Goodbye, silicon? On the way to new electronic materials with metal-organic networks

Silicon, a so called semiconductor, is currently widely employed for the development of components such as solar cells, LEDs or computer chips. High purity silicon, a highly ordered material which is expensive to produce, is required to develop such applications.

This is due to the fact that the electrical properties of a semiconductor (as silicon) are strongly affected by disorder. The scientists around group leader Enrique Cánovas (MPI-P, Department of Prof. Dr. Mischa Bonn) have now developed a novel low cost metal-organic framework (MOF), which has similar electrical properties.

The MOF, produced by Xinliang Feng´s group in Dresden, is a highly crystalline solid made of iron ions linked by organic molecules, hence the name metal-organic framework. In contrast to silicon, the material can be made at room temperature, and the sample chemistry, morphology and electronic properties are easily customizable during the manufacturing process.

MOFs produced in the past showed no or very little electrical conductivity, this prevented MOFs to be employed in optoelectronic applications, where an “easy” displacement of electrons under an applied electrical field is required. With the novel MOF, the researchers from Mainz have now demonstrated – for an organic-based material – a behavior of the conductivity similar to silicon, a so-called “Drude behavior” (named after the physicist Paul Drude).

A Drude behavior implies that electrons within the material are almost free to move under the application of an external electric field, i.e. if a voltage is applied. This behavior, a common observable for highly ordered inorganic crystals like silicon, has been hardly seen in organic based materials; organic based materials are generally disordered in nature, unlike MOFs.

To characterize the unique properties for the developed MOF, the scientists at MPI-P employed ultrafast terahertz spectroscopy, a tool that allows measuring the conductivity of the material without requiring physical – perturbative – contacts. Using this tool, energy is transferred to the electrons in the semicondcuting material due to the absorption of light (mediated by an ultrafast laser pump pulse in the visible region).

A second laser pulse – a so-called terahertz probe, which oscillates about a factor of 1000 slower than visible light – allows to interrogate the conductivity for the photo-generated electrons. This results in a frequency dependent conductivity signal from which the scientists were able to model by “Drude” the nature of the transport of the electrons in the MOF.

“From this response, we obtained record mobilities of the electrons in such materials which exceed the mobilities measured in insulating MOFs by a factor of 10000”, says Dr. Enrique Canovas of the MPI-P. This means that electrons can easily drift under an electric field over long distances in MOFs, an aspect that was independently verified in ~1000 micron length devices. Thus, the new material paves the way for the use of MOFs in optoelectronics.

In the future, the researchers will work on the ability to directly set and predict the electronic properties via tuning the composition of their produced metal-organic framework. Their research results have now been published in the renowned journal Nature Materials.

About Enrique Cánovas
Enrique Cánovas graduated on Applied Physics at the Universidad Autónoma de Madrid and holds a PhD, also in Physics, at the Polytechnic University of Madrid where he focused on the exploitation of quantum dots for the development of high-efficiency solar cells. After a 2 years postdoc at FOM Institute AMOLF (The Netherlands), in 2012, he was appointed group leader in the Department of Molecular Spectroscopy (Prof. Mischa Bonn) at the Max Planck Institute for Polymer Research. Since April 2018, he is Research Professor (tenure track) at IMDEA Nanoscience in Madrid (Spain). His research interests cover all aspects of nanotechnology, solar energy conversion and charge carrier dynamics.

About the Max Planck Institute for Polymer Research
The Max Planck Institute for Polymer Research (MPI-P) ranks among the globally leading research centers in the field of polymer research since its foundation in 1984. The focus on soft materials and macromolecular materials has resulted in the unique worldwide position of the MPI-P and its research focus. Fundamental polymers research on both production and characterization as well as analysis of physical and chemical properties are conducted by scientific collaborators from all over the world. Presently over 500 people are working at the MPI-P, the vast majority of whom are engaged in scientific research.

Dr. Enrique Canovas
Max Planck Institute for Polymer Research
Ackermannweg 10
D-55128 Mainz
Germany
Tel. +49 (6131) 379-326
Email: canovas@mpip-mainz.mpg.de

High-mobility band-like charge transport in a semiconducting two-dimensional metal–organic framework

Renhao Dong, Peng Han, Himani Arora, Marco Ballabio, Melike Karakus, Zhe Zhang, Chandra Shekhar, Peter Adler, Petko St. Petkov, Artur Erbe, Stefan C. B. Mannsfeld, Claudia Felser, Thomas Heine, Mischa Bonn, Xinliang Feng & Enrique Cánovas

https://www.nature.com/articles/s41563-018-0189-z

http://www.mpip-mainz.mpg.de/88633/Dr_Enrique_Canovas – Website of Dr. Enrique Canovas
http://www.mpip-mainz.mpg.de – Website of the Max Planck Institute for Polymer Research