Properties and Applications
Graphene film © bonninturina / fotolia.com
The material graphene consists of pure carbon and is predicted to enable promising applications in nanotechnology. Therefore, the European Commission funds research on this "wonder" material with a total of up to 1 billion euros within its biggest ever EU-funded research project "Graphene flagship".
Strictly speaking, graphene is a one atom layer of pure carbon. It is one of several so-called (crystallographic) modifications of carbon. Two modifications show different properties, despite the same chemical composition, because the carbon atoms are arranged in a different way (see also Crystal Structures of Nanomaterials). In addition, a distinction must be made between monolayer graphene, graphene with a few layers (few-layer graphene), graphene with up to 10 layers and Graphene with a thickness above 10 layers, which is called graphite as described below.
The thickness of a graphene layer is approximately 0.3 nanometers and this value represents approximately a hundred thousandth of the thickness of a human scalp hair. The lateral spread of a layer most often usually is much greater. Each carbon atom in a graphene monolayer is chemically bonded to three other carbon atoms. This leads to a (two-dimensional) structure with honeycomb layering.
Tip of a pencil: Graphite is responsible for the gray color of pencil leads © WimL / fotolia.com
Graphene is closely related to graphite, which is a further modification of carbon and best known as ingredient of pencil lead. Graphite consists of many stacked carbon layers and therefore has a three-dimensional-layered structure. In addition to graphene and graphite, other modifications of carbon exist and include diamond and fullerenes. Graphene is also found as so-called amorphous carbon, e.g. in carbon black. The different graphene types have not been named precisely even in the technical literature. Thus, -depending on the context- the same name is used for different graphene types .
Graphene and graphite react well with oxygen to form so-called graphene oxide (GO). Graphene oxides can contain different amounts of bound oxygen. Such graphene oxides can react with other molecules or atoms, which is called functionalization. In practice, it is very difficult to distinguish graphene from graphene oxide and therefore, the term graphene-based materials (GBMs) is used to include various mixtures of graphene and graphene oxide .Graphene-based materials have outstanding properties in various ways .
Works as a flame retardant: mixed with plastics, multi-layer graphene is hard to inflame and expands upon heat treatment, thus it can act in this manner as thermal insulation.
Increases strength of polymers: Graphene can bear high mechanical loads and at the same time it is stretchable. With the highest-ever measured tensile strength of approximately 130 GPa (109 Pa), a monolayer of graphene, for the same mass, has a 25 to 250 times higher tensile strength tensile strength than steel .
Use in touchscreens: Thin sheets of graphene are optically transparent and electrically conductive and may be used in solar cells and touch screen displays as optically transparent electrodes. Broadly, this would be an alternative to the so far used but expensive materials such as silver and indium tin oxides (ITO).
Use in packaging, fuel tanks, tires, thermal insulation: The incorporation of Graphenes into plastics prevents that gases and liquids can penetrate them. This improves the shelf-life of foods or leads to cost-effective thermal insulation, which significantly improves the energy efficiency of buildings.
Applications in micro- and nanoelectronics: Single layer graphene has both, good heat conductance and electrical conductance. Therefore graphene may form the basis of new carbon based microprocessors which are significantly more powerful than silicon-based processors.
Applicatons in medical science: Future applications using functionalized graphenes could enable drug delivery through cell membranes, e.g. for cancer treatment. Furthermore, novel analysis techniques in medical applications may make use of graphene based materials.
In addition, a large number of other applications are planned , e.g., capacitors for energy storage, organic light-emitting diodes (OLEDs), batteries, catalysts or tailor-made agents.
For the end user there are not (yet) commercial products with graphene available on the market. However, it is expected that the abovementioned properties of graphene based materials will open up a variety of possibilities to develop novel products which show a highly improved efficiency or stem from more sustainable production processes.
Graphene is not self-inflammable. As a mixture with air (dust) under the influence of an ignition source, graphene can be possibly ignited (dust explosion). The behaviour in a dust explosion is similar to that of other, carbon-based materials.
Andre Geim, Kostya Novoselov © U. Montan
Single-layer graphene was first isolated in 2004 by a group of physicists at the University of Manchester, under the direction of Andre Geim and Kostya Novoselov, and in 2010, the Nobel prize was awarded to them for their discovery .
So far, it has only been possible to produce graphene at laboratory scale or in small quantities. There are large efforts worldwide to develop processes for the mass-production of graphene. Depending on the scope of the intended application the fabrication process varies distinctively. Single-layer and few-layer graphene can be produced through either bottom-up methods or top-down exfoliation methods.
Using the top-down exfoliation method a single-layer of graphene is separated from a graphite crystal. This is done mostly mechanically by means of adhesive tape. A second way is the production of graphene from graphene oxide. This is done by the conversion of graphene oxide into graphene or few layer graphene using chemical or thermal means and by applying mechanical forces. This can be done in a liquid at an elevated temperature under strong stirring or ultrasonic treatment (i.e. sonic exfoliation). After chemical functionalization few-layer graphene, can be mixed with the starting materials of synthetic materials (for example plastics) for combined processing in subsequent steps.
An example of a bottom-up process for graphene production, uses a mixture of argon and methane gas, which is blown over a clean or coated metal surface (which functions as carrier or substrate). Through decay of the methane gas graphene is formed on the surface of the carrier. For the production of electrical components, the graphene layer is transferred from the first carrier to a second carrier. By further processing steps various electronic components can be manufactured eventually.
More detailed information on the production and processing methods for graphene based materials can be found in the literature .