Defects - Page 5

Researchers at the University of Illinois Urbana-Champaign and the Oak Ridge National Laboratory declared their discovery on heating of the grain boundaries in a graphene sheet (the nanoscale defects where individual grains of graphene stitch the sheet together) when the material is made into a functioning transistor. While this new discovery implies potential for early device failure, the defects themselves might be exploited to make phase-change memories and better graphene-enhanced sensors.

The team measured the nanoscale temperature increases in hexagonal grains of graphene grown by CVD using a technique called Joule Expansion Microscopy (SJEM) - a thermometry technique that works using an atomic force microscope (AFM). The tested graphene transistor was coated with a layer of polymer that expands upon transistor heating, measuring graphene’s expansion and converting the expansion into temperature increase by careful calculations.

Researchers from Rice and Osaka Universities discovered a simple method to measure contaminants on graphene sheets using terahertz spectroscopy.

The researchers placed the graphene on a layer of Indium Phosphide, and then used a laser pulse on the graphene. This causes the materials to emit terahertz waves, which can then be used to map contaminants (which change graphene's electrical properties) on the graphene sheet.

Researchers from Rice University and Georgia Tech measured the fracture toughness of imperfect graphene for the first time and found it to be somewhat brittle. It turns out that graphene is really only as strong as its weakest link, and this can make a defected graphene substantially less strong than a perfect graphene.

According to the researchers, graphene follows the century-old Griffith theory that quantifies the useful strength of brittle materials. Imperfections in graphene drastically lessen its strength (which has an upper limit of about 100 gigapascals for a perfect graphene).

Researchers from Rice University and Tsinghua University discovered (using theoretical calculations) that defects in graphene can cause it to become weak - if they occur at the grain boundaries of graphene sheets. Those grain boundaries occur because graphene grown in labs (usually using CVD) are not perfect and this creates several "islands" of graphene that merge together. The researchers say that at these points, graphene is about half as strong as perfect graphene.

The atoms on the lines that connect those islands are called grain boundaries - and the atoms at those lines usually bond in five- and seven- atom rings. These are weaker than the normal hexagon rings of graphene. The weakest points are seven-atom rings. These are found at junctions of three islands, and that's where cracks in graphene are most likely to form.

Nobel Prize-winner (together with Andre Geim) Professor and Kostya Novoselov Professor Volodya Falko from Lancaster University have released a graphene roadmap. The roadmap discusses the different possible applications for graphene and also the different ways to produce the material.

The authors says that the first key application is conductors for touch-screen displays (replacing ITO), where they expect can be commercialized within 3-5 years. They also see rollable e-paper displays soon - prototypes could appear in 2015. Come 2020, we can expect graphene-based devices such as photo-detectors, wireless communications and THz generators. Replacing silicon and delivering anti-cancer drugs are interesting applications too - but these will only be possible at around 2030.

NIST researchers say that defects in Graphene may appear due to the movement of the carbon atoms at high temperatures when producing graphene by heating silicon carbide under ultrahigh vacuum. Graphene tend to rearrange from six-sided rings to five or seven atoms. Stringing five and seven member rings together in closed loops creates a new type of defect or grain boundary loop in the honeycomb lattice. These defects might allow it a little flexibility, making Graphene even more resilient to tearing or fracturing.

The researchers say that we should be able to either avoid defects entirely or produce them at will by variations in growth conditions.

Researchers from the Naval Research Laboratory (NRL) shown that the valley degree of freedom in graphene can be polarized through scattering off a line defect. This makes valley-based electronics (valleytronics) one step closer to reality. Valleytronics may present a middle-ground between spintronics and electronics using the valley degree of freedom (which exists in certain crystals, including graphene).

The NRL research shows that an extended line defect in graphene acts as a natural valley filter. "As the structure is already available, we are hopeful that valley-polarized currents could be generated in the near future" said Dr. Daniel Gunlycke who made the discovery together with Dr. Carter White. Both work in NRL's Chemistry Division.