Material May Enable Single-Source White LEDs
Anne L. Fischer
White LEDs are commonly made by mixing blue light from an LED chip with yellow light from a stimulated YAG phosphor. This approach works, but just a single source would streamline the process. Researchers in a team at National Tsing Hua University in Hsinchu, Taiwan, have investigated a microporous metal phosphate in an attempt to develop such a white LED.
The structure of NTHU-4 features extra-large channels; the disordered part of the framework is shown in pink. NTHU-4Y and NTHU-4W emit yellow and white light, respectively. © 2005 American Chemical Society. Reprinted with Permission.
Using a large trimethylenedipyridine amine, they developed a material with extra-large channels that has distinctive photoluminescence properties. This zinc gallophosphate, NTHU-4, has two analogues that emit yellow or white luminescence upon exposure to 365-nm radiation.
In their experiments, the investigators took light brown crystals of NTHU-4Y (the yellow analogue) and illuminated them with UV radiation to produce yellow fluorescence. They found that NTHU-4Y can be excited by radiation with a wavelength ranging from 280 to 520 nm to emit 550-nm light. No metal phosphate or silicate lattices have done this.
Using NTHU-4W (the white analogue), the group achieved the same excitation range, but with tunable qualities. If tuned to 390 nm, the result is nearly perfect white light.
One challenge was that the disorderliness of the NTHU-4 lattices induced defect sites (meaning oxygen deficiency and reduced zinc ions in lattice), causing yellow emission in both analogues. After the scientists heated NTHU-4Y at 280 °C for four hours, it emitted white light. Heating NTHU-4Y tends to heal the defect sites, decreasing the disorderliness and converting it into NTHU-4W.
The intrinsic white phosphor is tunable from yellow to a combination of blue and yellow, when synthesized in a solvent. The scientists plan to look further into the mechanism for the unusual yellow and yellow-to-white photoluminescence and the possible correlation between microporosity and luminescence. These findings may help advance LED technology by employing a new class of advanced inorganic phosphors.
Journal of the American Chemical Society, June 21, 2005 (online), doi:10.1021/ja0512879.
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