Graphene is one of the crystalline forms of carbon, alongside diamond, graphite, carbon nanotubes and fullerenes. In this material, carbon atoms are arranged in a regular hexagonal pattern. Graphene can be described as a one-atom thick layer of the layered mineral graphite. High quality graphene is very strong, light, nearly transparent, an excellent conductor of heat and electricity. Its interaction with other materials and with light, and its inherently two-dimensional nature, produce unique properties.
At the time of its isolation, many researchers studying carbon nanotubes were already well familiar with the composition, structure and properties of graphene which had been calculated decades earlier. The combination of familiarity, extraordinary properties and surprising ease of isolation enabled an explosion in graphene research. The Nobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin Novoselov at the University of Manchester “for groundbreaking experiments regarding the two-dimensional material graphene”
Graphene is an allotrope of carbon whose structure is a single planar sheet of sp2-bonded carbon atoms, that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer carbon foils in 1962. Graphene is most easily visualized as an atomic-scale chicken wire made of carbon atoms and their bonds.
The carbon-carbon bond length in graphene is about 0.142 nanometers. Graphene sheets stack to form graphite with an interplanar spacing of 0.335 nm. Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes and fullerenes. It can also be considered as an indefinitely large aromatic molecule, the limiting case of the family of flat polycyclic aromatic hydrocarbons.
The IUPAC compendium of technology states: “previously, descriptions such as graphite layers, carbon layers, or carbon sheets have been used for the term graphene… it is incorrect to use for a single layer a term which includes the term graphite, which would imply a three-dimensional structure. The term graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed.” In this regard, graphene has been referred to as an infinite alternant (only six-member carbon ring) polycyclic aromatic hydrocarbon (PAH). The largest known isolated molecule of this type consists of 222 atoms and is 10 benzene rings across. It has proven difficult to synthesize even slightly bigger molecules, and they still remain “a dream of many organic and polymer chemists”.
Furthermore, ab initio calculations show that a graphene sheet is thermodynamically unstable with respect to other fullerene structures if its size is less than about 20 nm (“graphene is the least stable structure until about 6000 atoms”) and becomes the most stable one (as within graphite) only for sizes larger than 24,000 carbon atoms. The flat graphene sheet is also known to be unstable with respect to scrolling i.e. curling up, which is its lower-energy state.
A definition of “isolated or free-standing graphene” has also recently been proposed: “graphene is a single atomic plane of graphite, which – and this is essential – is sufficiently isolated from its environment to be considered free-standing.” This definition is narrower than the definitions given above and refers to cleaved, transferred and suspended graphene monolayers.
Other forms of graphene, such as graphene grown on various metals, can also become free-standing if, for example, suspended or transferred to silicon dioxide (SiO2). A new example of isolated graphene is graphene on silicon carbide (SiC) after its passivation with hydrogen.