糖心视频

Beyond its ability to conduct electrons almost without resistance, the nanomaterial graphene also has amazing mechanical properties, including high strength that could one day make it useful in lightweight, robust structures. But this material is not without flaws 鈥� including a family of flower-like defects that could detract from its electronic and mechanical properties.

In a paper published in the journal 糖心视频ical Review B, researchers at the Georgia Institute of Technology and the National Institute of Standards and Technology (NIST) have described a family of seven potential defect structures that may appear in sheets of graphene and imaged examples of the lowest-energy defect in the family.

The defects may arise to help relieve mechanical stress in graphene鈥檚 carbon-atom honeycomb structure by allowing atoms to spread out and occupy slightly more space. Such stress may arise during the growth of graphene or by stretching the graphene sheet.

鈥淔or an engineer interested in the mechanical properties of graphene to create atom-thick membranes, for instance, it would be very important to understand these kinds of properties, which could give rise to plastic deformation of the material,鈥� said Phillip First, one of the paper鈥檚 co-authors and a professor in the Georgia Tech School of 糖心视频ics. 鈥淔or instance, it may be that these defects are just one part of the kinetic pathway to failure for a strained sheet of graphene.鈥�

For electronic applications, the defects could deflect electrons and cause backscattering that would increase the resistance of the material 鈥� like a rock in a stream slows the flow of water. However, First says improved growth techniques developed since the defect study began may eliminate that concern.

鈥淲ith the growth techniques that have now been developed using silicon carbide, we typically do not see these defects,鈥� he noted. 鈥淭he defects occur on material that we know to be of a lower quality because of the growth conditions or substrate preparation.鈥�

Defects can appear due to the movement of carbon atoms at high temperatures, explained NIST Fellow Joseph Stroscio. Rearrangements of graphene that require the least amount of energy involve switching from the standard six-member carbon rings to structures containing either five or seven atoms. The NIST researchers have discovered that stringing five- and seven-member rings together in closed loops creates a new type of defect or grain boundary loop in the honeycomb lattice.

According to NIST researcher Eric Cockayne, the fabrication process plays a big role in creating the defects.

鈥淎s the graphene forms under high heat, sections of the lattice can come loose and rotate,鈥� he said. 鈥淎s the graphene cools, these rotated sections link back up with the lattice, but in an irregular way. It鈥檚 almost as if patches of the graphene were cut out with scissors, turned clockwise, and made to fit back into the same place. Only it really doesn鈥檛 fit, which is why we get these flowers.鈥�

So far, only the flower defect, which is composed of six pairs of five- and seven-atom rings, has been observed. Modeling of graphene鈥檚 atomic structure by the NIST team suggests there might be a veritable bouquet of flower-like configurations. These configurations 鈥� seven in all 鈥� would each possess its own unique mechanical and electrical properties, Cockayne said.

First hopes the team can continue studying the defects, both to learn whether their formation can be controlled and to clarify the role of defects in the material鈥檚 mechanical properties.

鈥淕raphene is strong and light, so the mechanical properties are of great interest,鈥� he noted. 鈥淯nderstanding just how it rips apart is an interesting question that has important implications. But even with these defects, graphene is still spectacularly strong.鈥�