Portland, Ore. A promise of future lightbulb replacements that are almost 100 percent efficient came to light in a recent proof-of-concept experiment carried out by the National Nuclear Security Administration's Los Alamos National Laboratory and Sandia National Laboratories.
In demonstrating these "light engines," experimental quantum wells emitted ultraviolet energy so rapidly that before that energy could become radiation, it was absorbed by integrated nanocrystals that glowed like a fluorescent tube.
Now, "the process is 55 percent efficient," reported Sandia researcher Daniel Koleske, "because, unlike the fluorescent bulb, which must radiate its ultraviolet energy to the phosphor in the form of photons, the quantum well here delivers its ultraviolet energy to the nanocrystal very rapidly, before photons even form."
As a consequence of the conversion process from UV photons to visible photons, normal fluorescent lightbulbs are less than 10 percent efficient. But in the future, light engines currently under development by companies working with the national laboratories could increase that efficiency to almost 100 percent.
The prototype's 55 percent measured efficiency is just a start, according to Koleske.
The principle investigator on the project at Los Alamos National Laboratory, Victor Klimov, cautioned that this is still merely a proof-of-concept, not the basis for a commercial product.
The work was performed by a joint team at Los Alamos and Sandia. The thin-film quantum well crystal film was grown at Sandia by chemist Koleske. Los Alamos researchers Klimov, Marc Achermann, Melissa Petruska, Simon Kos and Darryl Smith fabricated the semiconductor nanocrystals into a device, made the measurements and developed the theory.
Fluorescent lightbulbs work by arching electricity through a noble gas, thereby turning it into a plasma that emits UV light. The UV energy is absorbed by the phosphors coating the inside of the bulb. As a result, the glowing phosphors radiate visible white light.
Similarly, the energy from the quantum wells shifts the frequency of the emitted radiation to one determined by the nanocrystal's size. Different-sized nanocrystals can produce nearly any color light. This capability would let future nanocrystals pumped by quantum wells to combine all three colors to produce white-light emitters.
Quantum wells in laser diodes use the region between layers of gallium arsenide and aluminum gallium arsenide, where the density of electrons is high, to increase their lasing efficiency and reduce the generation of heat.
Such semiconductor heterostructures enabling quantum effects use an ultrathin layer of narrower bandgap semiconductor, sandwiched between two layers of larger bandgap semiconductor. For the quantum-well-pumped nanocrystals, layers of gallium nitride and indium gallium nitride were used instead of gallium arsenide and aluminum gallium arsenide.
Quantum effects resulted from the confinement of electrons and holes, which are free to move only in the direction that's perpendicular to the direction in which the crystal grows. As a result, electrons and holes are confined to two dimensions, thereby enabling the quantum effects to dominate.