Researchers in Japan created some of the world's smallest semiconducting nanotubes, structures 100,000 times thinner than a human hair. By growing molybdenum disulfide inside protective tubes of boron nitride, researchers, including those from the University of Tokyo, produced highly uniform tubes just 1 nanometer wide, a scale at which it's difficult to make stable nanotube structures. The work confirms decades-old theoretical predictions about how these ultrafine materials behave and could also provide a new route toward miniaturized electronic devices.
A few years ago, carbon nanotubes were attracting a lot of press attention. But there's a new contender in the ring, and it offers some advantages over its carbon counterpart that could tempt engineers to design products around it. Molybdenum disulfide (MoS2) nanotubes, though still experimental in nature, point to applications in semiconductor electronics, high-resolution sensing and quantum-scale physics research.
"We achieved the synthesis of atomically precise semiconducting nanotubes with nanometer diameters. The coaxial structure, where a semiconducting MoS2 nanotube is surrounded by an insulating boron nitride (BN) nanotube, is attractive for gate-all-around transistors, one of the most advanced transistor architectures," said Associate Professor Yusuke Nakanishi from the Department of Advanced Materials Science at the University of Tokyo. "Our paper demonstrates a way for structural control of inorganic semiconducting nanotubes at the atomic scale. And we experimentally demonstrated that the bandgap (related to how materials work as semiconductors) of the nanotubes decreases as their diameters become smaller, in agreement with theoretical predictions proposed more than a quarter century ago."
Conventional methods to produce nanotubes are usually limited to diameters above 10 nanometers, multiwalled concentric tubes, and poorly controlled or irregular atomic structures. Nakanishi and his team synthesized 1-nanometer-wide, single-walled MoS2 nanotubes, with well-defined atomic structures. They managed this using chemical reactions inside the narrow space of BN nanotubes. The confined space constrains the MoS2 nanotubes, which would otherwise be difficult to form, and promotes well-defined atomic arrangements, essential for engineered applications.
"In nanotubes, even small structural differences can strongly affect their properties. If the structure can be precisely controlled, the properties are more consistent, which is essential for reliable and reproducible transistor performance. Their biggest advantage is atomic-level structural control," said Nakanishi. "Current silicon transistors are typically made by etching bulk silicon, but It's increasingly difficult to keep their structures perfect at smaller sizes, where defects have a big impact. Carbon nanotubes also face a challenge for transistor applications, since even tiny structural differences can change how they behave, including whether they act more like metals or semiconductors. Our nanotubes could offer a more reliable way to build ultrasmall semiconductor channels with consistent properties." Practical applications are likely still some years away, and important challenges remain before working transistor devices can be made."
In particular, the team wishes to increase the nanotube length from the current limit of several hundred nanometers to around 1 micrometer (which is 1,000 nanometers, and one-thousandth of a millimeter). Another future direction relates to materials: The method could also allow for other inorganic nanotubes, including magnetic and superconducting materials. The researchers hope the work will help expand nanotube science beyond carbon-based systems and open the door to a broader class of atomically accurate nanotube materials for research, sensing and smaller, faster devices.
