Graphene is an atomic-scale hexagonal lattice made of carbon atoms.
Graphene /ˈɡræfiːn/[1] is an allotrope of carbon consisting of a single layer of atoms arranged in a two-dimensional honeycomb lattice.[2][3] The name is a portmanteau of "graphite" and the suffix ene, reflecting the fact that the graphite allotrope of carbon consists of stacked graphene layers.[4][5]
Each atom in a graphene sheet is connected to its three nearest neighbors by a σ-bond, and contributes one electron to a conduction band that extends over the whole sheet. This is the same type bonding seen in carbon nanotubes and polycyclic aromatic hydrocarbons, and (partially) in fullerenes and glassy carbon.[6][7] These conduction bands make graphene a semimetal with unusual electronic properties that are best described by theories for massless relativistic particles.[2] Charge carriers in graphene show linear, rather than quadratic, dependence of energy on momentum, and field-effect transistors with graphene can be made that show bipolar conduction. Charge transport is ballistic over long distances; the material exhibits large quantum oscillations and large and nonlinear diamagnetism.[8] Graphene conducts heat and electricity very efficiently along its plane. The material strongly absorbs light of all visible wavelengths,[9][10] which accounts for the black color of graphite; yet a single graphene sheet is nearly transparent because of its extreme thinness. The material is also about 100 times stronger than would be the strongest steel of the same thickness.[11][12]
Photograph of a suspended graphene membrane in transmitted light. This one-atom-thick material can be seen with the naked eye because it absorbs approximately 2.3% of light.[10][9]
Scientists have theorized about graphene for decades. It has likely been unknowingly produced in small quantities for centuries, through the use of pencils and other similar applications of graphite. It was originally observed in electron microscopes in 1962, but only studied while supported on metal surfaces.[4] The material was later rediscovered, isolated and characterized in 2004 by Andre Geim and Konstantin Novoselov at the University of Manchester,[13][14] who were awarded the Nobel Prize in Physics in 2010 for their research on the material. High-quality graphene proved to be surprisingly easy to isolate.
The global market for graphene was $9 million in 2012,[15] with most of the demand from research and development in semiconductor, electronics, electric batteries,[16] and composites. In 2019, it was predicted to reach over $150 million by 2021.[17]
The IUPAC recommends use of the name "graphite" for the three-dimensional material, and "graphene" only when the reactions, structural relations or other properties of individual layers are discussed.[18] A narrower definition, of "isolated or free-standing graphene" requires that the layer be sufficiently isolated from its environment,[19] but would include layers suspended or transferred to silicon dioxide or silicon carbide.[20]
A lump of graphite, a graphene transistor, and a tape dispenser. Donated to the Nobel Museum in Stockholm by Andre Geim and Konstantin Novoselov in 2010.
In 1859 Benjamin Brodie noted the highly lamellar structure of thermally reduced graphite oxide.[21][22] In 1916, Peter Debije and P. Scherrer determined the structure of graphite by powder X-ray diffraction.[23][24][25] The structure was studied in more detail by V. Kohlschütter and P. Haenni in 1918, who also described the properties of graphite oxide paper.[26] Its structure was determined from single-crystal diffraction in 1924.[27][28]
The theory of graphene was first explored by P. R. Wallace in 1947 as a starting point for understanding the electronic properties of 3D graphite. The emergent massless Dirac equation was first pointed out in 1984 by Gordon Walter Semenoff,David P. DiVincenzo, and Eugene J. Mele.[29] Semenoff emphasized the occurrence in a magnetic field of an electronic Landau level precisely at the Dirac point. This level is responsible for the anomalous integer quantum Hall effect.[30][31][32]
Transmission electron microscopy (TEM) images of thin graphite samples consisting of a few graphene layers were published by G. Ruess and F. Vogt in 1948.[33]) Eventually, single layers were also observed directly.[34] Single layers of graphite were also observed by transmission electron microscopy within bulk materials, in particular inside soot obtained by chemical exfoliation.[7]
In 1961–1962, Hanns-Peter Boehm published a study of extremely thin flakes of graphite, and coined the term "graphene" for the hypothetical single-layer structure.[35] This paper reports graphitic flakes that give an additional contrast equivalent of down to ~0.4 nm or 3 atomic layers of amorphous carbon. This was the best possible resolution for 1960 TEMs. However, neither then nor today is it possible to argue how many layers were in those flakes. Now we know that the TEM contrast of graphene most strongly depends on focusing conditions.[34] For example, it is impossible to distinguish between suspended monolayer and multilayer graphene by their TEM contrasts, and the only known way is to analyze the relative intensities of various diffraction spots. The first reliable TEM observations of monolayers are probably given in refs. 24 and 26 of Geim and Vovoselov's 2007 review.[2]
Starting in the 1970s, C. Oshima and others described single layers of carbon atoms that were grown epitaxially on top of other materials.[36][37] This "epitaxial graphene" consists of a single-atom-thick hexagonal lattice of sp2bonded carbon atoms, as in free-standing graphene. However, there is significant charge transfer between the two materials, and, in some cases, hybridization between the d-orbitals of the substrate atoms and π orbitals of graphene; which significantly alter the electronic structure compared to that of free-standing graphene.