Sandia Boosts Quantum Dot Phosphors
"This accomplishment brings quantum dot technology from the laboratory demonstration phase to a packaged component," says principal investigator Lauren Rohwer (pictured with white and blue light sources).
The encapsulated dots are produced with specially engineered surfaces so that they act as phosphors that efficiently emit visible light when illuminated by near ultraviolet (UV) light emitting diodes (LEDs). The dot size and surface chemistry determine the colour of the emitted light. The alternative of creating separate red, green and blue LEDs is expensive compared with the cost of fluorescent lighting. The quantum dot encapsulation material is either epoxy or silicone.
The nanometre-size dots are synthesised in a solvent containing soap-like molecules called surfactants as stabilisers. The small size of the quantum dots - much smaller than the wavelength of visible light - eliminates all light scattering and associated optical losses. Optical backscattering losses using larger conventional phosphors reduce the package efficiency by as much as 50%.
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 colour. The small dot size also means that, typically, more than 70% of the atoms are at surface sites so that chemical changes at these sites allow tuning of the light-emitting properties of the dots, permitting the emission of multiple colours from a single size dot.
Quantum dot phosphors are integrated with a commercial LED chip that emits at a wavelength of 400nm by encapsulating the chip with a dot-filled epoxy, creating a dome. The quantum dots in the dome absorb the invisible 400nm light from the LED and reemit it in the visible region - a principle similar to that used in fluorescent lighting.
A key technical issue in the encapsulation process was to stop the quantum dots "clumping up" or agglomerating when altering the environment of the dots from a solvent to an encapsulant. Clumping causes the dots to lose their light-emitting properties. Attaching the quantum dots to the "backbone" of the encapsulating polymer, they become close, but not touching. This allows for an increase in efficiency from the usual 10-20% for such efforts up to 60%.
The team notes that other people working in the field of quantum dots have reported conversion efficiencies of nearly 50% in dilute solutions. However, to their knowledge, Sandia's team is the first to make an encapsulated quantum dot device with such high efficiencies.
To date, Sandia's quantum dot devices have largely been composed of the semiconductor material cadmium sulphide. Cadmium is a toxic heavy metal similar to lead so alternative nanophosphor materials are desirable. Fortunately, quantum dot phosphors can also be made from other types of materials, including non-toxic nanosize silicon or germanium semiconductors with light-emitting ions like manganese on the quantum dot surface.
Stephen Woessner, one of the investigators, comments: "The scientific insights gained through the team's success with cadmium sulphide quantum dots will enable this next step in nanophosphor development."
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.
While the researchers investigate the use of quantum dots as phosphors as part of an internally funded research project they also have a grant from the US Department of Energy (DOE) Office of Building Technologies. This is for a collaborative project with Lumileds Lighting, a joint venture between Agilent Technologies and Philips Lighting. In this project they are helping Lumileds measure the quantum efficiency for light emission from various types of dots.
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