Making nanotubes respond to rainbows

Hank Hogan, hank@hankhogan.com

Taking their cue from the eye, researchers have constructed a carbon nanotube that responds to all the colors of the rainbow. With some changes and more work, they could even have a nanometer-size photovoltaic device.

The eye detects light through a photon-induced isomerization of a molecule. In its work, the group from Sandia National Laboratories attached azobenzene chromophores to a single-walled nanotube, with the chromophores serving as photoabsorbers and the nanotube as an electronic readout.

Shown is a representation of chromophores attaching to a transistor made from a single carbon nanotube. The resulting device acts as a photodetector with a response similar to the chromophores’ spectral response.

In some ways, the approach improves upon nature – at least as far as potentially maximizing photoabsorption. As Sandia chemist Andrew L. Vance noted, “In the eye, the light actually has to pass through the sensor to get to the molecule being isomerized. We have the sensor behind the molecule.”

In the study, which was described in a Nano Letters paper in February, the investigators used 0.8- to 2.0-nm-diameter single-walled carbon nanotubes from CheapTubes Inc. of Brattleboro, Vt. They deposited these on silicon wafers that had an array of electron sources and drains. As a result, many of the source-drain gaps were bridged by a single nanotube, creating a field-effect transistor.

Sandia researcher Xinjian Zhou measures the electronic and optical properties of carbon nanotube devices in a probe station. The monitor shows the electrode layout on the device wafer; the nanotubes are positioned in the small horizontal gaps. Photo by Randy Wong. Photos courtesy of Sandia National Laboratories.

They then prepared chromophores and attached them to the nanotubes. When light of the right wavelength struck the chromophore, it changed the molecule’s structure and dipole moment. Because the nanotubes in the experiment were in field-effect transistors, the effect of that small molecular change was not insignificant.

“The transistor has an amplification effect, so we have a large signal response,” said postdoctoral researcher and lead author Xinjian Zhou.

Zhou also developed the instrumentation used in the research. He modified a Zeiss optical microscope, taking the light from a xenon arc lamp and feeding it through an Acton monochromator to produce a 3-nm-bandwidth source that could be focused down to a 35-μm spot. This enabled illumination of a single device with light anywhere in the visible spectrum.

By functionalizing the nanotubes with different chromophores, the researchers created photodetectors that responded to different parts of the spectrum. In their study, they used disperse red 1, disperse orange 3 and nitrophenyl azophenol. They obtained response curves from the functionalized nanotubes that correlated with the absorption spectra of the respective molecules.

The current devices work well enough to study fundamental properties of chromophore-nanotube hybrids. In the future, the group would like to improve the sensitivity of the device. This could be done by having multiple layers of chromophores attached to the nanotubes or by having more than one nanotube running between the source/drain on each transistor. Applications of these improved devices could include single-molecule detection.

It might even be feasible to create a photovoltaic device, which would require fabricating a p-n semiconductor junction. That would take more research, but such a device is a possibility. It could even have multiple configurations, Vance said. “Depending on how the chromophores are applied, you could use it as a transistor, or you could set it up as a p-n junction.”