Mica, a well-known insulator, has been found to behave like a semiconductor when thinned down to a few molecular layers — ScienceDaily

In 2004, researchers at the University of Manchester used adhesive tape to pull sheets of single carbon atoms from graphite to make graphene, a material that is 1,000 times thinner than a human hair but stronger than steel. This innovative exfoliation technique paved the way for the development of a wide range of two-dimensional materials with different electrical and physical properties for the next generation of electronic devices.

One such interesting material is 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 attracted 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 completely clear. In particular, the properties of single and few molecular layer thick MuMs are not clearly understood. This is because in all studies that have investigated the electrical properties of MuM so far, the conductivity has been dominated by a quantum phenomenon called “tunneling”. This makes it difficult to understand the conductive nature of the thin MuM.

In a recent study published in the journal A physical examination is attachedProfessor Muralidhar Miryala of the Shibaura Institute of Technology (SIT), Japan, along with Professors MS Ramachandra Rao, Ananth Krishnan and Mr. Ankit Arora, Ph.D., of the Indian Institute of Technology Madras, India, have already observed semiconductor behavior in thin MuM flakes characterized by electrical conductivity that is 1000 times greater than that of thick MuM. “Mica is one of the most popular electrical insulators used in industry for decades. However, this semiconductor-like behavior has not been reported previously,” says Prof. Miryala.

In their study, the researchers exfoliated thin MuM flakes of varying thickness on silicon (SiO2/Si) substrates and, to avoid tunneling, a distance of 1 µm was maintained between the contact electrodes. When measuring the electrical conductivity, they noticed that the transition to a conductive state occurred gradually as the flakes thinned to fewer layers. They found that for MuM flakes below 20 nm the current is thickness dependent, becoming 1000 times larger for 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 “conductance hopping model,” which suggests that the observed conductivity is due to an increase in the carrier density of the conduction band with decreasing thickness. Simply put, as the thickness of the MuM flakes is reduced, the energy required to move electrons from the solid bulk to the surface is reduced, allowing the electrons to more easily pass into the “conduction band” where they are free to move. to conduct electricity. As for why the carrier density increases, the researchers attribute it to the effects of the surface doping contribution (adding impurities) of K+ ions and relaxation of the crystal structure of MuM.

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 shows that MuM has the potential to pave the way for the development of robust electronics,” says Prof. Miryala.

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Materials provided by Shibaura Institute of Technology. Note: Content may be edited for style and length.

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