A nanotube-based photodetector that gathers light in and beyond visible wavelengths promises to make possible a unique set of optoelectronic devices, solar cells and maybe even specialized cameras. Traditional cameras are light detectors that capture a record, in chemicals, of what they see. Modern digital cameras replaced film with semiconductor-based detectors. But the detector developed by researchers at Rice University and Sandia National Laboratories is based on extra-long 300-µm carbon nanotubes. That boots the broadband detector into what Rice physicist Junichiro Kono considers a macroscopic device, easily attached to electrodes for testing. The nanotubes are grown as a very thin “carpet” by the lab of Rice chemist Robert Hauge and pressed horizontally to be formed into a thin sheet of hundreds of thousands of well-aligned tubes. This illustration shows an array of parallel carbon nanotubes 300 µm long, which are attached to electrodes and display unique qualities as a photodetector, according to researchers at Rice University and Sandia National Laboratories. Courtesy of Sandia National Laboratories. Although each nanotube is the same length, the nanotubes have different widths and are a mix of conductors and semiconductors, each of which is sensitive to different wavelengths of light, Kono said. "Earlier devices were either a single nanotube, which are sensitive to only limited wavelengths," he said, "or they were random networks of nanotubes that worked, but it was very difficult to understand why." "Our device combines the two techniques," said Sébastien Nanot, a former postdoctoral researcher in Kono's group and first author of the paper. "It's simple in the sense that each nanotube is connected to both electrodes, like in the single-nanotube experiments. But we have many nanotubes, which gives us the quality of a macroscopic device." With so many nanotubes of varying types, the array can detect light from the infrared to the ultraviolet and all of the visible wavelengths in between. Its ability to absorb light across the spectrum should make the detector of great interest for solar energy, and its IR capabilities may make it suitable for military imaging, Kono said. "In the visible range, there are many good detectors already," he said. "But in the IR, only low-temperature detectors exist, and they are not convenient for military purposes. Our detector works at room temperature and doesn't need to operate in a special vacuum." The detector is also sensitive to polarized light and absorbs light that hits it parallel to the nanotubes, but not if the device is turned 90 degrees. The work is the first successful outcome of a collaboration between Rice and Sandia under Sandia's National Institute for Nano Engineering program funded by the Department of Energy. François Léonard's group at Sandia developed a novel theoretical model that correctly and quantitatively explained all characteristics of the nanotube photodetector. "Understanding the fundamental principles that govern these photodetectors is important to optimize their design and performance," he said. The initial device, according to Léonard, merely demonstrates the potential for nanotube photodetectors. The investigators plan to build new configurations that extend their range to the terahertz, and they also plan to test their abilities as imaging devices. "There is potential here to make real and useful devices from this fundamental research," Kono said. The study appeared in Scientific Reports (doi: 10.1038/srep01335). For more information, visit: www.rice.edu