Researchers have used a coating of nanocrystals to make LEDs that produce a warm white light:
Their LEDs have a high "colour rendering index" of more than 80 out of 100, meaning objects will tend to appear their usual colour under the light. That is similar to the best fluorescent lights, but behind incandescent bulbs which define the index with a benchmark of 100.
To accomplish this, Hilmi Volkan Demir and colleagues at Bilkent University in Ankara, Turkey, coated blue LEDs with a layer of nanocrystals. These crystals are made from a core of cadmium selenide with a surrounding layer of zinc sulphide.
The crystals absorb some of the LED's blue output and emit their own red and green light. That combines with the remaining blue light to produce a soft white glow.
Existing commercial white LEDs are also based on blue LEDs. But they use a phosphor coating that converts some blue light into a broad spectrum of yellow light. When mixed with remaining blue light the result is a harsh blue-hued white.
Nanocrystals emit light in a much tighter range of wavelengths than phosphor, making it possible to fine-tune the colour produced, Demir says. "Using combinations of nanocrystals, one can generate any emission spectrum as desired," he told New Scientist.
The efficiency is also remarkable. The scientists say their ned LEDs give off more than 300 lumens per watt which is ten times more than commercial white LEDs and twenty times more than traditional incandescent bulbs. More info at NewScientist.