Researchers at Sandia National Laboratories have blended quantum dots with an LED to produce a solid-state white-emitting device that does not depend on phosphors or multiple light sources.
While a variety of white-emitting diodes are now on the market, challenging traditional incandescent or fluorescent lighting, they are hobbled by complex phosphor-based structures that make them expensive and difficult to manufacture. “Highly efficient, low-cost quantum dot-based lighting would represent a revolution in lighting technology,” said Lauren Rohwer, principal investigator at Sandia (Albuquerque, N.M.).
Quantum dots confine the energy entering them to dimensions smaller than the wavelength of light they are to emit. When stimulated externally, in this case by ultraviolet energy from a light-emitting diode, nanoparticle energy levels can be pushed over the dots’ bandgap so that as their electrons fall back to normal energy levels, they emit light. Rohwer’s team designed quantum dots that emit in the visible light range as long as they are stimulated with UV.
According to team member Jess Wilcoxon, the team engineered the surfaces of the nanoparticles — here cadmium sulfide — to emit the exact spectrum of visible light to which the eye is sensitive. The team has succeeded in engineering quantum dots as both blue and white light emitters, with the white emitting a sun-like spectrum centered on 570-nanometer wavelengths.
Since the nanoparticles are akin to the phosphors in a fluorescent bulb, the researchers call them nanophosphors.
So-called white LEDs actually use three color light sources — a blue LED that stimulates red and green phosphors — to simulate a white emitter. But because the blue LED is highly directional and the phosphors are omnidirectional, the effect is dependent on the viewing angle and thus is uneven.
Nanophosphors, by contrast, are engineered to emit the visible band using a single LED as the source and thus “provide an even, consistent spectrum that you can tailor at the surface of the nanoparticles with chemistry,” Wilcoxon said. The team claims its nanophosphors will be cheaper in the long term because they only require a single LED per emitter.
Conventional phosphors’ efficiency can be as low as 50 percent because of light scattering. But nanophosphors virtually eliminate light scattering by confining it in a sub-wavelength space, according to Sandia. The nanophosphors are also easily synthesized using wet chemistry.
The team is researching how to change the surface chemistry of the dots’ molecules dynamically to tailor the wavelength of the light emitted. That would permit users to “tune in” the color they want.
To build “light bulbs,” the team embedded the quantum dots into a dome of epoxy and encapsulated the dome atop a commercial UV (400-nm) LED. The dots absorb the UV and emit visible light in a manner similar to fluorescent bulbs, but solid-state.
“We had to take care not to alter the surface chemistry of the quantum dots in transition from solvent to encapsulant,” said researcher Steven Thoma.
Early prototypes had only about 20 percent efficiency, but Thoma was able to boost that to over 60 percent by embedding the quantum dots in a polymer scaffolding. Thus, instead of clumping up, the dots are now spread out relatively evenly, never touching each other, within a polymer matrix.
Next, Sandia will research semiconducting-material alternatives to cadmium sulfide. In theory, silicon and germanium can work as well as cadmium sulfide, with less impact on the environment.
“We used cadmium sulfide because we already have a lot of experience with it, but for the future we think it will be a relatively easy engineering task to transfer our knowledge to using silicon or germanium instead,” said Wilcoxon.
The team also plans to increase the concentration of quantum dots within the encapsulant, to boost the total output of each device.
