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Nanotube Films Suggest Alternative to ITO

Photonics Spectra
Nov 2004
Daniel S. Burgess

Transparent electrically conductive materials have found application in a variety of products, from solar cells and detectors to flat panel displays. Currently, indium tin oxide is the material of choice, but its brittleness has left researchers hoping for alternatives. A team at the University of Florida in Gainesville and at MTA SZFKI, the research institute for solid-state physics and optics at the Hungarian Academy of Sciences in Budapest, thinks it has found such an alternative in thin films of single-walled carbon nanotubes.

The films' transparency in the visible compares well to commercially produced ITO, said Andrew G. Rinzler of the university's department of physics. The nanotube films perform even better in the 2- to 5-µm spectral band. Most importantly, he said, they show no degradation in conductivity with mechanical deformation. This at least makes them suited for the development of flexible or foldable organic LED displays and of so-called electronic paper.

Nanotube Films Suggest Alternative to ITO
Thin films of single-walled carbon nanotubes may offer an alternative to indium tin oxide as a transparent electrical conductor for various applications. The flexible films show no degradation in conductivity with mechanical deformation. Courtesy of Andrew G. Rinzler.

To fabricate the films, the scientists employ a three-step process. First, they vacuum-filter a suspension of single-walled carbon nanotubes onto a high-porosity filtration membrane. Because the accumulation of nanotubes affects the local permeation rate, this step is self-regulating, yielding a homogeneous film. Second, they remove the surfactant with a purified water wash. Finally, they dissolve the filtration membrane in acetone, leaving only the thin films, which may be used freestanding or mounted on a substrate. Additional doping and heating steps may be used to tune the transmittance spectra of the films.

In a demonstration of the suitability of the thin films for application, the scientists constructed an electrically activated optical modulator from two 150-nm-thick films, saturated with an ionic liquid and deposited on sapphire. Measurements revealed a modulation in transmittance at 1676 nm between 44 and 92 percent with the application of gate potentials of ±1.8 V.

Rinzler predicted that the first commercial application of the transparent single-walled nanotube films would be with flexible organic LED displays, perhaps within the next couple of years. Other potential uses, he suggested, might include solid-state lighting, infrared emitters and detectors, and electrochromic devices, such as windows with variable transparency.


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