Few photodetector materials can discern polarized light — individual electromagnetic waves oscillating parallel to one another — directly without the use of a grate or a filter. For a newly created carbon-based broadband photodetector, however, polarimetry is intrinsic to the active material. A team from Rice University and Sandia National Laboratories used carpets of aligned carbon nanotubes to create a solid-state electronic device that is hardwired to detect polarized light across a broad swath of the visible and infrared spectra. Such light is important for a number of applications, from remote sensing and communications to astronomy. "Our photodetector discerns polarized light intrinsically, much like the photoreceptors in the eyes of animals and insects that see polarized light," said François Léonard at Sandia National Lab, one of the lead researchers. The photodetector is the latest outcome of a collaboration between Rice and Sandia under Sandia's National Institute for Nano Engineering program, funded by the Department of Energy. In February, Rice professor Junichiro Kono, Léonard and colleagues described a new method for making photodetectors from carpets of carbon nanotubes. These schematic diagrams depict the process for fabricating p-n junction photodectors using flattened, highly aligned nanotube carpets. Courtesy of X. He/Rice University. These "carpets" are grown in the lab of Rice chemist Robert Hauge, who pioneered a process for growing densely packed nanotubes on flat surfaces. Xiaowei He, a graduate student in Kono's group, found a way to use Teflon film to flatten these tightly packed nanotubes so that they are aligned in the same direction. Each carpet contains dozens of varieties of nanotubes; about two-thirds of the varieties are semiconductors. Because each semiconducting variety interacts with a specific wavelength of light, Kono's team was able to show in its earlier work that the flattened, aligned carpets of nanotubes could serve as broad-spectrum photodetectors. In the new study, lead author He used dopants to alter the electrical properties of the nanotube carpets. He created two types of carpet, one with carriers that are positively charged (p-type) and another with carriers that are negatively charged (n-type), and overlapped them to create a fundamental building block of microelectronics, a p-n junction. The team observed a responsivity of up to 1 V/W in the devices, with a broadband spectral response spanning the visible to the mid-infrared — about 35 times larger than that of previous devices without p-n junctions. "Our work provides a new path for the realization of polarization-sensitive photodetectors that could be enabled on flexible or nonplanar surfaces," He said. Study co-authors include Hauge, Xuan Wang, Kankan Cong and Qijia Jiang, all of Rice; Léonard, Alexander Kane and John Goldsmith, all of Sandia; and Sébastien Nanot, formerly of Rice and now with the Institute of Photonic Sciences in Barcelona, Spain. The research appears in the American Chemical Society journal ACS Nano (doi: 10.1021/nn402679). For more information, visit: www.rice.edu