Scientists use a surfactant to help make “inert

image: Chemical vapor deposition of molybdenum disulfide on a well-insulated boron nitride nanotube creates a structure of coaxial nanotubes.
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Credit: Tokyo Metropolitan University

Tokyo, Japan – Researchers at Tokyo Metropolitan University have used a surfactant to disperse insulating boron nitride nanotubes and coat them onto surfaces without clumping them together. The team demonstrated that heat treatment could remove surfactant to reveal clean nanoscale patterns; chemical vapor deposition could then form coaxial nanotubes on the jig using a range of materials. The ability to embed nanotubes on “inert” insulating structures gives scientists unprecedented access to the properties of new nanotube materials.

Breakthroughs in nanotechnology have made nanotubes and nanosheets easier to find for materials scientists. But studying them in isolation is far from easy. Because they are often grouped or aggregated, it is difficult to target the exotic optical and electronic properties that arise from their reduced dimensionality.

Recent work has shown that nanotube materials can be grown on the surface of a carbon nanotube, providing well-separated structures that could potentially be characterized. But carbon nanotubes have conductive properties and strongly absorb light, which makes it difficult to distinguish the electrical and optical properties of the coated material from those of the original nanotube.

Now, a team led by Assistant Professor Yusuke Nakanishi, Assistant Professor Yohei Yomogida and Associate Professor Yasumitsu Miyata of Tokyo Metropolitan University has used insulating boron nitride (BN) nanotubes instead as models for growth. of nanotubes. No small feat: boron nitride nanotubes are notoriously sticky. Although they can be dispersed with a surfactant that helps keep the tubes separate, it was unclear if the surfactant could be removed to reveal a clean template. Now the team has managed to find a surfactant that does not stick to the tubes; they also perfected a vacuum heat treatment that leaves clean, well-insulated insulating nanotube models.

Using chemical vapor deposition, a range of materials could be coated onto the jigs. The new tube wraps around the original BN tubes, forming something like a nanoscale coaxial cable. Importantly, because BN is an insulating material, the electrical properties of all coated materials can be studied to an unprecedented depth. This includes a property known as chirality, the “sensitivity” in the structure of atoms in the nanotube that gives rise to a whole range of exotic electronic properties.

In principle, the team believe their “nano test tubes” can be used to model the growth of a whole range of different materials. They have already been successful with molybdenum disulfide and carbon, with room for many more. Add to that the optical and electrical inertia of their BN model, and their new platform promises not only the discovery of materials, but also unfettered access to their exotic physico-chemical properties.

This work was supported by JSPS KAKENHI Grant Numbers JP19H02543, JP19K15392, JP20H00220, JP20H02572, JP20H02573, JP20H02605, JP20KK0114, JP21H05232, JP21H05234, JP22H00280, JP22H00283, JP22H01911, JP22K04886, JP22H04957, JP22H05468, and JP22H05469, JST CREST Grant Numbers JPMJCR17I5, JPMJCR20B1, and JPMJCR20B5, and JST FOREST program grant number JPMJFR213X.

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