MOF conceptual diagram

Physicists have developed a photodetector that is completely based on layers of metal-organic frameworks. Since this compound can detect and transform a broad range of light wavelengths into electrical signals, it could become a novel detector material. [Credit: HZDR / Juniks]

Semiconductor photodetectors have found a home in devices large and small, ranging from the smartphone camera in your pocket to the imaging instrumentation on the Hubble Space Telescope. But the high costs of processing traditional semiconductor materials have led researchers to search for cheaper alternatives.

Metal-organic frameworks (MOFs), a type of compound that consists of metal ions linked to organic ligands, have been proposed as a possible alternative. Now, a team of scientists from Germany and Spain has reported a MOF-based photodetector that can detect a broad spectral range extending from blue to near-infrared without losing functionality (Adv. Mater., doi: 10.1002/adma.201907063). The work demonstrates the feasibility of integrating MOFs as an active element into most cost-effective photodetectors.

Semiconducting 2D MOF

MOFs are highly regarded for their tunability, structural diversity, and subsequently wide range of chemical and physical properties. They are highly porous materials that have emerging applications in gas storage and separation, catalysis, chemical sensing and drug delivery.

In 2018, researchers at Technische Universität Dresden, Germany, developed a novel two-dimensional MOF using cost-effective production methods. The hybrid material, like graphene, has a crystalline structure consisting of only a single layer of atoms.

“On testing its intrinsic properties, the material revealed semiconducting properties comparable to conventional semiconducting materials, such as high mobility, small band gap and chemical and thermal stability,” said Himani Arora, study author and physics graduate student at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany. “After characterizing the material, we were intrigued to test its potential as an active element in photodetectors.”

From blue to near-infrared

Arora and her colleagues set out to build a proof-of-concept photodetector using their semiconducting MOF thin films. To synthesize the MOF layer, iron atoms and 2,3,6,7,10,11-triphenylenehexathiol (THT) organic ligands were bonded with sulfur in a honeycomb structure. The researchers then used electron-beam lithography to fabricate indium metal electrodes directly onto the MOF sheet, which was affixed to an insulating glass substrate.

They tested the photoresponse of the MOF-based detector to laser light at various wavelengths, including blue (405 nm), red (633 nm) and near-infrared (785 nm and 1575 nm). Experiments also checked the detector’s functionality at different power settings and temperatures (from 25 °C down to −196 °C).

The prototype photodetector operated successfully throughout the range of wavelengths tested, and the results also show that the light sensitivity of the MOF layer can be modulated with temperature. The researchers saw significant improvements in the performance of the device at lower temperatures, and much weaker light signals could be detected.

Bringing MOFs to industry

According to the researchers, their work represents the first time such a broadband photodetector fully based on a MOF layer has been demonstrated. It suggests the potential of MOF applications in optoelectronic technology including photovoltaics, solar cells and phototransistors, as well as sensors for cameras and infrared detectors.

“At the moment, we are interested in scaling down the device dimensions to make it as compact as possible, while maintaining the performance to meet the requirements of future electronics,” said Arora. “The optimization of the device fabrication techniques to be able to meet industry requirements is also underway.”