"There are only so many transparent, electrical conductors available and they all have their limitations," said Andrew Rinzler of the University of Florida. "In the near- to mid-infrared the nanotube films appear to have the highest transparency for given electrical conductivity of anything available. In the visible spectrum the best laboratory-grown indium tin oxide (ITO) has better electrical conductivity for given transparency but evaluated against commercially available ITO the nanotube films compare pretty well."

To make the films, Rinzler and colleagues vacuum-filtered a suspension of nanotubes in surfactant onto a filtration membrane. Then they washed away the surfactant with purified water and dissolved the filtration membrane in a solvent. In this way, the team made transparent carbon nanotube films up to 10 cm in diameter. They were also able to transfer the films to various substrates.

"We needed a way to deposit nanotubes in a layer thin enough to be optically transparent while maintaining electrical contact throughout the layer," said Rinzler. "Years earlier, at Rice University, I devised a method to make freestanding films of pure nanotubes by depositing them on Teflon filters in a layer thick enough to peel up as free-standing films (buckypaper). These were electrically conducting because of the metallic nanotubes in the mixture. I realized that what we needed now was essentially an ultrathin buckypaper."

Rinzler says that he and graduate student Zhihong Chen concluded that if they used the right filter material they could dissolve it away after forming the nanotube layer on the filter. "That set us looking through the Millipore catalogue for the chemical compatibility of the available filters," added Rinzler. "Usually one looks at these to see what won't dissolve your filter. We settled on the mixed cellulose ester membrane because of its incompatibility with acetone and its high porosity, which we reasoned would give uniform films."

According to the scientists, the nanotube films have tremendous potential wherever transparent conductors are needed, especially in the near- to mid-infrared bands. "This includes light-emitting diodes (e.g. solid-state lighting), optical and infrared detectors, photovoltaics, touch screens, electrochromics...the list goes on," said Rinzler.

The team also used the films to make an optical nanotube-based field-effect transistor. This device, which incorporated two 150 nm thick transparent nanotube films and an ionic liquid, modulated light transmission according to its gate voltage. Such a modulator could have applications as a tunable heat shield.

"Others have previously demonstrated charging-induced transparency modulation for nanotubes (in electrochemical cells)," said Rinzler. "However, those earlier studies only showed modulation within the nanotube absorption bands associated with inter-band transitions. We have gone further into the infrared, demonstrating that transparency modulation is possible there as well due to field-induced changes in the free carrier density."

The scientists say their use of an ionic liquid rather than an electrolyte allowed them to demonstrate a device that need not be carefully shielded from ambient oxygen and that was only as thick as the substrate, the nanotube film and the low vapour pressure ionic liquid layer saturating the film.

Now, the researchers are exploring electronic transport across junctions between the nanotube films and other materials, and plan to use a magnetic nanotube alignment process to create ultra-thin aligned films. "The uniform, thin nanotube films also have potential applications as electrically addressable filtration membranes in their own right," said Rinzler. "[And] with catalyst loading the films should provide a wonderful electrically conducting cathode for fuel cells."

The researchers reported their work in Science.