Defects - Page 2

Researchers at the University of Illinois examined how tiny defects in graphene membranes, formed during fabrication, could be used to improve molecule transport. They found that the defects make a big difference in how molecules move along a membrane surface. Instead of trying to fix these flaws, the team set out to use them to help direct molecules into the membrane pores.

Nanopore membranes have generated interest in biomedical research because they help researchers investigate individual molecules - atom by atom - by pulling them through pores for physical and chemical characterization. This technology could ultimately lead to devices that can quickly sequence DNA, RNA or proteins.

Researchers at MIT have utilized an everyday material - wax- to protect graphene from performance-impairing wrinkles and contaminants. Removing graphene from the substrate it’s grown on and transferring it to a new substrate is known t be challenging. Traditional methods encase the graphene in a polymer that protects against breakage but also introduces defects and particles onto graphene’s surface. These interrupt electrical flow and stifle performance.

a Schematics showing the process of paraffin-assisted graphene transfer. b Schematics showing the effect of paraffin’s thermal expansion on graphene wrinkle. c A typical paraffin-supported graphene film floated on water at different temperatures

The MIT team describes a fabrication technique that applies a wax coating to a graphene sheet and heats it up. Heat causes the wax to expand, which smooths out the graphene to reduce wrinkles. Moreover, the coating can be washed away without leaving behind much residue.

Researchers at MIT have produced a catalog of the exact sizes and shapes of defects and holes that would most likely be observed (as opposed to the many more that are theoretically possible) when a given number of atoms is removed from the atomic lattice. The MIT team collaborated on this project with researchers at Lockheed Martin Space and Oxford University.

The 12 different forms that six-atom vacancy defects in graphene can have, as determined by the researchers

It’s been a longstanding problem in the graphene field, what we call the isomer cataloging problem for nanopores, Michael Strano from MIT says. "For those who want to use graphene or similar two-dimensional, sheet-like materials for applications including chemical separation or filtration", he says, we just need to understand the kinds of atomic defects that can occur, compared to the vastly larger number that are never seen".

A team of NYU researchers has tackled the longstanding question of how to build ultra-sensitive, ultra-small electrochemical sensors with homogeneous and predictable properties, by discovering how to engineer graphene structure on an atomic level. The team's findings could benefit biochemical detection, environmental monitoring, and lab-on-a-chip applications

Finely tuned electrochemical sensors (also referred to as electrodes) that are as small as biological cells have tremendous potential for medical diagnostics and environmental monitoring systems. However, efforts to develop them have encountered obstacles, like the lack of quantitative principles to guide the precise engineering of the electrode sensitivity to biochemical molecules.

Researchers at Royal Melbourne Institute of Technology (RMIT) have found that graphene could better fulfill its potential when purified to remove silicon, doubling its electrical performance.

Despite researchers demonstrating countless possible applications of graphene, many people feel that graphene is thus far showing rather sluggish industrial adoption. Now, researchers based at RMIT have proposed a possible reason for this and suggested how graphene's full potential could be unlocked.

Researchers at MIT have found a way to directly pinprick microscopic holes into graphene as the material is grown in the lab. Using this technique, they have fabricated relatively large sheets of graphene (roughly the size of a postage stamp), with pores that could make filtering certain molecules out of solutions vastly more efficient.

Holes would typically be considered unwanted defects, but the MIT team has found that certain defects in graphene can be an advantage in fields such as dialysis. Typically, much thicker polymer membranes are used in laboratories to filter out specific molecules from solution, such as proteins, amino acids, chemicals, and salts. If it could be tailored with selectively-sized pores that let through certain molecules but not others, graphene could substantially improve separation membrane technology.

Researchers at the Japanese Tsukuba University described a graphene-based electrode that can produce hydrogen under acidic conditions. The electrolysis of water to generate hydrogen is vital for energy storage in a green economy. One of the major obstacles, however, is the high cost of noble-metal electrodes. Cheaper non-noble electrodes function well in driving the hydrogen evolution reaction (HER), but mainly in alkaline conditions, where the reaction is electricity-hungry. The more efficient acid-phase reaction requires precious metals such as platinum. Worse still, the acid electrolytes are corrosive and eat away at the core metal.

The researchers have found that holey graphene offers a way around this problem. They used nitrogen-doped graphene sheets to encapsulate a nickelmolybdenum (NiMo) electrode alloy. The graphene was punched full of nanometer-size holes. The researchers showed that in acid conditions, their HER system dramatically outperforms an electrode using regular non-holey graphene. The use of graphene in HER electrodes is not new—this flexible, conductive carbon sheet is ideal for wrapping around the core metal. However, although it protects the metal against corrosion, graphene also suppresses its chemical activity. In the Tsukuba system, the holes promote the reaction in two ways, while the intact graphene part protects the metal.

A team of researchers from China has designed a new growth method that produces smooth and pristine graphene. Using a carefully engineered substrate, the researchers can grow high-quality graphene free of wrinkles that often form during manufacture. The team reports that the super-smooth graphene has shown improved electrical properties over rumpled graphene grown by the usual methods.

Existing methods usually use copper foil as a growth substrate to form a sheet of graphene. However, the research team hypothesized that a mismatch in material properties between between graphene and the copper growth substrate may be the cause of wrinkling that often damage the resulting graphene's properties. Graphene and the form of copper usually used as substrate expand at different rates at a given temperature, leading to mechanical strain and causing wrinkling. So, the team searched for copper substrates with a crystalline structure that’s a better match.

Two interesting projects focused on coating single-layer graphene with metal-oxide nanolayers were presented at the latest Thin Films and Coating Technologies for Science and Industry event in the UK. Researchers from Cranfield University, UK, together with collaborators from University of Cambridge and the Centre for Process Innovation (CPI), applied alumina to form a composite barrier layer, while a team from Imperial College London, UK, used the unique properties of strontium titanate to fabricate a tuneable capacitor.

The researchers of the first project explained that in theory, graphene should represent an ideal ultrathin barrier layer, as the pores between carbon atoms are smaller even than the radius of a helium atom. In practice, however, crystal boundaries and missing atoms allow vapor to permeate through the material, and the weak van der Waals bonds between planes mean that even stacks of multiple graphene layers can be penetrated. The solution reported by the team is to take a graphene monolayer formed by CVD, and to then use atomic layer deposition (ALD) to coat it with a 2550 nm thick layer of alumina. Achieving conformal coatings on single-layer graphene is known to be difficult due to the material’s strong hydrophobicity.

Researchers at Clemson University in the U.S have designed a prototype Aluminum-ion battery (AIB) that uses a graphene electrode to intercalate tetrachloroaluminate (AlCl4). The researchers have used the device to investigate the effect of defects and doping on battery performance.

Aluminum-ion batteries are gaining recognition in the scientific community as a potential alternative to Li-ion battery systems, but so far there have been many obstacles. Unlike in LIBs, where the mobile ion is Li+, aluminum forms a complex with chloride in most electrolytes and generates an anionic mobile charge carrier, usually AlCl4 or Al2Cl7. The team at Clemson University's Nanomaterials Institute have elucidated the intercalation mechanism of the AlCl4 anion in graphene electrodes, and provided a unique insight into the influence of defects and doping on the intercalation process.