The width of a human hair is about 100,000 nanometers (a nanometer is one-billionth of a meter). A single bacterium is just 1,000 nanometers wide. And a quantum dot is a mere few nanometers across. These tiny crystals, however, might one day provide big energy.
For nearly 30 years, Los Alamos National Laboratory has helped to pioneer quantum dot research, raising it from a purely theoretical, even science fiction-esque idea, to a technology that is already in use around the country.
“When we started this work, our understanding of quantum dots was very minimal and there were a lot of challenges,” says Victor Klimov, leader of the Nanotechnology and Advanced Spectroscopy team in the Lab’s Chemistry division. “Today we have many patents and papers on quantum dot technologies; we have helped the world understand the power of these nanocrystals.”
Scientists at the Lab create quantum dots through colloidal synthesis, during which precursor materials are reacted in a chemical solution. “It’s pretty much like cooking at a moderate temperature, about 200 to 300 degrees Celsius,” Klimov says. “You do this and the materials—such as cadmium selenide—nucleate into tiny crystals. Depending on the temperature and time, we can control how large they grow.”
The resulting nanocrystals share many properties with atoms, including the ability to absorb or emit energy and light. The wavelength of light a quantum dot grabs, or generates, depends on its size, and because scientists have full control over this sizing, developers can create quantum dots that react with the entire spectrum of light, from the infrared to visible and further ultraviolet ranges.
Quantum dots are already used in televisions, where blue light from a display panel excites the nanocrystals and they glow red or green, depending on their size, to provide the clearest picture available.
A similar effect can be used to harness solar energy, and the Lab has dedicated much of its focus to this research. Quantum dots can enable so-called luminescent solar concentrators, which harvest sunlight for photovoltaic panels. Because quantum dots are so small, they can be manufactured into a thin roll of film, akin to the tint used on car windows—except that this tint creates energy. This technology is already being used on windows in homes and buildings, and it helps supplement traditional sources of electricity.
“Present-day photovoltaic panels, most of which rely on crystalline silicon to convert solar energy into electricity, top out at an efficiency around 25 percent,” Klimov says. “But with quantum dots, we can potentially exceed this limit by employing carrier multiplication.”
A typical photovoltaic solar panel absorbs one photon of light and releases one electron, which becomes energy. But with advances developed at the Lab, including carrier multiplication and incorporating magnetic manganese ions, quantum dot solar panels generate two or more electrons for each photon absorbed.
Klimov’s team is also researching how quantum dots can be used in energy-demanding chemistries such as ammonia production, which currently accounts for more than 2 percent of global energy use (ammonia is used for everything from plastic production to fertilizer). In this particular application, Klimov’s team uses quantum dots to generate not light but “free electrons” through the process of photoemission. The current method of producing ammonia requires laser pulses to generate these free electrons, but Klimov and his team are working to achieve this reaction with nothing more than quantum dots and solar energy.
“When I first came to Los Alamos from Russia in 1995, I brought with me a handful of fragments of semiconductor-doped colored glass—the predecessors of modern quantum dot samples,” Klimov says. “Since then, Los Alamos has built a world-class quantum dot program that brings together efforts in synthesis, spectroscopy, theory, and devices. Many more exciting applications of these fascinating structures are under development in areas spanning from solar energy and radiation detection to novel lasers and quantum information.” ★