Researchers have achieved a significant breakthrough in materials science by creating stable metallic nanotubes from niobium disulfide—a feat long considered elusive. The unexpected key ingredient? Ordinary table salt. This discovery, published in ACS Nano, opens doors to faster electronics, superconductor wires, and potentially even future quantum computers.
Nanotubes are minuscule cylinders of rolled-up atoms, thousands of which could fit across a human hair. Their unique size and structure give them extraordinary properties compared to traditional bulk materials. They can be stronger than steel but lighter than plastic, conduct electricity efficiently with minimal resistance, transfer heat effectively, and even exhibit unusual quantum effects.
These characteristics have made nanotubes highly sought-after building blocks for advanced technologies. However, previous efforts primarily focused on creating nanotubes from carbon (semiconductor or semimetal) and boron nitride (insulating). Crafting metallic nanotubes, which behave differently at the atomic level, proved significantly challenging.
Metallic nanotubes hold immense promise due to their potential to exhibit superconductivity – allowing electricity to flow with zero resistance – and magnetism. “These shells can, in principle, show phenomena like superconductivity and magnetism, which are impossible in insulating or semiconducting versions,” explains Slava V. Rotkin, professor of engineering science and mechanics and a lead researcher at Penn State. “Previous attempts with carbon nanotubes didn’t achieve these properties due to insufficient electron density.”
The team focused on niobium disulfide, a metal known for its superconductivity in bulk form. They successfully coaxed this metal into incredibly thin tubes – billionths of a meter wide – by wrapping it around templates made from carbon and boron nitride nanotubes.
This shaping process proved the key breakthrough: normally, niobium disulfide prefers to form flat sheets.
The unexpected solution? A minuscule addition of table salt at a precise point in the growth process. “In a sense, it was like alchemy,” Rotkin says. “You add this tiny ingredient and suddenly the reaction changes. Without salt, the niobium disulfide grows flat; with it, it envelops the nanotube and forms the shells we need.”
Further surprises emerged during observation. Instead of primarily forming single-layer tubes, these nanotubes favored a double-shell structure – akin to nested straws.
Rotkin postulates that this unusual formation is driven by electrical activity between the layers. “With two layers, electrons can hop from one to the other,” he explains, “acting like an atomic-sized capacitor that stabilizes the entire structure.” Computational models support this theory.
This unique rolled shape also addresses a persistent challenge in working with flat 2D materials. To create nanowires from these sheets, scientists typically employ lithography – akin to etching patterns on silicon chips. However, at such minuscule scales, carving leaves jagged edges that disrupt the material’s properties.
“If you roll it up,” Rotkin notes, “you have a shell with no dangling bonds. The diameter of the shell tells you exactly what the behavior will be. Nanotubes are much less random than nanowires cut from two-dimensional sheets.” This precision could make metallic nanotubes invaluable for applications requiring reliable performance at the nanoscale.
While research is still in its early stages, this proof-of-concept offers a glimpse into exciting possibilities. “These are early results,” Rotkin states, “but they show we can grow metallic nanotubes and begin to understand their stability. From here, we can start thinking about how to integrate them into technologies.”
The project underscores the power of international collaboration. “This is not work that can be done in isolation,” Rotkin emphasizes. “It takes a team with diverse expertise, and I was fortunate to be part of such a team.”
Future research could pave the way for superconductor wires enabling faster electronics, as well as exploring applications in quantum computing – technologies reliant on harnessing the unique properties of materials at the nanoscale
