Oxygen-activated Growth and Bandgap Tunability of Large Single-Crystal Bilayer Graphene
Fig. 2: BLG domain size and stacking order control*
[Reprinted by permission from Macmillan Publishers Ltd: Nature Nanotechnology ©2016]
Bernal (AB)-stacked bilayer graphene (BLG) is a semiconductor whose bandgap can be tuned by a transverse electric field, making it a unique material for a number of electronic and photonic devices. A scalable approach to synthesize high-quality BLG is therefore critical and requires minimal crystalline defects in both graphene layers and maximal area of Bernal stacking, which is necessary for bandgap tunability.
In a Letter* poublished in the latest issue of Nature Nanotechnology, an international group of scientists, including Prof. Philip Kim, demonstrates that Bernal-stacked BLG single crystals can be synthesized on copper (Cu) in an oxygen-activated chemical vapour deposition (CVD) process, half-millimetre in size. Besides the traditional 'surface-limited' growth mechanism for SLG (1st layer), the scientists discovered new microscopic steps governing the growth of the 2nd graphene layer below the 1st layer as the diffusion of carbon atoms through the Cu bulk after complete dehydrogenation of hydrocarbon molecules on the Cu surface, which does not occur in the absence of oxygen. They found that the efficient diffusion of the carbon atoms present at the interface between Cu and the 1st graphene layer further facilitates growth of large domains of the 2nd layer. The CVD BLG has superior electrical quality, with a device on/off ratio greater than 104, and a tunable bandgap up to ∼100 meV at a displacement field of 0.9 V nm−1.
* See Yufeng Hao, et al., "Oxygen-activated growth and bandgap tunability of large single-crystal bilayer graphene," Nature Nanotechnology 11, 426–431 (2016) doi:10.1038/nnano.2015.322.