Graphene (/ˈɡræfiːn/)[1] is a carbon allotrope consisting of a single layer of atoms arranged in a honeycomb planar nanostructure.[2][3] The name "graphene" is derived from "graphite" and the suffix -ene, indicating the presence of double bonds within the carbon structure. Graphene is known for its exceptionally high tensile strength, electrical conductivity, transparency, and being the thinnest two-dimensional material in the world.[4] Despite the nearly transparent nature of a single graphene sheet, graphite (formed from stacked layers of graphene) appears black because it absorbs all visible light wavelengths.[5][6] On a microscopic scale, graphene is the strongest material ever measured.[7][8] 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.[6][5] The existence of graphene was first theorized in 1947 by Philip R. Wallace during his research on graphite's electronic properties.[9] In 2004, the material was isolated and characterized by Andre Geim and Konstantin Novoselov at the University of Manchester[10][11] using a piece of graphite and adhesive tape.[12] In 2010, Geim and Novoselov were awarded the Nobel Prize in Physics for their "groundbreaking experiments regarding the two-dimensional material graphene".[13] While small amounts of graphene are easy to produce using the method by which it was originally isolated, attempts to scale and automate the manufacturing process for mass production have had limited success due to cost-effectiveness and quality control concerns.[14][15] The global graphene market was $9 million in 2012, [16] with most of the demand from research and development in semiconductors, electronics, electric batteries, [17] and composites. The IUPAC (International Union of Pure and Applied Chemistry) advises using the term "graphite" for the three-dimensional material and reserving "graphene" for discussions about the properties or reactions of single-atom layers.[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] History Main article: Discovery of graphene 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. Structure of graphite and its intercalation compounds In 1859, Benjamin Brodie noted the highly lamellar structure of thermally reduced graphite oxide.[21][22] Pioneers in X-ray crystallography attempted to determine the structure of graphite. The lack of large single crystal graphite specimens contributed to the independent development of X-ray powder diffraction by Peter Debye and Paul Scherrer in 1915, and Albert Hull in 1916.[23][24][25] However, neither of their proposed structures was correct. In 1918, Volkmar Kohlschütter and P. Haenni described the properties of graphite oxide paper.[26] The structure of graphite was successfully determined from single-crystal X-ray diffraction by J. D. Bernal in 1924, [27] although subsequent research has made small modifications to the unit cell parameters.[28][29] 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 separately pointed out in 1984 by Gordon Walter Semenoff, [30] and by David P. Vincenzo and Eugene J. Mele.[31] 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.[32][33][34] Observations of thin graphite layers and related structures 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.[35] Eventually, single layers were also observed directly.[36] Single layers of graphite were also observed by transmission electron microscopy within bulk materials, particularly inside soot obtained by chemical exfoliation.[37] From 1961 to 1962, Hanns-Peter Boehm published a study of extremely thin flakes of graphite.[38] The study measured flakes as small as ~0.4 nm, which is around 3 atomic layers of amorphous carbon. This was the best possible resolution for TEMs in the 1960s. However, it is impossible to distinguish between suspended monolayer and multilayer graphene by their TEM contrasts, and the only known method is to analyze the relative intensities of various diffraction spots.[36] The first reliable TEM observations of monolayers are likely given in references 24 and 26 of Geim and Novoselov's 2007 review.[2] In 1975, van Bommel et al. epitaxially grew a single layer of graphite on top of silicon carbide.[39] Others grew single layers of carbon atoms on other materials.[40][41] This "epitaxial graphene" consists of a single-atom-thick hexagonal lattice of sp2-bonded 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. Boehm et al. coined the term "graphene" for the hypothetical single-layer structure in 1986.[42] The term was used again in 1987 to describe single sheets of graphite as a constituent of graphite intercalation compounds, [43] which can be seen as crystalline salts of the intercalant and graphene. It was also used in the descriptions of carbon nanotubes by R. Saito and Mildred and Gene Dresselhaus in 1992, [44] and in the description of polycyclic aromatic hydrocarbons in 2000 by S. Wang and others.[45] Efforts to make thin films of graphite by mechanical exfoliation started in 1990.[46] Initial attempts employed exfoliation techniques similar to the drawing method. Multilayer samples down to 10 nm in thickness were obtained.[2] In 2002, Robert B. Rutherford and Richard L. Dudman filed for a patent in the US on a method to produce graphene by repeatedly peeling off layers from a graphite flake adhered to a substrate, achieving a graphite thickness of 0.00001 inches (0.00025 millimetres). The key to success was the ability to quickly and efficiently identify graphene flakes on the substrate using optical microscopy, which provided a small but visible contrast between the graphene and the substrate.[47] Another U.S. patent was filed in the same year by Bor Z. Jang and Wen C. Huang for a method to produce graphene-based on exfoliation followed by attrition.[48] In 2014, inventor Larry Fullerton patented a process for producing single-layer graphene sheets