By Patti Koning
To construct a nanoscale color detector, a team of Sandia researchers took inspiration from the human eye, and in a sense, improved on the model.
When light strikes the retina, it initiates a cascade of chemical and electrical impulses that ultimately trigger nerve impulses. In the nanoscale color detector, light strikes a chromophore and causes a conformational change in the molecule, which in turn causes a threshold shift on a transistor made from a single-walled carbon nanotube.
“In our eyes the neuron is in front of the retinal molecule, so the light has to transmit through the neuron to hit the molecule,” says Xinjian Zhou (8656). “We placed the nanotube transistor behind the molecule — a more efficient design.”
Detecting the entire visible spectrum
Zhou, François Léonard (8656), Andy Vance, Karen Krafcik, Tom Zifer, and Bryan Wong (all 8223) created the first carbon nanotube device that can detect the entire visible spectrum of light. The team recently published a paper, “Color Detection Using Chromophore-Nanotube Hybrid Devices,” in the journal Nano Letters. The research has garnered attention in industry press, with stories appearing in physicsworld.com, Technology Review, and Nature Photonics.
The idea of carbon nanotubes being light-sensitive has been around for a long time, but earlier efforts using an individual nanotube were only able to detect light in narrow wavelength ranges at laser intensities. The Sandia team found that their nanodetector was orders of magnitude more sensitive, down to about 40 watts per square meter — about 3 percent of the density of sunshine reaching the ground. “Because the dye is so close to the nanotube, a little change turns into a big signal on the device,” explains Xinjian.
The research is a Laboratory Directed Research and Development (LDRD) project, now in its second year, based on François’ collaboration with the University of Wisconsin to explain the theoretical mechanism of carbon nanotube light detection. If you’re going to work on carbon nanotubes, François is the guy to have on your team because he literally wrote the book on carbon nanotubes — The Physics of Carbon Nanotubes, published September 2008 (Lab News, April 25, 2008).
He points out that a key component of the project was bringing together the right people and equipment, as the LDRD draws upon Sandia’s expertise in both materials physics and materials chemistry. In fact, when asked what the most difficult part of the project was, the different team members pointed to their counterparts in other disciplines; Andy says he felt it was the device fabrication, while Xinjian thought it was the chemical synthesis.
“This is an example of the sum of the parts being more than the individuals,” says Andy.
François and Bryan laid the groundwork for the project with their theoretical research. Bryan did the first-principles calculations that supported the hypothesis of how the chromophores were arranged on the nanotubes and how the chromophore isomerizations affected electronic properties of the devices.
To construct the device, Xinjian and Karen first had to create a tiny transistor made from a single carbon nanotube. They deposited carbon nanotubes on a silicon wafer and then used photolithography to define electrical patterns to make contacts.
The final piece came from Andy and Tom, who synthesized molecules to create three types of chromophores that respond to either the red, green, or orange bands of the visible spectrum. Xinjian immersed the wafer in the dye solution and waited a few minutes while the chromophores attached themselves to the nanotubes.
The team reached their goal of detecting visible light faster than they expected; they thought the entire first year of the LDRD would be spent testing UV light. Now they are looking to increase the efficiency by creating a device with multiple nanotubes.
Larger size more practical for applications
“Detection is now limited to about 3 percent of sunlight, which isn’t bad compared with a commercially available digital camera,” says Xinjian. “I hope to add some antennas to increase light absorption.”
A device made with multiple carbon nanotubes would be easier to construct and the resulting larger area would be more sensitive to light. A larger size is also more practical for applications.
The team is now setting its sights on detecting infrared light. “We think this principle can be applied to infrared light and there is a lot of interest in infrared detection,” says Andy. “So we’re in the process of looking for dyes that work in infrared.”
This research eventually could be used for a number of applications, such as an optical detector with nanometer-scale resolution, ultra-tiny digital cameras, solar cells with more light absorption capability, or even genome sequencing. The near-term purpose, however, is basic science, to understand the fundamental interactions between the molecules and nanotubes.
“A large part of why we are doing this is not to invent a photo detector, but to understand the processes involved in controlling carbon nanotube devices,” explains François. “We can use a nanotube to probe single molecule transformations and study how individual molecules respond to light and change shapes.”
The next step in the LDRD is to create a nanometer-scale photovoltaic device. Such a device on a larger scale could be used as an unpowered photo detector or for solar energy. “Instead of monitoring current changes, we’d actually generate current,” says Andy. “We have an idea of how to do it, but it will be a more challenging fabrication process. -- Patti Koning
Over the past five decades Sandia has developed, fielded, and operated 140 satellite payloads without a critical mission system failure, making its satellite endeavors among the largest enduring programs at the Labs.
Sandia is now on the verge of a major program deliverable that has involved hundreds of people and hundreds of millions of dollars over the past several years.
“We are proud of our satellite efforts and their contributions to national security,” says Bruce Walker, director of Monitoring Systems and Technology Center 5700. “Many people from across the Labs, particularly from divisions 1000, 2000, and 5000, have contributed to these complicated programs that are built on past successes. The work they do continues to amaze me.”
Today’s systems, he says, draw on many unique Sandia competencies, including space system engineering, high-bandwidth data processing, advanced software development, thermal management, high-reliability software, and hardware engineering. Like other emerging technologies, the systems have grown in size, complexity, and mission importance with each new generation.
“The complexity and sophistication of the newest systems constantly challenge Sandia’s engineering and management talents,” says Mike Vahle, director of Systems Mission Engineering Center 5500. “Through the years Sandia has pushed the state-of-art in space-based instrumentation and ground-based command, control, and processing, which is why customers have continued to trust us with important projects.”
Current satellite programs have budgets that rival investments made in programs like MESA (Microsystems & Engineering Science Applications).
Role began with Vela program
Sandia’s role in space began with the Vela program of the 1960s, a result of a nuclear test moratorium of 1958 and the Limited Test Ban Treaty of 1963. Vela was designed to detect testing in the atmosphere and space. Los Alamos and Sandia national laboratories both had roles in Vela. Also, Sandia and Los Alamos jointly developed and installed atmospheric and space nuclear burst detectors and logic systems on Air Force space satellites.
After Vela was fully approved in 1961, Sandia became responsible for power handling, logic and data storage systems, ground checkout equipment, and computer analysis of data coming from detectors designed and built by LANL. In addition, for the later eight Vela satellites, Sandia designed and built the various optical detectors and optical burst locators that have become, with exponential complexity, the mainstay of the programs supporting the US Nuclear Detection System (USNDS) mission.
Following Vela’s successes, payloads on two other “bread and butter” satellite programs — Defense Support Program (DSP) and Global Positioning System (GPS) — employed hundreds of people throughout Sandia, Bruce says. Sandia and LANL designed payloads to be flown on both systems. For the GPS satellites, Sandia continues to design the sophisticated optical radiometers, power systems, and data processing logic as well as radiation-hardened, large-scale integrated circuits. Sandia also provides the essential payload testing, system integration, launch support, and orbital technical systems for these payloads.
Four decades, 23 DSP launches
The DSP satellite systems have flown for nearly four decades with 23 launches; the first was launched in November 1970 and the last in November 2007. The DSP USNDS packages, which incorporated novel, state-of-the-art designs, leveraged other space-based designs and have provided nuclear burst detection, nuclear burst location, and various space and Earth environment monitoring data.
In support of space-based resources, large-scale ground-based processing systems — designed, built, and maintained by Sandia — have provided the essential interface for customers and analysts in numerous agencies and departments.
Through the years Sandia has been involved in numerous satellite initiatives. For many, Sandia added experimental and operational packages to existing payloads. In the case of the MTI (Multi-Spectral Thermal Imager), Sandia was responsible for the entire satellite, doing all the design, assembly, system integration, and construction work, as well as supporting launch of the satellite and providing integral parts to the ground processing system.
All of these projects involved the development, deployment, and operation of very complicated systems that have extensive hardware and software elements both on the ground and on orbit. That complexity has implications for the engineering administrative and management talent required, the underlying physical and information technology, the extraordinary level of reliability required, and the need for extended operations in very extreme environments.
Mike notes that current projects require sophisticated equipment. To meet these needs two buildings were renovated. A Class 100 clean room integration facility was created in one building while another building was modified to create a facility for the development and testing of large, high-performance computing systems.
Jerry McDowell, VP of Defense Systems & Assessments Div. 5000, says satellite sensor programs are “vital to Sandia’s future.”
“The paradigm of strategic national security is changing, and space will grow in importance as a leverage point for our nation’s enduring security,” Jerry says. “Success with these programs demonstrates that Sandia’s engineering, science, and technology base is strong and relevant to the new challenges we face and that Sandia can deliver products.
“Remote sensing and verification is the new face of Sandia. Staff from across the Labs combine talents from multiple disciplines to deliver real capability for decision makers. It’s an exciting time in the Defense Systems and Assessment SMU [Strategic Management Unit], and I applaud all Sandians who are forging a ‘new order for the ages’ by their successful work on our ongoing sensor programs.” -- Chris Burroughs
Sandia’s hopping robots may soon be in combat.
Boston Dynamics, developer of advanced dynamic robots such as BigDog and PETMAN, has been awarded a contract by Sandia to develop the next generation of the Precision Urban Hopper.
When fully operational, the four-wheeled robots with one mighty leg will navigate autonomously using their wheels and will jump onto or over obstacles when they meet them. The hopper will be able to jump more than 25 feet into the air, says Jon Salton (6473), program manager.
“The Precision Urban Hopper is part of a broad effort to bolster the capabilities of troops and special forces engaged in urban combat, giving them new ways to operate unfettered in the urban canyon,” says Jon.
The development program, funded by DARPA, DoD’s advanced technology organization, has a nine-month design phase followed by a nine-month build phase, with testing and delivery in late 2010.
As part of the ongoing DARPA project, Sandia developed the shoebox-sized, GPS- guided, unmanned ground robots.
Their demonstrated hopping capability allows the small unmanned ground vehicles to overcome up to 30 obstacles that are 40-60 times their own size. Hopping mobility has been shown to be five times more efficient than hovering for obstacles at heights under 10 meters, which allows longer station-keeping time for the same amount of fuel.
The wheeled robotic platform adapts to the urban environment in real time and provides precision payload deployment to any point of the urban jungle while remaining lightweight and small. Researchers addressed several technical challenges, including appropriate management of shock forces during landing; controlling hop height from varying terrain including concrete, asphalt, sand, and vegetation; and controlling landings to limit tumbling.
An overall goal of the robots is to decrease the number of causalities in combat. To that end, the hopping robots will provide enhanced situational awareness for shaping the outcome of the immediate local combat situation, says Jon. Their compact, lightweight design makes them portable, and their semiautonomous capability greatly reduces the workload burden of the operator.
In addition to providing military assistance, the hopping capabilities of the robots could be used in law enforcement, homeland security, search and rescue applications in challenging terrain, and in planetary exploration, says Jon.
“We are delighted to win this project and get a chance to work with Sandia on such a novel and potentially useful robot,” says Marc Raibert, president and founder of Boston Dynamics. “The program gives us a chance to apply our special brand of advanced controls and stabilization to a system that can help our warfighters in the near future.” -- Michael Padilla