Researchers using the Advanced Light Source (ALS) at the US Department of Energy's Lawrence Berkeley National Laboratory have discovered subtle misalignments in the stacking of graphene sheets which disproportionately affect graphene's electronic properties.
Graphene has hitherto failed to live up to its enormous promise in electonic and photonic devices because monolayers of graphene have no bandgaps – meaning that it is impossible to control or modulate the electron current. Berkeley Lab researchers had engineered precisely controlled bandgaps in bilayer graphene by applying an external electric field. However, even when devices were made with these engineered bandgaps, the devices behaved strangely, as if conduction in those bandgaps had not been stopped completely.
Now, an ALS research team led by Aaron Bostwick has discovered that in the stacking of graphene monolayers, misalignments arise, creating an almost imperceptible twist in the final bilayer graphene. Tiny as it is – as small as 0.1 degree – this twist can change the bilayer graphene's electronic properties.
'The introduction of the twist generates a completely new electronic structure in the bilayer graphene that produces massive and massless Dirac fermions,' said Bostwick. 'The massless Dirac fermion branch produced by this new structure prevents bilayer graphene from becoming fully insulating even under a very strong electric field. This explains why bilayer graphene has not lived up to theoretical predictions in actual devices that were based on perfect or untwisted bilayer graphene.'
Massless Dirac fermions are electrons that essentially behave as if they were photons, and they are not subject to the same bandgap constraints as conventional electrons. The researchers believe that the twists that generate this massless Dirac fermion spectrum may be nearly inevitable in the making of bilayer graphene and can be introduced as a result of only 10 atomic misfits in a square micron of bilayer graphene.
The Advanced Light Source is a third-generation synchrotron light source producing light in the x-ray region. The team performed a series of angle-resolved photoemission spectroscopy (ARPES) experiments at the facility, resulting in a paper published in Nature Materials: 'Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene'.