May 2006

Nanocrystal Displays

Continued from page 1

By Kevin Bullis

"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.