Better than graphene

When asked why he chose hexagonal boron nitride (hBN) for his latest project instead of graphene that he worked with before, PhD candidate Santiago Cartamil-Bueno revealed that graphene, the single-atomic layer of carbon, can only survive in shielded environments because of its organic nature. Expose it to ultraviolet radiation and it will get damaged. Put in in contact with air and it will either oxidise or it will bind floating hydrocarbons. Cleaning graphene is also difficult because it will disintegrate at over 450 degrees Celsius or be damaged by aggressive chemicals. Despite the hype, graphene seems too fragile for real-world applications.

Enter boron nitride, an artificial material that has the same hexagonal form as graphene, but with completely different properties. Whereas graphene is black, boron nitride is white. Unlike graphene, it does not conduct electricity, it is chemically inert, and heat resistant. Alas, it is also hard to make (Ruizhi Wang and Professor Stephan Hofmann from Cambridge University produced the material) and it is difficult to handle because separate sheets stick to each other and, once separated, the material has become optically invisible.

Freezing membrane

Nonetheless, Cartamil-Bueno and his colleagues from the Faculty of Applied Sciences succeeded in covering five-micron-sized silicon wells with a single layer of boron nitride and recorded the resonance frequency (about 13 MHz) over a large range of temperature from room temperature down to almost the absolute zero (minus 273 degrees Celsius). They published their results in Nature Partner Journals last month.

Just like graphene, a boron nitride sheet expands with decreasing temperatures because of its negative temperature coefficient. Therefore, one would expect the resonance frequency to decrease with temperature as the membrane gets sloppier. Instead, measurements show a stiffening (higher resonance frequency) as the mercury drops. The authors explain this by postulating that this behaviour is governed by polymer remains (‘glue’). And indeed, after intensive cleaning of the boron-nitride (at high temperatures in the presence of ozone) the stiffening disappears.

Dozens of new single-layer materials

Electron microscopy picture of the covered 5-micron well (Photo: TU Delft)

But shouldn’t the resonance frequency go down with temperature as the membrane expands? It should, says Cartamil-Bueno, but it doesn’t. Instead, the expanding material clings to the inside of the silicon well, keeping the membrane tension equal. Thus the resonance frequency remains constant over the entire temperature range.

So can we expect micron-sized barometers in our smartphones any time soon? That may take some time, says Cartamil-Bueno smiling. “If you’d use boron nitride, you would have to add a metallic layer to make capacitive sensing possible. Besides, boron nitride sheets are still difficult to manufacture. The good news is that, since the discovery of graphene, dozens of new materials have been made that exist in single-atom layers have been made.”

The multiplicity in single-layer materials may be a blessing, but it also complicates matters considerably.