Assembly Lines: Researchers Use Quantum Dots For Lighting
ALBUQUERQUE, NM—In a different approach to creating white light, researchers at Sandia National Laboratories have developed the first 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 nanotechnology.
“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, principal investigator. “Highly efficient, low-cost quantum dot-based lighting would represent a revolution in lighting technology through nanoscience.”
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 absorb light in the near UV range and re-emit visible light. The color of the light is determined by the size and surface chemistry of the dots.
The nanometer-size quantum dots are synthesized in a solvent containing 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 over 70 percent of the atoms are at surface sites. 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.
For the quantum dots to be used for lighting, they need to be encapsulated, usually in epoxy or silicone. Quantum dot phosphors are integrated with a commercial LED chip that emits in the near ultraviolet spectrum. The chip is encapsulated with a dot-filled epoxy, creating a dome. The quantum dots in the dome absorb the invisible 400-nanometer light from the LED and re-emit it in the visible region—a principle similar to that used in fluorescent lighting.
To date, 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 also 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.
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.