Cornell materials scientist Chekesha Liddell uses an old science to create new crystals.

By Kenny Berkowitz and Robert Emro

MSE graduate student Ian Hosein prepares a column to separate colloids by size as Assistant Professor Chekesha Liddell looks on. University Photo

"I loved chemistry from the first class that I took at Spelman," says Liddell, now an assistant professor in materials science and engineering at Cornell. "So when I went to Georgia Tech, I started reading through the course catalog, looking for a discipline where I’d be able to do the kind of analytical investigations that excite me, and I found materials science, which is really a mixture of chemistry, physics, and engineering. I could see exactly how to apply the chemistry that I had learned, so I took my first materials class, and I just loved it. From then on, I knew it was exactly what I wanted to do."

It’s a cold January morning, with a dusting of snow on the Engineering Quad, and in the white-cinderblock quiet of her Bard Hall office, Liddell pages through her laptop, explaining the pleasures of self-assembly. Still jet-lagged, she’s recently returned from a conference to promote nanoscience collaborations between India and the United States. Before that, she was in Japan for the National Academy of Sciences Frontiers of Science Symposium, and before that, at the White House, where she was honored as one of 20 winners of a Presidential Early Career Award for Scientists and Engineers from the National Science Foundation. (Cornell’s Brian Kirby, assistant professor of mechanical and aerospace engineering, received the same award from the Department of Energy.)

It’s the latest in a series of honors that began in high school with a National Achievement Scholarship and continued in college with a NASA Women in Science and Engineering Scholarship. There, at NASA’s Kennedy Space Center, two summers spent as an intern in the microchemical analysis laboratories helped point her toward nanoscience.

"That’s really where I got excited about understanding materials at small scales and being able to solve problems with that understanding," says Liddell. "We actually investigated some space shuttle tiles, which were a very big issue at the time, in the wake of the Challenger disaster. It was like being in an episode of CSI, where we were assigned to figure out why these tiles hadn’t stayed bonded. We were detectives, discovering how engineering could mean the difference between life and death."

Ten years later, Liddell keeps a piece of that tile in a drawer behind her desk. After graduating summa cum laude in 1999 with two bachelor’s degrees—one in chemistry and one in materials science engineering—she stayed at Georgia Tech to earn her doctorate in 2003. Faced with a choice between academia and industry, she accepted an assistant professorship at Cornell, arriving in November 2003 and quickly going to work.

Confocal microscope image of fluorescent hollow-silica shell, peanut-shaped particles.  Chekesha Liddell

In her dissertation work at Georgia Tech, Liddell made colloids of non-spherical particles with the idea that they could be processed into photonic crystals. Because of their highly ordered microstructure, these crystals interact with light in ways that give them useful properties. Opal is a naturally occurring example, consisting of roughly spherical particles. Synthetic photonic crystals can be created with complex nanofabrication machinery, or more simply with colloids, the science of which was already established when Cornell was founded in 1868.

Colloidal crystal self-assembled by controlled drying (convective assembly) of a suspension of "mushroom-cap" shaped polystyrene particles. The particle size is 1.2 micrometers in outer diameter and 890 nanometers in height. Chekesha Liddell

In theory, researchers should be able to create photonic structures from these non-spherical shapes as well. Computer modeling shows that the additional complexity of the non-spherical building blocks should give the material the desired photonic properties even if its structure is not quite perfect. And the different shapes give rise to new packing patterns which affect the ways the new materials interact with light. In practice, however, preparing uniform non-spherical particles and drying the liquid slowly enough to allow them to self-assemble into highly ordered structures has been challenging. For the past five years, that’s what Liddell has been trying to do.

Liddell holds up a small vial with the thin film that remains after the liquid has been evaporated from one of her colloids. It shines with the iridescence of mother of pearl. "My central interest is in understanding the role of symmetry and crystal structure in the interaction between light and colloidal materials," says Liddell. "I’m interested in expanding understanding from the opal-type structures through a range of structures with new geometries."

The goal is to create low-cost packings over large areas that control light far better than anything that currently exists in the photonic crystal field, and if the theory is right, the applications for these photonic structures are numerous. Using titanium dioxide, these new geometries could conceivably power the next generation of solar cells. In medicine and biotechnology, photonic crystals of metals like silver could greatly increase the sensitivity of tests designed to detect the presence of proteins, DNA, or pesticides. And crystals made with germanium and silicon could ultimately advance optical communications, leading to a new world of photonic computers.

"The implications are enormous," says graduate student Ian Hosein, a member of Liddell’s research group. "People are already familiar with semiconductor technology and the fact that so many of our devices, like cell phones and computers, work on semiconductors that control electrical current and transform it into a form of electronic information. We’re trying to do a similar thing with light: to make a semiconductor that can channel light into optical information. So in the same way that we have electronic switches and electronic memory storage, we can envision optical switches and optical storage."

Though it’s too early to know the full impact of their efforts, Liddell and her group have been working steadily, producing eight papers and 43 presentations in the last four years. In the cover story of the October 2007 issue of Langmuir, Liddell’s group showed how they created two-dimensional packings predicted to respond strongly to light from asymmetric, peanut-shaped particles.

Asymmetric dimers in an assembly. Chekesha Liddell

"When you are just starting your career, it’s all too easy to focus on a specific problem and miss the big picture," says collaborator Fernando Escobedo, associate professor of chemical and biomolecular engineering. "I am very impressed that Chekesha has a vision of what she wants to do and a good balance of the scientific and technological understanding she needs. She has great potential, and you can see the ability she has to ask the right questions and integrate that new knowledge. She’s very humble, very nice, very polite—and also very, very busy."

Liddell teaches two courses a year, an undergraduate class in the atomic and molecular structure of matter and an upper-level seminar in colloid assembly that culminates in each student writing an NSF-styled grant proposal for funding new work in colloid research. "I try to really engage students in their own learning, instead of having them just tacitly listening to a lecture," she says. "That’s the fun part for me, the creative part. It leads to a much stronger interaction with the material, and really draws students into the process."

For the students around her, the underlying lessons are clear. "Sometimes people don’t try things because they think, ‘Well, that will never work,’" says graduate student Stephanie Lee, a member of the Liddell group. "But Professor Liddell is not that kind of person. She thinks, ‘I don’t see why it shouldn’t work.’ So even though people said we couldn’t make assemblies with non-spherical particles, we aggressively pursued different processing strategies until we found something that worked. She just goes after what she thinks is right. She’s not intimidated by the hard things."

As the daughter of a college administrator and a social worker, Liddell takes her responsibilities as a mentor very seriously. "That was one of the big draws of this job, that I’d be helping people prepare for the future," she continues. "So I try to spend time emphasizing communication skills, sending students to conferences early on, and talking with them often about professional issues. When I think about this career, I ask myself, ‘What do I want to do with my life? How do I want to contribute to the world?’ Developing people is a very worthwhile goal—that’s what drew me to education."

Hosein, Liddell, and MSE graduate student Stephanie Lee evaluate the first batch of particles collected from the column. University Photo

Liddell would like to introduce more young people to engineering, using home science kits to produce materials like the irridescent film in the vial and jigsaw puzzles with enlarged images of her photonic crystals to teach the basics of materials science. "We want to take these images and use them as tools to talk about the differences between self-assembly and directed assembly or microfabrication," says Liddell. " We’re working on colloidal materials that children can make at home, by adding water and then letting it dry out. Then, with a laser pointer, they could see the diffraction patterns and recognize how the light is scattered into different patterns by different structures."

Writing her proposal for the Presidential Early Career Award helped focus her thinking about outreach, and now that she’s won it, with the opportunity to publicly recognize some of her most important mentors at the ceremony, Liddell has found herself doubly committed to making a contribution to the world. "These structures we’re building and the principles behind them could be fundamental for many different research areas," she says. "There are always going to be new questions, new problems that people haven’t thought of before, and my goal is to develop the fundamental understandings that can be passed down to the next generation of researchers. Tools change, so just as we can go back to contributions people made twenty years ago, we can pass on our new ideas to the next generation of researchers—who knows the exciting places they will lead?"