Description
One definition given in a recent review on graphene is:
Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite.
A previous definition is:
A single carbon layer of the graphitic structure can be considered as the final member of the series naphthalene, anthracene, coronene, etc. and the term graphene should therefore be used to designate the individual carbon layers in graphite intercalation compounds. Use of the term "graphene layer" is also considered for the general terminology of carbons.
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.
Read more about this topic: Graphene
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