Graphene

Graphene is an allotrope of carbon. Its 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 and the suffix -ene by Hanns-Peter Boehm, who described single-layer carbon foils in 1962. Graphene is most easily visualized as an atomic-scale chicken wire made of carbon atoms and their bonds. The 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 with an interplanar spacing of 0.335 nm. Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes and fullerenes. It can also be considered as an indefinitely large aromatic molecule, the limiting case of the family of flat polycyclic aromatic hydrocarbons.

The Nobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin Novoselov at the University of Manchester “for groundbreaking experiments regarding the two-dimensional material graphene”.

Description

One definition given in a recent review on graphene is:

Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, and is a basic building block for graphitic materials of all other dimensionalities. It can be wrapped up into 0D fullerenes, rolled into 1D nanotubes or stacked into 3D graphite.

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 compounds (GICs); conceptually a GIC is a crystalline salt of the intercalant and graphene. The term was also used in early descriptions of carbon nanotubes, 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.

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 within bulk materials, 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.

Occurrence and production of Graphene

In essence, graphene is an isolated atomic plane of graphite. From this perspective, graphene has been known since the invention of X-ray crystallography. Graphene planes become even better separated in intercalated graphite compounds. In 2004, physicists at the University of Manchester and the Institute for Microelectronics Technology, Chernogolovka, 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 demonstrated that quasiparticles in graphene were massless Dirac fermions. These discoveries led to an explosion of interest in graphene.

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

Drawing method

In 2004, the Manchester group obtained graphene by micro-mechanical alleviation 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 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.

Epitaxial growth of Graphene on silicon carbide

Another method of obtaining graphene is to heat silicon carbide (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.

Epitaxial growth of Graphene on metal substrates

This method uses source and the atomic structure of a metal substrate to seed the growth of the graphene (epitaxial growth). Graphene grown on ruthenium 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 is very weakly bonded, uniform in thickness, and can be made highly ordered.

Synthesis of Graphene by reduction of Graphite oxide

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 of Graphene 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.

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

Synthesis of Graphene From nanotubes

Experimental methods for the production of graphene ribbons are reported consisting of cutting open nanotubes. In one such method multi-walled carbon nanotubes are cut open in solution by action of potassium permanganate and sulfuric acid. In another method graphene nanoribbons are produced by plasma etching of nanotubes partly embedded in a polymer film.

Production of Graphene from graphite by sonication

It consists in dispersing graphite in a proper liquid medium that is then sonicated. Non exfoliated graphite is eventually separated from graphene by centrifugation.

Synthesis of Graphene by reduction of Carbon dioxide

Graphene Technologies, a dba of High Temperature Physics LLC, has discovered and developed a novel 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.

Laser scribing method for production of Graphene

Maher F. El-Kady, Veronica Strong, Sergey Dubin and Richard Kaner from the University of California, Los Angeles, discovered a very efficient and low cost way to produce graphene. Applying a layer of graphite oxide film to a DVD disc and burning it in a DVD writer resulted in a thin graphene film with high electrical conductivity (1738 siemens per meter) and specific surface area (1520 square meters per gram), besides being highly resistant and malleable.

Properties of Graphene

Atomic structure of Graphene

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.

Electronic properties of Graphen

Graphene differs from most conventional three-dimensional materials. Intrinsic graphene is a semi-metal or zero-gap semiconductor. Understanding the electronic structure of graphene is the starting point for finding the band structure of graphite. It was realized that the E-k relation is linear for low energies near the six corners of the two-dimensional hexagonal Brillouin zone, leading to zero effective mass for electrons and holes.

Electronic transport of Graphene

Experimental results from transport measurements show that graphene has a remarkably high electron mobility at room temperature, with reported values in excess of 15,000 cm2V−1s−1. Additionally, the symmetry of the experimentally measured conductance indicates that the mobilities for holes and electrons should be nearly the same.

Nanostripes

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

Optical properties of Graphene

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, where α is the fine-structure constant. This is “a consequence of the unusual low-energy electronic structure of monolayer graphene that features electron and hole conical bands meeting each other at the Dirac point… [which] is qualitatively different from more common quadratic massive bands”.

The band gap 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 (FET) at room temperature. The optical response of graphene nanoribbons 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.

Spin transport

Graphene is thought to be an ideal material for spintronics due to small spin-orbit interaction and near absence of nuclear magnetic moments in carbon.

Graphite 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. The maximally oxidized bulk product is a yellow solid with C:O ratio between 2.1 and 2.9, that retains the layer structure of graphite but with a much larger and irregular spacing.

The bulk material disperses in basic solutions to yield monomolecular sheets, known as graphene oxide by analogy to graphene, the single-layer form of graphite. Graphene oxide sheets have been used to prepare a strong paper-like material, and have recently attracted substantial interest as a possible intermediate for the manufacture of graphene. However, as of 2010 this goal remains elusive since graphene obtained by this route still has many chemical and structural defects.

Mechanical properties of Graphene

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, with a tensile modulus (stiffness) of 1 TPa (150,000,000 psi). 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.

Thermal properties of Graphene

The near-room temperature thermal conductivity of graphene was 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. The isotopic composition, the ratio of 12C to 13C, has a significant impact on thermal conductivity, where isotopically pure 12C graphene has higher conductivity than either a 50:50 isotope ratio or the naturally occurring 99:1 ratio.

Applications of Graphene

Room temperature distillation of ethanol for fuel and human consumption

Graphene oxide membranes have been shown to be impermeable to all gases including helium, while simultaneously allowing water vapour to pass through the membrane as though no barrier were there. This phenomenon has been used for further distilling vodka sitting in a room-temperature laboratory to higher alcohol concentrations without the application of heat or vacuum normally used in traditional distillation methods. Further development and commercialization of such membranes could revolutionize the economics of biofuel production and the alcoholic beverage industry.

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 adsorbed 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 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 (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 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.

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 transistors. Graphene exhibits a pronounced response to perpendicular external electric fields, allowing one to build FETs (field-effect transistors).

Facing the fact that current graphene transistors show a very poor on-off ratio, researchers are trying to find ways for improvement.

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 field-effect transistor. 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 onto a target substrate of interest.

Solar cells

The USC Viterbi School of Engineering lab reported the large scale production of highly transparent graphene films by chemical vapor deposition 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 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 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.

Anti-bacterial

The Chinese Academy of Sciences has found that sheets of graphene oxide are highly effective at killing bacteria such as Escherichia coli. 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.

Study of liquids

Graphene can form a clear “window” to see liquids at higher resolution than was previously possible using transmission electron microscopes.