Home      Discussion      Topics      Dictionary      Almanac
Signup       Login
Graphene

Graphene

Overview
Graphene is an allotrope of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 and the suffix -ene
-ene
The suffix -ene is used in organic chemistry to form names of organic compounds where the -C=C- group has been attributed the highest priority according to the rules of organic nomenclature. Sometimes a number between hyphens is inserted before it to say that the double bond is between that atom...

 by Hanns-Peter Boehm
Hanns-Peter Boehm
Hanns-Peter Boehm is a German chemist and professor emeritus at Ludwig-Maximilians-Universität in Munich, Germany. Boehm is considered a pioneer of graphene research.- Biography :...

, who described single-layer carbon foils in 1962. Graphene is most easily visualized as an atomic-scale chicken wire
Chicken wire (chemistry)
The term chicken wire in chemistry is used in different contexts. Most of them relate to the similarity of the regular hexagonal patterns found in certain chemical compounds to the mesh structure commonly seen in real chicken wire....

 made of carbon atoms and their bonds.
Discussion
Ask a question about 'Graphene'
Start a new discussion about 'Graphene'
Answer questions from other users
Full Discussion Forum
 
Unanswered Questions
Recent Discussions
Encyclopedia
Graphene is an allotrope of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 and the suffix -ene
-ene
The suffix -ene is used in organic chemistry to form names of organic compounds where the -C=C- group has been attributed the highest priority according to the rules of organic nomenclature. Sometimes a number between hyphens is inserted before it to say that the double bond is between that atom...

 by Hanns-Peter Boehm
Hanns-Peter Boehm
Hanns-Peter Boehm is a German chemist and professor emeritus at Ludwig-Maximilians-Universität in Munich, Germany. Boehm is considered a pioneer of graphene research.- Biography :...

, who described single-layer carbon foils in 1962. Graphene is most easily visualized as an atomic-scale chicken wire
Chicken wire (chemistry)
The term chicken wire in chemistry is used in different contexts. Most of them relate to the similarity of the regular hexagonal patterns found in certain chemical compounds to the mesh structure commonly seen in real chicken wire....

 made of carbon atoms and their bonds. The crystalline or "flake" form of graphite consists of many graphene sheets stacked together.

The carbon-carbon bond length in graphene is about 0.142 nanometers. Graphene sheets stack to form graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 with an interplanar spacing of 0.335 nm, which means that a stack of three million sheets would be only one millimeter thick. Graphene is the basic structural element of some carbon allotropes
Allotropy
Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements...

 including graphite, charcoal
Charcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...

, carbon nanotubes and fullerenes. It can also be considered as an indefinitely large aromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

 molecule, the limiting case of the family of flat polycyclic aromatic hydrocarbons.

The Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...

 for 2010 was awarded to Andre Geim
Andre Geim
Andre Konstantin Geim, FRS is a Dutch-Russian-British physicist working at the University of Manchester. Geim was awarded the 2010 Nobel Prize in Physics jointly with Konstantin Novoselov for his work on graphene...

 and Konstantin Novoselov
Konstantin Novoselov
Konstantin Sergeevich Novoselov FRS is a Russo-British physicist, most notably known for his works on graphene together with Andre Geim, which earned them the Nobel Prize in Physics in 2010. Novoselov is currently a member of the mesoscopic physics research group at the University of Manchester as...

 "for groundbreaking experiments regarding the two-dimensional
Two-dimensional space
- Details :Bi-dimensional space is a geometric model of the planar projection of the physical universe in which we live.The two dimensions are commonly called length and width .Both directions lies in the same plane....

 material graphene".

Description


One definition given in a recent review on graphene is:

A previous definition is:

The IUPAC
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...

 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
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...

 (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
Ab initio quantum chemistry methods
Ab initio quantum chemistry methods are computational chemistry methods based on quantum chemistry. The term ab initiowas first used in quantum chemistry by Robert Parr and coworkers, including David Craig in a semiempirical study on the excited states of benzene.The background is described by Parr...

 show that a graphene sheet is thermodynamically unstable with respect to other fullerene
Fullerene
A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...

 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
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

 (SiO2). A new example of isolated graphene is graphene on silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...

 (SiC) after its passivation
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...

 with hydrogen.

Occurrence and production


In essence, graphene is an isolated atomic plane of graphite. From this perspective, graphene has been known since the invention of X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

. Graphene planes become even better separated in intercalated
Intercalation (chemistry)
In chemistry, intercalation is the reversible inclusion of a molecule between two other molecules . Examples include DNA intercalation and graphite intercalation compounds.- DNA intercalation :...

 graphite compounds. In 2004 physicists at the University of Manchester
University of Manchester
The University of Manchester is a public research university located in Manchester, United Kingdom. It is a "red brick" university and a member of the Russell Group of research-intensive British universities and the N8 Group...

 and the Institute for Microelectronics Technology, Chernogolovka
Chernogolovka
Chernogolovka is a town in Moscow Oblast, Russia, located northeast from Moscow border. Population: Chernogolovka does not have a rail link but long distance buses link the town to Moscow, Noginsk and Fryanovo.-Research facilities:...

, Russia, first isolated individual graphene planes by using adhesive tape. They also measured electronic properties of the obtained flakes and showed their unique properties. In 2005 the same Manchester Geim group together with the Philip Kim group from Columbia University (see the History section) demonstrated that quasiparticle
Quasiparticle
In physics, quasiparticles are emergent phenomena that occur when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in free space...

s in graphene were massless Dirac fermion
Dirac fermion
In particle physics, a Dirac fermion is a fermion which is not its own anti-particle. It is named for Paul Dirac. All fermions in the standard model, except possibly neutrinos, are Dirac fermions...

s. These discoveries led to an explosion of interest in graphene.

Since then, hundreds of researchers have entered the area, resulting in an extensive search for relevant earlier papers. The Manchester researchers themselves published the first literature review. They cite several papers in which graphene or ultra-thin graphitic layers were epitaxially grown on various substrates. Also, they note a number of pre-2004 reports in which intercalated graphite compounds were studied in a transmission electron microscope
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...

. In the latter case, researchers occasionally observed extremely thin graphitic flakes ("few-layer graphene" and possibly even individual layers). An early detailed study on few-layer graphene dates back to 1962. The earliest TEM images of few-layer graphene were published by G. Ruess and F. Vogt in 1948. However, already D.C. Brodie was aware of the highly lamellar structure of thermally reduced graphite oxide in 1859. It was studied in detail by V. Kohlschütter and P. Haenni in 1918, who also described the properties of graphite oxide paper
Graphene Oxide Paper
Graphene oxide paper or graphite oxide paper is a composite material fabricated from graphite oxide.The material has exceptional stiffness and strength, due to the intrinsic strength of the two-dimensional graphene backbone and to its interwoven layer structure which distributes loads.The starting...

.

It is now well known that tiny fragments of graphene sheets are produced (along with quantities of other debris) whenever graphite is abraded, such as when drawing a line with a pencil. There was little interest in this graphitic residue before 2004/05 and, therefore, the discovery of graphene is often attributed to Andre Geim
Andre Geim
Andre Konstantin Geim, FRS is a Dutch-Russian-British physicist working at the University of Manchester. Geim was awarded the 2010 Nobel Prize in Physics jointly with Konstantin Novoselov for his work on graphene...

 and colleagues who introduced graphene in its modern incarnation.

In 2008, graphene produced by exfoliation was one of the most expensive materials on Earth, with a sample that can be placed at the cross section of a human hair costing more than $1,000 as of April 2008 (about $100,000,000/cm2). Since then, exfoliation procedures have been scaled up, and now companies sell graphene in large quantities. On the other hand, the price of epitaxial graphene on SiC is dominated by the substrate price, which is approximately $100/cm2 as of 2009. Even cheaper graphene has been produced by transfer from nickel by Korean researchers, with wafer sizes up to 30 inches (76.2 cm) reported.

In 2011 the Institute of Electronic Materials Technology and Department of Physics, Warsaw University announced a joint development of acquisition technology of large pieces of graphene with the best quality so far. In April the same year, Polish
Poland
Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...

 scientist
Scientist
A scientist in a broad sense is one engaging in a systematic activity to acquire knowledge. In a more restricted sense, a scientist is an individual who uses the scientific method. The person may be an expert in one or more areas of science. This article focuses on the more restricted use of the word...

s with support from the Polish Ministry of Economy began the procedure for granting a patent to their discovery around the world.

In the literature, specifically that of the surface science community, graphene has also been commonly referred to as monolayer graphite. This community has intensely studied epitaxial graphene on various surfaces (over 300 articles prior to 2004). In some cases, these graphene layers are coupled to the surfaces weakly enough (by Van der Waals force
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...

s) to retain the two dimensional electronic band structure
Electronic band structure
In solid-state physics, the electronic band structure of a solid describes those ranges of energy an electron is "forbidden" or "allowed" to have. Band structure derives from the diffraction of the quantum mechanical electron waves in a periodic crystal lattice with a specific crystal system and...

 of isolated graphene, as also happens with exfoliated graphene flakes with regard to SiO2. An example of weakly coupled epitaxial graphene is the one grown on SiC (see below).

Drawing method


In 2004, the Manchester group obtained graphene by mechanical exfoliation of graphite. They used adhesive tape to repeatedly split graphite crystals into increasingly thinner pieces. The tape with attached optically transparent flakes was dissolved in acetone, and, after a few further steps, the flakes including monolayers were sedimented on a silicon wafer. Individual atomic planes were then hunted in an optical microscope. A year later, the researchers simplified the technique and started using dry deposition, avoiding the stage when graphene floated in a liquid. Relatively large crystallites (first, only a few micrometres in size but, eventually, larger than 1 mm and visible by the naked eye) were obtained by the technique. It is often referred to as a scotch tape
Scotch Tape
Scotch Tape is a brand name used for certain pressure sensitive tapes manufactured by 3M as part of the company's Scotch brand.- History :The precursor to the current tapes was developed in the 1930s in Minneapolis, Minnesota by Richard Drew to seal a then-new transparent material known as...

 or drawing method. The latter name appeared because the dry deposition resembles drawing with a piece of graphite. The key for the success probably was the use of high-throughput visual recognition of graphene on a properly chosen substrate, which provides a small but noticeable optical contrast. The optical properties section below contains a photograph of what graphene looks like.

The isolation of graphene led to the current research boom. Previously, free-standing atomic planes were often "presumed not to exist" because they are thermodynamically unstable on a nanometer scale and, if unsupported, have a tendency to scroll and buckle. It is currently believed that intrinsic microscopic roughening on the scale of 1 nm could be important for the stability of purely 2D crystals.

There were a number of previous attempts to make atomically thin graphitic films by using exfoliation techniques similar to the drawing method. Multilayer samples down to 10 nm in thickness were obtained. These efforts were reviewed in 2007. Furthermore, a couple of very old papers were recently unearthed in which researchers tried to isolate graphene starting with intercalated compounds (see History and experimental discovery). These papers reported the observation of very thin graphitic fragments (possibly monolayers) by transmission electron microscopy. Neither of the earlier observations was sufficient to "spark the graphene gold rush", until the Science paper did so by reporting not only macroscopic samples of extracted atomic planes but, importantly, their unusual properties such as the bipolar-transistor
Bipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...

 effect, ballistic transport of charges, large quantum oscillations, etc. The discovery of such interesting qualities intrinsic to graphene gave an immediate boost to further research and several groups quickly repeated the initial result and moved further. These breakthroughs also helped to attract attention to other production techniques, such as epitaxial growth of ultra-thin graphitic films. In particular, it has later been found that graphene monolayers grown on SiC and Ir are weakly coupled to these substrates (how weakly remains debated) and the graphene–substrate interaction can be passivated further.

Not only graphene but also free-standing atomic planes of boron nitride
Boron nitride
Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms...

, mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

, dichalcogenides
Chalcogenide
A chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element. Although all group 16 elements of the periodic table are defined as chalcogens, the term is more commonly reserved for sulfides, selenides, and tellurides, rather than...

 and complex oxides were obtained by using the drawing method. Unlike graphene, the other 2D materials have so far attracted surprisingly little attention.

Epitaxial growth on silicon carbide


Another method of obtaining graphene is to heat silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...

 (SiC) to high temperatures (>1100 °C) to reduce it to graphene. This process produces epitaxial graphene with dimensions dependent upon the size of the SiC substrate (wafer). The face of the SiC used for graphene formation, silicon- or carbon-terminated, highly influences the thickness, mobility and carrier density of the graphene.

Many important graphene properties have been identified in graphene produced by this method. For example, the electronic band-structure (so-called Dirac cone structure) has been first visualized in this material. Weak anti-localization is observed in this material and not in exfoliated graphene produced by the pencil-trace method. Extremely large, temperature-independent mobilities have been observed in SiC-epitaxial graphene. They approach those in exfoliated graphene placed on silicon oxide but still much lower than mobilities in suspended graphene produced by the drawing method. It was recently shown that even without being transferred graphene on SiC exhibits the properties of massless Dirac fermions such as the anomalous quantum Hall effect.

The weak van der Waals force that provides the cohesion of multilayer graphene stacks does not always affect the electronic properties of the individual graphene layers in the stack. That is, while the electronic properties of certain multilayered epitaxial graphenes are identical to that of a single graphene layer, in other cases the properties are affected as they are for graphene layers in bulk graphite. This effect is theoretically well understood and is related to the symmetry of the interlayer interactions.

Epitaxial graphene on SiC can be patterned using standard microelectronics methods. The possibility of large integrated electronics on SiC-epitaxial graphene was first proposed in 2004, and a patent for graphene-based electronics was filed provisionally in 2003 and issued in 2006. Since then, important advances have been made. In 2008, researchers at MIT Lincoln Lab produced hundreds of transistors on a single chip and in 2009, very high frequency transistors were produced at the Hughes Research Laboratories on monolayer graphene on SiC. Band gap of the epitaxial graphene can be tuned by irradiating with laser beams; modified graphene has a lot of advantages in device application.(Laser Patterning of Epitaxial Graphene for Schottky Junction Photodetectors)

Epitaxial growth on metal substrates


This method uses the atomic structure of a metal substrate to seed the growth of the graphene (epitaxial growth). Graphene grown on ruthenium
Ruthenium
Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...

 doesn't typically yield a sample with a uniform thickness of graphene layers, and bonding between the bottom graphene layer and the substrate may affect the properties of the carbon layers.
On the other hand, graphene grown on iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

 is very weakly bonded, uniform in thickness, and can be made highly ordered. Like on many other substrates, graphene on iridium is slightly rippled. Due to the long-range order of these ripples, generation of minigaps in the electronic band-structure (Dirac cone) becomes visible.
High-quality sheets of few-layer graphene exceeding 1 cm² (0.15500031000062 sq in) in area have been synthesized via chemical vapor deposition on thin nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

 films. These sheets have been successfully transferred to various substrates, demonstrating viability for numerous electronic applications.

An improvement of this technique has been found in copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 foil; at very low pressure, the growth automatically stops after a single graphene layer forms, and arbitrarily large graphene films can be created. However, in atmospheric-pressure CVD growth, multilayer graphene may form on copper (similar to that seen on nickel films). Weatherup and co-workers showed that graphene could be grown at temperatures compatible with conventional CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...

 processing using nickel-based alloy with gold as catalysts.

Graphite oxide reduction


Graphite oxide reduction was probably historically the first method of graphene synthesis. P. Boehm reported monolayer flakes of reduced graphene oxide already in 1962. In this early work existence of monolayer reduced graphene oxide flakes was demonstrated. The contribution of Boehm was recently acknowledged by Nobel prize winner for graphene research, Andre Geim
Andre Geim
Andre Konstantin Geim, FRS is a Dutch-Russian-British physicist working at the University of Manchester. Geim was awarded the 2010 Nobel Prize in Physics jointly with Konstantin Novoselov for his work on graphene...

: (Many Pioneers in Graphene Discovery). Graphite oxide exfoliation can be achieved by rapid heating and yields highly dispersed carbon powder with a few percent of graphene flakes. Reduction of graphite oxide monolayer films e.g. by hydrazine, annealing in argon/hydrogen was reported to yield graphene films. However, the quality of graphene produced by graphite oxide reduction is lower compared to e.g. scotch-tape graphene due to incomplete removal of various functional groups by existing reduction methods. Recently, reduction and exfoliation of graphite oxide by focused solar radiation was reported with less oxygen functionalities.

Growth from metal-carbon melts


The general idea in this process is to dissolve carbon atoms inside a transition metal melt at a certain temperature, and then allow the dissolved carbon to precipitate out at lower temperatures as single layer graphene (SLG). The metal is first melted in contact with a carbon source. This source could be the graphite crucible inside which the melting process is carried out or it could be the graphite powder or chunk sources which are simply placed in contact with the melt. Keeping the melt in contact with carbon source at a given temperature will give rise to dissolution and saturation of carbon atoms in the melt based on the binary phase diagram of metal-carbon. Upon lowering the temperature, solubility of the carbon in the molten metal will decrease and the excess amount of carbon will precipitate on top of the melt. The floating layer can be either skimmed or allowed to freeze for removal afterwards. Different morphology including thick graphite, few layer graphene (FLG) and SLG were observed on metal substrate. The Raman spectroscopy proved that SLG has been successfully grown on nickel substrate. The SLG Raman spectrum featured no D and D' band, indicating the pristine and high-quality nature of SLG. Among transition metals, nickel provides a better substrate for growing SLG. Since nickel is not Raman active, the direct Raman spectroscopy of graphene layers on top of the nickel is achievable. The graphene-metal composite could be utilized in thermal interface materials for thermal management applications.

Pyrolysis of sodium ethoxide


A 2008 publication described a process for producing gram-quantities of graphene, by the reduction of ethanol by sodium metal, followed by pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

 of the ethoxide product, and washing with water to remove sodium salts.

From nanotubes


Experimental methods for the production of graphene ribbons are reported consisting of cutting open nanotubes
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...

. In one such method multi-walled carbon nanotube
Mwnt
Mwnt is a very small community and ancient parish in south Ceredigion, Wales, on the Irish Sea coast about 4.5 miles from Cardigan. It gets its name from the prominent steep conical hill, a landmark from much of Cardigan Bay, that rises above the beach....

s are cut open in solution by action of potassium permanganate
Potassium permanganate
Potassium permanganate is an inorganic chemical compound with the formula KMnO4. It is a salt consisting of K+ and MnO4− ions. Formerly known as permanganate of potash or Condy's crystals, it is a strong oxidizing agent. It dissolves in water to give intensely purple solutions, the...

 and sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

. In another method graphene nanoribbons are produced by plasma etching
Plasma etching
Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge of an appropriate gas mixture being shot at a sample. The plasma source, known as etch species, can be either charged or neutral...

 of nanotubes partly embedded in a polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 film.

From sugar


Sucrose among other substances such as Plexiglas have been turned into graphene via application to a copper or nickel substrate and being subjected to 800 °C under low pressure with exposure to argon and hydrogen gas. The process can be completed in ten minutes and is scalable for industrial production. It also allows the manufacturer to modulate the purity of the graphene for applications that require electrical switching.

From graphite by sonication


It consists in dispersing graphite in a proper liquid medium that is then sonicated
Sonication
thumb|right|A sonicator at the [[Weizmann Institute of Science]] during sonicationSonication is the act of applying sound energy to agitate particles in a sample, for various purposes. In the laboratory, it is usually applied using an ultrasonic bath or an ultrasonic probe, colloquially known as...

. Non exfoliated graphite is eventually separated from graphene by centrifugation.
This method was first proposed by Hernandez et al. who obtained graphene concentration up to 0.01 mg/ml in N-methylpyrrolidone (NMP).
The method was then largely improved by several groups. In particular, it was greatly developed by the Italian group of Alberto Mariani. Mariani et al. reached the concentration of 2.1 mg/ml in NMP (the highest in this solvent). The same group published the highest graphene concentrations reported so far in any liquid and obtained by any method. An example is the use of a suitable ionic liquid as the dispersing liquid medium for graphite exfoliation; in this medium the very high concentration of 5.33 mg/ml was obtained.

Carbon Dioxide Reduction Method


Graphene Technologies has discovered, developed and applied for patents on a process for scalable production of single to few layer graphene employing an exothermic combustion reaction of certain Group I and II metals, including magnesium, and carbon bearing gases, including carbon dioxide. The first of a series of patent applications by Graphene Technologies' owner, High Temperature Physics, was published by the USPTO in October 2011. Graphene Technologies has worked with Evans Analytical Group to characterize its graphene product material over a long series of experiments and production cycles using the full range of state-of-the-art testing methods. Others have subsequently published work on versions of this process including Narayan Hosmane
Narayan Hosmane
Narayan S. Hosmane is an Indian-born cancer research scientist who made the featured article in NRI Achievers magazine and is currently distinguished research professor of Chemistry and Biochemistry and the inaugural board of trustees professor at Northern Illinois University, Dekalb. He received...

 and co-workers at Northern Illinois University
Northern Illinois University
Northern Illinois University is a state university and research institution located in DeKalb, Illinois, with satellite centers in Hoffman Estates, Naperville, Rockford, and Oregon. It was originally founded as Northern Illinois State Normal School on May 22, 1895 by Illinois Governor John P...

 in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, who published a simple method for producing high yields of graphene by burning magnesium metal in dry ice. The NIU team has shown that graphene was formed in few-layer nanosheets up to 10 atoms thick.

Atomic structure


The atomic structure of isolated, single-layer graphene was studied by transmission electron microscopy (TEM) on sheets of graphene suspended between bars of a metallic grid. Electron diffraction patterns showed the expected hexagonal lattice of graphene. Suspended graphene also showed "rippling" of the flat sheet, with amplitude of about one nanometer. These ripples may be intrinsic to graphene as a result of the instability of two-dimensional crystals, or may be extrinsic, originating from the ubiquitous dirt seen in all TEM images of graphene. Atomic resolution real-space images of isolated, single-layer graphene on SiO2 substrates were obtained by scanning tunneling microscopy
Scanning tunneling microscope
A scanning tunneling microscope is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer , the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and...

. Graphene processed using lithographic techniques is covered by photoresist
Photoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....

 residue, which must be cleaned to obtain atomic-resolution images. Such residue may be the "adsorbates" observed in TEM images, and may explain the rippling of suspended graphene. Rippling of graphene on the SiO2 surface was determined to be caused by conformation of graphene to the underlying SiO2, and not an intrinsic effect.

Graphene sheets in solid form (density > 1 g/cm3) usually show evidence in diffraction for graphite's 0.34 nm (002) layering. This is true even of some single-walled carbon nanostructures. However, unlayered graphene with only (hk0) rings has been found in the core of presolar
Presolar grains
Presolar grains are isotopically-distinct clusters of material found in the fine-grained matrix of primitive meteorites, such as chondrites, whose differences from the surrounding meteorite suggest that they are older than the solar system...

 graphite onions. Transmission electron microscope studies show faceting at defects in flat graphene sheets, and suggest a possible role in this unlayered-graphene for two-dimensional crystallization from a melt.

Electronic properties


Graphene differs from most conventional three-dimensional materials. Intrinsic graphene is a semi-metal or zero-gap semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

. Understanding the electronic structure of graphene is the starting point for finding the band structure of graphite. It was realized as early as 1947 by P. R. Wallace
P. R. Wallace
P. R. Wallace was a Canadian theoretical physicist and long-time professor at McGill University. He was a Fellow of the Royal Society of Canada and Fellow of the National Academy of Sciences...

 that the E-k relation is linear for low energies near the six corners of the two-dimensional hexagonal Brillouin zone
Brillouin zone
In mathematics and solid state physics, the first Brillouin zone is a uniquely defined primitive cell in reciprocal space. The boundaries of this cell are given by planes related to points on the reciprocal lattice. It is found by the same method as for the Wigner–Seitz cell in the Bravais lattice...

, leading to zero effective mass for electrons and holes
Electron hole
An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics, chemistry, and electrical engineering. The concept describes the lack of an electron at a position where one could exist in an atom or atomic lattice...

.
Due to this linear (or “conical
Conical intersection
In quantum chemistry, a conical intersection of two potential energy surfaces of the same spatial and spin symmetries is the set of molecular geometry points where the two potential energy surfaces are degenerate . Conical intersections are ubiquitous in both trivial and non-trivial chemical...

") dispersion relation at low energies, electrons and holes near these six points, two of which are inequivalent, behave like relativistic
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

 particles described by the Dirac equation
Dirac equation
The Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928. It provided a description of elementary spin-½ particles, such as electrons, consistent with both the principles of quantum mechanics and the theory of special relativity, and...

 for spin 1/2 particles. Hence, the electrons and holes are called Dirac fermions, and the six corners of the Brillouin zone are called the Dirac points. The equation describing the E-k relation is ; where the Fermi velocity vF ~ 106 m/s.

Electronic transport


Experimental results from transport measurements show that graphene has a remarkably high electron mobility
Electron mobility
In solid-state physics, the electron mobility characterizes how quickly an electron can move through a metal or semiconductor, when pulled by an electric field. In semiconductors, there is an analogous quantity for holes, called hole mobility...

 at room temperature, with reported values in excess of . Additionally, the symmetry of the experimentally measured conductance indicates that the mobilities for holes and electrons should be nearly the same. The mobility is nearly independent of temperature between 10 K and 100 K, which implies that the dominant scattering mechanism is defect scattering. Scattering by the acoustic phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

s of graphene places intrinsic limits on the room temperature mobility to at a carrier density of 1012 cm−2. The corresponding resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...

 of the graphene sheet would be 10−6 Ω·cm. This is less than the resistivity of silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

, the lowest resistivity substance known at room temperature. However, for graphene on SiO2 substrates, scattering of electrons by optical phonons of the substrate is a larger effect at room temperature than scattering by graphene’s own phonons. This limits the mobility to .

Despite the zero carrier density near the Dirac points, graphene exhibits a minimum conductivity on the order of . The origin of this minimum conductivity is still unclear. However, rippling of the graphene sheet or ionized impurities in the SiO2 substrate may lead to local puddles of carriers that allow conduction. Several theories suggest that the minimum conductivity should be 4e2/πh; however, most measurements are of order 4e2/πh or greater and depend on impurity concentration.

Recent experiments have probed the influence of chemical dopants on the carrier mobility in graphene. Schedin et al. doped graphene with various gaseous species (some acceptors, some donors), and found the initial undoped state of a graphene structure can be recovered by gently heating the graphene in vacuum. They reported that even for chemical dopant concentrations in excess of 1012 cm−2 there is no observable change in the carrier mobility. Chen, et al. doped graphene with potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

 in ultra high vacuum
Ultra high vacuum
Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...

 at low temperature. They found that potassium ions act as expected for charged impurities in graphene, and can reduce the mobility 20-fold. The mobility reduction is reversible on heating the graphene to remove the potassium.

Due to its two-dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

al property, charge fractionalization (where the apparent charge of individual pseudoparticles in low-dimensional systems is less than a single quantum) is thought to occur in graphene. It may therefore be a suitable material for the construction of quantum computer
Quantum computer
A quantum computer is a device for computation that makes direct use of quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. Quantum computers are different from traditional computers based on transistors...

s using anyon
Anyon
In physics, an anyon is a type of particle that occurs only in two-dimensional systems. It is a generalization of the fermion and boson concept.-From theory to reality:...

ic circuits.

Optical properties



Graphene's unique optical properties produce an unexpectedly high opacity for an atomic monolayer, with a startlingly simple value: it absorbs πα ≈ 2.3% of white light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

, where α is the fine-structure constant
Fine-structure constant
In physics, the fine-structure constant is a fundamental physical constant, namely the coupling constant characterizing the strength of the electromagnetic interaction. Being a dimensionless quantity, it has constant numerical value in all systems of units...

. This is "a consequence of the unusual low-energy electronic structure of monolayer graphene that features electron and hole conical band
Conical intersection
In quantum chemistry, a conical intersection of two potential energy surfaces of the same spatial and spin symmetries is the set of molecular geometry points where the two potential energy surfaces are degenerate . Conical intersections are ubiquitous in both trivial and non-trivial chemical...

s meeting each other at the Dirac point... [which] is qualitatively different from more common quadratic massive bands". Based on the Slonczewski-Weiss-McClure (SWMcC) band model of graphite, the interatomic distance, hopping value and frequency cancel when the optical conductance is calculated using the Fresnel equations
Fresnel equations
The Fresnel equations , deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between media of differing refractive indices...

 in the thin-film limit.

This has been confirmed experimentally, but the measurement is not precise enough to improve on other techniques for determining the fine-structure constant.

Recently it has been demonstrated that the band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...

 of graphene can be tuned from 0 to 0.25 eV (about 5 micrometre wavelength) by applying voltage to a dual-gate bilayer graphene field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...

 (FET) at room temperature. The optical response of graphene nanoribbons
Graphene nanoribbons
Graphene nanoribbons , often abbreviated GNRs, are thin strips of graphene or unrolled single-walled carbon nanotubes...

 has also been shown to be tunable into the terahertz regime by an applied magnetic field. It has been shown that graphene/graphene oxide system exhibits electrochromic behavior, allowing tuning of both linear and ultrafast optical properties.

Saturable absorption


It is further confirmed that such unique absorption could become saturated when the input optical intensity is above a threshold value. This nonlinear optical behavior is termed saturable absorption
Saturable absorption
Saturable absorption is a property of materials where the absorption of light decreases with increasing light intensity. Most materials show some saturable absorption, but often only at very high optical intensities ....

 and the threshold value is called the saturation fluence. Graphene can be saturated readily under strong excitation over the visible to near-infrared region, due to the universal optical absorption and zero band gap. This has relevance for the mode locking of fiber laser
Fiber laser
A fiber laser or fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium. They are related to doped fiber amplifiers, which provide light amplification without lasing...

s, where fullband mode locking has been achieved by graphene-based saturable absorber. Due to this special property, graphene has wide application in ultrafast photonics
Photonics
The science of photonics includes the generation, emission, transmission, modulation, signal processing, switching, amplification, detection and sensing of light. The term photonics thereby emphasizes that photons are neither particles nor waves — they are different in that they have both particle...

. Moreover, the ultrafast optical response of graphene/graphene oxide layers can be tuned electrically.

Spin transport


Graphene is thought to be an ideal material for spintronics
Spintronics
Spintronics , also known as magnetoelectronics, is an emerging technology that exploits both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.An additional effect occurs when a spin-polarized current is...

 due to small spin-orbit interaction
Spin-orbit interaction
In quantum physics, the spin-orbit interaction is any interaction of a particle's spin with its motion. The first and best known example of this is that spin-orbit interaction causes shifts in an electron's atomic energy levels due to electromagnetic interaction between the electron's spin and...

 and near absence of nuclear magnetic moments in carbon. Electrical spin-current injection and detection in graphene was recently demonstrated up to room temperature. Spin coherence length above 1 micrometre at room temperature was observed, and control of the spin current polarity with an electrical gate was observed at low temperature.

Anomalous quantum Hall effect


The quantum Hall effect is relevant for accurate measuring standards of electrical quantities, and in 1985 Klaus von Klitzing
Klaus von Klitzing
Klaus von Klitzing is a German physicist known for discovery of the integer quantum Hall Effect, for which he was awarded the 1985 Nobel Prize in Physics....

 received the Nobel prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

 for its discovery. The effect concerns the dependence of a transverse conductivity on a magnetic field, which is perpendicular to a current-carrying stripe. Usually the phenomenon, the quantization of the so-called Hall conductivity
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...

  at integer multiples of the basic quantity (where e is the elementary electric charge and h is Planck's constant) can be observed only in very clean Si or GaAs solids, and at very low temperatures around 3 K, and at very high magnetic fields.

Graphene in contrast, besides its high mobility and minimum conductivity, and because of certain peculiarities explained in Pseudo-relativistic theory below, shows particularly interesting behavior just in the presence of a magnetic field and just with respect to the conductivity-quantization: it displays an anomalous quantum Hall effect with the sequence of steps shifted by 1/2 with respect to the standard sequence, and with an additional factor of 4. Thus, in graphene the Hall conductivity is , where N is the above-mentioned integer "Landau level" index, and the double valley and double spin degeneracies give the factor of 4. Moreover, in graphene these remarkable anomalies can even be measured at room temperature, i.e. at roughly 20 °C. This anomalous behavior is a direct result of the emergent massless Dirac electrons in graphene. In a magnetic field, their spectrum has a Landau level with energy precisely at the Dirac point. This level is a consequence of the Atiyah-Singer index theorem and is half-filled in neutral graphene, leading to the "+1/2" in the
Hall conductivity. Bilayer graphene also shows the quantum Hall effect, but with the standard sequence (with ), i.e. with only one of the two anomalies. Interestingly, concerning the second anomaly, the first plateau at N = 0 is absent, indicating that bilayer graphene stays metallic at the neutrality point.

Unlike normal metals, the longitudinal resistance of graphene shows maxima rather than minima for integral values of the Landau filling factor in measurements of the Shubnikov–De Haas oscillations, which show a phase shift of π, known as Berry’s phase
Geometric phase
In classical and quantum mechanics, the geometric phase, Pancharatnam–Berry phase , Pancharatnam phase or most commonly Berry phase, is a phase acquired over...

. The Berry’s phase arises due to the zero effective carrier mass near the Dirac points. Study of the temperature dependence of the Shubnikov-de Haas oscillations in graphene reveals that the carriers have a non-zero cyclotron mass, despite their zero effective mass from the E-k relation.

Anomalous quantum Hall effect in strong magnetic fields


The quantum Hall effect in graphene in sufficiently strong magnetic fields (above 10 Teslas or so) reveals additional interesting features. Additional plateaus of the Hall conductivity at with are observed. Also, the observation of a plateau at and the fractional quantum Hall effect at were reported.

These observations of the quantum Hall effect with indicate that the four-fold degeneracy (two valley and two spin degrees of freedom) of the Landau energy levels is partially or completely lifted. According to one of the suggestions, it is the magnetic catalysis
Magnetic catalysis
Magnetic catalysis is a phenomenon in quantum field theory which explains a spontaneous breaking of flavor or chiral symmetry, triggered by the presence of an external magnetic field.-General description:...

 of symmetry breaking that is responsible for lifting the degeneracy of the Landau levels.

Nanostripes: Spin-polarized edge currents


Nanostripes of graphene (in the "zig-zag" orientation), at low temperatures, show spin-polarized metallic edge currents, which also suggests applications in the new field of spintronics
Spintronics
Spintronics , also known as magnetoelectronics, is an emerging technology that exploits both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.An additional effect occurs when a spin-polarized current is...

. (In the "armchair" orientation, the edges behave like semiconductors.)

Graphene oxide



By dispersing oxidized and chemically processed graphite in water, and using paper-making techniques, the monolayer flakes form a single sheet and bond very powerfully. These sheets, called graphene oxide paper
Graphene Oxide Paper
Graphene oxide paper or graphite oxide paper is a composite material fabricated from graphite oxide.The material has exceptional stiffness and strength, due to the intrinsic strength of the two-dimensional graphene backbone and to its interwoven layer structure which distributes loads.The starting...

 have a measured tensile modulus of 32 GPa. The peculiar chemical property of graphite oxide is related to the functional groups attached to graphene sheets. They even can significantly change the pathway of polymerization and similar chemical processes. Graphene Oxide flakes in polymers also shown enhanced photo-conducting properties.

Chemical modification


Soluble fragments of graphene can be prepared in the laboratory through chemical modification of graphite. First, microcrystalline graphite is treated with a strongly acidic mixture of sulfuric acid and nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

. A series of steps involving oxidation and exfoliation result in small graphene plates with carboxyl groups at their edges. These are converted to acid chloride groups by treatment with thionyl chloride
Thionyl chloride
Thionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...

; next, they are converted to the corresponding graphene amide
Amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...

 via treatment with octadecylamine. The resulting material (circular graphene layers of 5.3 angstrom
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

 thickness) is soluble in tetrahydrofuran
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...

, tetrachloromethane and dichloroethane
1,2-Dichloroethane
The chemical compound 1,2-dichloroethane, commonly known by its old name of ethylene dichloride , is a chlorinated hydrocarbon, mainly used to produce vinyl chloride monomer , the major precursor for PVC production. It is a colourless liquid with a chloroform-like odour...

.
Refluxing single-layer graphene oxide (SLGO) in solvents leads to size reduction and folding of the individual sheets as well as loss of carboxylic group functionality, by up to 20%, indicating thermal instabilities of SLGO sheets dependant on their preparation methodology. When using thionyl chloride, acyl chloride groups result, which can then form aliphatic and aromatic amides with a reactivity conversion of around 70-80%. Hydrazine reflux is commonly used for reducing SLGO to SLG(R), but titrations show that only around 20-30% of the carboxylic groups are lost leaving a significant number of COOH groups available for chemical attachment. Analysis of SLG(R) generated by this route reveals that the system is unstable and using a room temperature stirring with HCl (< 1.0 M) leads to around 60% loss of COOH functionality. Room temperature treatment of SLGO with carbodiimides leads to the collapse of the individual sheets into star-like clusters, which exhibited poor subsequent reactivity with amines (ca. 3-5% conversion of the intermediate to the final amide). It is apparent that conventional chemical treatment of carboxylic groups on SLGO generates morphological changes of individual sheets that leads to a reduction in chemical reactivity, which may potentially limit their use in composite synthesis. Therefore, other types of chemical reactions have been explored. SLGO has also been grafted with polyallylamine, cross-linked through epoxy groups. When filtered into graphene oxide paper, these composites exhibit sheets increased stiffness and strength relative to unmodified graphene oxide paper.

Full hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...

 from both sides of graphene sheet results in graphane
Graphane
Graphane is a 2-dimensional polymer of carbon and hydrogen with the formula unit n where n is large. Graphane's carbon bonds are in sp3 configuration, as opposed to graphene's sp2 bond configuration, thus graphane is a 2-D analog of cubic diamond. Graphane is a form of hydrogenated graphene...

, but partial hydrogenation leads to hydrogenated graphene

Thermal properties


The near-room temperature thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

 of graphene was recently measured to be between (4.84±0.44) ×103 to (5.30±0.48) ×103 Wm−1K−1. These measurements, made by a non-contact optical technique, are in excess of those measured for carbon nanotubes or diamond. It can be shown by using the Wiedemann-Franz law, that the thermal conduction is phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

-dominated. However, for a gated graphene strip, an applied gate bias causing a Fermi energy
Fermi energy
The Fermi energy is a concept in quantum mechanics usually referring to the energy of the highest occupied quantum state in a system of fermions at absolute zero temperature....

 shift much larger than kBT can cause the electronic contribution to increase and dominate over the phonon
Phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

 contribution at low temperatures. The ballistic thermal conductance of graphene is isotropic.

Potential for this high conductivity can be seen by considering graphite, a 3D version of graphene that has basal plane thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

 of over a 1000 Wm−1K−1 (comparable to diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

). In graphite, the c-axis (out of plane) thermal conductivity is over a factor of ~100 smaller due to the weak binding forces between basal planes as well as the larger lattice spacing. In addition, the ballistic thermal conductance of a graphene is shown to give the lower limit of the ballistic thermal conductances, per unit circumference, length of carbon nanotubes.

Despite its 2-D nature, graphene has 3 acoustic phonon modes. The two in-plane modes (LA, TA) have a linear dispersion relation
Dispersion relation
In physics and electrical engineering, dispersion most often refers to frequency-dependent effects in wave propagation. Note, however, that there are several other uses of the word "dispersion" in the physical sciences....

, whereas the out of plane mode (ZA) has a quadratic dispersion relation. Due to this, the T2 dependent thermal conductivity contribution of the linear modes is dominated at low temperatures by the T1.5 contribution of the out of plane mode. Some graphene phonon bands display negative Grüneisen parameter
Grüneisen Parameter
The Grüneisen parameter, γ, named after Eduard Grüneisen, describes the effect that changing the volume of a crystal lattice has on its vibrational properties, and, as a consequence, the effect that changing temperature has on the size or dynamics of the lattice...

s. At low temperatures (where most optical modes with positive Grüneisen parameters are still not
excited) the contribution from the negative Grüneisen parameters will be dominant and thermal expansion coefficient (which is directly proportional to Grüneisen parameters) negative. The lowest negative Grüneisen parameters correspond to the lowest transversal acoustic ZA modes. Phonon frequencies for such modes increase with the in-plane lattice parameter since atoms in the layer upon stretching will be less free to move in the z direction. This is similar to the behavior of a string which is being stretched will have vibrations of smaller amplitude and higher frequency. This phenomenon, named "membrane effect", was predicted by Lifshitz in 1952.

Mechanical properties


As of 2009, graphene appears to be one of the strongest materials ever tested. Measurements have shown that graphene has a breaking strength 200 times greater than steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

, with a tensile strength of 130 GPa (19,000,000 psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...

). However, the process of separating it from graphite, where it occurs naturally, will require some technological development before it is economical enough to be used in industrial processes, though this may be changing soon.

Graphene paper or GP has recently been developed by a research department from the University of Technology Sydney by Guoxiu Wang, that can be processed, reshaped and reformed from its original raw material state. Researchers have successfully milled the raw graphite by purifying and filtering it with chemicals to reshape and reform it into nano-structured configurations which are then processed into sheets as thin as paper, according to a university statement. Lead researcher Ali Reza Ranjbartoreh said: 'Not only is it lighter, stronger, harder and more flexible than steel, it is also a recyclable and sustainably manufacturable product that is eco-friendly and cost effective in its use.' Ranjbartoreh said the results would allow the development of lighter and stronger cars and planes that use less fuel, generate less pollution, are cheaper to run and ecologically sustainable. He said large aerospace companies have already started to replace metals with carbon fibres and carbon-based materials, and graphene paper with its incomparable mechanical properties would be the next material for them to explore.

Using an atomic force microscope
Atomic force microscope
Atomic force microscopy or scanning force microscopy is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit...

 (AFM), the spring constant of suspended graphene sheets has been measured. Graphene sheets, held together by van der Waals forces, were suspended over SiO2 cavities where an AFM tip was probed to test its mechanical properties. Its spring constant was in the range 1–5 N/m and the Young's modulus
Young's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...

 was 0.5 TPa, which differs from that of the bulk graphite. These high values make graphene very strong and rigid. These intrinsic properties could lead to using graphene for NEMS
Nanoelectromechanical systems
Nanoelectromechanical systems are devices integrating electrical and mechanical functionality on the nanoscale. NEMS form the logical next miniaturization step from so-called microelectromechanical systems, or MEMS devices...

 applications such as pressure sensors and resonators.

As is true of all materials, regions of graphene are subject to thermal and quantum fluctuations in relative displacement. Although the amplitude of these fluctuations is bounded in 3D structures (even in the limit of infinite size), the Mermin-Wagner theorem
Mermin-Wagner theorem
In quantum field theory and statistical mechanics, the Mermin–Wagner theorem states that continuous symmetries cannot be spontaneously broken at finite temperature in systems with sufficiently short-range interactions in dimensions d ≤ 2...

 shows that the amplitude of long-wavelength fluctuations will grow logarithmically with the scale of a 2D structure, and would therefore be unbounded in structures of infinite size. Local deformation and elastic strain are negligibly affected by this long-range divergence in relative displacement. It is believed that a sufficiently large 2D structure, in the absence of applied lateral tension, will bend and crumple to form a fluctuating 3D structure. Researchers have observed ripples in suspended layers of graphene, and it has been proposed that the ripples are caused by thermal fluctuations in the material. As a consequence of these dynamical deformations, it is debatable whether graphene is truly a 2D structure.

Casimir effect


The Casimir effect
Casimir effect
In quantum field theory, the Casimir effect and the Casimir–Polder force are physical forces arising from a quantized field. The typical example is of two uncharged metallic plates in a vacuum, like capacitors placed a few micrometers apart, without any external electromagnetic field...

 is generated by extremely thin plates of electrically conducting materials. Since graphene fulfils those criteria, it probably can be used for generating a Casimir effect as well.

Bilayer graphene



Bilayer graphene is two layers of graphene, which can be separated by a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

, and it has been shown to have interesting electrical properties, such as the quantum hall effect, a tunable band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...

, and potential for exictonic condensation.

Single-molecule gas detection


Theoretically graphene makes an excellent sensor due to its 2D structure. The fact that its entire volume is exposed to its surrounding makes it very efficient to detect adsorb
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...

ed molecules. However, similar to carbon nanotubes, graphene has no dangling bonds on its surface. Gaseous molecules cannot be readily adsorbed onto graphene surface. So intrinsically graphene is insensitive. The sensitivity of graphene chemical gas sensors can be dramatically enhanced by functionalizing graphene, for example, coating with a thin layer of certain polymers. The thin polymer layer acts like a concentrator that absorbs gaseous molecules. The molecule absorption introduces a local change in electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 of graphene sensors. While this effect occurs in other materials, graphene is superior due to its high electrical conductivity (even when few carriers are present) and low noise which makes this change in resistance detectable.

Graphene nanoribbons


Graphene nanoribbons
Graphene nanoribbons
Graphene nanoribbons , often abbreviated GNRs, are thin strips of graphene or unrolled single-walled carbon nanotubes...

 (GNRs) are essentially single layers of graphene that are cut in a particular pattern to give them certain electrical properties. Depending on how the un-bonded edges are configured, they can either be in a zigzag or armchair configuration. Calculations based on tight binding predict that zigzag GNRs are always metallic while armchairs can be either metallic or semiconducting, depending on their width. However, recent density functional theory
Density functional theory
Density functional theory is a quantum mechanical modelling method used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases. With this theory, the properties of a many-electron system can be determined by...

 calculations show that armchair nanoribbons are semiconducting with an energy gap scaling with the inverse of the GNR width. Indeed, experimental results show that the energy gaps do increase with decreasing GNR width. However, as of February 2008, no experimental results have measured the energy gap of a GNR and identified the exact edge structure. Zigzag nanoribbons are also semiconducting and present spin-polarized edges.
Their 2D structure, high electrical and thermal conductivity, and low noise also make GNRs a possible alternative to copper for integrated circuit interconnects. Some research is also being done to create quantum dots by changing the width of GNRs at select points along the ribbon, creating quantum confinement.

Graphene transistors


Due to its high electronic quality, graphene has also attracted the interest of technologists who see it as a way of constructing ballistic transistor
Ballistic transistor
Ballistic deflection transistors are electronic devices being developed for high-speed integrated circuits. Instead of switching the flow of several electrons using gates, as it is done in field-effect transistors, they try to manipulate the course of single electrons using electromagnetic forces...

s. Graphene exhibits a pronounced response to perpendicular external electric fields, allowing one to build FETs (field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...

s). In their 2004 paper, the Manchester group demonstrated FETs with a "rather modest" on-off ratio of ~30 at room temperature. In 2006, Georgia Tech researchers, led by Walter de Heer
Walter de Heer
Walter Alexander "Walt" de Heer is a Dutch physicist and nanoscience researcher known for discoveries in the electronic shell structure of metal clusters, magnetism in transition metal clusters, field emission and ballistic conduction in carbon nanotubes, and graphene-based electronics.-Academic...

, announced that they had successfully built an all-graphene planar FET with side gates. Their devices showed changes of 2% at cryogenic temperatures. The first top-gated FET (on-off ratio of <2) was demonstrated by researchers of AMICA and RWTH Aachen University
RWTH Aachen
RWTH Aachen University is a research university located in Aachen, North Rhine-Westphalia, Germany with roughly 33,000 students enrolled in 101 study programs....

 in 2007. Graphene nanoribbons may prove generally capable of replacing silicon as a semiconductor in modern technology.

Facing the fact that current graphene transistors show a very poor on-off ratio, researchers are trying to find ways for improvement. In 2008, researchers of AMICA and University of Manchester demonstrated a new switching effect in graphene field-effect devices. This switching effect is based on a reversible chemical modification of the graphene layer and gives an on-off ratio of greater than six orders of magnitude. These reversible switches could potentially be applied to nonvolatile memories.

In 2009, researchers at the Politecnico di Milano demonstrated four different types of logic gates, each composed of a single graphene transistor. In the same year, the Massachusetts Institute of Technology
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...

 researchers built an experimental graphene chip known as a frequency multiplier. It is capable of taking an incoming electrical signal of a certain frequency and producing an output signal that is a multiple of that frequency. Although these graphene chips open up a range of new applications, their practical use is limited by a very small voltage gain (typically, the amplitude of the output signal is about 40 times less than that of the input signal). Moreover, none of these circuits was demonstrated to operate at frequencies higher than 25 kHz.

In February 2010, researchers at IBM reported that they have been able to create graphene transistors with an on and off rate of 100 gigahertz, far exceeding the rates of previous attempts, and exceeding the speed of silicon transistors with an equal gate length. The 240 nm graphene transistors made at IBM were made using extant silicon-manufacturing equipment, meaning that for the first time graphene transistors are a conceivable—though still fanciful—replacement for silicon.
In November 2011, researchers at Cambridge University demonstrated the feasibility of ink-jet printing as a method for fabricating graphene devices.

Graphene optical modulators


When the Fermi level of graphene is tuned, the optical absorption of graphene can be changed. In 2011, researchers at UC Berkeley reported the first graphene-based optical modulator. Operating at 1.2 GHz without any temperature controller, this modulator has a broad bandwidth (from 1.3 to 1.6 μm) and small footprint (~25 μm2).

Integrated circuits


Graphene has the ideal properties to be an excellent component of integrated circuits. Graphene has a high carrier mobility, as well as low noise, allowing it to be used as the channel in a FET. The issue is that single sheets of graphene are hard to produce, and even harder to make on top of an appropriate substrate. Researchers are looking into methods of transferring single graphene sheets from their source of origin (mechanical exfoliation on SiO2 / Si or thermal graphitization of a SiC surface) onto a target substrate of interest. In 2008, the smallest transistor so far, one atom thick, 10 atoms wide was made of graphene. IBM announced in December 2008 that they fabricated and characterized graphene transistors operating at GHz frequencies. In May 2009, an n-type transistor was announced meaning that both n and p-type transistors have now been created with graphene. A functional graphene integrated circuit was also demonstrated – a complementary inverter
Inverter (logic gate)
In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. The truth table is shown on the right.This represents perfect switching behavior, which is the defining assumption in Digital electronics. In practice, actual devices have electrical characteristics that...

 consisting of one p- and one n-type graphene transistor. However, this inverter also suffered from a very low voltage gain.

According to a January 2010 report,
graphene was epitaxially grown on SiC in a quantity and with quality suitable for mass production of integrated circuits. At high temperatures, the Quantum Hall effect could be measured in these samples. See also the 2010 work by IBM in the transistor section above in which 'processors' of fast transistors on 2 inches (50.8 mm) graphene sheets were made.

In June 2011, IBM researchers announced that they had succeeded in creating the first graphene-based integrated circuit, a broadband radio mixer. The circuit handled frequencies up to 10 GHz, and its performance was unaffected by temperatures up to 127 degrees Celsius.

Electrochromic devices


Graphene oxide
Graphene oxide
Graphite oxide, formerly called graphitic oxide or graphitic acid, is a compound of carbon, oxygen, and hydrogen in variable ratios, obtained by treating graphite with strong oxidizers...

 can be reversibly reduced and oxidized using electrical stimulus. Controlled reduction and oxidation in two-terminal devices containing multilayer graphene oxide films are shown to result in switching between partially reduced graphene oxide and graphene, a process which modifies the electronic and optical properties. Oxidation and reduction are also shown to be related to resistive switching.

Transparent conducting electrodes


Graphene's high electrical conductivity and high optical transparency make it a candidate for transparent conducting electrodes, required for such applications as touchscreen
Touchscreen
A touchscreen is an electronic visual display that can detect the presence and location of a touch within the display area. The term generally refers to touching the display of the device with a finger or hand. Touchscreens can also sense other passive objects, such as a stylus...

s, liquid crystal display
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....

s, organic photovoltaic cells
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....

, and organic light-emitting diode
Organic light-emitting diode
An OLED is a light-emitting diode in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes...

s. In particular, graphene's mechanical strength and flexibility are advantageous compared to indium tin oxide
Indium tin oxide
Indium tin oxide is a solid solution of indium oxide and tin oxide , typically 90% In2O3, 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey...

, which is brittle, and graphene films may be deposited from solution over large areas.

Large-area, continuous, transparent, and highly conducting few-layered graphene films were produced by chemical vapor deposition and used as anodes for application in photovoltaic devices. A power conversion efficiency (PCE) up to 1.71% was demonstrated, which is 55.2% of the PCE of a control device based on indium-tin-oxide.

Organic light-emitting diode
Organic light-emitting diode
An OLED is a light-emitting diode in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes...

s (OLEDs) with graphene anodes have also been demonstrated. The electronic and optical performance of devices based on graphene are shown to be similar to devices made with indium-tin-oxide.

An all carbon-based device called a light-emitting electrochemical cell
Light-emitting Electrochemical Cell
A light-emitting electrochemical cell is a solid-state device that generates light from an electric current . LEC's are usually composed of two metal electrodes connected by an organic semiconductor containing mobile ions...

 (LEC) was demonstrated with chemically derived graphene as the cathode and the conductive polymer
Conductive polymer
Conductive polymers or, more precisely, intrinsically conducting polymers are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive...

 PEDOT
Pedot
Pedot is the eleventh album of the Finnish rock band CMX. "Pedot" means "Beasts" in Finnish.The opening track "Eteläisen tähtitaivaan kartoitus" breaks a CMX tradition of starting their albums with powerful rock songs. It is a minimalistic piano-based ballad with no drums...

 as the anode by Matyba et al. Unlike its predecessors, this device contains no metal, but only carbon-based electrodes. The use of graphene as the anode in LECs was also verified in the same publication.

Reference material for characterizing electroconductive and transparent materials


One layer of graphene absorbs 2.3 % of white light.
This property was used to define the Conductivity of Transparency
Conductivity of transparency
The conductivity of transparency describes the combination of the sheet resistance and the transparency and utilizes the properties of graphene as the reference.- Description:...

that combines the sheet resistance
Sheet resistance
Sheet resistance is a measure of resistance of thin films that are namely uniform in thickness. It is commonly used to characterize materials made by semiconductor doping, metal deposition, resistive paste printing, and glass coating. Examples of these processes are: doped semiconductor regions...

 and the transparency
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...

. This parameter was used to compare different materials without the use of two independent parameters.

Thermal management materials-Thermal interfacial materials


In 2011, researchers in Georgia Institute of Technology
Georgia Institute of Technology
The Georgia Institute of Technology is a public research university in Atlanta, Georgia, in the United States...

 reported that a three-dimensional vertically aligned functionalized multilayer graphene architecture can be an approach for graphene-based thermal interfacial materials (TIMs
TIMS
TIMS is a four-letter combination that may refer to:* The International Molinological Society* Thermal ionization mass spectrometry - see Thermal ionization* Thermal infrared multispectral scanner - see Thermal infrared spectroscopy and 1982 in archaeology...

) with superior equivalent thermal conductivity and ultra-low interfacial thermal resistance
Interfacial thermal resistance
Interfacial thermal resistance, also known as thermal boundary resistance, or Kapitza resistance, is a measure of an interface's resistance to thermal flow. This thermal resistance differs from contact resistance, as it exists even at atomically perfect interfaces...

 between graphene and metal.

Solar cells


The USC Viterbi School of Engineering lab reported the large scale production of highly transparent graphene films by chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...

 in 2008. In this process, researchers create ultra-thin graphene sheets by first depositing carbon atoms in the form of graphene films on a nickel plate from methane gas. Then they lay down a protective layer of thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

 over the graphene layer and dissolve the nickel underneath in an acid bath. In the final step they attach the plastic-protected graphene to a very flexible polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 sheet, which can then be incorporated into an OPV cell (graphene photovoltaics). Graphene/polymer sheets have been produced that range in size up to 150 square centimeters and can be used to create dense arrays of flexible OPV cells. It may eventually be possible to run printing presses laying extensive areas covered with inexpensive solar cells, much like newspaper presses print newspapers (roll-to-roll)

Ultracapacitors


Due to the extremely high surface area to mass ratio of graphene, one potential application is in the conductive plates of ultracapacitors. It is believed that graphene could be used to produce ultracapacitors with a greater energy storage density than is currently available.

Graphene biodevices


Graphene's modifiable chemistry, large surface area, atomic thickness and molecularly-gatable structure make antibody-functionalized graphene sheets excellent candidates for mammalian and microbial detection and diagnosis devices.

The most ambitious biological application of graphene is for rapid, inexpensive electronic DNA sequencing. Integration of graphene (thickness of 0.34 nm) layers as nanoelectrodes into a nanopore can solve one of the bottleneck issues of nanopore-based single-molecule DNA sequencing.

Anti-bacterial


The Chinese Academy of Sciences
Chinese Academy of Sciences
The Chinese Academy of Sciences , formerly known as Academia Sinica, is the national academy for the natural sciences of the People's Republic of China. It is an institution of the State Council of China. It is headquartered in Beijing, with institutes all over the People's Republic of China...

 has found that sheets of graphene oxide are highly effective at killing bacteria such as Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

. This means graphene could be useful in applications such as hygiene products or packaging that will help keep food fresh for longer periods of time.

Pseudo-relativistic theory


The electrical properties of graphene can be described by a conventional tight-binding model; in this model the energy of the electrons with wave vector k is


with the nearest-neighbor hopping energy γ0 ≈ 2.8 eV and the lattice constant
Lattice constant
The lattice constant [or lattice parameter] refers to the constant distance between unit cells in a crystal lattice. Lattices in three dimensions generally have three lattice constants, referred to as a, b, and c. However, in the special case of cubic crystal structures, all of the constants are...

 a ≈ 2.46 Å. Conduction
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...

 and valence band
Valence band
In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature....

, respectively, correspond to the different signs in the above dispersion relation
Dispersion relation
In physics and electrical engineering, dispersion most often refers to frequency-dependent effects in wave propagation. Note, however, that there are several other uses of the word "dispersion" in the physical sciences....

; they touch each other in six points, the "K-values". However, only two of these six points are independent, whereas the rest is equivalent by symmetry. In the vicinity of the K-points the energy depends linearly on the wave vector, similar to a relativistic particle. Since an elementary cell of the lattice has a basis of two atoms, the wave function even has an effective 2-spinor structure
Spinor
In mathematics and physics, in particular in the theory of the orthogonal groups , spinors are elements of a complex vector space introduced to expand the notion of spatial vector. Unlike tensors, the space of spinors cannot be built up in a unique and natural way from spatial vectors...

. As a consequence, at low energies, even neglecting the true spin, the electrons can be described by an equation which is formally equivalent to the massless Dirac equation
Dirac equation
The Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928. It provided a description of elementary spin-½ particles, such as electrons, consistent with both the principles of quantum mechanics and the theory of special relativity, and...

. Moreover, in the present case this pseudo-relativistic description is restricted to the chiral limit
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

, i.e., to vanishing rest mass M0, which leads to interesting additional features:


Here vF ~ 106 is the Fermi velocity in graphene which replaces the velocity of light in the Dirac theory; is the vector of the Pauli matrices
Pauli matrices
The Pauli matrices are a set of three 2 × 2 complex matrices which are Hermitian and unitary. Usually indicated by the Greek letter "sigma" , they are occasionally denoted with a "tau" when used in connection with isospin symmetries...

, is the two-component wave function of the electrons, and E is their energy.

History and experimental discovery


The term graphene first appeared in 1987 to describe single sheets of graphite as one of the constituents of graphite intercalation compound
Graphite intercalation compound
Graphite intercalation compounds are complex materials having formula XCy where element or molecule X is inserted between the graphite layers. In this type of compound, the graphite layers remain largely intact and the guest molecules or atoms are located in between...

s (GICs); conceptually a GIC is a crystalline salt of the intercalant and graphene. The term was also used in early descriptions of carbon nanotube
Carbon nanotube
Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...

s, as well as for epitaxial graphene, and polycyclic aromatic hydrocarbons.

Larger graphene molecules or sheets (so that they can be considered as true isolated 2D crystals) cannot be grown even in principle. An article in Physics Today
Physics Today
Physics Today, created in 1948, is the membership journal of the American Institute of Physics. It is provided to 130,000 members of twelve physics societies, including the American Physical Society...

 reads:
Single layers of graphite were previously (starting from the 1970s) grown epitaxially on top of other materials. 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 from the substrate to the epitaxial graphene, and, in some cases, hybridization between the d orbitals of the substrate atoms and π orbitals of graphene, which significantly alters the electronic structure of the epitaxial graphene.

Single layers of graphite were also observed by transmission electron microscopy
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...

 within bulk materials (see section Occurrence), in particular inside soot obtained by chemical exfoliation. There have also been a number of efforts to make very thin films of graphite by mechanical exfoliation (starting from 1990 and continuing until after 2004) but nothing thinner than 50 to 100 layers was produced during these years.

A key advance in the science of graphene came when Andre Geim
Andre Geim
Andre Konstantin Geim, FRS is a Dutch-Russian-British physicist working at the University of Manchester. Geim was awarded the 2010 Nobel Prize in Physics jointly with Konstantin Novoselov for his work on graphene...

 and Kostya Novoselov at Manchester University managed to extract single-atom-thick crystallites (graphene) from bulk graphite in 2004. The Manchester researchers pulled out graphene layers from graphite and transferred them onto thin SiO2 on a silicon wafer in a process sometimes called micromechanical cleavage or, simply, the Scotch tape technique. The SiO2 electrically isolated the graphene, and was weakly interacting with the graphene, providing nearly charge-neutral graphene layers. The silicon beneath the SiO2 could be used as a "back gate" electrode to vary the charge density in the graphene layer over a wide range.

The micromechanical cleavage technique led directly to the first observation of the anomalous quantum Hall effect in graphene, which provided direct evidence of the theoretically predicted pi Berry's phase of massless Dirac fermions in graphene. The anomalous quantum Hall effect in graphene was reported around the same time by Geim and Novoselov and by Philip Kim
Philip Kim
Philip Kim is a condensed matter physicist known for study of quantum transport in carbon nanotubes and graphene, including observations of quantum Hall effects in graphene.-Academic career:...

 and Yuanbo Zhang in 2005.

Geim has received several awards for his pioneering research on graphene, including the 2007 Mott medal for the "discovery of a new class of materials – free-standing two-dimensional crystals – in particular graphene", the 2008 EuroPhysics Prize (together with Novoselov) "for discovering and isolating a single free-standing atomic layer of carbon (graphene) and elucidating its remarkable electronic properties", and the 2009 Körber Prize for "develop[ing] the first two-dimensional crystals made of carbon atoms". In 2008 and 2009, Reuters (which also runs a bibliometric service Web of Science) tipped him as one of the front-runners for a Nobel prize in Physics. On October 5, 2010, the Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...

 in Physics for the year was awarded to Andre Geim
Andre Geim
Andre Konstantin Geim, FRS is a Dutch-Russian-British physicist working at the University of Manchester. Geim was awarded the 2010 Nobel Prize in Physics jointly with Konstantin Novoselov for his work on graphene...

 and Konstantin Novoselov
Konstantin Novoselov
Konstantin Sergeevich Novoselov FRS is a Russo-British physicist, most notably known for his works on graphene together with Andre Geim, which earned them the Nobel Prize in Physics in 2010. Novoselov is currently a member of the mesoscopic physics research group at the University of Manchester as...

 from the University of Manchester for their work on graphene.

The theory of graphene was first explored by P. R. Wallace
P. R. Wallace
P. R. Wallace was a Canadian theoretical physicist and long-time professor at McGill University. He was a Fellow of the Royal Society of Canada and Fellow of the National Academy of Sciences...

 in 1947 as a starting point for understanding the electronic properties of more complex, 3D graphite. The emergent massless Dirac equation was first pointed out by Gordon Walter Semenoff
Gordon Walter Semenoff
Gordon Walter Semenoff is a theoretical physicist and Professor of Physics at University of British Columbia. He is known for his research on quantum mechanics, quantum field theory, statistical mechanics and string theory and is particularly famous for his co-invention, together with Antti Niemi,...

 and David P. DeVincenzo and Eugene J. Mele.
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. Later, single graphene layers were also observed directly by electron microscopy.

More recently, graphene samples prepared on nickel films, and on both the silicon face and carbon face of silicon carbide, have shown the anomalous quantum Hall effect directly in electrical measurements.
Graphitic layers on the carbon face of silicon carbide show a clear Dirac spectrum
Dirac spectrum
In mathematics, a Dirac spectrum, named after Paul Dirac, is the spectrum of eigenvalues of a Dirac operator on a Riemannian manifold with a spin structure. The isospectral problem for the Dirac spectrum asks whether two Riemannian spin manifolds have identical spectra...

 in angle-resolved photoemission
ARPES
Angle-resolved photoemission spectroscopy , also known as ARUPS , is a direct experimental technique to observe the distribution of the electrons in the reciprocal space of solids...

 experiments, and the anomalous quantum Hall effect is observed in cyclotron resonance and tunneling experiments.
Even though graphene on nickel and on silicon carbide have both existed in the laboratory for decades, it was graphene mechanically exfoliated on SiO2 that provided the first proof of the Dirac fermion nature of electrons in graphene.

See also


  • Aromaticity
    Aromaticity
    In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...

  • Exfoliated graphite nano-platelets
    Exfoliated Graphite Nano-Platelets
    Exfoliated graphite nano-platelets are new types of nanoparticles made from graphite. These nanoparticles consist of small stacks of graphene that are 1 to 15 nanometers thick, with diameters ranging from sub-micrometre to 100 micrometres...

  • Fullerene
    Fullerene
    A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes...

    s
  • Polycyclic aromatic hydrocarbon
    Polycyclic aromatic hydrocarbon
    Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...

    s
  • Carbon nanotube
    Carbon nanotube
    Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...

    s
  • Graphene nanoribbons
    Graphene nanoribbons
    Graphene nanoribbons , often abbreviated GNRs, are thin strips of graphene or unrolled single-walled carbon nanotubes...

  • Graphene oxide paper
    Graphene Oxide Paper
    Graphene oxide paper or graphite oxide paper is a composite material fabricated from graphite oxide.The material has exceptional stiffness and strength, due to the intrinsic strength of the two-dimensional graphene backbone and to its interwoven layer structure which distributes loads.The starting...

  • Graphane
    Graphane
    Graphane is a 2-dimensional polymer of carbon and hydrogen with the formula unit n where n is large. Graphane's carbon bonds are in sp3 configuration, as opposed to graphene's sp2 bond configuration, thus graphane is a 2-D analog of cubic diamond. Graphane is a form of hydrogenated graphene...

  • Silicene
    Silicene
    Silicene is a two-dimensional allotrope of silicon, similar to graphene.-History:Although theorists have speculated about the existence and possible properties of silicene, it has not been observed. In 2010, researchers have observed for the first time silicon structures that are suggestive of...



External links