Graphene can become a semiconductor when formed into nanoribbons, meaning it has a sufficiently large band gap such that it can be turned on and off - and thus may become a key component of nanotransistors.
How well graphene nanoribbons perform depends on their width and on their edge structure. If the edge is zigzagged, they behave like metals; if ‘armchair shaped’, they become semiconductors.
Empa says this will be a major challenge for the production of nanoribbons: if they are cut from a layer of graphene or made by cutting carbon nanotubes, the edges may be irregular and may not exhibit the desired electrical properties.
Swiss laboratory Empa, in collaboration with the Max Planck Institute for Polymer Research in Mainz and the University of California at Berkeley, has succeeded in growing ribbons exactly nine atoms wide using precursor molecules. The ribbons also have a regular ‘armchair’ edge. After several process steps, they are combined like puzzle pieces on a gold base to form 1nm wide nanoribbons up to 50nm long with a precisely defined energy gap.
Initial tests showed the difference in current flow between the on and off states was too small; something to do with the silicon oxide dielectric layer. In order to have the desired properties, it needed to be 50nm, which influenced electron behaviour. However, by replacing this with a 1.5nm thick layer of hafnium oxide, the on current is said to be orders of magnitudes higher.
The team also determined that ribbons should be aligned exactly along the transistor channel, reducing the current high level of non-functioning nanotransistors.
