Graphene – the versatile wonder material
Source: The Graphene Flagship

Graphene – the versatile wonder material

In 2010, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics jointly to Sir Andre Geim and Sir Konstantin Novoselov of the University of Manchester “for groundbreaking experiments regarding the two-dimensional material graphene”. But what exactly is graphene and what makes it so special?

Report: Sascha Keutel

There have been sporadic attempts to study graphene since 1859, but the biggest breakthrough in the research of the material came in 2004 when Geim and Novoselov discovered and extracted graphene from a piece of graphite as found in ordinary pencils. In what is now known as the ‘Scotch tape method’, they used a regular adhesive tape to obtain a flake of carbon with a thickness of just one atom. Back then, it was believed to be impossible for such a thin crystalline material to be stable.

Stronger, thinner, lighter

Materials as thin as just a single atomic layer are known as two-dimensional, or 2D, materials – of which graphene is probably the best known. It is an atomic crystal made up of carbon atoms arranged in a hexagonal lattice. The versatile material has a set of unique and outstanding properties. It is considered the thinnest (around 0.34 nanometer), the strongest (between 100-300 times stronger than steel) and the lightest material discovered (weighing approximately 0.77 milligrams per square meter). It is also one of the most stretchable crystals: it can be stretched up to 20 percent of its initial size without breaking it. It is also highly impermeable; in fact it is so dense that even helium, the smallest gas atom, cannot pass through it.

Recently, a team of researchers at Columbia University and the University of Washington discovered that a variety of exotic electronic states can arise in a three-layer graphene structure. The work was inspired by previous studies of twisted monolayers or twisted bilayers of graphene, comprising either two or four total sheets. These materials were found to host an array of unusual electronic states driven by strong interactions between electrons. ”We wondered what would happen if we combined graphene monolayers and bilayers into a twisted three-layer system,” said Cory Dean, a professor of physics at Columbia University. ”We found that varying the number of graphene layers endows these composite materials with some exciting new properties that had not been seen before.”

Stacking monolayer and bilayer graphene sheets with a twist leads to new...
Stacking monolayer and bilayer graphene sheets with a twist leads to new collective electronic states, including a rare form of magnetism.
Source: Columbia University

The team also discovered new magnetic states in the system. Unlike conventional magnets, which are driven by a quantum mechanical property of electrons called ‘spin’, a collective swirling motion of the electrons in the team's three-layer structure underlies the magnetism, they observed.

This form of magnetism was discovered recently by other researchers in various structures of graphene resting on crystals of boron nitride. The team has now demonstrated that it can also be observed in a simpler system constructed entirely with graphene. ”Pure carbon is not magnetic,” said Assistant Professor Matthew Yankowitz from the University of Washington. ”Remarkably, we can engineer this property by arranging our three graphene sheets at just the right twist angles.”

In addition to the magnetism, the study uncovered signs of topology in the structure. Akin to tying different types of knots in a rope, the topological properties of the material may lead to new forms of information storage, which ”may be a platform for quantum computation or new types of energy-efficient data storage applications,” Professor Xiaodong Xu from the University of Washington said.

Graphene ink can power flexible and wearable skin sensors

Graphene has a vast variety of practical applications in the creation of new materials and the manufacture of innovative electronics. For example, the strong and flexible nature of graphene makes flexible displays and bendable batteries possible. Since it is practically transparent and a good conductor, graphene is suitable for producing transparent touch screens or light panels.

Due to its large surface area, high electrical conductivity, unique optical properties and high thermal conductivity, graphene is extremely suitable for the development of the next generation of wearables and sensors. The biological compatibility of graphene enables it to be used in biosensors capable of sensing molecules such as DNA or glucose, glutamate, cholesterol, hemoglobin.

A team of physicists from the University of Arkansas has successfully developed a circuit capable of capturing graphene’s thermal motion and converting it into an electrical current. “An energy-harvesting circuit based on graphene could be incorporated into a chip to provide clean, limitless, low-voltage power for small devices or sensors,” said Paul Thibado, professor of physics and lead researcher in the discovery.

Graphene chip - A sample harvesting chip under development.
Graphene chip - A sample harvesting chip under development.
Source: University of Arkansas

Researchers at the Daegu Gyeongbuk Institute of Science and Technology in South Korea have developed a low-cost energy storage device that can effectively power flexible and wearable skin sensors along with other electronic devices, paving the way towards remote medical monitoring and diagnoses and wearable devices. A key for their success is spraying a specific amount of graphene ink onto flexible substrates at a specific angle and temperature.

They sprayed ten milliliters of graphene ink at a 45° angle and 80°C temperature onto a flexible substrate. This led to the formation of porous, multi-layered electrodes. The team’s micro-supercapacitor is 23 micrometers thin, ten times thinner than paper, and retains its mechanical stability after 10,000 bends. It can store around 8.4 microfarads of charge per square centimeter (2 times higher than that of the value reported today) and has a power density of about 1.13 kilowatts per kilogram (4 times higher than that of the Li-ion batteries). The team demonstrated it could be used in wearable devices that adhere to the skin. “Our work shows that it’s possible to reduce the thickness of micro-supercapacitors for use in flexible devices, without degrading their performance”, says materials scientist Sungwon Lee.

The trick to extremely thin supercapacitors with improved performance is...
The trick to extremely thin supercapacitors with improved performance is spraying graphene ink at an angle.
Source: DGIST

Biomedical application

Its inherent properties make graphene well suited for several biomedical applications, such as disease and tumour detection, drug delivery or tissue engineering.

An interdisciplinary team of researchers from The University of Manchester have developed a new graphene-based testing system for disease-related antibodies, initially targeting a kidney disease called Membranous Nephropathy. The new instrument, based on the principle of a quartz-crystal microbalance (QCM) combined with a graphene-based bio-interface, offers a cheap, fast, simple and sensitive alternative to currently available antibody tests. ”Our research has the potential to make antibody testing for various diseases more widely available, at the point of care like GP clinics or care homes, rather than just in specialist testing centers," said Dr. Aravind Vijayaraghavan, from the University of Manchester, who is the lead investigator on this project.

The scientists developed a method to form a protein-based layer on graphene,...
The scientists developed a method to form a protein-based layer on graphene, which can then be functionalised with specific receptor molecules for antibodies. These receptors ensure that only the targeted antibody will bind to the graphene surface, and nothing else.
Source: The University of Manchester

Fighting Covid-19

Graphene is set to play an important role in combating Covid-19. According to Professor Henry Yi Li, Chair of Textile Science and Engineering at the University of Manchester, the use of smart fabrics will have a key role in helping safeguard us from Covid-19. He said that wearable technology also has a vital role to play in “detection, testing and diagnosis”.

Yi Li explains that by combining e-textile antennas and fabric sensors that feature graphene or other 2D materials, it is possible to create a wearable device to monitor different types of physiological health indicators, such as breathing rates, heart rates, body temperature and sweating rates. This biodata could then be assessed and results fed back to an individual using an appropriate mobile phone app linked to a cloud-based digital health service. This could be helpful to monitor an individual’s wellbeing and also, if permission were given, the data could be used more widely to provide public health observers with real-time indicators on the potential spread or retreat of a virus and help them better manage the public health response.

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