Graphene - Online Article

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It can be viewed as an atomic-scale chicken wire made of carbon atoms and their bonds. The name comes from GRAPHITE + -ENE; graphite itself consists of many graphene sheets stacked together.

The carbon-carbon bond length in graphene is approximately 1.42 Å. Graphene is the basic structural element of all other graphitic materials including graphite, carbon nanotubes and fullerenes. It can also be considered as an infinitely large aromatic molecule, the limiting case of the family of flat polycyclic aromatic hydrocarbons called graphenes.

Description

Perfect graphenes consist exclusively of hexagonal cells; pentagonal and heptagonal cells constitute defects. If an isolated pentagonal cell is present, then the plane warps into a cone shape; insertion of 12 pentagons would create a fullerene. Likewise, insertion of an isolated heptagon causes the sheet to become saddle-shaped. Controlled addition of pentagons and heptagons would allow a wide variety of complex shapes to be made, for instance carbon NanoBuds. Single-walled carbon nanotubes may be considered to be graphene cylinders; some have a hemispherical graphene cap (that includes 6 pentagons) at each end.

The largest molecule of this type consists of 222 atoms and is 10 benzene rings across. The onset of graphene properties, as compared to those of a polycyclic aromatic hydrocarbon (PAH) are not known. PAHs of 60, 78, and 120 carbon atoms have UV absorbance spectra that show a discrete PAH electronic structure, but a PAH of 222 carbon atoms has Raman bands similar to those in graphite.

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Properties

  • No rest mass: The electrons in graphene behave like they have no rest mass at all. This makes them travel as fast as a million metres per second. In effect, this would allow manipulation of electrons as waves rather than as particles, much like photonic systems control light waves. We thus have a novel form of carbon that is an excellent conductor with very little resistance. And unlike carbon nanotubes, where consistent sizes and electron properties have been problematic, and also where electrical resistance is higher, graphene turns out to be the ace material.
  • 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.
  • Electronic properties: While electron transport in most condensed matter systems are accurately described by the non-relativistic Schrödinger equation, graphene is quite different from most conventional three-dimensional materials. Intrinsic graphene is a semi-metal or zero-gap semiconductor. 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.
  • Thermal properties: The near-room temperature thermal conductivity of 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-dominated. However, for a gated graphene strip, an applied gate bias causing a Fermi Energy shift much larger than kBT can cause the electronic contribution to increase and dominate over the phonon contribution at low temperatures.

Applications

  • Grid: It is the technology in which the speed of the internet is 10000 faster than the current internet speed by which we can download the movies in second and 1000 gamers play the online games.In this technology graphene is used.
  • GNRs: Graphene nanoribbons (GNRs) are essentially single layers of graphene that are cut in a particular pattern to give it certain electrical properties. Depending on how the un-bonded edges are configured, they can either be in a Z (zigzag) or Armchair configuration. According to recent theoretical simulations, zigzag GNRs are always metallic while armchairs can either be metallic or semiconducting, depending on their width, with the energy gap scaling with the inverse of the GNR width. Due to its high electronic quality, graphene has also attracted the interest of technologists who see them as a way of constructing ballistic transistors. Graphene exhibits a pronounced response to perpendicular external electric field allowing one to built FETs (field-effect transistors).
  • 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 utilized 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.

Advantages

  • Carbon material called graphene is fast replacing silicon (the material at the heart of all computer chips) in electronics. Graphene - a single layer of carbon atoms arranged in a honeycomb lattice, could allow electronics to process information and produce radio transmissions 10 times better than silicon-based devices. Usage of graphene produces faster and more powerful cell phones, computers as well as other electronics.
  • Graphene could also replace indium tin oxide as an electrode material in displays. Transparent conducting films are an essential part of many gadgets including common liquid crystal displays for computers, TVs and mobile phones. The underlying technology uses thin metal-oxide films based on indium. But indium is becoming an increasingly expensive commodity and, moreover, its supply is expected to be exhausted within just 10 years.
  • It believes the new material, which is light but stiff and tough, could be used to make fuselages for aircraft, as well as in electronics and potentially in paints and coatings.

Disadvantage

The switch from silicon to carbon had not been possible because technologists believed they needed graphene in the same form as the silicon used to make chips: a single crystal of material 8 or 12 inches wide. The hurdle in achieving these is that the largest single-crystal graphene sheets made to date have been no wider than a couple millimeters, not big enough for a single chip. However it has many advantage over the disadvantage and its disadvantage can be remove by simple technologies so it is very good for our future.

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