March 27, 2000
Vol. 89, No. 21

Senior thesis leads to Nature paper

By Steven Schultz 

In a finding that could benefit the manufacture of ultra-small electronic devices, Princeton scientists have discovered a simple and inexpensive way to make microscopic patterns with particles of plastic and other materials.

The research, which is published in the March 2 issue of Nature, was done by an undergraduate working on his senior thesis.

Ryan Hayward '99 was trying to understand some basic mysteries concerning the way particles arrange themselves on surfaces, when he discovered that he could direct the process with a beam of ultraviolet light. It turned out that it was much easier to control the process than he or his advisers, professors of chemical engineering Ilhan Aksay and Dudley Saville, had imagined.

"This could have been the nucleus of an excellent PhD thesis," says Aksay, one of the coauthors on the Nature paper. "It attests to the quality of undergraduate students here at Princeton."

Particles on a thin film

Aksay and Saville had published results in 1996 showing that they could pack particles onto a thin film of metal oxide by applying an electric current. They did not fully understand the details of this process and were looking for a student who would work on explaining it.

By the fall of 1998, Aksay and Saville had found the student they wanted; it was just a question of convincing Hayward that it was the best thesis topic for him.

"They played a little psychology with me," recalls Hayward, who is now a PhD candidate at the University of California, Santa Barbara. "They told me how challenging a problem it was. They figured I couldn't turn down a challenge, and they were right."

When Aksay and Saville handed Hayward the problem, they mentioned that light seemed to have some effect on how the particles stuck together. In the first few weeks, Hayward realized that this seemingly obscure point could be exploited to great advantage, and he moved quickly to develop the idea.

Hayward started by applying an electric field to a random collection of particles to make them assemble on an electrode. He then showed that when he directed an ultraviolet light through a tiny mesh onto the electrode, the small beads formed a pattern that mirrored the mesh.

The technique is attractive because making almost any pattern is simply a question of constructing a mask with the desired pattern of openings. The lithographic techniques for making the mask are expensive, but once that is done, an endless number of copies can be made. Hayward's technique also could allow scientists to "draw" the desired pattern with a laser. A computer-controlled ultraviolet laser could scan across the film of particles, creating a thin line as it moves.

Hayward mainly used polystyrene particles that measured about two microns in diameter. That is about one hundred times smaller than a human hair. He also showed the process works with submicron-sized pieces of silica, a ubiquitous component of electronic circuitry.

Electronic, optical devices

The electronics industry is interested in ways of making patterns out of such particles for use in a variety of electronic and optical devices, says Aksay. For example, the patterns could form channels that guide light within an electronic device. A trend in electronics is to use beams of light, rather than electricity in wires, to transmit information. Over long distances, light can be transmitted through long strands of glass called fiber optic cables. Inside a small device, however, these fibers can be too bulky. Minute channels on a circuit board could be a more compact way to guide the light.

These details were not on Hayward's mind when he started the project; he just knew it was interesting, but very difficult work.

"It's hard to say what I was expecting," he says. "I tried to go in with an open mind and then take the most interesting path. Both advisers were very good about encouraging me to take whatever path seemed most promising, rather than making me follow some predetermined course."

As a graduate student, Hayward is continuing to tinker in the world of the very small. He has not decided what his dissertation research will be, but he has already been working on other techniques for making particles self-assemble into very small structures.

"I like the idea of being able to control the properties of material on very small scales," he says. "It's neat because it's a domain that you don't normally think you can control."