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Nanoparticles generate white light

18 Jul 2003

Could cadmium sulfide quantum dots replace phosphors in solid-state lighting?

Scientists at Sandia National Laboratories, US, have developed a solid-state white-light emitting device based on quantum dots.

Lauren Rohwer and colleagues encapsulated the quantum dots in epoxy. When excited by ultraviolet light from an LED, these nanoparticles re-emit light at a longer wavelength, just like a phosphor.

The problem with conventional phosphors is that careful mixing of red, green and blue light is needed to produce white illumination, demanding a relatively complicated chip design.

But the color of the light re-emitted by a quantum dot depends largely on its diameter, with smaller particles emitting at a shorter wavelength. Rohwer encapsulated nanoparticles of different diameters, tailored to emit in the red, green or blue, within a single device to form the white-light source.

There is one serious drawback to the Sandia work, however. The material used in the quantum dots is cadmium sulfide. Cadmium is a toxic heavy metal and the researchers say that they need to replicate the work with an alternative material, ideally silicon.

Despite this problem, Rohwer says that the work could lead to an entirely new approach in solid-state lighting: “Highly efficient, low-cost quantum dot-based lighting would represent a revolution in lighting technology through nanoscience,” she said.

Rohwer and the Sandia team encapsulated commercial LED chips emitting at 400nm in a dome-shaped quantum dot-filled epoxy. In their scheme, the dots are attached to the encapsulating polymer. This stops the nanoparticles clumping together, which tends to reduce the overall efficiency of the device.

Michael Hatcher is technology editor of Opto & Laser Europe magazine.

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