Defects - Page 3

Researchers from Korea and China have developed a method to synthesize large sheets of monolayer single-crystal graphene.

Polycrystalline graphene is formed by randomly oriented graphene islands, which decreases its quality. Currently, scientists can grow meter-sized polycrystalline graphene and smaller sizes of the usually higher-quality single-crystal graphene, ranging from 0.01 mm² to a few square centimeters. The synthesis of large single-crystal graphene at a low cost is considered a desirable goal. In this study, the team reported the synthesis of a large sheet of monolayer single-crystal graphene.

Researchers at Rice University, who invented laser-induced graphene (LIG), in collaboration with researchers at Ben-Gurion University in Israel, have designed a way to make the spongy graphene either superhydrophobic or superhydrophilic.

Until recently, the Rice lab made LIG in open air only, using a laser to burn part of the way through a flexible polyimide sheet to get interconnected flakes of graphene. However, putting the polymer in a closed environment with various gases changed the product’s properties. Forming LIG in argon or hydrogen makes it superhydrophobic (extremely water-avoiding), a property highly beneficial for separating water from oil or de-icing surfaces. Forming it in oxygen or air makes it superhydrophilic (extremely water-attracting), making it highly soluble.

Researchers from the University of Hamburg and Graphenea have succeeded in upscaling high-quality graphene devices to the 100-micron scale and beyond. By perfecting CVD graphene production, transfer and patterning processes, the team managed to observe the quantum Hall effect in devices longer than 100 micrometers, with electronic properties on par with micromechanically exfoliated devices.

The work started from graphene grown by chemical vapor deposition (CVD) on a copper substrate. Since graphene on metal is not useful for applications in electronics, the material is usually transferred onto another substrate before use. The transfer process has proven to be a challenge, in many cases leading to cracks, defects, and chemical impurities that reduce the quality of the graphene.

Researchers from the Clemson Nanomaterials Institute and the Ural Federal University in Russia have discovered a way to make an extremely thin oxygen selective membrane using graphene. Such membranes allow only oxygen into Li-O₂ batteries while stopping or slowing water vapor intake. This could impede corrosion caused by ambient water vapor from air and push forward the usability of much-awaited Li-O₂ batteries in electric vehicles and more.

The team has developed an in situ technique to induce pores in graphene by doping it with nitrogen during the growth process. Doping the graphene sheet with nitrogen inevitably breaks some carbon bonds in graphene, opening nanoscopic pores. The researchers observed that such pores in doped graphene selectively allow oxygen, leading to oxidation of the underlying copper foil, unlike pristine graphene.

Scientists at MIT, along with researchers from IBM, the University of California at Los Angeles, and Kyungpook National University in South Korea, have found a way to produce graphene with fewer wrinkles, and to iron out the wrinkles that do appear. The team reports that the techniques successfully produce wafer-scale, "single-domain" graphene - single layers of graphene that are uniform in both atomic arrangement and electronic performance.

After fabricating and then flattening out the graphene, the researchers tested its electrical conductivity. They found each wafer exhibited uniform performance, meaning that electrons flowed freely across each wafer, at similar speeds, even across previously wrinkled regions.

Researchers at the German FAU have created defect-free graphene directly from graphite. Using the additive benzonitrile, the team designed a technique to produce defect-free graphene directly from a solution that enables selective electronic properties to be set through the various charge carriers and enables the production of efficient and cost-effective graphene.

The solution benzonitrile (grey circle) removes the causes of possible defects and turns red, resulting in defect-free graphene (red circle).

With the addition of a solvent called benzonitrile, defect-free graphene can be obtained without the formation of any additional functional groups. In addition, the benzonitrile molecule formed as a byproduct of the reaction remains red unless it comes into contact with water or oxygen. This color change helps to easily determine the number of charge carriers in the system with the help of absorption measurements. This could give battery and graphene researchers a new way to determine the charge state, as previously could only be done by measuring voltage.

Researchers from the University of Illinois at Chicago have used rod-shaped bacteria - precisely aligned in an electric field, then vacuum-shrunk under a graphene sheet - to cause nanoscale ripples in the material, causing it to conduct electrons differently in perpendicular directions. The resulting material can be applied to a silicon chip and may led to various applications in electronics and nanotechnology.

The team explains that the current across the graphene wrinkles is less than the current along them; The key to formation of these wrinkles is graphene's extreme flexibility at the nanometer scale, which allows formation of carbon nanotubes. The wrinkle opens a 'V' in the electron cloud around each carbon atom, creating a dipole moment, which can open an electronic band gap that flat graphene does not have.

A collaborative theoretical and experimental study suggested that graphene sheets efficiently shield chemical interactions. This may hold promise for applications like quality improvement of 2D materials by "de-charging" charged defect centers located on the surface of carbon materials. Another important feature is the ability to control selectivity and activity of the supported metallic catalysts on the carbon substrate.

The team studied carbon materials with surface defects - an active species, that need to be protected. The experiments showed that the defect areas were reactive and retained high activity towards various molecules. However, as soon as the defects were covered with few layers of graphene flakes, the distribution of reactive centers became uniform (without localized reactivity centers typical for defect areas). Put simply, covering surface defects with graphene layers has decreased the influence of charged defects and made them "invisible" for chemical interactions at the molecular level.

Researchers at the US Department of Energy's Lawrence Berkeley National Laboratory have found that polycrystalline graphene has quite low toughness, or resistance to fracture, despite having very high strength. 

The researchers say that while the extremely high strength is impressive, it can't necessarily be utilized unless it has resistance to fracture. The senior scientist in the Materials Sciences Division of Berkeley Lab developed a statistical model for the toughness of polycrystalline graphene to better understand and predict failure in the material. This mathematical model found that the strength varies with the grain size up to a certain extent, but most importantly it defined graphene's fracture resistance. 

Researchers at Lawrence Livermore National Laboratory (LLNL) have found a way to make lithium ion batteries last longer and charge faster, by using graphene nanofoam electrodes and treating them with hydrogen. The calculations and experiments carried out as part of the research revealed that when defect-rich graphene was intentionally treated with hydrogen at a low-temperature, it enhanced the rate capacity of the graphene, so that the interaction of the two opened small gaps in the coating that resulted in better binding between the electrode and lithium ions. 

Using these new electrodes, charging rates went up to 40% faster, with less energy waste during charging and higher power output. The scientists say, however, that here is still a lot to achieve before the work finds its way into commercial batteries. The study also reveals that controlled hydrogen treatment could help optimize the transport of lithium and reversible storage in other materials that are based on graphene.