A band gap in semiconductor terminology is not the difference between two rock groups. Semiconductors – like the silicon of computer chips – are structured in bands of energy where electrons flow along the bands but may or may not be able to move between bands. Two such bands are the valence band (the highest band filled with electrons) and the conduction band (the lowest unfilled band to accept electrons). The band gap is the energy difference between these two bands, or put another way, it is the amount of energy needed for an electron to jump from the valence band to the conductor band. Materials with a large band gap are insulators. Semiconductors have very small band gaps that act like insulators at absolute zero, but allow electrons to jump at higher temperatures. Materials with no band gap are conductors, such as copper.
This seems like esoteric stuff, but it’s the band gap that makes a semiconductor the key material for modern electronics. Graphene, nanoscale sheets of carbon one atom thick, has no band gap – making it effectively a conductive material and not useful for electronics. However, it’s possible to manufacture band gaps in graphene by cutting it into a matrix of ribbons (with gaps between ribbons). Unfortunately, this arrangement in graphene is difficult to manufacture and not very efficient. That’s where new work by Yu Huang, a professor of materials science and engineering at the UCLA, and her team may have found a better approach.
They created a new graphene nanostructure called a graphene nanomesh by taking one or more layers of graphene and punching a high-density array of nanoscale holes. The distance between the holes determines the properties of the band gap created – and importantly, can be manufactured to a high degree of precision. The graphene nanomesh can handle far more electrical energy than standard graphene sheets, while retaining about the same on-off ratio (the ratio of current in the on or off position). Control of the distance between holes of the mesh also controls this on-off ratio.
Graphene in the nanomesh configuration, produced by new techniques that create large sheets of graphene, make graphene a very viable candidate to replace traditional (and more expensive) silicon semiconductor material. Graphene also has electrical properties that improve its value for electronics.
“The concept of the GNM therefore points to a clear pathway towards practical application of graphene as a semiconductor material for future electronics. The unique structural and electronic characteristics of the GNMs may also open up exciting opportunities in highly sensitive biosensors and a new generation of spintronics, from magnetic sensing to storage,” said Huang.
[Source: Nanotechnology Today]