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[Sandia Lab News]

Vol. 55, No. 14           July 11, 2003
[Sandia National Laboratories]

Albuquerque, New Mexico 87185-0165    ||   Livermore, California 94550-0969
Tonopah, Nevada; Nevada Test Site; Amarillo, Texas

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Quantum dots being used in solid-state lighting Energy security team works to make military bases 'grid-free' Sandia helps small businesses in New Mexico program



Sandia researchers use quantum dots as a new approach to white, blue solid-state lighting

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By Chris Burroughs

In a different approach to creating white light several Sandia researchers have developed the first solid-state white light-emitting device using quantum dots. In the future, the use of quantum dots as light-emitting phosphors may represent a major application of nano-technology.

"Understanding the physics of luminescence at the nanoscale and applying this knowledge to develop quantum dot-based light sources is the focus of this work," says Lauren Rohwer (1745), principal investigator. "Highly efficient, low-cost quantum dot-based lighting would represent a revolution in lighting technology through nanoscience."

The project is part of Sandia's internally funded Laboratory Directed Research and Development (LDRD) Solid State Lighting Grand Challenge. The approach is based on encapsulating semiconductor quantum dots -- nanoparticles approximately one billionth of a meter in size -- and engineering their surfaces so they efficiently emit visible light when excited by near-ultraviolet (UV) light-emitting diodes (LEDs). The quantum dots strongly absorb light in the near-UV range and re-emit visible light that has its color determined by both their size and surface chemistry.

This nanophosphor-based device is quite different from an alternative approach based upon growth of blue, green, and red emitting semiconductor materials that requires careful mixing of the those primary colors to produce white illumination. Efficiently extracting all three colors in such a device requires costly chip designs, which likely cannot compete with conventional fluorescent lighting but can be attractive for more specialized lighting applications.

Lauren and the quantum dot team -- Jess Wilcoxon (1122), Stephen Woessner (1122), Billie Abrams (1123), Steven Thoma (14172), and Arturo Sanchez (14172) -- started on the project two-and-a-half years ago. Subsequently, their research has advanced significantly, including recently reaching a major milestone of creating white and blue lighting devices using encapsulated quantum dots.

"This accomplishment brings quantum dot technology from the laboratory demonstration phase to a packaged component," Lauren says.

LEDs for solid-state lighting typically emit in the near UV to the blue part of the spectrum, around 380-420 nanometers. Conventional phosphors used in fluorescent lighting are not ideal for solid state lighting because they have poor absorption for these energies. So researchers worldwide have been investigating other chemical compounds for their suitability as phosphors for solid state lighting.

Quantum dots represent a new approach. The nanometer-size quantum dots are synthesized in a solvent containing soap-like molecules called surfactants as stabilizers. The small size of the quantum dots -- much smaller than the wavelength of visible light -- eliminates all light scattering and the associated optical losses. Optical backscattering losses using larger conventional phosphors reduce the package efficiency by as much as 50 percent.

Nanophosphors based upon quantum dots have two significant advantages over the use of conventional bulk phosphor powders. First, while the optical properties of conventional bulk phosphor powders are determined solely by the phosphor's chemical composition, in quantum dots the optical properties such as light absorbance are determined by the size of the dot. Changing the size produces dramatic changes in color. The small dot size also means that, typically, more than 70 percent of the atoms are at surface sites so that chemical changes at these sites allow tuning of the light-emitting properties of the dots, permitting the emission of multiple colors from a single size dot.

"This provides two additional ways to tune the optical properties in addition to chemical composition of the quantum dot material itself," Jess says.

For the quantum dots to be used for lighting, they need to be encapsulated, usually in epoxy or silicone.

"Doing this, we had to take care not to alter the surface chemistry of the quantum dots in transition from solvent to encapsulant," says Steven, who worked on the encapsulation portion of the project.

Quantum dot phosphors are integrated with a commercial LED chip that emits in the near ultraviolet at 400 nanometers by encapsulating the chip with a dot-filled epoxy, creating a dome. The quantum dots in the dome absorb the invisible 400 nanometer light from the LED and reemit it in the visible region -- a principle similar to that used in fluorescent lighting.

However, a key technical issue in the encapsulation process had to be solved first. When altering the environment of the dots from a solvent to an encapsulant, the quantum dots would clump up or agglomerate, causing them to lose their light-emitting properties. By attaching the quantum dots to the "backbone" of the encapsulating polymer they are close, but not touching. This allows for an increase in efficiency from 10-20 percent to an "amazing" 60 percent, Steven says.

The team notes that other people working in the field of quantum dots have reported conversion efficiencies of nearly 50 percent in dilute solutions. However, to their knowledge, Sandia's team is the first to make an encapsulated quantum dot device with such high efficiencies.

To date, the Sandia's quantum dot devices have largely been composed of the semiconductor material cadmium sulfide. Cadmium is a toxic heavy metal similar to lead, so alternative nanophosphor materials are desired. Fortunately, quantum dot phosphors can be made from other types of materials, including nontoxic nanosize silicon or germanium semiconductors with light-emitting ions like manganese on the quantum dot surface.

"Silicon, which is abundant, cheap, and nontoxic, would be an ideal material," says Stephen. "The scientific insights gained through the team's success with cadmium sulfide quantum dots will enable this next step in nanophosphor development."

In the next year the researchers will increase the concentration of the quantum dots in the encapsulant to obtain further increases in light output while extending the understanding of quantum dot electronic interactions at high concentrations.

While the researchers investigate the use of quantum dots as phosphors as part of the LDRD grand challenge, they also have a grant from the DOE Office of Building Technologies for a collaborative project with Lumileds Lighting, a joint venture between Agilent Technologies and Philips Lighting. In this project they are helping Lumileds measure quantum efficiency of light emission from various types of dots.

Jerry Simmons (1123), who with James Gee (6200) heads up the Sandia's Solid State Lighting grand challenge, says the quantum dot research is an integral part of the work at Sandia.

"We are very proud of these accomplishments," he says. "The team has come a long way in a short time." - - Chris Burroughs

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Sandia energy security team works to make key military bases 'grid-free' in the future

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By Will Keener

Dave Menicucci thinks that it was Mother Nature as terrorist who ultimately pushed the Department of Defense toward improving its energy infrastructure security. He knows September 11, 2001, had a role as well. "Prior to that date, most experts felt border security was sufficient and concern about infrastructure security at domestic military bases was minimal," Dave says. After September 11, that changed.

And then in May 2002, a fierce wildfire cut both feeder lines supplying a remote military base with power. Completely off the grid, the base went to its back-up mode. Although some facilities were equipped with diesel generators, officials soon realized the base would need more fuel for the significant amount of time needed to restore grid power. But by the time they turned attention to this problem, the generators had run out of diesel.

The result: the base was at a diminished state of readiness for 16 hours. Costs attributed to the downtime were approximately $3 million. True, this strike was from nature, but it illustrates how effective a coordinated attempt at sabotage could be.

"This really raised awareness as to how vulnerable these bases are," says Dave, project lead and staff member in Energy Infrastructure and Distributed Energy Resources Dept. 6251.

In the months since, the Army has asked Sandia to look at three pilot forts and make recommendations for an optimal mix of generating technologies to help them achieve energy security. In addition, a US Marines Corps base is also very interested in the Sandia approach, Dave reports. They want to bring their generation inside their fences and distribute it around so that there won't be any one clear target. Sandia is working to help them develop a "grid-free" plan that will be in place by 2007.

Bill Black of Solar Technologies Dept. 6218 is project manager for several of the military energy security projects and has visited three pilot Army bases. "This is a natural extension of our work with nuclear weapons security," he says. "It builds on what Sandia has done with dams, transmission lines, and other vulnerability assessments."

The Sandia goal is not just to assess and implement distributed energy resources for the three pilot bases, but to develop a methodology that can be used by others as well. "We want to be able to say, 'Here's a workbook you can apply to any military base to provide combined heat and power systems,'" Bill says. Nonmilitary applications, such as support for emergency services in smaller communities, also are a possibility for this approach, he says.

The Marines have asked Sandia to integrate a 7.6-megawatt cogeneration power plant with a one-megawatt photovoltaic system. "They want a plan to show how they can get off the grid if they need to and still have the mission capabilities they need to respond," says Dave.

The Sandia approach will be to provide solutions that are based on full-time generation by a distributed energy system, which can be isolated for security reasons when the grid goes away. "This is not a diesel backup approach. We want to consider a broad spectrum of energy generation capabilities ranging from conventional diesel to photovoltaics, fuel cells, wind, batteries, micro-turbines, or whatever it takes," says Dave.

Part of the solution to moving to a self-sufficient mode involves the concept of a "microgrid." This is a grid dedicated to a campus, military base, or even a single building, explains John Stevens (6251). The microgrid must generate enough power to meet normal loads, have safeguards to prevent disruption of power, and offer a way to make fast disconnects from the power grid when necessary.

Sandia is working with the Army Construction Engineering Research Lab in Illinois on microgrid technology right now, John says. "The microgrid is largely an engineering problem, although there are a couple of places where we are using existing devices in new ways," he says.

Another goal of the projects is to work with private sector vendors, who can install and operate energy systems that compete economically with the grid and still meet military needs. "The trick is to develop solutions that not only meet the base's needs from a security perspective but also meet the private sector's needs from an investment perspective," says Dave. "It's identifying whatever makes sense depending on the resources of a given facility."

Sandia is providing essential expertise, evolved from Labs' efforts to leverage the use of renewable and emerging energy systems, not available commercially, notes Dave. Through a Laboratory Directed Research and Development proposal, the team hopes to reach out to tap other expertise within Sandia, as well. "We're sure there's related work out there where we can contribute technical value and vice versa," says Dave. "This fits at the heart of what Sandia is all about." - - Will Keener

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Sandia helps 310 state businesses in NMSBA program

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Sandia assisted 310 small businesses throughout New Mexico in 2002, helping to solve pressing technical problems ranging from how to detect and extract oil from the ground more efficiently to developing better packaging to prevent fragile ceramic tiles from breaking during shipping.

The Labs provided the assistance under a program that went into effect in mid-2000 after its approval by the New Mexico Legislature. The New Mexico Small Business Assistance (NMSBA) program allows Sandia to give up to $5,000 a year worth of technical advice and assistance to small businesses in the Albuquerque/Rio Rancho area, and up to $10,000 a year worth of help to small businesses outside that area.

"We are excited to see the program grow each year; in fact we were oversubscribed last year. We are especially happy to help small businesses that would find if difficult, if not impossible, to find help solving difficult technical problems," says Lenny Martinez (14000), VP for Manufacturing Systems, Science & Technology, and the NMSBA program director. "And this assistance, in turn, is helping small businesses grow, increasing the businesses' capability to support not only the labs, but to become suppliers to companies outside New Mexico. And, it is generating additional employment opportunities for New Mexico."

What makes the program unique is that the state does not allocate money for the program. Instead, in exchange for helping small businesses, the state "forgives" Sandia a portion of the gross receipts taxes it pays each year. During 2002, Sandia received $1.67 million in tax credit, 77 percent of which went to small businesses in rural New Mexico and 23 percent to small businesses in Bernalillo County. Sandia is the state's single biggest gross receipts taxpayer at an estimated $62 million for FY03.

There are few requirements for small-business participation -- mainly that companies must be bona fide for-profit New Mexico small businesses (500 or fewer employees), and Sandia can help only when such help isn't available for a reasonable cost through private sources.

Success stories

A few examples:

Providence Technologies Inc. of Roswell, which leads a consortium of small New Mexico oil producers, sought assistance in locating hard-to-find oil deposits using seismic modeling. Marianne Walck, Manager of Geophysical Technology Dept. 6116, led Sandia's effort. Her department focuses on solving problems in fossil energy, hazardous and nuclear waste management and restoration, defense, nonproliferation, and energy-related basic research. Sandia was able to provide $40,000 in assistance in 2002 since the consortium consisted of multiple companies, and that amount is growing this year as the project continues.

"When people think of oil companies they often think of the Shells and Exxons, but there are many small independent companies out there that are left out in the cold," Marianne says. "We are helping the small independents, which helps the domestic energy supply. This is also good because we're helping New Mexico at the same time."

El Kabode Tile of Las Cruces makes custom handmade ceramic tiles using old-world methods that they then ship throughout the country. Each tile is usually part of a larger pattern and therefore unique. If any tile breaks during shipping, the entire pattern is spoiled. David Szklarz of Materials Mechanics Dept. 9123 helped oversee a series of stress tests and container-design analyses that led to the design of a custom container. The container consists of foam-lined interior walls. Each tile was then wrapped with bubble shrink wrap and then the tiles were placed on end in the container like a loaf of bread.

"This turned out to be both effective and user-friendly," David says. "It wasn't a highly engineered operation, but it did solve the problem, and our work helped let the community know that Sandia has a community partnership with outside industry."

Results

Results of the 2001 NMSBA program show for each dollar spent on an assistance project, 98 cents was recovered by the state through taxes within the first year. Over a one-year period, 44 jobs were retained and 68 new jobs were created. The results also showed that participating New Mexico small businesses reported a total of $3,380,000 of increased revenue and a $1,703,500 decrease in operating costs, and participating small businesses spent $2,147,500 to expand operations and $828,600 on local goods and services.

More information on the NMSBA program is available from Mariann Johnston in Sandia's Regional & Small Business Partnering Dept. 1302, 284-9548.

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Last modified: July 10 , 2003

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