Nanotube photodetector built by Rice University & Sandia Labs

This illustration shows an array of parallel carbon nanotubes 300 micrometers long that are attached to electrodes and display unique qualities as a photodetector, according to researchers at Rice University and Sandia National Laboratories. Credit: Sandia National Laboratories

Read more at: http://phys.org/news/2013-02-nanotube-photodetector-built.html#jCp
http://phys.org/news/2013-02-nanotube-photodetector-built.html#jCp
Researchers at Rice University and Sandia National Laboratories have made a nanotube-based photodetector that gathers light in and beyond visible wavelengths. It promises to make possible a unique set of optoelectronic devices, solar cells and perhaps even specialized cameras.

A traditional camera is a light detector that captures a record, in chemicals, of what it sees. Modern digital cameras replaced film with semiconductor-based detectors. But the Rice detector, the focus of a paper that appeared today in the online Nature journal Scientific Reports, is based on extra-long carbon nanotubes. At 300 micrometers, the nanotubes are still only about 100th of an inch long, but each tube is thousands of times longer than it is wide. 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 turn them into a thin sheet of hundreds of thousands of well-aligned tubes. They're all the same length, Kono said, but the nanotubes have different widths and are a mix of conductors and semiconductors, each of which is sensitive to different wavelengths of light. "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 so many types, the array can detect light from the infrared (IR) to the ultraviolet, and all the visible wavelengths in between. That it can 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 applications, 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," Léonard said. Kono expects many more papers to spring from the collaboration. The initial device, according to Léonard, merely demonstrates the potential for nanotube photodetectors. They plan to build new configurations that extend their range to the terahertz and to test their abilities as imaging devices. "There is potential here to make real and useful devices from this fundamental research," Kono said. More information: www.nature.com/srep/2013/130226/srep01335/full/srep01335.html

Rice University demonstrates electrified graphene used as a shutter for light

My Note: The abstract relating to this article was posted earlier this week. This provides a better understanding for those of us in the lay community.http://www.nanowerk.com/news/newsid=25608.php (Nanowerk News) An applied electric voltage can prompt a centimeter-square slice of graphene to change and control the transmission of electromagnetic radiation with wavelengths from the terahertz to the midinfrared.

The experiment at Rice University advances the science of manipulating particular wavelengths of light in ways that could be useful in advanced electronics and optoelectronic sensing devices.

Experiments at Rice University showed that voltage applied to a sheet of graphene on a silicon-based substrate can turn it into a shutter for both terahertz and infrared wavelengths of light. Changing the voltage alters the Fermi energy (Ef) of the graphene, which controls the transmission or absorption of the beam. The Fermi energy divides the conduction band (CB), which contains electrons that absorb the waves, and the valance band (VB), which contains the holes to which the electrons flow. (Graphic by Lei Ren/Rice University)

In previous work, the Rice lab of physicist Junichiro Kono found a way to use arrays of carbon nanotubes as a near-perfect terahertz polarizer. This time, the team led by Kono is working on an even more basic level; the researchers are wiring a sheet of graphene – the one-atom-thick form of carbon – to apply an electric voltage and thus manipulate what’s known as Fermi energy. That, in turn, lets the graphene serve as a sieve or a shutter for light.

The discovery by Kono and his colleagues at Rice and the Institute of Laser Engineering at Osaka University was reported online this month in the American Chemical Society journal Nano Letters ("Terahertz and Infrared Spectroscopy of Gated Large-Area Graphene").

In graphene, “electrons move like photons, or light. It’s the fastest material for moving electrons at room temperature,” said Kono, a professor of electrical and computer engineering and of physics and astronomy. He noted many groups have investigated the exotic electrical properties of graphene at zero- or low frequencies.

“There have been theoretical predictions about the unusual terahertz and midinfrared properties of electrons in graphene in the literature, but almost nothing had been done in this range experimentally,” Kono said.

Key to the new work, he said, are the words “large area” and “gated.”

“Large because infrared and terahertz have long wavelengths and are difficult to focus on a small area,” Kono said. “Gated simply means we attached electrodes, and by applying a voltage between the electrodes and (silicon) substrate, we can tune the Fermi energy.”

Fermi energy is the energy of the highest occupied quantum state of electrons within a material. In other words, it defines a line that separates quantum states that are occupied by electrons from the empty states. “Depending on the value of the Fermi energy, graphene can be either p-type (positive) or n-type (negative),” he said.

Making fine measurements required what is considered in the nano world to be a very large sheet of graphene, even though it was a little smaller than a postage stamp. The square centimeter of atom-thick carbon was grown in the lab of Rice chemist James Tour, a co-author of the paper, and gold electrodes were attached to the corners.

Raising or lowering the applied voltage tuned the Fermi energy in the graphene sheet, which in turn changed the density of free carriers that are good absorbers of terahertz and infrared waves. This gave the graphene sheet the ability to either absorb some or all of the terahertz or infrared waves or let them pass. With a spectrometer, the team found that terahertz transmission peaked at near-zero Fermi energy, around plus-30 volts; with more or less voltage, the graphene became more opaque. For infrared, the effect was the opposite, he said, as absorption was large when the Fermi energy was near zero.

“This experiment is interesting because it lets us study the basic terahertz properties of free carriers with electrons (supplied by the gate voltage) or without,” Kono said. The research extended to analysis of the two methods by which graphene absorbs light: through interband (for infrared) and intraband (for terahertz) absorption. Kono and his team found that varying the wavelength of light containing both terahertz and infrared frequencies enabled a transition from the absorption of one to the other. “When we vary the photon energy, we can smoothly transition from the intraband terahertz regime into the interband-dominated infrared. This helps us understand the physics underlying the process,” he said.

They also found that thermal annealing – heating – of the graphene cleans it of impurities and alters its Fermi energy, he said.

Kono said his lab will begin building devices while investigating new ways to manipulate light, perhaps by combining graphene with plasmonic elements that would allow a finer degree of control.

Co-authors of the paper include former Rice graduate students Lei Ren, Jun Yao and Zhengzong Sun; Rice graduate student Qi Zhang; Rice postdoctoral researchers Zheng Yan and Sébastien Nanot; former Rice postdoctoral researcher Zhong Jin; and graduate student Ryosuke Kaneko, assistant professor Iwao Kawayama and Professor Masayoshi Tonouchi of the Laser Engineering Institute, Osaka University.

The research was supported by the Department of Energy, the National Science Foundation, the Robert A. Welch Foundation and the Japan Society for the Promotion of Science Core-to-Core Program. Support for the Tour Group came from the Office of Naval Research and the Air Force Office of Scientific Research.

Source: By Mike Williams, Rice University

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