Scientists have long been intrigued by what happens to a metal, like gold, when it is shrunk down to a thin sheet (a two-dimensional system) or a very narrow wire (a one-dimensional system). Unfortunately, while it is possible to theorize about what such low-dimensional metals would look like, actually making them so they are stable for long enough to be measured is a major challenge for materials scientists.

Now, Ryo Kitaura, Hisanori Shinohara and colleagues from Nagoya University in Japan1 are taking advantage of the tiny cavities inside carbon nanotubes to mold metal atoms into the ultimate thin wire. Unlike the common method of making atomically thin wires by pulling apart a wire until it is necked by just a few atoms, this method produces wires that are chemically and structurally stable for months.

“Usually, fabrication of ultrathin metal wire is very difficult,” says Kitaura. “So our work paves the way to elucidate fundamental properties of ultrathin nanowires.”

Their method is simple: they place carbon nanotubes and powders of a metal, such as europium, into a sealed glass tube. They then heat the tube for about one day at 500–600 °C. This heat treatment, called ‘nanofilling’, causes the europium to vaporize and fill the carbon nanotubes.

Fig. 1: Europium atoms (dark spheres in this transmission electron micrograph) fill the narrow inner region of a carbon nanotube to make an atomically thin wire.© 2009 R. Kituara

With a high-resolution transmission electron microscope, the researchers could see individual europium atoms lined up inside the carbon nanotubes, like a strand of beads (Fig. 1). The europium wires are long — on the order a micrometer — but just one atom wide, which means they are truly one-dimensional systems.

By choosing carbon nanotubes with different inner diameters, the group can make wires from one to four atomic strands thick. The carbon tube also acts like a protective shield that provides structural stability and keeps the europium, a highly reactive material, from oxidizing.

“Our method can be applied to any metal that boils at low temperature,” says Kitaura. “In fact, we have already fabricated ytterbium, samarium, potassium, calcium and strontium nanowires inside carbon nanotubes.” The team plans to use this synthesis technique to study the fundamental properties of metals in one-dimensional form.