Graphene is an allotrope of carbon, 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 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 University of Manchester has proved graphene is the fastest semiconductor.
“We knew this material is exceptionally good, but we couldn’t put a number on it,” Professor Andre Geim, director of the University’s Centre for Mesoscience and Nanotechnology told Electronics Weekly.His team has proved mobility is above 200,000cm2/Vs at room temperature. “Greater than 100 times that of silicon, 30 times GaAs, and larger even than carbon nanotubes,” he said. “It is the only material where electrons at room temperature can move thousands of interatomic distances without scattering.”
Graphene is a chicken wire-like array of carbon atoms – effectively a single layer of graphite.
Manchester was behind the technique of isolating graphene by rubbing a lump of graphite to flake it off in sheets. “This technique will always remain the technique of choice for research and proof-of-concept,” said Geim. “Now people are finding ways of making graphene in bulk, but they are still not single layer.” Ideally one layer only, or two at the most, of graphene would be grown on a substrate. More than this and the astounding mobility does not appear.
“Graphene which is now grown epitaxially using SiC underneath is very thin, with perhaps five to seven layers: good enough for chemical sensors, but still not good enough for electronic circuits,” said Geim.
According to him, powdered graphene is available and looking useful. “Many groups are making powder and spinning it on to surfaces for interconnect,” he said. The resulting film is transparent and could be used in displays.
Emphasising that work is “very preliminary”, Geim said: “Its nearest equivalent is ITO [indium tin oxide] and graphene is a slight electrical improvement, but the biggest advantage is indium is too expensive. Powdered graphene is certainly less expensive, easier to produce, and spinning is easier than vacuum deposition.”
Geim believes graphene-based devices like chemical gas sensors, and THz sources and detectors, could begin to materialise within three to five years.
Manchester has been working with The Institute for Microelectronics Technology in Russia, The University of Nijmegen in the Netherlands and The Department of Physics at Michigan Technological University.
Graphene reveals yet more electronic complexity
Graphene research has discovered hidden interactions that will affect the way components are designed from the super-fast material.
Scientists from the Georgia Institute of Technology and the US National Institute of Standards and Technology (NIST) have determined how the orbits of electrons interact with magnetic fields applied to epitaxial graphene.
“Understanding such interference will be important for bi-layer graphene devices that have been proposed, and may be important for other lattice-matched substrates used to support graphene and graphene devices,” said Professor Phillip First of Georgia Tech.Findings include that energy states follow contours of constant electric potential, and that there are energy gaps within isolated patches on the surface.
“The regular pattern of energy gaps in the graphene surface creates regions where electron transport is not allowed. Electron waves would have to go around these regions, requiring new patterns of electron wave interference,” First explained. “By examining their distribution over the surface for different magnetic fields, we determined that the energy gap is due to a subtle interaction with the substrate, which consists of multilayer graphene grown on a silicon carbide wafer,” .In multi-layer epitaxial graphene, each layer’s symmetrical sub-lattice is rotated slightly with respect to the next.
Prior studies, said Georgia Tech, indicated that the rotations decouple the electronic properties of each graphene layer.
“Our findings hold the first indications of a small position-dependent interaction between the layers,” said First’s colleague David Miller.
Results came from a custom-built scanning-tunnelling microscope at NIST, scanning 100 square nanometre of graphene by taking spectroscopic data every 0.4nm.
According to First, the study raises a number of questions, including whether the new phenomenon can be controlled.
“This study is really a stepping stone in long path to understanding the subtleties of graphene’s interesting properties,” he said. “This material is different from anything we have worked with before in electronics.” Graphene reveals yet more electronic complexity..