May 2006

Nanocrystal Displays

QD Vision's Seth Coe-Sullivan is using quantum dots to make vibrant, flexible screens.

By Kevin Bullis

Coe-Sullivan holds a prototype quantum dot display; such displays emit extremely pure colors and could eventually be scaled up to compete with conventional screens. (Photo credit: Porter Gifford.)

Seth Coe-Sullivan, chief technology officer at Watertown, MA, startup QD Vision, fastens alligator clips to two edges of a transparent wafer the size of a cell-phone screen and flips a switch: a rectangle filling the center of the wafer suddenly turns from reflective silver to faint red. A lab worker turns off the room lights to heighten the effect -- but this isn't necessary. Coe-Sullivan turns a knob and the device begins glowing brilliantly.

[For images of this research, the team, equipment, and prototypes, click here.]

This is QD Vision's first display -- a monochromatic 32-by-64-pixel test bed for a technology Coe-Sullivan hopes will replace those used in today's high-definition TVs. Thin and flexible, the next-generation display will be easy to see in sunlight and less power hungry than the one in your current laptop, he says. It will also cover more of the visible color spectrum than current displays and produce such high-contrast images that today's flat-screen displays will look dull and washed out by comparison.

At its heart are nanoparticles called quantum dots, nanoscale semiconductor crystals. By altering the size of the particles, researchers can change the color they emit: for example, a six-nanometer-diameter particle would glow red, while another of the same material but only two nanometers wide would glow blue.

Where these particles really shine is in the purity of the colors they emit. Displays create millions of colors from a palette of just three: each pixel is made of a red, a green, and a blue subpixel, and varying their relative intensities varies the pixel's apparent color. In LCDs and organic light-emitting devices (OLEDs), a new kind of display, the subpixel colors are impure. The red, for example, while made mostly of red light, also contains smaller amounts of other colors. With quantum dots, however, the red subpixel emits only red.

This purity means quantum dot-based displays have more-saturated color than LCDs, OLEDs, and even bulky cathode-ray tubes (CRTs), which are still prized for their excellent color rendition. What's more, Coe-Sullivan says, the range of colors possible in a quantum dot display is 30 percent greater than in CRTs: "We're increasing the depth of the green that screens can display, and the depth of the blue-green, et cetera. It's actually a different color than can be seen on an LCD, OLED, or CRT."

Perhaps what is most exciting about quantum dot LEDs (QD-LEDs) is that they use much less power than LCDs. In LCDs, a backlight illuminates every pixel on the screen. Dark pixels are simply blocking this light, in effect wasting energy. In part because quantum dots emit light rather than filtering it, a QD-LED display could potentially use one-30th the power of an LCD.

And there's another benefit to not having a backlight, according to Vladimir Bulovic, an expert at MIT in OLED displays. Because in LCDs the dark pixels don't block light perfectly, Bulovic says, the "black" pixels on LCDs are really just dark grey. With quantum dots, on the other hand, black pixels emit no light. "What makes the picture crisp and really jump out at you is that the black is really, really dark," he says.