Synthesising perfect crystalline structures is very challenging, with most methods yielding polycrystals – crystals with defects or with multiple different basic units,” according to Dongguk University.
Liquid gold works because the surface tension of gold interacts with the boron and nitrogen, causing circular hBN grains form.
As these grains grow, they can rotate freely on the liquid, and tend to align when they meet other circular grains, eventually bonding together, then the gaps between fill in – see diagram, and this video.
“They can easily rotate when they are about to come into contact with another grain so as to assume the best possible orientation before joining the lattice,” said the university. “A very promising application of such films is using them as a substrate for synthesising other crystalline thin-film materials on top of them, such as graphene.”
Another use, according to Professor Ki Kang Kim, it to act as a protecting layer against metal oxidation and as a gas-diffusion barrier for water vapour transmission.
The work is described in ‘Wafer-scale single-crystal hexagonal boron nitride film via self-collimated grain formation‘, a paper in Science.
Synthesizing Single-Crystalline Hexagonal Boron Nitride (hBN) Film That Uniformly Self-Assembles
Dongguk University worked with Korea Institute of Science and Technology (KIST), Chonbuk National University, CINAP Institute for Basic Science, Sungkyunkwan University and Ewha Womans University.