Terahertz Detectors with Carbon Nanotubes Designed to Improve MRI Technology and Other Image-Detection Applications

American and Japanese scientists are developing new terahertz detectors based on carbon nanotubes that could lead to significant enhancements in medical imaging, food inspection, airport passenger screening, and other applications.

The study was published in a letter online, on May 29, 2014, in the journal Nano Letters. It described a technique that uses carbon nanotubes to detect light in the terahertz frequency range without cooling. Historically, the terahertz frequency range—which falls between the more conventional ranges used for electronics on one end and optics on another—has presented great promise along with puzzling challenges for researchers, according to Sandia National Laboratories’ (Albuquerque, NM, USA) Dr. François Léonard, one of the authors. “The photonic energy in the terahertz range is much smaller than for visible light, and we simply don’t have a lot of materials to absorb that light efficiently and convert it into an electronic signal,” said Dr. Léonard. “So we need to look for other approaches.”

Researchers from Sandia National Laboratories, Rice University (Houston, TX, USA), and the Tokyo Institute of Technology (Japan) have developed a terahertz detector using several nanoscopic-sized tubes, creating a macroscopic thin film that contains a mix of metallic and semiconducting carbon nanotubes.

Researchers need to resolve this technical problem to exploit the many beneficial applications for terahertz radiation, said coauthor Dr. Junichiro Kono of Rice University. Terahertz waves, for instance, can easily penetrate fabric and other substances and could provide less intrusive ways for security screenings of people and cargo. Terahertz imaging could also be used in food inspection without adversely impacting food quality.

Possibly the most exciting application offered by terahertz technology, according to Dr. Kono, is as a potential replacement for magnetic resonance imaging (MRI) technology in screening for cancer and other diseases. “The potential improvements in size, ease, cost, and mobility of a terahertz-based detector are phenomenal,” he said. “With this technology, you could conceivably design a hand-held terahertz detection camera that images tumors in real-time, with pinpoint accuracy. And it could be done without the intimidating nature of MRI technology.”

Sandia, its collaborators and Dr. Léonard, particularly, have been studying carbon nanotubes and related nanomaterials for quite some time. In 2008, Léonard authored a study that looked at the research and theoretical aspects of carbon nanotube devices.

Carbon nanotubes are long, thin cylinders made up completely of carbon atoms. While their diameters are in the 1–10-nm range, they can be up to several centimeters-long. The carbon-carbon bond is very strong, so it resists any kind of deformation. Scientists have long been interested in the terahertz characteristic of carbon nanotubes, noted Dr. Léonard, but nearly all of the research to date has been theoretical or computer-model based. Several studies have explored terahertz sensing using carbon nanotubes, but those have focused principally on the use of a single or single bundle of nanotubes.

The difficulty, according to the investigators, is that terahertz radiation usually requires an antenna to achieve coupling into a single nanotube due to the relatively large size of terahertz waves. The Sandia, Rice University, and Tokyo Institute of Technology research team, however, found a way to create a small but visible-to-the-naked eye detector, developed by Rice researcher Robert Hauge and graduate student Xiaowei He, which uses carbon nanotube thin films without requiring an antenna.

The technique is therefore amenable to simple fabrication and represents one of the team’s most important achievements, according to Dr. Léonard. “Carbon nanotube thin films are extremely good absorbers of electromagnetic light,” he explained. In the terahertz range, it appears that thin films of these nanotubes will capture up all of the incoming terahertz radiation. Nanotube films have even been called “the blackest material” for their ability to absorb light effectively.

The researchers were able to combine several nanoscopic-sized tubes to create a macroscopic thin film that contains a mix of metallic and semiconducting carbon nanotubes. “Trying to do that with a different kind of material would be nearly impossible, since a semiconductor and a metal couldn’t coexist at the nanoscale at high density,” explained Dr. Kono. “But that's what we’ve achieved with the carbon nanotubes.”

The technique is significant because it combines the remarkable terahertz absorption characteristics of the metallic nanotubes and the unique electronic properties of the semiconducting carbon nanotubes. This allows researchers to achieve a photodetector that does not require power to operate, with performance comparable to existing technology.

The next step for researchers, according to Dr. Léonard, is to enhance the design, engineering and performance of the terahertz detector. For instance, they need to integrate an independent terahertz radiation source with the detector for applications that require a source, reported Dr. Léonard. The researchers also need to integrate electronics into the system and to additionally optimize properties of the carbon nanotube material. “We have some very clear ideas about how we can achieve these technical goals,” concluded Dr. Léonard, adding that new collaborations with industry or government agencies are welcome. Our technical accomplishments open up a new path for terahertz technology, and I am particularly proud of the multidisciplinary and collaborative nature of this work across three institutions.”

Related Links:
Sandia National Laboratories 
Rice University 
Tokyo Institute of Technology