Graphene is nature's thinnest elastic material and displays exceptional mechanical and electronic properties. Its one-atom thickness, planar geometry, high current-carrying capacity and thermal conductivity make it ideally suited for further miniaturizing electronics through ultra-small devices and components for semiconductor circuits and computers.
A new research could lead to the development of transparent and stretchy electrodes made from graphene, which could result in bendy displays. The research is being undertaken by Byung Hee Hong from Sungkyunkwan University in Suwon, Korea and his colleagues. They transferred a wafer-thin layer of graphene, etched into the shape needed to make an electrode, onto pieces of polymer. The polymers they used are transparent, and one-polyethylene terephthalate (PET)-can be bent, whereas the other-polydimethylsiloxane (PDMS)-is stretchable. The resulting films conduct electricity better than any other sample of graphene produced in the past. Until recently, high-quality graphene has been hard to make on a large scale. To produce their graphene, the team used a technique that is well known in the semiconductor industry-chemical vapour deposition. This involves exposing a substrate to a number of chemicals, often at high temperatures. These chemicals then react on the surface to give a thin layer of the desired product. The results were relatively large; high-quality films of graphene just a few atoms thick and several centimeters wide. The team made the electrodes by using nickel as a catalyst on which to react methane and hydrogen. Nickel usually catalyses the formation of thick layers of graphite. But, by using a layer of nickel less than 300 nanometers thick and by cooling the sample quickly after the reaction, the researchers could produce up to ten single-atom layers of carbon in graphene's signature honeycomb pattern. The samples aren't perfect, as each layer covers only around two-thirds of the sample, but the team is working to improve this. The graphene samples can be chemically etched into specific shapes, and when stamped onto the polymer, they can be bent or stretched by as much as 11% without losing their conductivity. Because the layers of graphene are so thin, the resulting electrodes are transparent, and that makes the material ideal for use in applications such as portable displays. It could, for instance, be used to replace indium titanium oxide, which is expensive and inflexible. The team is also looking at using the graphene electrodes in photovoltaic cells. The electrodes are likely to be incorporated in niche applications such as individual ultra-high-frequency transistors. The large scale manufacturing process can be commercialized by 2013 if not earlier.
The first artificial graphene has been created at the NEST laboratory of the Italian Institute for the Physics of Matter (INFM-CNR) in Pisa. It is sculpted on the surface of a gallium-arsenide semiconductor, to which it grants the extraordinary properties of the original graphene. The "artificial graphene", the very first ever created, seems ready to raise the interests of both industry and research. This amazing copy promises to render available the incredible electronic qualities of graphene, and thus, it offers a way to overcome the closing physical limits that plague silicon.
Natural graphene is an interesting but elusive material, observed for the first time in 2004. It has a very peculiar structure, being composed of a single layer of carbon atoms (only one atom thick) arranged in a grid which resembles common chicken wire. This structure grants graphene its exciting electronic properties: over this two-dimensional carbon nanoworld, electrons move almost freely at very high speeds, acting like massless particles. For the electronic industry, this means more efficient devices that will be able to be built a lot smaller than what silicon allows. Such an innovation, however, is yet far away to come, because production of graphene with sizes and reproducibility needed by the semiconductor industry is not possible yet.
The new innovation is based on the idea that replicating graphene's structure on a different material might endow it with graphene's extraordinary properties. And this is exactly what that scientists at NEST tried with the help of gallium-arsenide semiconductors, objects widespread in the production of transistors and lasers. They carved a semiconductor with the help of an ion beam, creating a nanopattern on its surface that replicates the exact graphen's structure. And the idea proved to be a complete success: modified in this way, the nanosculptured semiconductor exhibits the properties of the famous material it imitates, thus becoming the very first artificial graphene. With an added advantage: the overall procedure does not rely on exotic equipments, but on tools and instruments that the nanofabrication industry already possesses and masters, meaning that the artificial graphene can already enable the development of high-mobility transistors and lasers.
One of graphene's intrinsic features is ripples, similar to those seen on plastic wrap tightly pulled over a clamped edge. Induced by pre-existing strains in graphene, these ripples can strongly affect graphene's electronic properties, and not always favorably. If the ripples can be controlled, however, they can be used to advantage in nanoscale devices and electronics, opening up a new arena in graphene engineering: strain-based devices. UC Riverside's Chun Ning (Jeanie) Lau and colleagues have reported the first direct observation and controlled creation of one- and two-dimensional ripples in graphene sheets. Using simple thermal manipulation, the researchers produced the ripples, and controlled their orientation, wavelength and amplitude. When the graphene sheets are stretched across a pair of parallel trenches, they spontaneously form nearly periodic ripples. When these sheets are heated up, they actually contract, so the ripples disappear. When they are cooled down to room temperature, the ripples re-appear, with ridges at right angle to the edges of the trenches. This phenomenon is similar to what happens to a piece of thin plastic wrap that covers a container and heated in a microwave oven.
The unusual thermal contraction of graphene had been predicted theoretically, but Lau's lab is the first to demonstrate and quantify the phenomenon experimentally. Because graphene is both an excellent conductor and the thinnest elastic membrane, the ripples could have profound implications for graphene-based electronics. This is because graphene's ability to conduct electricity is expected to vary with the local shape of the membrane. The ripples may produce effective magnetic fields that can be used to steer and manipulate electrons in a nanoscale device without an external magnet. Lau, an associate professor of physics and a member of UCR's Center for Nanoscale Science and Engineering, and her colleagues examined the ripples' morphology using a scanning electron microscope and an atomic force microscope. They found that almost all the graphene membranes underwent dramatic morphological changes when heated, displaying significant alterations in the ripple geometry, a buckling of the graphene membrane, or both. Their experimental system, which involved a stage inside a scanning electron microscope (SEM) with a built-in heater, thermometer and several electrical feed-throughs, enabled them to image graphene while it was being heated and explore the interplay between graphene's mechanical, thermal and electrical properties. The result has important implications for controlling thermally induced stress in graphene electronics. Ability to control and manipulate the ripples in graphene sheets represents the first step towards strain-based graphene engineering. The suspended graphene is almost invariably rippled, and this may need to be considered in the interpretation of a broad array of existing and future research.
|
|