Graphene has long been touted as the answer to all of the world’s problems, with particular emphasis on more efficient electronics. While it shows great promise in this area, creating this two-dimensional material in the correct dimensions for such applications is proving to be a real challenge. Physical cutting of graphite blocks using fine diamond blades has been possible, but the isolation of individual layers of graphene in long, wide sections known as nanoribbons requires further isolation.
Zoomed-in image of the graphene nanoribbons. (Arnold Research Group and Guisinger Research Group)
Graphene can be manufactured in layers using surface-assisted thin film synthesis methods that result from the reaction of two pre-cursor chemicals however this can currently only be achieved on metal substrates. In electronics, the materials that require the use of graphene nanoribbons are semiconductor materials such as geranium. In order for the graphene nanoribbons to replace the silicon that is currently used, it must be very smooth, with specific width, and smooth edges. This seemed impossible to achieve until recently.
Engineers at the University of Wisconsin – Madison have found a way to deposit perfectly proportioned graphene nanotubes directly onto a semiconductor wafer of germanium. They achieved this through a chemical vapour deposition (CVD) method, which is similar in some ways to surface assisted synthesis in that precursor chemicals react to produce a thin film, however the reactants are introduced to the reaction chamber in a gaseous state. In this particular method of chemical vapour deposition however, only one reactant is necessary – methane. When methane is introduced to the reaction chamber, under the correct temperature and pressure the methane sticks to the surface of the germanium wafer. Here it decomposes into other organic molecules, which further react with one another to produce a layer of graphene.
While this method of CVD has long been used to grow sheets of graphene, the team behind this discovery noticed that if the amount of methane present in the reaction chamber was reduced, long nanoribbons of graphene with the desired length and required smooth edges formed, perfect for their desired use in electronics.
The limitations at the moment are that patches of graphene are growing in one of two orientations. Further work will be required to reconcile these directional difficulties, but this discovery takes us one step closer to replacing silicon in computer chips with energy efficient graphene.
