Imagine chipping a small sliver from a crystal of silicon, then chipping that sliver itself into ever smaller fragments until you end up with a particle composed of only a few thousand atoms of the original material.
What have you got? A quantum dot.
By itself this speck of silicon won't change the world, but large numbers embedded in a host material could revolutionize the flow of information between computers, telephones, and other such devices. To that end Kirk D. Kolenbrander, an assistant professor in materials science and engineering, and others at MIT are working to create and characterize these tiny particles.
The key to it all is the dots' behavior with light. Because they are so small, these particles of silicon or other semiconductors have special optical qualities. Specifically, explained Professor Kolenbrander, the atoms in each dot are no longer part of an infinite (to them) array of atoms, so "each atom can recognize that it's part of a finite system. It can see edges."
And that disrupts the band gap of the material, a property unique to every semiconductor and one that determines color. "So now this 5,000-atom particle doesn't know what color it is," Professor Kolenbrander said.
But scientists have observed a strange phenomenon when the dots are exposed to laser light: they show a non-linear response to the light, or change color depending on how much light strikes them. "And that behavior you can exploit in a device," Professor Kolenbrander said.
Specifically, quantum dots could be used to create optical switches that are turned on and off by light.
Currently the switches that regulate telephone calls and other forms of data transmission work electronically: a flow of electrons is turned on and off. But with our ever-increasing need for data, eventually even electronic switches will not be able to keep up.
"Just as a telegraph key had fundamental limits as to how fast it could be switched, so too does an electronic device," explained Professor Kolenbrander.
For example, a great deal of information currently travels across oceans and countries over strands of hair-thin optical fibers. But because information transmitted via light is still regulated at either end by electronics, we have hardly begun to take advantage of fiber optics' potential.
"It's like a 1,000-lane super highway with a one-lane toll booth at either end," Professor Kolenbrander said.
And even though microelectronics are here to stay, said Professor Kolenbrander, electronic switches will never be able to approach a data flow of that magnitude.
Optical switches, however, could break the technological barriers and allow a number of marvels. For example, engineers could tie together all of the nation's supercomputers to form a monster network. "That can't happen with anything approaching the existing rates [of data flow]," Professor Kolenbrander said.
Also, an extraordinary amount of information could be made available to individuals via optical fibers run to each home. "Imagine being able to select any movie ever made while in your home," Professor Kolenbrander said. "These movies would be available all the time-you just take them off [the fiber]."
First, however, scientists have to develop materials like quantum dots to do optical switching. And "such materials are in their infancy," Professor Kolenbrander said. "There are certainly materials in 1992 that can do it, but none that are robust, highly processable, and, frankly, useful. So it is a materials problem."
Professor Kolenbrander and others at MIT are focusing on quantum dots as one possible answer. Although all are working on ways to make the dots, characterize their optical properties, and place them in some matrix for a device, they are approaching the problem from different directions.
For example, some are creating the dots and host matrices by chemical synthesis processes. These researchers include Professors Richard R. Schrock of chemistry in collaboration with Robert E. Cohen of chemical engineering and Robert J. Silbey of chemistry. In addition, Professor Moungi G. Bawendi of chemistry is working on two different projects, one in collaboration with Professors Michael J. Cima of materials science and engineering and Alan T. Hatton of chemical engineering.
Professor Kolenbrander and graduate students Arun A. Seraphin and Leon A. Chiu are literally chipping small particles from a crystal of silicon with a laser, then doing a size selection to sort out the quantum dots (they are the only group working with silicon).
The dots are then incorporated into a host matrix by shooting them down at a cold plate of steel. At the same time, the host material is sprayed onto the plate. "It's roughly analogous to tossing marshmallows (quantum dots) into a jello salad (the host)," Professor Kolenbrander said.
He notes that the current dot/host material resulting from this process is "not technologically useful because the host is not presently technologically useful, but we have ideas to modify the processing scheme to get the dots into an immediately useful host."
Quantum dots and their optical qualities are still relatively new and untested, but hold great promise. Who knows. The next generation could rely on optical switches much as we rely on electronics, and our ancestors relied on the telegraph.
A version of this
article appeared in the
June 3, 1992
issue of MIT Tech Talk (Volume