"The biggest challenge was overcoming the general belief that mercury and other pure metals do not wet carbon nanotubes," researchers Konstantinos Giapis and Pat Collier told nanotechweb.org. "There exist over 100 refereed papers stating the latter, thus one must think against the grain in proposing that wetting is possible and then cover all bases in proving it beyond doubt."
Giapis, Collier and colleagues introduced mercury into open single-walled carbon nanotubes with an average diameter of 5 nm. To achieve this, they attached the nanotubes to gold-coated silicon atomic-force microscope tips. Next, electric-pulse etching against a fresh highly oriented pyrolytic graphite surface shortened the nanotubes and opened up their free ends. Finally, the team immersed the nanotubes in a 200 µm diameter droplet of mercury to a depth of 17 nm and applied an electric potential to the nanotube.
Increasing the voltage led to a sudden large rise in the nanotube probe's conductivity. The team believes this is the point at which mercury filled the nanotube. It's likely that applying an electric field lowers mercury's surface tension by inducing repulsion between similar electric charges at its surface.
According to the researchers, metal-filled nanotubes could have applications in nanolithography, nanoprobes and the production of continuous one-dimensional metal nanowires with unique electrical and magnetic properties.
"In addition, we have invented a method for pumping fluids controllably that could lead to nanofluidic devices," said Giapis. "We have demonstrated pumping mercury, but can also imagine being able to pump other liquids, including nonmetallic liquids, with this technique. We envision constructing nano-inkjet printers that will use metal inks to print text and circuitry with nanometre precision. These devices could be scaled up to operate in a massively parallel manner."
Now the researchers are starting work on driving other metals, such as gallium, into the nanotubes by using the technique at higher temperatures. "Our hope is that the liquid metal will freeze inside the nanotube and remain intact when the voltage is turned off," said Giapis.
The researchers reported their work in Science.