Two-dimensional materials have garnered much interest since the successful isolation of graphene — atomic layers of carbon exhibiting novel electronic properties. Boron nitride (BN) is a structural analog of carbon, having cubic and layered structures, and BN nanosheets (BNNSs) have recently come under the spotlight for potential use in a range of applications. Previously, however, BNNSs have not been produced in quantities sufficient for practical use. Now, Chunyi Zhi and colleagues1 in Japan have demonstrated a ‘sonication–centrifugation’ method that allows BNNSs to be produced in large quanitites.

Fig. 1: Schematic diagram of boron nitride nanosheets, showing the atomic structure (top) and a three-layered BNNS (bottom).

The researchers prepared milligram quantities of BNNSs by a two-step process that involved exfoliation of BN powder into nanosheets by sonication (agitation using sound energy) in a strong polar solvent (dimethyl formamide), followed by centrifugation to isolate the BNNSs. Zhi and his team examined the structure of the resulting nanosheets by scanning and transmission electron microscopy, and found that the average nanosheet thickness depended on the speed of centrifugation — the faster the speed, the thinner the sheets. The BNNSs obtained by centrifugation at 8,000 r.p.m. were 3 nm thick (<10 atomic layers), whereas those obtained at 5,000 r.p.m. were up to 7 nm thick (Fig. 1).

The researchers then demonstrated the utility of BNNSs for improving the mechanical properties of a polymer composite. When added to polymethylmethacrylate (PMMA), even at low concentrations (0.3 wt%), the BNNSs significantly improved the mechanical properties of the PMMA–BNNS composite, with little effect on the inherent transparency of the PMMA at wavelengths greater than 600 nm — an important property because PMMA is used as an organic glass. The PMMA’s propensity to expand on heating was also reduced by more than half at temperatures above the glass transition, indicating that the nanosheets restrict the mobility of the polymer chains. The composite was 11% stronger and 22% less likely to deform under stress compared with pure PMMA. The researchers suggest that the nanosheets in the composite bear much of the mechanical load.

Zhi says that the BN nanosheets could find applications in optoelectronic devices and heat-releasing composite materials. They now plan to investigate highly thermoconductive insulating composites using BNNSs as fillers. They are also improving the dispersibility of the nanosheets with polymers, which may require the nanosheets to be chemically functionalized, and investigating how to make very thin, single-atomic-layer sheets.