Graphene

Graphs [ gʁa'fe ː n] (English graphene ) is the name for a form of carbon with two-dimensional structure in which each carbon atom is surrounded by three other, so that forms a honeycomb- like pattern. Since carbon is tetravalent, it must ever " honeycomb" three double bonds occur, but these are not localized. It is a chain of benzene rings, as in the aromatic chemistry occurs frequently. At the " edge " of the honeycomb lattice other atomic groups need to be docked, but - adversely affect the properties of the graph hardly - depending on its size.

In theory, were single-layer carbon films, Graphene, first used to describe the structure and electronic properties of complex carbon existing materials can.

Infinitely extended flat and everywhere strictly two-dimensional structures, however, are based on a rigorous mathematical theorem, the Mermin -Wagner theorem and its variants, not possible, because they are proven to be thermodynamically unstable.

Therefore, when chemists and physicists was general astonishment when Konstantin Novoselov, Andre Geim and their employees in 2004 announced the presentation of free, single-layer graphene crystals. Their unexpected stability might be the existence of metastable states or by forming an irregular ripple (English crumpling ) of the graphene layer are explained.

2010 were honored for their studies with the Nobel Prize for Physics, after they had made ​​decisive not only for the representation of these systems, but had also discovers many of their unusual properties Geim and Novoselov.

Brainstorming can be created by stacking such monolayers the three-dimensional structure of graphite, so it is structurally closely related to the graph. If you imagine the monolayers on the other hand rolled before, one gets stretched carbon nanotubes. Also theoretically possible to replace some of the six-membered rings by five-membered rings, which the flat surface bulges out into a sphere surface and in certain numerical ratios fullerenes arise: For example, changing 12 of 32 rings, the smallest fullerene is formed ( C60). In theory, single-ply layers of other tetravalent elements such as silicon and germanium are possible. 2012 were in fact called silicene layers, in the form of a slightly corrugated single layer of silicon, as demonstrated experimentally.

• 4.1 pseudo - relativistic behavior
• 4.2 Unusual quantum Hall effect
• 4.3 stiffness and temperature dependence
• 4.4 Elastic behavior and pseudo- magnetic field
• 4.5 spin currents
• 4.6 Chemical Functionalization

History

In 1859, Benjamin Collins Brodie, Jr. described the lamellar structure of thermally reduced graphite oxide. This was in 1918 investigated intensively by Volkmar Kohlschütter and P. Haenni. They reported also about the production of Graphitoxidpapier. The first transmission electron micrographs ( TEM ) of the graph of a low number were published by G. Ruess and F. Vogt in 1948. Among the pioneers of graphene research Hanns -Peter Boehm heard. He already reported 1962 on single-layer carbon foils and also coined the term graph.

Structure

All carbon atoms of graphene are sp2 hybridized, that is, each carbon atom can form three equivalent σ - bonds to other carbon atoms. This results in a well-known and from the layers of graphite honeycomb structure. The carbon-carbon bond lengths are all equal and are 142 pm. The third, unhybridized 2p orbitals are as in graphite perpendicular to the graphene plane and form a delocalized π - bond system.

Graph thus consists of two layers of atoms equivalent to A and B, where the carbon atoms are associated. The atomic layers are shifted by the bond length from each other. Diatomic the unit cell is defined by the grating vectors and. These show it to the respective next-nearest neighbors. The length of the vectors and therefore the lattice constant a can be calculated

Graphs on the one hand be, on the other hand understood as a single crystal as a giant molecule. Likewise, small molecules such as Benzene, naphthalene or hexabenzocoronene be seen as a hydrogen- substituted graphene fragments.

Production of graphene

Mechanically

The first graphene flakes were recovered from Novoselov by exfoliation ( exfoliation ) of HOPG ( Highly Oriented Pyrolytic Graphite English, German highly ordered pyrolytic graphite). The method used is similar to the so-called tape test; while a tape is pressed onto a surface - in this case, a block of graphite - will be deducted and then quickly, so leachable elements ( here graphite ) are left in the adhesive. This tape is then pressed onto a photoresist coated silicon wafer and stripped again. After removal of the tape thin graphite particles are left on the surface of the photoresist layer. Subsequently, the photoresist layer is dissolved with propanone and then rinsed the wafer with water and 2- propanol. Dissolving the photoresist layer in some exemplary graphite particles on the wafer surface, which may additionally be coated with thin silica. In this way, locally thin graphite films can be produced. Interesting for investigations graph layers which are thinner than 50 nm, are optically almost transparent. The additional layer alters the reflection characteristics of the substrate, so that the change caused by interference effects of the silicon dioxide to blue -violet color. At the edges of these 50 -nm layers can then search with the scanning tunneling or atomic force microscope for thinner graphene.

In another Exfoliationsmethode be etched with an oxygen plasma depressions in the HOPG before Exfoliationsprozess, leave the isolated plateaus ( mesas ). Thereafter, a glass substrate is wetted with adhesive and stripped down on the surface. The adhesive in the adhesive mesas are now also debited for as long with tape until only a residue remains. Thereafter, the adhesive is dissolved in propanone and fished propanone dissolved in the graphene flakes with a silicon wafer and, in turn, scanned with an optical microscope and scanning tunneling or atomic force microscope.

In both of these methods are very time-consuming processes in which one obtains indeed quality, but very few samples.

Chemical

The most promising method is the production of graphene by reduction of graphene. For example, reported the California NanoSystems Institute ( CNSI ) in 2008 about a " mass production method ", based on the reduction of graphite in liquid hydrazine. In this way graphene monolayers were the size of 20 microns x 40 microns are produced. In addition was also reported on the gradual construction of polycyclic aromatics and a chemical exfoliation of graphite by organic solvents.

In gram-scale graphene can also be produced in a two step reaction. Here, sodium and ethanol are reacted in the first step in a solvothermal reaction. Under heating for several hours under high pressure, thereby creating a complex mixture with sodium ethoxide as a main component. In the second step, the reaction mixture is strongly heated with exclusion of air (pyrolysis ), which can be isolated among other graphs after a final sonication.

Epitaxial Growth

Graphs can grow epitaxially on metallic substrates. A presented in the literature method is the decomposition of ethylene on iridium. In another method, the solubility of carbon is used in transition metals. When heated, the carbon dissolved in the metal, occurs out again on cooling and assigns itself as the graph on the surface.

Another possibility for the representation of individual graphene sheets is the thermal decomposition of hexagonal silicon carbide surfaces. At temperatures above the melting point of silicon, the silicon evaporates due to its (relative to carbon), higher vapor pressure. On the surface then form thin layers of single-crystal graphite, which consist of a few graphene monolayers. This method is suitable for processes in the event a vacuum and in an inert atmosphere of argon. The thickness and structure of the epitaxially grown graphene depends sensitively on the set process parameters, in particular on the choice of atmosphere, the structure of the substrate surface as well as the polarization of the silicon carbide surface.

Large areas of the graph is used to produce, characterized in that applying a mono-atomic layer of carbon on a sheet of inert support material, such as copper, by chemical vapor deposition (CVD ), and then dissolving the carrier material.

Properties

Graph has unusual properties that make it both for basic research and for applications of interest, notably in physics.

For example, graphene Flächeneinkristalle within the areas extremely stiff and strong. The elastic modulus corresponds with about 1020 GPa that of normal graphite along the basal planes and is almost as large as that of diamond. Its tensile strength of 1.25 × 1011 Pa is the highest that has ever been found, and about 125 times higher than steel. A band of graphs of 1 m width and 3.35 × 10-10 m thickness, ie from one atomic layer, thus has a tensile strength of 42 N. A tie for a space elevator from graphs with constant cross-sectional area would be of the height of the geostationary orbit 35,786 km will be charged only 87.3 % of its tensile strength.

Based on single-crystal graphite with a density of 2260 kg · m- 3 and a layer spacing of 3.35 × 10-10 m, is calculated by multiplying for graphs a basis weight of 7.57 × 10-7 kg · m -2. A square kilometers thus weighs 757 g

Graph has, in contrast to semiconductors, no band gap, according to the below reproduced color image. An artificial band gap in the graph, however, can be generated by " cuts " in the layer is a maximum of 10 nm wide so-called gate.

Measurements have shown that a single graphene layer attenuates the light by πα ≈ 2.3% ( with the fine structure constant α ), and indeed across the entire visible spectrum.

Pseudo - relativistic behavior

The electrical properties of graphene can be described well by a tight-binding model. Under this model, the energy of the electrons with wavenumber results (see wave vector ) to

With the nearest neighbor hopping energy and the lattice constant. Conduction and valence bands correspond to plus and minus signs in the above dispersion relation. They touch one another in the graph exactly six excellent points, the so-called K- points, of which, however, only two independent ( the other are the lattice symmetry of these two equivalent). In its vicinity, the energy depends linearly as in a relativistic particles (cf. Photon: ). Since the base is diatomic, the wave function even has a formal Spinorstruktur. This means that the electrons can be described at low energies by a relation which is equivalent to the Dirac equation, and additionally the so-called chiral limit, ie for vanishing rest mass, which results in some peculiarities:

Here, the Fermi velocity in the graph, which takes the place of the speed of light; denotes the Pauli matrices, the two-component wave function of the electrons and their energy.

Unusual quantum Hall effect

Because of the peculiarities in the dispersion in this material, the staircase structure of the integer quantum Hall plateaus, " shifted by 1/2" for all steps exactly, The two- Valley - structure ( formal " pseudo- spin " ) and the "real" spin degeneracy together result in an additional factor of 4 Remarkably, you can do this - in contrast to the conventional quantum Hall effect - even at room temperature observed.

Stiffness and temperature dependence

Graph is extremely stiff in the slice direction, because the sp2 bond between adjacent atoms on the strength of her with the sp3 bonding of diamond is comparable. Accordingly, it is generally expected - and that corresponds to the experiment - that interesting for the application properties of graphene are not only at the absolute zero temperature, ie remain valid at -273.15 ° C, but at room temperature.

One such property is the thermopower: a temperature gradient in the graphene planes causing an electric field strength due to a decoupling of the temperature of the electrons from the grid. Voltage in light of graphs had already been observed earlier, but as the cause of photovoltaics was suspected. That decoupling can be observed at room temperature, is due to the rigidity of the grid: the fundamental vibration excitation of the grid ( phonons) is so high in energy that the electrons rarely generate such a phonon.

Elastic behavior and pseudo- magnetic field

In July 2010 it was reported in an article in the American journal Science of extremely strong pseudo - magnetic fields. Due to elastic deformation tiny triangular bubbles were generated 4-10 nanometer size, in which the electrons as moving as if a 300 tesla strong magnetic field acting on them in graphs. It was found that the observed effect, in contrast to the effect of a real magnetic field, the actual spin of the electron is not affected, but that is instead influenced the just mentioned pseudo - spin, which with the existence of two different equivalent base atoms in the honeycomb structure related. This pseudo spin has a similar interaction with the magnetic field, such as pseudo real pins with true magnetic fields, since these " two-level system " to generate. The experiments the so- generated "pseudo quantum Hall effect" based on theoretical predictions that have been confirmed.

Spin currents

In April 2011, A. Geim and co-workers have published an article in which they describe strong spin currents and current-induced magnetism in the vicinity of the Dirac point, ie near the meeting point of the conduction band and valence band. This opens up the prospect of applications in spintronics.

Chemical functionalization

2013 has announced a major new project ( " flagship project" ) on Graphene, will collaborate on the research in many Member States, especially physicists and chemists the EU. One can, for example, double bonds of the graph - every other bond is one such - " break " and replace it with two single bonds, to which one then attaches various organic molecules: This way you can influence the properties of the system specifically, a possibility already is evaluated as a journalist on a large scale.

Generalizations

Generalizations are obvious. Some created by folding or rolling processes structures, such as the so-called carbon nanotubes (English, German, carbon nanotubes ' or ' carbon nanotubes ') and fullerenes, have already been mentioned. But closer to home is to first examine two-layer systems of graphs. They have interesting additional properties: they show semiconducting behavior similar to silicon, but having a band gap, which can be systematically altered by electric fields.

Graphane

With the help of atomic hydrogen, which is generated by an electric discharge in a hydrogen-argon mixture, graphene in graphane can be converted. In graphane each carbon atom is associated with a hydrogen atom, and the bond structure is similar to the chair-shaped cyclohexane. Graphane decomposes above 450 ° C in graphene and hydrogen. Graphane is an electrical insulator, in contrast to the graph.

Basic research and possible application

Because of the high electrical conductivity of graphene is currently being researched whether graphene could replace silicon as the transistor material. The first successes as the representation of a graphite microchips has already been achieved. With graph- based transistors clock rates in the range of 500 to 1000 GHz should be possible, while crossing with silicon-based clock rate of 5 GHz hardly. IBM succeeded in early 2010, for the first time to produce a 100 - GHz transistor on the basis of graphene. According to studies of Yanqing Wu and employees from April 2011 carbon with diamond structure seems to provide suitable substrates.

In the basic research graph serves as a model substance for two-dimensional crystals: It is difficult to get the system in the form of individual layers; only in 2004 could be obtained the first contactable "Graphene Flakes ".

• Possible use in supercapacitors and batteries
• As graphene is gas-tight and at the same time permeable to H ₂ O molecules. Thus, a use as a water filter, distiller and sealant mainly as a hermetic seal is ( even for dense helium ) are suitable.
• Possible use in photovoltaic solar cell as the third generation with an efficiency up to 60 %. However, this approach, according to new research appears not possible.

The European Commission has decided 2013 initiative within the European flagship to promote the study of graphs with 1 billion euros.