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By changing the size of the silicon nanocrystals, the color of the light emitted can be varied.

Researchers at Karlsruhe Institute of Technology (KIT, Germany) and University of Toronto (Canada) built efficient LEDs based on silicon nanocrystals and organic materials that emit colors from red all the way to yellow (Nano Letters DOI: 10.1021/nl3038689). That's not a complete spectrum, but it's getting there. Moreover, the LEDs improve on previous nanocrystalline Si LED results by offering a longer life and less sensitivity to the applied drive voltage.
Hybrid quantum-dot LEDs give off bright colors from inorganic emitters with easy processing from spin-cast organic layers. Most research has been focused on the use of II-VI quantum dots, which are made using toxic and expensive materials such as cadmium selenide, cadmium sulfide or lead sulfide. However, silicon—the workhorse of microelectronics—is fairly benign, common and cheap.
The color emitted by the nanocrystals depends on their size. The researchers separated out the nanocrystals, ranging from 1 to 3 nm in diameter. They found that this separation increases the lifetime of the devices by reducing short circuits from oversized particles. The devices produce photoluminescent quantum yields up to 43 percent.
If the researchers can get silicon to emit the rest of the visible spectrum, this could offer a way to make large area Si LEDs at low cost.