Shortly after their discovery, carbon nanotubes seemed to be a material wonder. There were metallic and semiconducting forms; they were tiny and incredibly light; and they could only be broken by tearing apart chemical bonds. The ideas for using them seemed endless.
But then the reality of working with them set in. It was hard to get a pure population of metallic or semiconducting forms. Synthesis techniques tended to produce a tangle of mostly short nanotubes; those that extended for more than a couple of centimeters remain rare. And while the metallic version offered little resistance to carrying electric current, it was hard to send many electrons down the nanotube.
Materials scientists, however, are a stubborn bunch, and they’re still trying to get them to work. Today’s issue of Science includes a paper describing the addition of a chemical to carbon nanotube bundles to boost their ability to carry current to levels closer to those of copper. While the more conductive nanotubes weren’t stable, the discovery may point the way toward something with a longer shelf life.
Doped nanotubes
Carbon nanotubes come in various forms. In the case of single-walled nanotubes, you can think of them as taking a sheet of graphene, rolling it up into a circle, and linking together the two opposite ends you just brought together. These can also be different diameters. There are also multi-walled carbon nanotubes, where a second nanotube (and maybe third, and maybe more beyond that) is wrapped around the first.
When metallic, these offer little resistance to electron flow along the nanotube. But, because most of their electrons are tied up in the chemical bonding needed to form the nanotube, there’s not a lot of them available to carry current. So, a lot of people have tried developing dopants—chemicals that can be mixed in small quantities that change the behavior of the bulk material. In this case, the goal was to find chemicals that would act as electron donors, adding to the amount of current that could potentially be sent down the nanotube.
Obviously, isolated nanotubes can’t really have dopants, since they’re pretty self-contained. But the team behind the new work, based in Spain, was working with bulk nanotube fibers, which are a mixture of nanotubes of various lengths bundled into a larger fiber, with most individual nanotubes oriented along the fiber’s long axis. In this case, the fiber was made from double-walled nanotubes, given its interior a pretty consistent structure.
You can think of the interior space of these fibers as a bit like what you’d get if you were packing spherical objects into a box. Even under the most efficient packing arrangement, there will be gaps between neighboring spheres. In the same way, these fibers have internal spaces that can allow additional chemicals to be incorporated inside the fiber.
The nanotube fibers themselves came from a commercial supplier. To dope these fibers, the researchers decided to use tetrachloroaluminate, or AlCl4–, a charged molecule that has electrons to spare. To get it into the spaces between the nanotubes, they used a vapor composed of aluminum trichloride plus a source of additional chlorine. This seeped into the fibers themselves and formed the charged tetrachloroaluminate in place.
Current carrying
A large chunk of the paper simply consists of imaging and spectroscopy that confirms the expected chemical is present in the spaces between the nanotubes. There was also a fair bit of modeling using Density functional theory to confirm that the resulting doping would be expected to make additional electrons available to carry current. Overall, they estimate that the resulting material has a chemical formula of C39AlCl4 and that the chemical changes occur without altering the fiber’s physical size.
The interesting results come when the researchers start looking into the material’s current-carrying capacity. Doping with the aluminum stuff boosted the mean conductivity by a factor of 10. That is about as high as any previously tested dopant achieved. The highest individual fiber they tested saw this rise to an over 15x improvement and is about 70 percent as conductive as aluminum (which makes it a bit less than half as good as copper).
However, a key feature of this is that the doping doesn’t add much mass to what’s a very light material to start with. So, normalized by density, the doped carbon nanotube fibers actually outperformed copper.
This may sound like an artificial standard, but it could actually matter in applications where space isn’t a concern, and/or where weight is. So, if you could tolerate the wiring being a bit over twice the thickness, then it should be an option to just use a nanotube fiber that’s thicker than the copper wire you’d otherwise need. Another application might be high-capacity transmission lines, where getting the same performance with a lower weight could save money on the support towers needed.
Relevant to this last application, the doping doesn’t alter the durability of the (very tough) carbon nanotube fibers. They have higher tensile strength than either copper or aluminum, and they are closer to steel.
Before you rush out to invest in carbon nanotube futures, however, there is a major issue: The tetrachloroaluminate isn’t stable under normal environmental conditions, as it will react with water molecules in the air. The researchers could extend its useful life by sealing the fibers in a polymer coating, but it still had a lifetime measured in weeks rather than the decades we would want to see.
That doesn’t mean this research is useless. It clearly demonstrates the potential of these materials if the price of carbon nanotube fibers could be brought down. It has identified the structural and chemical features of a highly effective dopant that boosts conductivity, which may ultimately allow us to identify a similar yet more stable chemical to replace it.
Science, 2026. DOI: 10.1126/science.aeb0673 (About DOIs).
John is Ars Technica's science editor. He has a Bachelor of Arts in Biochemistry from Columbia University, and a Ph.D. in Molecular and Cell Biology from the University of California, Berkeley. When physically separated from his keyboard, he tends to seek out a bicycle, or a scenic location for communing with his hiking boots.
