The Twist in Studying Graphene

By Allison Kubo

Graphene is a comprised of a one-atom-thick layer of carbon atoms arranged in a honeycomb structure. This sheet can be wrapped into fullerenes, rolled into nanotubes, or stacked to form graphite the same thing uses ing pencils. All of these are made of carbon: diamonds, graphite, graphene are all different arrangements of carbon, The potential of graphene can is on the level of some of the most out-there science fiction novels but the last decade it has become fact.

The first strange thing about graphene is that it shouldn’t exist. Graphene is a two-dimensional crystal (crystals in physics are arrangements of atoms in a lattice structure). It was theorized that two-dimensional crystals would be too unstable to maintain their crystal lattice [1]. However, in 2004, Professors Sir Andre Geim and Sir Kostya Novoselov experimentally isolated graphene for the first time [2]. They were acknowledged for this discovery with the Nobel Prize in Physics in 2010. Their method utilizes a shocking simple tool: Scotch tape. Although new methods have emerged, Geim and Novoselov originally separated the thin graphene sheets using tape to pull off thin layers from a piece of graphite until reaching the required thickness. Their other innovation was developing a specific method for observing graphene by placing the sheet on a thin glass sheet of a very specific thickness which makes it visible to optical microscopy.

Surprisingly, the crystal is extremely stable and exhibits a high quality-crystal lattice with no defects in the structure. This structure, which was previously thought to be impossible, has many puzzling properties.

Graphene is extremely strong. Despite being perhaps the thinnest possible structure, it is extremely resistant to tearing. It is one million times thinner than paper and completely transparent to the human eye, yet it is the strongest material in the world. Sound like science fiction yet? There’s so much more.

Electrons in graphene behave like massless particles. Electrons, although very small, do have mass. In normal solids, the electrons are constantly being pushed and pulled by the positively charged protons or by other electrons. They all have different speeds and race around chaotically. However, in graphene, all electrons move at the same speed near the speed of light and behave as if they have no mass. This can allow scientists to study how extremely small particles like seen in the Large Hadron Collider on a desktop.

Graphene is an amazing conductor. Conductors are materials that allow electrons to move through them and transmit electricity. However, most materials have some sort of resistance due to impurities that interrupt the flow of electrons and some of the electricity is lost to heat. Graphene has few impurities and the strange behavior of electrons yields an excellent conductor. Graphene could be utilized to replace silicon in many computer chips and make more energy-efficient and smaller electronic components. If one layer isn’t enough for you, it gets more complicated when you stack graphene on top of each other.

Moire pattern in bilayer graphene. Photo by APS/Alan Stonebraker.

It was theorized in 2011 and then proved in 2018 that graphene layered on top of each other but misaligned slightly formed a superconductor [3, 4]. Misaligned by exactly 1.1 degrees, bilayer graphene is able to conduct electrical current with no loss of energy. Superconductors are materials whose resistance drops to zero after it has been cooled to a specific temperature. The emergence of superconductivity is one of the interesting properties of twisted bilayer graphene (TBLG). Studying the twist angle is away for investigators to delve into the interactions between electrons in the graphene layers and the nature of superconductors. The twister layers form moire patterns that have only fascinated artists and mathematicians alike.

By manipulating the twist angle, carrier-density, magnetic-field, pressure, and gate electric field of the TBLG, you can control the electronic properties. In addition, there are many strange states that can emerge from TBLG which can act like both insulators and exhibit magnetism similar to a feromagnet. This work could be applied in the creation of quantum computers or in making more efficient traditional computer parts.

The secrets of monolayer and bilayer graphene dig into the fields of quantum mechanics, microelectronics, and semiconductor stages. It spans the real tangible of since into the abstract and confusing. So the next time your pencil tip breaks, instead of being frustrated you should remember that there are some strange physics when you separate graphite into single atom thick layers.

[1] Peierls, R. E. Quelques proprietes typiques des corpses solides. Ann. I. H. Poincare 5, 177-222 (1935).
[2] Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, 666-669 (2004).
[3] Bistritzer, R., & MacDonald, A. H. (2011). Moiré bands in twisted double-layer graphene. Proceedings of the National Academy of Sciences, 108(30), 12233-12237.
[4] Y. Cao, V. Fatemi, A. Demir, S. Fang, SL. Tomarken, JY. Luo, J. D. Sanchez-Yamagishi, K Watanabe, T Taniguchi, E Kaxiras, R C Ashoori, P Jarillo-Herrero (2018). “Correlated insulator behaviour at half-filling in magic-angle graphene superlattices”. Nature. 556 (7699): 80–84.

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