A photodetector that uses carbon nanotube films of varying widths can gather light in and beyond visible wavelengths, a finding that promises to make possible a unique set of optoelectronic devices, solar cells and maybe even specialized cameras. “There has been growing interest in the past few years in harnessing the properties of carbon nanomaterials, such as carbon nanotubes and graphene, for photodetector applications,” François Léonard of Sandia National Laboratories told Photonics Spectra. “Our photodetector is the first to use optically thick films of carbon nanotubes where the nanotubes are macroscopically long, aligned and dense, such that each nanotube in the film bridges both electrodes.” Previous work used single carbon nanotubes, two-dimensional sheets of nanotubes or single graphene layers. Although these devices have demonstrated the photodetection capability of such materials, they have small optical absorption because of the materials’ atomically thin nature, he said. “Other researchers have addressed this problem using disordered films of carbon nanotubes, but these are dominated by charge transport between nanotubes and thus lose the unique properties of charge transport entirely along individual nanotubes.” This illustration shows an array of parallel carbon nanotubes 300 µm long that are attached to electrodes and display unique qualities as a photodetector, according to researchers at Rice University and Sandia National Laboratories. But the detector developed by Léonard’s group and researchers at Rice University is based on extra-long 300-µm carbon nanotubes grown as a very thin “carpet” by the lab of Rice chemist Robert Hauge and pressed horizontally to be formed into thin sheets of hundreds of thousands of well-aligned tubes. With so many nanotubes, 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, Rice physicist Junichiro Kono said in a university release. “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. “We think that the polarization sensitivity is actually an advantage of these devices,” Léonard told Photonics Spectra. “Detectors that can simultaneously detect light and its polarization are of interest because they allow new information to be obtained when imaging a scene.” The investigators will continue to develop working principles through theory and modeling and plan to “test the photodetector response at longer wavelengths, including the terahertz,” he said. “In addition, the current nanotube thin film contains both metallic and semiconducting nanotubes, and we are working on approaches to eliminate the metallic nanotubes.” 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 US Department of Energy (DOE). The study – with funding from the DOE, Office of Science, National Institute for Nanoengineering, Lockheed-Martin Lancer Program, National Science Foundation, Robert A. Welsh Foundation and the Center for Engineering Education Development at Hokkaido University – appeared in Scientific Reports (doi: 10.1038/srep01335).