Tuesday, November 25, 2008

Color Quantum-Dot Displays

A new way to print quantum dots could lead to brighter, more power-efficient displays.

By Prachi Patel-Predd

Dot matrix: Using a new stamping technique, researchers can imprint a surface with a grid of red, green, and blue light-emitting semiconductor nanocrystals, called quantum dots. Quantum-dot displays would have brighter colors than LCDs and consume only a fraction as much power. Credit: Bulovic group, MIT

Researchers at MIT have shown that they can use rubber stamps to deposit quantum dots--tiny light-emitting crystals--on a surface. The technique lets them put rows of different-colored dots next to each other, a crucial step in the development of color quantum-dot displays, which promise to be thinner, more flexible, brighter, sharper, and more power efficient than flat-panel LCDs.

Startup QD Vision, based in Watertown, MA, is commercializing the approach, which was described online in Nano Letters. Affordable displays based on the technique could be on the market as soon as 2011, says the company's chief technology officer, Seth Coe-Sullivan, one of the coauthors of the Nano Letters paper.

Quantum dots are 3- to 12-nanometer-wide crystals of semiconductors, which emit different colors depending on their composition and size. Quantum dots are appealing for displays because they can ultimately use one-fifth to one-tenth as much power as LCDs, which require backlights to illuminate their pixels.

In this respect, quantum dots are similar to organic light-emitting diodes (OLEDs), another display technology, which is used chiefly in cell phones and MP3 players. But quantum dots outdo OLEDs in the purity of the colors that they emit. "A typical OLED that glows green also gives aqua and yellowish photons [that] make it look whitish green, so it'll be more washed out," says MIT electrical-engineering professor Vladimir Bulovic, who led the new work. "Quantum dots give a very narrow emission spectrum, so the perception of color coming from them appears to be much more rich."

In a standard quantum-dot display, nanocrystals of a semiconductor like cadmium selenide are sandwiched in a single layer between two organic films. Until now, depositing the quantum-dot layer generally involved making a solution, coating the organic film with it, and then evaporating the solvent. That method didn't allow swaths of different-colored crystals to be placed next to each other, says Bulovic, impeding the development of multicolor displays. Each pixel in a color display is divided into three subpixels--red, green, and blue--that are mixed in varying intensities to produce millions of colors. In desktop-sized displays, the subpixels are 20 to 50 micrometers across.

For about two years now, Bulovic and his colleagues have been experimenting with a new deposition technique, which involves a simple twist on the old one. Instead of coating a surface directly, they first coat a rubber stamp that has been premolded with ridges. After evaporating the solvent, the researchers press the stamp on the desired surface to transfer the dots. The technique creates stripes of quantum dots that are 25 micrometers wide. By printing red, green, and blue stripes that criss-cross each other, the researchers produce pixels with 25-micrometer-wide subpixels.