Another story that appeared in the June issue of Materials Today journal….. Originals can be accessed for free here: http://www.sciencedirect.com/science/journal/13697021/16/6
An international team of scientists, led by the University of Manchester, has tuned the electronic properties of graphene by combining it with boron nitride; a discovery that they believe will herald a new generation of novel electronic devices.
Since its isolation in 2004, the race to explore the properties and promises of graphene has been top of the materials research agenda. Now, a team led by Roman Gorbachev at Manchester have demonstrated that by using the wonder material, it is possible to build materials with specific and tuneable electronic properties. In doing so, the team has also observed the elusive Hofstadter butterfly, a rare fractal pattern predicted by quantum theory, but one which has eluded experimentalists for four decades. The work appears in Nature, alongside a paper from Columbia University, who have also observed the rare butterfly pattern in two layers of graphene.
Graphene is a one-atom-thick layer of carbon whose remarkably high electrical conductivity is due to the behaviour of its electrons. Intrinsic graphene is a zero-gap semiconductor, meaning that its bands appear as cones which meet at a single point, and its electrons behave relativistically and are called Dirac fermions.
To alter the electronic properties of graphene, the Manchester team sandwiched a single sheet of the material between two larger crystals of boron nitride (BN). By aligning each of the material layers and then twisting the stack, an interference pattern (called a moiré pattern) of electrons was produced. The final superlattice was found to have a profoundly different electronic spectrum than for either graphene or BN alone, confirming that it is possible to control the electronic properties of graphene by aligning it to other materials.
The team then applied a large magnetic field (17 T) to their superlattice and measured its energy spectrum. The graphene’s fermions were found to have rearranged themselves, aligning to produce the complex pattern known as the Hofstadter butterfly. Douglas Hofstadter produced his fractal model in 1976, to describe the behaviour of electrons in a magnetic field, but this is the first time it has been witnessed experimentally.
The discovery has not only confirmed a 40-year-old prediction, but has fundamentally improved understanding of the physics of low-dimensional material systems. Gorbachev and his team also believe that the ability to tune graphene’s properties will herald new generation of electronic and optoelectronic devices.
Nature (2013) doi:10.1038/nature12187