Nanocrystals give pure white light Optical materials
The amount of electricity used in lighting could be cut drastically by switching from incandescent and fluorescent sources to highly efficient white light-emitting diodes (LEDs). Making such LEDs requires high-quality phosphors that emit white light when excited by an ultraviolet source.
Researchers from the Indian Institute of Science in Bangalore, and the Indian Association for the Cultivation of Science in Kolkata have now developed a new class of white light phosphor based on doped semiconducting nanocrystals [Nag and Sarma, J. Phys. Chem. C (2007) 111, 13641].
White phosphors can be made by blending semiconducting nanocrystals that have different colored emissions. However, these tend to produce tinted, unstable shades of white light.
Instead, Angshuman Nag and Dipankar Das Sarma have used transition-metal-doped nanocrystals, where white light is generated by combining nanocrystal surface-state emissions and d–d transitions from dopant centers.
The approach was illustrated with 1.8 nm sized Mn2+-doped CdS nanocrystals. White light of differing shades was produced by controlling the dopant concentration, and the nanocrystals could be excited over a wide range of wavelengths without compromising the color produced. Furthermore, the large separation between the absorption and emission spectra minimizes self-absorption. “Self-absorption, which affects the purity of the white light produced, is a notorious problem with white LEDs. Our results show a way to get rid of this vexing problem,” says Sarma.
Work is now underway to boost the phosphor's quantum efficiency