Defects - Page 2

An international team of researchers in Korea, the UK, Japan, the US and France recently shed light on the mysterious ability of graphene (and other carbon materials) to change its structure and even self-heal defects, by showing that fast-moving carbon atoms catalyze many of the restructuring processes.

Until now, researchers typically explained the structural evolution of graphene defects via a mechanism known as a Stone-Thrower-Wales type bond rotation. This mechanism involves a change in the connectivity of atoms within the lattice, but it has a relatively large activation energy, which makes it seem unlikely to succeed without some form of assistance.

Researchers from Nanjing University in China have developed a method to make large graphene films free of any wrinkles. The ultra-smooth films could enable large-scale production of electronic devices that harness the unique physical and chemical properties of graphene and other 2D materials.

Wrinkles in graphene films grown via chemical vapor deposition appear as jagged white lines at the top of this atomic force microscope image (left), but they disappear when the material is treated with a hydrogen plasma (right). Credit: Nature

Chemical vapor deposition (CVD) is the best-known method for making high-quality graphene sheets. It typically involves growing graphene by pumping methane gas onto copper substrates heated to temperatures around 1,000 °C, and then transferring the graphene to another surface such as silicon. But some of the graphene sticks to the copper surface, and as the graphene and copper expand and contract at different rates, wrinkles form in the graphene sheets. Such wrinkles often present hurdles for charge carriers and lower the film’s conductivity. Other researchers have tried to reduce wrinkles using low growth temperatures or special copper substrates, but the wrinkles have proven difficult to eliminate entirely, according to Libo Gao, a physicist at Nanjing University.

In order for CVD graphene to be used in its intended application, it needs to be transferred from the growth substrate to a target substrate a challenging but extremely important process step. Typically the transfer is done by spin-coating a supporting polymer layer and then chemically dissolving away the copper to release the graphene film from the substrate. The transferred graphene produced in this way is prone to contamination from the chemical agents used to remove the growth substrate as well as defective amorphous carbon generated during the high-temperature CVD growth. It also frequently leads to a substantial amount of polymer particle residue on the graphene generated during the transfer process. A third source of contamination could be airborne particles that are adsorbed onto the graphene surface.

Top: Schematic of the activated carbon-coated lint roller for cleaning the graphene surface. Bottom left: AFM image of unclean graphene on Cu foil. Bottom right: AFM image of superclean graphene on Cu foil. Image taken from Nanowerk

Researchers from Peking University and Tsinghua University in China and University of Manchester in the UK have recently demonstrated that the amorphous carbon contaminants on CVD-produced graphene, which could greatly degrade its properties, can be removed by an activated carbon-coated lint roller, relying on the strong interactions between the amorphous carbon and activated carbon.

According to our information, LG Electronics is aiming to start supplying CVD graphene materials worldwide soon, with an aim to accelerate the adoption of CVD graphene in various applications. LG is collaborating with research groups to identify new applications for graphene sheets.

LG Electronics developed its own roll-to-roll production process in addition to a specific quality control system for its graphene. LG says that its inspection system can manage uniformity deviations in crystal size, defects and electrical properties in its graphene to within 10%.

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.