Web Date: June 19, 2014
Carbon Nanotube Film Detects Terahertz Waves
Terahertz waves, the radiation that sits between microwaves and infrared light in the electromagnetic spectrum, could be used to spot bombs inside luggage or image patients’ internal organs. But sensors that work in this wavelength range have been limited to niche applications, such as analyzing light from the Big Bang, because the devices tend to be complex to operate. Now researchers report a terahertz sensor made from sheets of carbon nanotubes that works at room temperature and doesn’t require an external power source (Nano Lett. 2014, DOI: 10.1021/nl5012678).
One reason why it’s been difficult to make devices that detect terahertz frequencies (1012 Hz) is that researchers haven’t found materials that efficiently absorb these waves at room temperature. Some detectors must be cooled to temperatures as low as 0.1 K, says François Léonard, an engineer at Sandia National Laboratories.
But it’s been an intense area of research because the technology is so promising. For example, terahertz waves penetrate tissue and are absorbed by water, which suggests that terahertz imagers could provide medical information similar to magnetic resonance imaging. But unlike MRI machines, a terahertz scanner wouldn’t need bulky, high-power magnets. Instead, such a device would consist of a compact array of terahertz sensors—like the light-sensing chips inside cameras.
In recent years, scientists have explored carbon nanotubes as a terahertz-sensing material because they absorb light well and are electrically conductive. The trouble with nanotubes is that their size doesn’t pair well with terahertz wavelengths, which are on the order of hundreds of micrometers, Léonard says. Previous terahertz sensors based on these materials used one carbon nanotube or a small bundle of them coupled with an antenna to compress the waves so that they can interact with the detector. During this conversion process, the signal weakens, so “only a fraction of incoming light could be detected,” Léonard says.
Working with Junichiro Kono, an electrical engineer at Rice University, Léonard’s group made macroscopic sheets of carbon nanotubes to serve as terahertz detectors at room temperature. Because of the sheets’ relatively larger size, an antenna isn’t necessary, and the resulting detector absorbs terahertz radiation preferentially.
The team made a dense film of carbon nanotubes about 1 mm across, transferred it to flexible plastic, and deposited gold electrical contacts on either side. They doped half the film with a drop of benzyl viologen, making that half capable of conducting negative charges. The other half was not doped so it could conduct positive charges. This creates a so-called p-n junction where the doped and undoped halves meet and allows the film to generate a current. When terahertz waves hit the sheet, the p-n junction absorbs the photons, which heats up the film. Due to the thermoelectric effect, this heat generates a measurable current across the device.
The nanotube terahertz detector produces signals that are just 5% the strength of ones from existing detectors, Léonard says. However, he points out that those detectors have been optimized, and this is a prototype. He thinks they can improve their detectors by boosting the thermoelectric effect in the films. There is some evidence that this effect is stronger in disordered films of carbon nanotubes; this prototype used aligned ones.
Misha E. Portnoi, a physicist at the University of Exeter, in the U.K., says that up to now, scientists have focused their work on the theory behind carbon nanotube terahertz sensors. “The development of a powerless, compact, broadband, flexible, large-area detector that works at room temperature is a truly formidable achievement,” he says.
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