Nanocrystal Displays

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.

“Beakers of This Glowing Green Stuff”
The idea to use quantum dots in displays is not new. In the early 1990s, when chemists such as Moungi Bawendi, now an MIT professor of chemistry and scientific advisor at QD Vision, were perfecting techniques for forming precise, uniform quantum dots, some tried to make QD-LEDs but produced only dim, inefficient devices that required about a hundred thousand electrons to coax quantum dots to emit a single photon. In contrast, Coe-Sullivan’s QD-LEDs require only about 50 electrons per photon.

Achieving this advance required the right people to come together at the right time. That happened in 2000, when Coe-Sullivan came to MIT as a graduate student and met Bawendi and a brand new MIT electrical-engineering professor who had arrived a few weeks before – Vladimir Bulovic.

Just inside the door to QD Vision’s lab is a row of flasks containing a bubbling red liquid – a solution of recently formed quantum dots. The collaboration that led to the first efficient QD-LED display began after Bulovic, on a visit to MIT, stumbled upon a similar scene in the lab of one of Bawendi’s collaborators.

Bulovic says that before he encountered “beakers of this glowing green stuff” at MIT, he had “never heard of quantum dots.” Coe-Sullivan borrowed Bulovic’s knowledge of OLED fabrication tricks and Bawendi’s quantum dot expertise and also enlisted the help of fellow students Jonathan Steckel and Wing-Keung Woo.

Even with all this expertise, however, the breakthrough that enabled the device occurred partly by accident. The researchers had mixed quantum dots into a solution of organic molecules and spread the mixture into a thin film using a process called spin-casting, in the hope that the quantum dots would disperse evenly through the film. As it turned out, the quantum dots rose to the surface of the film and assembled in an orderly, uniform layer just one dot thick, an arrangement that turned out to be more efficient than the one the researchers had intended.

This layer of quantum dots became the core of a multilayer single-color QD-LED, sandwiched between electrodes and charge transport layers. Coe-Sullivan, along with Bulovic and Greg Moeller, director of business development, founded QD Vision in 2004 to move from this simple device to a full-color display that can be profitably manufactured.

A major step was arranging arrays of pixels. At QD Vision, Coe-Sullivan points to a glass-front cabinet carefully blocked off to hide part of a proprietary process for distributing quantum dots in the alternating three-color rectangular grids necessary for a working display. Already the technique, which Coe-Sullivan says should lead to relatively inexpensive manufacturing, has produced patterns with pixels smaller than those typical of current displays.

Coe-Sullivan says QD Vision should be able to borrow from OLED tech-nology one key component of displays, the “back plane” that controls the pixels. Now the company is focused on improving the efficiency of its device, which, while competitive with cell-phone displays, could still be improved.

In all, Coe-Sullivan says he expects that it will be about four years before the company has its first commercial product – probably a small display for a cell phone. But he says the colorful images will be worth the wait.

Home page image courtesy of Porter Gifford.