/Cu-BPyDC interface. Clues into this mechanism of conduction were obtained by the density functional theory (DFT) calculations carried out by Dr. Kartick Tarafder’s group at NITK Surathkal.  Structural refinements on the system were carried out by Dr. Anil Jain from BARC Mumbai. 

“This development clearly opens a new avenue and challenges the need of sophisticated deposition techniques in the field, in general” said Prof. Ballav.  

This work received grant support from Science and Engineering Research Board (SERB), India.

Article Citation

Charge-transfer interface of insulating metal-organic frameworks with metallic conduction Pooja Sindhu, K. S. Ananthram, Anil Jain, Kartick Tarafder and Nirmalya Ballav (2022) Nature Communications 13, 7665.


Related background

Metal-organic frameworks (MOFs) are a special class of polymers with nanoscale pores made up of metal nodes linked to organic ligands. The characteristics of MOFs make them ideal as catalysts and for applications such as water purification and drug delivery. Typically, MOFs are poor conductors of electricity. Over the last decade, drawn by the promise of enhancing the characteristics and utility of MOFs even further, researchers have contributed to improve the electrical transport properties of MOFs.

Many different approaches have been proposed and developed to impart electrical property to MOFs. Some of these include adding redox-active guest molecules, choosing metal ions or redox-active linkers strategically, forming composites with conducting polymers and many more.


- with inputs from Pooja Sindhu and Nirmalya Ballav