Graphene -- extremely thin layered sheet of carbon atoms -- is quintessentially slated to take electronics to the next level considering the number of the research projects undertaken in this field. These transparent graphene sheets are characterized by high-rate thermal properties, electrical conductivity, and mechanical stiffness which the research community believes, will shortly open doors to the rapid developments in flexible and foldable plastic electronics. In addition, these wonder sheets are also claimed by some to be used in parts requiring electronic interface in tomorrow�s photovoltaic cells and LCD screens.
As a major breakthrough in this field, researchers from South Korea developed a technique for making commercial scale thin flexible electrodes out of graphene. Previously, developing such graphene sheets of a usable size on a commercial scale was extremely cumbersome. The relatively difficult to use �sticky tape method� included peeling off tiny graphite crystals into graphene which sometimes led to imperfections. The Sungkyunkwan University team employed chemical vapor deposition (CVD) to develop these extremely flexible, conductive and transparent graphene electrodes. The process involved allowing of a hot gaseous mixture of methane and hydrogen to flow over heated nickel metal foil. As a result, carbon atoms from the methane get deposited on the nickel substrate. Allowing the same nickel substrate to cool down rapidly, several layers of graphene are formed. Subsequently, the nickel can be chemically dissolved away from the film by soaking it in etchants which only leaves thin, conductive graphene behind. The graphene layer is then, transferred onto optically clear and flexible plastic. Such graphene-etched polymer gives foldability in large displays, bendable PV panels and the like. The scientists indicate that the factors like nickel foil�s thickness, temperature and concentration of gases play an important role in controlling the thickness and stretchability of the graphene sheets. Further, the graphene can easily take the template pattern same as that of the nickel substrate. The graphene film thus obtained can be conveniently transferred to any other substrate like plastic as need demands. The process is relatively more convenient and scalable as compared to previous method since extremely large-sized single layer graphene films can be developed.
Furthering the scope of graphene, two research groups based in USA have been successful in splitting carbon nanotubes to produce flat graphene nanoribbons. Research by the Rice University revealed a room-temperature chemical process which makes it possible to produce the ultra-thin graphene ribbons in bulk quantities. The process involves sulfuric acid and potassium permanganate which attacks single and multiwalled carbon nanotubes, reacting with the carbon framework and unzipping them in a straight line. As a result, the carbon nanotubes turn into flat, straight-edged, water-soluble ribbons of graphene. When bulk-produced, these microscopic sheets can be applied onto a surface or combined with a polymer to let it conduct electricity. The study reported that single-walled carbon nanotubes convert to sheets at room temperature and are good for small electronic devices because the width of the unzipped sheet is highly controllable. However, the multiwalled nanotubes (characterized by concentric rings) are much economical starting materials, and the resulting nanoribbons would be useful in a host of applications including semi conducting thin films, an economical replacement for mono crystalline silicon in solar cells. These conductive graphene nanoribbons could in future provide a cost-effective replacement for indium tin oxide (ITO) commonly used in flat-panel displays, touch-panels, electronic ink and solar cells. Presently, small quantities of graphene nanoribbons can be devised by this method. According to the lead researcher, the study might take roughly couple of more years to scale the process up.
Another development in production of graphene ribbons from CNTs came from a team at Stanford University under Hongjie Dai who developed a new method that to allow relatively precise production of mass quantities of the tiny ribbons by slicing open carbon nanotubes. CNTs are placed on a silicon substrate, and later coated with a polymer film. The film covers the entire surface of each nanotube, except for a thin strip where the nanotube is in contact with the substrate. The polymer film is then peeled off from the substrate which takes away all the CNTs with it. Subsequently, the remaining polymer-nanotube material is then chemically etched using plasma which strips the carbon nanotube along the narrow strip. The duration of exposure to plasma controls whether the CNTs deep inside the polymer cut open into graphene or not. This is highly efficient process for creating tens of thousands of nanoribbons at one go which is governed by the surface area of CNTs coated with polymer films. Further, the ribbons can easily be removed from the polymer film and transferred onto any other substrate, making it easy to create items such as graphene transistors, which may hold promise as a way to possibly make high performance electronic devices. Another important plus point in this method is that the edges of the nanoribbons produced are very smooth, thereby showing a great scope for use in electronics.
Also of critical importance is a novel method to disperse chemically modified graphene in organic solvents which was developed by engineering team at University of Texas, Austin. The development is expected to bring out new avenues of using graphene in a host of important materials and applications such as conductive films, polymer composites, inks, plastic electronics to name a few. The researchers came up with a set of solubility parameters for chemically modified graphenes taking cue form the 'solubility parameters' applied by industry universally to determine the solvents most likely to dissolve certain materials or to create good colloids. Spelling boon for electronics, researchers at Rutgers University, New Jersey, developed a new semiconducting thin-film graphene-polystyrene composite which is slated to bring down cost of printed electronics applications. According to lead researcher Manish Chhowalla, the important aspect to be noted is that the composite is made employing ordinary plastic processing techniques and majority of the semiconducting material consists of commodity plastic polystyrene (PS). The research team processed this composite material from a solution of graphene and polystyrene in a common solvent, dimethylformamide (DMF). This solution is spin coated onto a substrate and the solvent is chemically removed, leaving behind a thin film of graphene-polystyrene. The composite material is semiconducting and can be electrostatically doped with electrons, just like graphene itself, and its field-effect mobility rivals those of the best organic thin-film transitors (TFTs). The composite material benefits from graphene's excellent thermal, mechanical and electrical properties. Although graphene itself is a zero bandgap semiconductor, this is the first time that such composites have been shown to be semiconducting. This graphene-polystyrene composite material will prove beneficial for thin-film printed electronics as the electrically active elements are solution-processed and later deposited on flexible plastic substrates. Further, this material can also be deposited uniformly over a large area at relatively low temperatures with high throughput thus bringing down the cost of printed electronics.