Although graphene sheets are difficult and expensive to produce, their use is on the increase, especially in nanoelectronics, electrochemistry and gas sensing. Graphene, a sheet of carbon that is only one atom thick, is the thinnest known and strongest material ever measured, is thought to be about 200 times stronger than steel, and has the ability to carry one million times more electricity than copper.
While there are various methods for fabricating graphene films, such as through epitaxial growth or self-assembly procedures where graphene oxide films are transferred to a substrate and reduced to graphene by chemical reaction or heating, the approach that was taken by the research team was that of chemical vapor decomposition. This involves the deposition of hydrocarbon molecules onto an iridium surface that is heated between room temperature and 1,000 degrees.
The scientists, from the University of Trieste, the Synchrotron light laboratory in Trieste and the University College London, whose study has been published in [Lacovig et al., Physical Review Letters, doi: 10.1103/PhysRevLett.103.166101], have shown how mechanisms of graphene growth have been found depending on the metal substrate, very different from those observed for two-dimensional metal islands on metals.
When these molecules hit the surface they lose their hydrogen atoms, leaving the remaining carbon atoms sticking to the iridium, where they start to self-assemble in small “nano-structures”. The nano-structures eventually develop into fully formed graphene sheets; the researchers are now starting to understand how the process takes place, and therefore how it might be controlled.
The study is concentrating on how the mechanism moves from a carbon-covered surface to the formation of a fully formed high-quality graphene sheet. As Alessandro Baraldi points out, “The growth of graphene starts with the formation of small islands of carbon with an unusual dome structure, in which only the atoms at the perimeter are bound to the iridium substrate while the central atoms detach from it, making the island bulge upwards at the centre.”
The team also found that the size of these geodesic carbon nanodomes depended on the temperature of the metallic substrate, and the manipulation procedure, suggesting a number of possible ways of controlling the size of graphene sheets at the nanoscale.