Defects

Researchers from the University of Illinois Chicago, Wayne State, and Northwestern have shown that random defects in graphene transistors can be harnessed for next-generation hardware security. Their work demonstrates how the intrinsic disorder in graphene can generate unique electromagnetic “fingerprints,” signals so tied to each device’s atomic structure that they cannot be copied or predicted.

Traditional digital encryption relies on stored keys that can be stolen or cracked. By contrast, this graphene-based system uses a physical unclonable function (PUF), a hardware identity formed by the material’s natural randomness. When probed wirelessly, each graphene transistor produces a distinctive radio signal encoding its physical quirks - residues, strain, charge variations - into a one-of-a-kind signature.

Researchers from several institutions. including the University of Nottingham, University of Warwick and Diamond Light Source, have developed a novel approach to controlling structural defects in graphene, enhancing its functionality for a range of technological applications.

Using a one-step chemical vapor deposition process, the scientists have grown graphene films from azupyrene - a molecule designed to mimic the defects known as Stone-Wales defects, which consist of adjacent five- and seven-membered carbon rings instead of graphene’s typical six-membered arrangement. By adjusting the growth temperature on a copper substrate, the concentration of these defects can be precisely controlled: higher temperatures yield graphene closer to its ideal, defect-free lattice, while lower temperatures facilitate the intentional inclusion of defects.

Researchers at the Institute of Science Tokyo, Japan Science and Technology Agency (JST) and Nagoya University have created a new way to study the mechanical behavior of graphene nanosheets. The technique enables direct measurement of bending rigidity in sheets with structural defects, without the need for laboratory experiments. 

Graphene's structure sometimes includes rings of five or seven carbon atoms instead of the usual six. These "defects" change the sheet's shape - five-membered rings make it cone-like, while seven-membered rings give it a saddle shape. Until now, it was hard to measure how these defects affect the sheet's ability to bend without using complicated experiments. The new approach combines powerful computer simulations and a theory used for describing the bending of biological membranes. This makes it possible to predict how sheets with different types or arrangements of defects will bend, just by looking at their atomic structure.

Würzburg University researchers have created a defect in graphene that allows ions to pass through, which could lead to new applications in water filtration or sensor technology.

The Würzburg model system consisting of two nanographene layers that can absorb and bind chloride ions (green) through a defect in the crystal lattice. (Image: Kazutaka Shoyama / Universität Würzburg)

Defects that allow scientists to control the permeability of graphene for different substances can be very useful: ‘So-called defects can be created in the carbon lattice of graphene. These can be thought of as small holes that make the lattice permeable to gases,’ says chemistry professor Frank Würthner from Julius-Maximilians-Universität (JMU) Würzburg in Germany.

Researchers at Sweden's Umeå University, Lund University and Denmark's Aarhus University have reported a new way to synthesize graphene oxide, which has significantly fewer defects compared to materials produced by the most common method. To date, graphene oxide of similarly good quality could only be synthesized by using a rather dangerous method involving extremely toxic fuming nitric acid.

Graphene oxide is often used to produce graphene by removing oxygen. However, if there are holes in graphene oxide, there will also be holes after it is converted to graphene. Therefore, the quality of the graphene oxide is very important. Umeå University's Alexandr Talyzin and his research group have now addressed the issue of how to safely make good graphene oxide. 

Researchers from The University of Warwick, the University of Manchester, Brazil's Universidade Federal do Ceara and Turkey's Izmir Institute of Technology have tackled the long-standing conundrum of why graphene is so much more permeable to protons than expected by theory. 

A decade ago, scientists at The University of Manchester demonstrated that graphene is permeable to protons, nuclei of hydrogen atoms. The unexpected result sparked a debate because theory predicted that it would be extremely hard for a proton to permeate through graphene's dense crystalline structure. This had led to suggestions that protons permeate not through the crystal lattice itself, but through the pinholes in its structure.

Researchers from  the Vinča Institute of Nuclear Sciences, University of Belgrade, Serbia, recently examined the rainbow scattering of photons passing through graphene and how it reveals the structure and imperfections of the material.

While other methods to examine the defects of graphene exist, these have drawbacks. For instance, Raman spectroscopy can not distinguish some defect types, while high-resolution transmission electron microscopy can characterize crystal structure defects with outstanding resolution, but the energetic electrons it uses can degrade the crystal lattice.

Researchers from the University of Amsterdam, New York University and Spain's CSIC have developed a way to build micrometer-size models of atomic graphene using 'patchy particles’ - particles which are large enough to be easily visible in a microscope but small enough to reproduce many of the properties of actual atoms, can interact with the same coordination as the atoms in graphene, and form the same structures.

Using these patchy particles, the team built a model system and used it to obtain insight into the defects in 2D materials, including their formation and evolution over time. 

Researchers from the Chinese Academy of Sciences, the University of Science and Technology of China and North China University of Water Resources and Electric Power have studied the effects of irradiation defects on the work function of graphene electrodes in thermionic energy converters (TECs) and found that the generation of defects in graphene through irradiation would increase the work function and reduce the electron emission capacity.

Schematic diagram of a thermionic energy converter. (Image by ZHAO Ming) 

Graphene has great potential as an electrode coating material for TECs of the microreactor, which can significantly improve the electron emission ability of electrode. Electrode materials will be exposed to irradiation by high-energy particles during TECs use. Previous studies have shown that the types of defects induced by irradiation in graphene are mainly Stone-Wales defects, doping defects, and carbon vacancies. The appearance of defects will affect the adsorption properties of alkali and alkaline earth metals on the graphene surface in the electrode gap, and then change the electron emission properties of the graphene coating.

A team of researchers, led by Director Rod Ruoff at the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS) and including graduate students at the Ulsan National Institute of Science and Technology (UNIST), has achieved growth and characterization of large area, single-crystal graphene totally free from wrinkles, folds, or adlayers. It was said to be 'the most perfect graphene that has been grown and characterized, to date'.

Director Ruoff notes: This pioneering breakthrough was due to many contributing factors, including human ingenuity and the ability of the CMCM researchers to reproducibly make large-area single-crystal Cu-Ni(111) foils, on which the graphene was grown by chemical vapor deposition (CVD) using a mixture of ethylene with hydrogen in a stream of argon gas. Student Meihui Wang, Dr. Ming Huang, and Dr. Da Luo along with Ruoff undertook a series of experiments of growing single-crystal and single-layer graphene on such ‘home-made’ Cu-Ni(111) foils under different temperatures.