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.
At QD Vision, older deposition techniques have led to either single-color or low-resolution displays in which each color pixel is millimeter scale, Coe-Sullivan says.
The new method also produces more energy-efficient light-emitting devices. That’s because the solvent is evaporated before the quantum dots are deposited. “Putting the material on a stamp and then transferring it dry is a great way of doing things,” says Ghassan Jabbour, a materials-engineering professor at Arizona State University. “It avoids any contact of the bottom layers”–the surface on which the dots are being deposited–“with the solvent.” That improves efficiency, Jabbour says, since “solvents can interact with the bottom organic layer and degrade the device performance.” Indeed, Bulovic says that the devices the researchers built using the stamps are the most efficient they’ve made in the lab.
Using the older deposition technique, QD Vision has already developed two commercial products–one for general illumination and one for consumer-electronics devices–that the company plans to launch by May 2009. But, Coe-Sullivan says, “to make high-information-content displays like TVs or cell phones, you need to be able to pattern at the 40- to 100-micrometer level. As we get to future product platforms that involve high-resolution multicolor displays … the stamp printing technique will be the enabler.”
Coe-Sullivan says that QD Vision should be able to use quantum-dot stamps to make displays as big as current LCDs, which could give quantum dots an advantage over OLEDs. Large OLED displays are difficult to make, because their manufacture involves spraying organic-semiconductor molecules through a stencil that has nanoscale pinholes in it. Although Samsung has demonstrated prototypes of 40-inch OLED TVs, the only commercially available model is Sony’s 11-inch TV, which sells for $2,500. Coe-Sullivan expects quantum-dot displays to be more cost-competitive with LCD TVs.
It might even be possible, Jabbour says, to use the new deposition technique for roll-to-roll printing, which would enable quick production of flexible displays. “A roll-to-roll printer is nothing but a stamp that is rolling around at 100 kilometers per hour,” Jabbour says, “so it doesn’t really matter if it is a flat or a round stamp.”
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