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Loops (seen above in blue) between graphene layers can be minimized using electron irradiation (bottom). (Credit: ORNL)

According to a press release, researchers at Oak Ridge National Lab have discovered how loops develop in graphene, an electrically conductive high-strength low-weight material that resembles an atomic-scale honeycomb. The nanoscale simulations are bringing scientists closer to using graphene in electronic applications.

“Graphene is a rising star in the materials world, given its potential for use in precise electronic components like transistors or other semiconductors,” says Bobby Sumpter, a staff scientist at ORNL.

Structural loops that sometimes form during a graphene cleaning process can render the material unsuitable for electronic applications.

However, when graphene was subjected to electron irradiation with a transmission electron microscope, it  prevented loop formation. The simulations showed that by injecting electrons to collect an image, the electrons were simultaneously changing the material’s structure.

“Taking a picture with a TEM is not merely taking a picture,” Sumpter says. “You might modify the picture at the same time that you’re looking at it.”

Graphene is only as good as the uniformity or cleanliness of its edges, which determine how effectively the material can transmit electrons. ORNL’s Vincent Meunier says the ability to efficiently clean graphene edges is crucial to using the material in electronics.

Recent experimental studies have shown that the Joule heating process can lead to undesirable loops that connect different graphene layers. Joule heating cleans graphene edges by running a current through the material. The team can show electron irradiation from a TEM prevents loop formation.

In the advanced online edition of Nature Nanotechnology, researchers have developed a new method to produce graphene sheets leading to a diagonal dimension of 30 in. - larger than ever before. This could result in cheaper electrodes for flexible displays. The graphene has already been used to construct a touchscreen that is twice as flexible as one made using indium titanium oxide, the material usually used in touchscreens.

Using chemical vapor deposition, researchers at University of Texas, Austin heated methane and hydrogen gas to 1000 °C above a flexible copper substrate, causing a reaction that left a layer of graphene deposited on the copper. Once the graphene cooled, they transferred it onto a piece of flexible plastic.

Now Jong-Hyun Ahn and Byung Hee Hong of Sungkyunkwan University in Suwon, South Korea, and colleagues, have adapted this approach to produce still larger graphene sheets. They ran the reactions inside a modified roll-to-roll machine through which they fed the flexible copper sheets. The result was a rectangular graphene sheet with a diagonal diameter of 30 in.

The team built a palm-sized touchscreen with the film. The touchscreen was able to withstand up to 6 percent combined compressive and tensile strain before breaking, compared with only about 3 percent for touch panels based on indium tin oxide.

Graphene could be a cheaper and more flexible alternative to indium tin oxide, and this work is a step towards producing commercially useful quantities. The researchers still have to show that their graphene sheets can be made to a consistently high quality, without introducing tears or discontinuities that could affect performance, Colombo says.

A group of researchers representing several institutions report in Science they have gained new abilities to “print” graphene oxide-based nano-scale replacements for IC wiring and some semiconductor devices using a method that employs an atomic force microscope to act as a printer head do the detailed work of tuning the conductivity of the material in precise patterns.

GO is an interesting material because it is more resilient to mechanical stresses than standard graphene. Furthermore, in a reduced form, GO becomes a semiconductor (reduced GO – rGO – has a conductivity that is 33,000 times higher than that of doped hydrogenated amorphous silicon).

The innovation the researchers are pioneering is the use of a heated AFM tip on GO to precisely create nanoribbons of rGO. The group – from Georgia Tech, Naval Research Lab, Chung Ang University (Korea), University of Illinois at Urbana-Champaign and CNRS-Institut Néel (France) – didn’t invent thermochemical nanolithography, but the were the first to employ TCNL, via an AFM probe tip, to reduce patterned regions of GO simply by varying the temperature of the tip.

They tested their TCNL method on both GO flakes on a SiO_x/Si substrate and large-area GO films (>15 mm²) formed from epitaxial graphene grown on the carbon face of silicon carbide. They were able to print the rGO nanoribbons at a rate of about  2 µm per second, forming ribbons as narrow as 25 nm. They were able to demonstrate the formation of nanoribbons in zigzag and cross-shaped patterns.

What’s down the road for this? The researchers envision graphene nanoelectronics made by using large arrays of independent heated probe tips that would “print” nanostructures on wafer-scale areas at high speed.

Credit: UPenn

A group out of University of Pennsylvania’s Department of Mechanical Engineering and Applied Mechanics thinks it knows why, at nanoscales, the friction encountered by, say, an atomic force microscope, increases as the number of layers decrease: The AFM tip pushes material in front into sort of a wrinkle or wave in front, and stretches it in the back. This pucker in the material creates a force that pushes back on the AFM  tip.

The effect is sort of like what would happen if you quickly tried to push an area rug on a slick floor. In the case of nanoscale materials, the fewer the layers, the easier it is for it to bunch up in front of the AFM.

The researchers, whose work is published in Science, say that after testing atomically thin samples of four  materials they think this may be a universal characteristic for any material at this scale. The materials tested were graphene, molybdenum disulfide, hexagonal-boron nitride and niobium diselenide. The researchers test a range of thickness, from several atomic layers all the down to a single layer, and then compared the friction to that found in bulk quantities of these materials.

Compared to the bulk material, the researchers found that friction progressively increased as the number of layers is decreased.

Research coleader Robert Carpick says in a Penn release that, “We call this mechanism, which leads to higher friction on thinner sheets, the ‘puckering effect.’ Interatomic forces, like the van der Waals force, cause attraction between the atomic sheet and the nanoscale tip of the atomic force microscope which measures friction at the nanometer scale.”

Here is a brief animation of what goes on:

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He says that thicker sheets cannot deflect as easily because they are much stiffer. The material can’t bunch up as easily so the increase in friction is less pronounced.

The researchers also found a logical to prevent the increase in friction. Continuing with my analogy, an area rug won’t bunch up if the special Home Depot double-sided tape is used. Likewise, if the atomic sheets are strongly bound to a substrate, such as mica, the problem disappears.

Carprick and his group say that this will have practical implications for the design of nanomechanical devices that use graphene, and will will shed more light on the macroscopic behavior of common lubricants, such as graphite, MoS2 and BN.

A) Previous thermal conductivity measurements were performed on suspended graphene. (B) Researchers have now studied graphene supported on a SiO2 substrate. The graphene layer makes contact on the summits of the rough surface, interacting with the substrate through van der Waals forces. Credit: E. E. Zumalt/Univ. of Texas at Austin

Those engineering electronic devices always have had to factor in the use of special materials that can conduct heat away from crucial components. Bulk copper’s thermal conductivity is pretty good at around 400 watts per meter per kelvin, but this ability decreases as the copper approaches film-size dimensions.

Diamond has long been known to have excellent thermal conductivity properties, so carbon forms are a logical area to focus on in the search for better materials. Indeed, researchers had shown in 2008 that a suspended monolayer of graphene and carbon nanotubes have thermal conductivity of 5000 W m^-1 K^-1 at room temperature.

Now an international team of researchers from the University of Texas at Austin, Boston University, Christopher Newport University and Commissariat à l’Énergie Atomique have taken the next logical step and tested a graphene monolayer on an SiO_² substrate (via mechanical exfoliation). As the graphic above indicates, the graphene doesn’t completely adhere to the substrate, but sits on its high points.

The team found that the graphene–SiO^₂ combination delivered very good thermal conductivity, around 600 W m^–1 K^–1 . As Ravi Prasher notes in a commentary about this work,  this is “an order of magnitude lower than that of suspended graphene. However, this is still higher than the thermal conductivities of bulk or thin-film copper.”

According to David Broido, a Boston College professor of physics, the decrease is the result of graphene’s interaction with the substrate. The combination interferes with the vibrational waves of graphene atoms as they bump against the adjacent substrate.

This suggests a promising route for creating a new generation of devices that will consume less energy, be cooler and more reliable, and operate faster than the current generation of silicon and copper devices.