Dec 18, 2013
Hexagonal boron nitride monolayers could make perfect coatings
Monolayer hexagonal boron nitride (h-BN), a single atomic layer consisting of sp2 hybridized boron and nitrogen arranged in a honeycomb lattice, is often referred to as “white graphene” because it has a similar structure to the carbon material. Compared to graphene, however, h-BN boasts a significantly higher thermal and chemical stability, is insulating and completely transparent to visible light. These properties mean that monolayer h-BN is promising as a unique coating material. Fabricating large area, high-quality monolayer h-BN remains a challenge though and its performance as a coating material has never been demonstrated experimentally. Characterizing the optical properties of h-BN is also difficult.
Researchers at the Nanjing University of Aeronautics and Astronautics in China have now successfully grown high-quality h-BN monolayer over large areas using chemical vapour deposition on copper foil with ammonia borane as the source in a two-heat zone system under low pressure. The team showed that monolayer h-BN can serve as a perfect coating layer to significantly improve the friction, oxidation, and electric resistance of surfaces. The researchers also found that the exceptional low friction and insulating properties of monolayer h-BN can help in the characterization of its optical properties using atomic force microscopy technqiues.
As shown in the figure, the friction of a copper surface coated with monolayer h-BN can be reduced by tens of times even at pressures of up to 46 MPa. The separated h-BN domains can be distinguished from the bare surface of the Cu substrate in both the friction and current imaging modes of an atomic force microscope thanks to the significant reduction in friction and contact current by the monolayer h-BN coating. Even grain boundaries of the h-BN monolayer grown on copper films can be distinguished in the images obtained. The h-BN monolayer coated Cu surface also appears to be resistant to oxidation at elevated temperatures.
These results provide a comprehensive understanding of monolayer h-BN and highlight its outstanding properties as a single atomic layer coating material that has remarkable thermal and chemical stability, and intrinsic wide spectrum transparency.
The researchers presented their results in the journal Nanotechnology (in press).
About the author
Xuemei Li is a PhD candidate at the Nanjing University of Aeronautics and Astronautics, China, under the supervision of Prof. Wanlin Guo, whose research focuses on intelligent nanomaterials and devices.