A micrograph of pyramid-shaped quantum dots grown from indium, gallium and arsonic.
Nearly Nothing
February / March 2005
Volume 3 Number 1
Claustrophobic Woody Allen once detoured one hundred miles to avoid a tunnel. One Native American legend places in the center of popcorn kernels a demon that gets angry and finally explodes. The praying mantis female eats its mate—€”but only in captivity.
It is far from a universal law, but things behave differently when confined in small spaces. Electrons are no exception. When an electron is trapped in a very small space, its momentum becomes less and less certain, and its energy must become higher. Quantum uncertainty dictates that electrons confined in a small enough space become more and more energetic. When electrons are confined to a box a nanometer—€”a billionth of a meter—€”across, the electrons become stuck in a single point. These nanoscale boxes that can pin electrons in a precise place in space are called "quantum dots." One practical use of quantum dots is that they can emit light of virtually any color. Relatively large quantum dots of cadmium selenide, for example, can emit a brilliant red light, while smaller quantum dots produce blue emissions. All one needs to do is supply energy into the boxes and the electrons do the rest. Quantum dots act, in essence, like the different pipes in a pipe organ. The low notes come from large pipes, and the high notes come from the small pipes.
Los Alamos National Laboratory has recently made available for commercial licensing its extensive portfolio of quantum dot technologies. These technologies may enable the next generation of high-efficiency lighting, solar cells and lasers. Because quantum dots can emit a wide spectrum of colors, some scientists believe quantum dots will one day replace most forms of lighting in homes, automobiles and offices.
How can you turn it on?
It has been recognized for the last 25 years that quantum dots could be extremely efficient light sources for interior lights, commercial signs and traffic signals. However, there remains one problem: How do you turn them on? This problem turned out to be more difficult to solve than one might have imagined. Powders made of quantum dots can be vividly colored, of course, but in this case the energy is provided by ambient light. For obvious reasons, a solar-powered flashlight is not much use to anyone.
For quantum dots to be turned into usable lighting, a way had to be invented to transfer electrical energy into the quantum dots. This appears to be a paradox. How does one transfer electrical energy through a barrier made specifically to trap electrons?
Victor Klimov and his colleagues at Los Alamos and Sandia recently took a major step toward solving this vexing problem. Klimov's team exploited noncontact energy transfer from a thin sheet of a semiconductor (known as a quantum well) to excite quantum dots. Such energy transfer relies on electrostatic coupling between the quantum well and the quantum dots and, therefore, is not hindered significantly by the quantum dot barrier. In their work, Klimov's team used the combination of the InGaN (indium gallium nitride) quantum well with the CdSe (cadmium selenide) quantum dots to experimentally demonstrate the feasibility of this new "pumping" scheme.
The energy transfer was remarkably efficient: more than 50 percent, even in this experimental system. Traditional incandescent light bulbs are only about 10 percent efficient. In theory, the energy transfer in a quantum dot light could be 100 percent efficient.
As Klimov's reported in the British science journal Nature last year, InGaN quantum wells are common semiconductor devices; therefore, electrically powered, hybrid quantum dot/quantum-well emitters may soon be a reality.
Quantum-dot sol-gel glasses
It is far easier to paint a house than to cover it with a colorful tile mosaic. Similarly, it would be much easier to spread a paste of quantum dots rather than etch quantum dots onto a solid surface. Machines that can etch quantum dots tend to do so only on small surfaces and the work is painstaking.
LANL has recently offered for license three new methods of taking quantum dot powders and incorporating them in sol-gel glasses. Rather than etch quantum dots in various surfaces, the laboratory's method produces quantum dots in powders and then spreads the powder on target surfaces by using a sol-gel process. The advantages of these systems are obvious. If a flat plane of material is needed, a flat plane can be painted. If a curved coating is needed, a quantum dot coating may be sprayed on a curved surface. If a pattern of lights is needed, a pattern can be applied.
The use of a sol-gel glass as a carrier medium may allow quantum dots to be used for simple-to-manufacture light-emitting diodes, optical switches, optical amplifiers and lasers.
Interestingly, the quantum-dot in appropriate carrier media may also find use as lotions and creams. It may be possible to use the ultraviolet absorbing properties of some quantum dots to make more efficient sunscreens than any of the sunscreens on the market today. These sunscreens would absorb dangerous ultraviolet light and re-emit the light in harmless colors such as infrared or transform to heat. The FDA approval process for sunscreens is very difficult, but some of the materials used in quantum dots are generally accepted as safe, so perhaps quantum dots will in time stand between the sun and our skin, between cancer and us.
Quantum dot holograms and solar cells
Holograms on credit cards and novelty toys tend to fade over time. Quantum dots can be used to create much better holograms. These quantum dot holograms are very efficient, respond quickly and are extremely photostabile. Using quantum dots, LANL has recently shown that it can create holographic material with the diffraction efficiency comparable to the highest results reported for polymer-based holograms.
Solar cells are well known to be inefficient. One reason the United States is dependent on oil is that the typical solar cell can cost more energy to manufacture than it delivers in electricity over its lifetime. One large source of energy loss in solar cells is the transfer of energy from high-energy, green and blue photons from the solar spectrum. Quantum dots, however, can harvest solar energy much more efficiently than traditional silicon-based solar cells.
Instead of producing just a single electron per absorbed photon, quantum dots can respond to incident light by producing multiple electrons. This effect, discovered at LANL, is called carrier multiplication and it can potentially double the power conversion efficiency of solar cells. LANL hopes that quantum-dot photocells will be able to push solar cell efficiency to the point where solar energy becomes practical. Carrier multiplication can in principle be used to create photodiodes with sensitivity ranging across the light spectrum ultraviolet to far-infrared wavelengths. The same technology could also be used to make practical optical switches and amplifiers for telecommunication and optical information processing.
Quantum dots may be the most exciting piece of nearly nothing to emerge from the nanotech revolution. At LANL, quantum dots are becoming one step closer from the bench to the bedside . . . lamp.
Additional general information about LANL quantum dot research is available at http://quantumdot.lanl.gov. For more information on partnering mechanisms, visit the technology transfer web site at http://www.lanl.gov/partnerships/. Specific licensing inquiries should be directed to Laura Barber at (505) 667-9266 or by email to tmt-2@lanl.gov.
Jeffrey J. Stewart is a business development executive, Technology Transfer, at Los Alamos National Laboratory.
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