UNITED STATES: It has been discovered that mica, a well-known insulator, behaves as a semiconductor when flattened to a few molecule layers.
In 2004, scientists at the University of Manchester created graphene. This substance is 1000 times thinner than a human hair but tougher than steel by using adhesive tape to pull sheets of single carbon atoms away from graphite. This innovative method of exfoliation cleared the way for the creation of a wide variety of two-dimensional materials with unique electrical and physical properties for the following generation of electronic devices.
Muscovite mica (MuM) is one such item that has attracted attention. These minerals are composed of layers of silicon (Si), aluminum (Al), and potassium (K) and have the general formula KAl2(AlSi3O10)(F, OH)2 (Si). Muscovite mica (MuM) has drawn interest as an ultra-flat substrate for creating flexible electrical devices, similar to graphene. MuM, on the other hand, is an insulator, unlike graphene.
MuM’s electrical characteristics are not entirely understood, though. Notably, it is unclear how the characteristics of MuMs with single and few molecule layer thicknesses work. This is due to the conductivity in all investigations that have examined the electrical characteristics of MuM is being dominated by a quantum phenomenon known as “tunnelling.” Because of this, it has been challenging to comprehend the conductivity of thin Muscovite mica (MuM).
Professor Muralidhar Miryala from Shibaura Institute of Technology (SIT), Japan, Professors M. S. Ramachandra Rao, Ananth Krishnan, and Mr Ankit Arora, a PhD student from Indian Institute of Technology Madras, India, has recently published a study in the journal Physical Review Applied that describes the semiconducting behaviour in thin MuM flakes, which is characterised by an electrical conductivity that is 1000 times greater than that of thick Mu “For many years, businesses have employed mica as one of their most common electrical insulators. But no prior reports of this semiconductor-like activity have been made,” Prof. Miryala explains.
In their investigation, the researchers used silicon (SiO2/Si) substrates to exfoliate thin MuM flakes of various thicknesses onto them while keeping the contact electrodes apart by 1 µm to prevent tunnelling. As the flakes were reduced to fewer layers and the electrical conductivity was measured, it was discovered that the change to a conducting condition happened gradually. They found that the current varied with thickness for MuM flakes less than 20 nm, growing 1,000 times greater for a 10 nm thick MuM (5 layers thick) than for a 20 nm thick MuM.
The “hopping conduction model,” which proposed that the observed conductance is caused by an increase in the conduction band carrier density with the reduction in thickness, was fitted to the experimental conductivity data by the researchers to make sense of this result. As MuM flakes become thinner, less energy is needed to move electrons from the solid bulk to the surface. This makes it simpler for electrons to enter the “conduction band,” where they can move around freely and conduct electricity. The researchers attributed the reason for the rise in carrier density to the impacts of K+ ion-contributed surface doping (impurity addition) and MuM crystal structure relaxation.
This discovery is significant because it reveals that the band structure of thin exfoliated sheets of MuM is comparable to that of wide bandgap semiconductors. This makes thin MuM flakes a perfect material for two-dimensional electronic devices that are both flexible and robust, together with its exceptional chemical stability.
“MuM is known for its exceptional stability in demanding environments, such as those characterised by high temperatures, high pressures, and electrical stress. Our study’s observation of semi-conductor like behaviour suggests that MuM may open the door to creating reliable electronics,” Professor Miryala said.
Ankit Arora, Kolla Lakshmi Ganapathi, Tejendra Dixit, Muralidhar Miryala, Murakami Masato, M.S. Ramachandra Rao, Ananth Krishnan. Thickness-Dependent Nonlinear Electrical Conductivity of Few-Layer Muscovite Mica. Physical Review Applied, 2022; 17 (6) DOI: 10.1103/PhysRevApplied.17.064042