In 2004, researchers from the University of Manchester used adhesive tape to pull sheets of single carbon atoms away from graphite to make graphene — a material that is 1000 times thinner than human hair yet stronger than steel. This ground-breaking exfoliation technique paved the way for the development of a wide range of two-dimensional materials with distinct electrical and physical characteristics for the next generation of electronic devices.
One such material of interest has been muscovite mica (MuM). These minerals have the general formula KAl2(AlSi3O10) (F, OH)2 and have a layered structure consisting of aluminum (Al), potassium (K), and silicon (Si). Like graphene, MuM has gained attention as an ultra-flat substrate for building flexible electronic devices. Unlike graphene, however, MuM is an insulator.
However, the electrical properties of MuM are not altogether clear. In particular, the properties of single and few-molecule-layer thick MuMs are not clearly understood. This is because in all the studies that have probed the electrical properties of MuM so far, the conductivity has been dominated by a quantum phenomenon called "tunneling." This has made it difficult to understand the conductive nature of thin MuM.
In a recent study published in the journal Physical Review Applied, Professor Muralidhar Miryala from Shibaura Institute of Technology (SIT), Japan, along with Professors M. S. Ramachandra Rao, Ananth Krishnan and Mr. Ankit Arora, a PhD student, from Indian Institute of Technology Madras, India, have now observed a semiconducting behavior in thin MuM flakes, characterized by an electrical conductivity that is 1000 times larger than that of thick MuM. "Mica has been one of the most popular electrical insulators used in industries for decades. However, this semiconductor-like behavior has not been reported earlier," says Prof. Miryala.
In their study, the researchers exfoliated thin MuM flakes of varying thickness onto silicon (SiO2/Si) substrates and, to avoid tunneling, maintained a separation of 1 µm between the contact electrodes. On measuring the electrical conductivity, they noticed that the transition to a conducting state occurred gradually as the flakes were thinned down to fewer layers. They found that for MuM flakes below 20 nm, the current depended on the thickness, becoming 1000 times larger for a 10 nm thick MuM (5 layers thick) compared to that in 20 nm MuM.
To make sense of this result, the researchers fitted the experimental conductivity data to a theoretical model called the "hopping conduction model," which suggested that the observed conductance is due to an increase in the conduction band carrier density with the reduction in thickness. Put simply, as the thickness of MuM flakes is reduced, the energy required to move electrons from the solid bulk to the surface decreases, allowing the electrons easier passage into the "conduction band," where they can freely move to conduct electricity. As to why the carrier density increases, the researchers attributed it to the effects of surface doping (impurity addition) contributions from K+ ions and relaxation of the MuM crystal structure.
The significance of this finding is that thin exfoliated sheets of MuM have a band structure similar to that of wide bandgap semiconductors. This, combined with its exceptional chemical stability, makes thin MuM flakes an ideal material for two-dimensional electronic devices that are both flexible and durable. "MuM is known for its exceptional stability in harsh environments such as those characterized by high temperatures, pressures, and electrical stress. The semiconductor-like behavior observed in our study indicates that MuM has the potential to pave the way for the development of robust electronics," says Prof. Miryala.
There is certainly more to mica than meets the eye!
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Reference
DOI: https://doi.org/10.1103/PhysRevApplied.17.064042
About Shibaura Institute of Technology (SIT), Japan
Shibaura Institute of Technology (SIT) is a private university with campuses in Tokyo and Saitama. Since the establishment of its predecessor, Tokyo Higher School of Industry and Commerce, in 1927, it has maintained "learning through practice" as its philosophy in the education of engineers. SIT was the only private science and engineering university selected for the Top Global University Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology and will receive support from the ministry for 10 years starting from the 2014 academic year. Its motto, "Nurturing engineers who learn from society and contribute to society," reflects its mission of fostering scientists and engineers who can contribute to the sustainable growth of the world by exposing their over 8,000 students to culturally diverse environments, where they learn to cope, collaborate, and relate with fellow students from around the world.
Website: https://www.shibaura-it.ac.jp/en/
About Professor Muralidhar Miryala from SIT, Japan
Dr. Muralidhar Miryala is a Professor at the College of Engineering / Graduate School of Science and Engineering and Board of Councilor at SIT. He is an active researcher in solid state physics and materials science with a focus on Materials for Energy and Environment, high-temperature superconductivity research group. He has over 500 publications so far, including patents, books, review-articles, articles etc. He has received several awards for his research contributions, including the prestigious 2021 Pravasi Bharatiya Samman Award by the President of India and SIT Excellent Education Award (2021) by Chairman of Board of Directors.
Funding Information
This study was funded in part by the Department of Science and Technology, Government of India (DST-GoI, sanction order no. DST/INSPIRE/04/2016/001865 under DST INSPIRE Faculty program) that led to the establishment of Nano Functional Material Technology Centre (NFMTC) (SR/NM/NAT/02-2005 and DST/NM/JIIT-01/2016(C)), in part by Japan Student Services Organization for the advanced Project Based Learning (aPBL), and in part by Shibaura Institute of Technology under the Top Global University Project, designed by Ministry of Education, Culture, Sports, Science & Technology in Japan.
