Synthesizing single-crystalline hexagonal boron nitride film that uniformly self-assembles

Researchers at South Korea’s Dongguk University (led by professor Ki Kang Kim) in collaboration with other institutes – the Korea Institute of Science and Technology (KIST) and Sungkyunkwan University – have developed a method for synthesizing hexagonal boron nitride (hBN) films with a nearly perfect single-crystalline structure (Joo Song Lee et al, ‘Wafer-scale single-crystal hexagonal boron nitride film via self-collimated grain formation’, Science; DOI: 10.1126/science.aau2132). This structure consistently self-assembles on top of liquid gold and can then be used as a platform for synthesizing other crystalline thin-film materials such as graphene.

Synthesizing perfect crystalline structures is very challenging, with most methods yielding crystals with defects or with multiple different basic units (polycrystals). Hexagonal boron nitride (also called white graphite) can be synthesized in the shape of crystalline films with the width of a single atom. These have an insulating effect that has found uses in various types of research. However, these films are polycrystalline and not single-crystalline. Therefore, a research team set out to find a method for synthesizing single-crystalline hBN films on a small scale.

The synthesis method they developed consists of letting the thin hBN film self-assemble on top of a liquid gold substrate. Because of the surface tension of liquid gold and the characteristics of its interaction with boron and nitrogen, circular hBN grains form automatically over time. These grains grow to a specific diameter and eventually form a lattice. 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.

The final product of this process is a nearly perfect single-crystalline hBN film, as demonstrated in many different experiments and via multiple measurements. A very promising application of such films is using them as a substrate for synthesizing other crystalline thin-film materials on top of them, such as graphene. Their applications go beyond that. “We demonstrated that our hBN films can serve as a protecting layer against metal oxidation and as a gas-diffusion barrier for water vapor transmission,” Kim says.

This innovative synthesis method could be exploited further as well. “Our strategy for the synthesis of single-crystalline hBN films opens a new horizon for the single-crystal growth of other diatomic 2D materials,” adds Kim. This would make many single-crystalline materials easier to fabricate, allowing them to naturally find a multitude of applications.

Tags: hBN Graphene

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