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Let there be Nanolight

The future is already bright for light-emitting diodes (LEDs), which are based on semi-conducting materials—mainly gallium nitride. In fact not a month goes by without engineers finding a new use for LEDs in television sets, remote controls, automobiles, cell phones, street and interior lighting, etc. “But we're now trying to incorporate nanocrystals into them, rather than the conventional layers that convert blue to yellow, as in fluorescent tubes,” explains Jean-Yves Duboz, director of CNRS' Research Center for Heteroepitaxy and its Applications (CRHEA).1

scanning electron

© E. Perrin/CNRS Photothèque

Scanning electron microscope image of micro-disks of aluminum gallium nitride, a semi-conductor used to make LEDs.



Nanocrystals emit photons (light) when electrons (i.e., an electric current) pass through them, and the color of the light emitted depends on the size of the nanocrystal. “White light is obtained by a clever mix of red, blue, and green photons, which means that you have to couple three LEDs,” explains Duboz. “What we're trying to do here is to develop LEDs that only use a single crystal that can directly emit all colors.
To do this, we insert nanometer-sized components in the crystal, which are little chunks of various sizes that emit blue, green, red, etc., together producing white light. On paper, it's an ideal solution, though for the moment our LEDs are still not powerful enough.” Although their output is actually very efficient, the white LEDs now on the market have a luminous efficacy of 100 lumens2 per watt (lm/W), as opposed to 60 to 80 lm/W for fluorescent lamps and 16 lm/W for incandescent bulbs. “There are prototypes that reach close to 200 lm/W,” Duboz continues. “But if LEDs are very efficient when they work at low currents, their efficacy falls off a little (75 lm/W) at the higher currents needed for cheaper street and industrial lighting.”
To get around this problem, researchers make use of photonic crystals, a type of light cage made up of a regular arrangement of nanostructures that works like reflective road signs. This should “improve the directionality of LEDs,” comments Henri Benisty, from LCFIO,3 “preventing them from dispersing light in all directions.”

Notes :

1. Centre de recherche sur l'hétéroépitaxie et ses applications (CNRS).
2. The lumen is the unit of light flux.
3. Laboratoire Charles Fabry de l'Institut d'Optique (CNRS / Université Paris-XI).

Contacts :

Jean-Yves Duboz,
jean.yves.duboz@crhea.cnrs.fr
Henri Benisty,
henri.benisty@institutoptique.fr


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