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Posted: Apr 18, 2011
Curing quantum dots with ultraviolet light causes a permanent increase in their light emission efficiency
(Nanowerk News) Semiconductor-based light-emitting devices that produce while light are beginning to replace incandescent light-globes in homes and offices around the world thanks to their high efficiency. Chang-Soo Han and colleagues at the Korea Institute of Machinery and Materials have now shown that curing polymer-embedded quantum dots with ultraviolet light can permanently increase the light-emitting efficiency of these elements as part of white-light devices ("Photoenhancement of a Quantum Dot Nanocomposite via UV Annealing and its Application to White LEDs").
Quantum dots are nanometer-scale particles of semiconducting material that tightly confine electrons. This characteristic endows them with a host of unusual properties, many of which are now being harnessed for optoelectronic applications. Han and his team fabricated quantum dots by covering a cadmium selenide core with 10 nm-thick onion-like layers of cadmium sulfide and zinc sulfide.
The researchers investigated the effect of exposing these quantum dots to ultraviolet light for 96 hours in a variety of situations: when in a powder form, suspended in a solution and contained within a polymer matrix. Curing or annealing adversely affected the intensity of the red light emitted from the powder dots, whereas it had no effect on the dots in solution. However, exposure to ultraviolet light had an extremely positive influence on the polymer-embedded quantum dots, seeing a 180% increase in photoluminescence with no change in the emission wavelength.
"We believe that this phenomenon is closely related to the polymer," the researchers explain. "Before ultraviolet illumination, the quantum dots were randomly dispersed in clumps throughout the polymer matrix. But the ultraviolet light organized the dots into a regular pattern."
Impressively, the researchers showed in their latest research that this enhancement was permanent, showing very little decrease after days of exposure to air. In contrast, the efficiency degraded quickly for quantum dots with only a single shell surrounding the cadmium selenide core, making them of limited use in practical light-emitting devices.
The team constructed a white-light source by adding a phosphor to the polymer and placing it on a blue light-emitting diode. The blue emission excited red light from the quantum dots and yellow light from the phosphor, which combined together to create white light.