Boron nitride (BN) can be made into atomically thin 2D crystals due to its hexagonal structure similar to graphite. Previously, BN nanosheets have been prepared by exfoliation of bulk crystals and solid-state chemical reaction. Continuous BN films, with few atomic layers, have also been synthesized by chemical vapour deposition (CVD ) using nickel or copper as catalysts.

Now, mono- and few-atomic-layer BN sheets can be grown directly by CVD without using catalysts, which makes the large-scale production of BN nanosheets simple and eliminates the problem of contamination by any residual catalyst.

As well as being catalyst-free, the grown BN sheets are well separated and grow vertically. This wonderful growth process contrasts with common sense that suggests that the CVD process generates flat films on the substrate surface.

This work was carried out by researchers at Shenzhen Graduate School, Harbin Institute of Technology (SGS-HIT) and the Institute of Physics, Chinese Academy of Sciences.

Formation and growth

The BN nanosheets are synthesized in a microwave plasma CVD system from a gas mixture of BF3–H2–N2. Formation of the BN nanosheets is due to the strong etching reactivity of F on the BN phase. The increase in the number of atomic layers during growth is by nucleation on the surfaces of previously grown BN sheets and the increase in the layer size occurs due to the formation of covalent bonds between B and N atoms at the layer edges. Because the new nuclei are bonded with the previously grown BN sheets by van der Waals forces they are more easily etched than the covalently bonded edge atoms of the BN sheets. The result is the formation of 2D BN sheets.

Currently the research group is tailoring the properties of the material by doping the structure with carbon as well as investigating ways to precisely control the number of layers and shape of the BN sheets.

Additional information can be found in the journal Nanotechnology.