For Andrew Koltonow, a Northwestern graduate student in materials science, it was just another typical day in the lab. He and another student were preparing slides with samples of various materials to further their research into improving solar cells. Koltonow loaded a slide into the microscope and looked through the lens. What he saw transformed his day from average to astounding. On the slide glowed a golden starburst surrounded by brilliant colors. It was so breathtaking that Koltonow photographed the image. That picture went on to win first place in the 2011 Northwestern Scientific Images Contest.
Can you tell me about the research that’s represented in this image?
The image came out of a project to make solar cells using a number of different carbon-based materials, such as graphene oxide, carbon nanotubes, and fullerenes. In the course of making this solar cell, we needed to come up with a blocking layer that would allow light-generated electrons to flow through the surface while blocking the lower energy electrons. This part wasn’t novel research on our end; we were just trying to adapt a common material—zinc oxide nanoparticles. We were trying to work a thin film of the particles into our system to see if it would improve it. Someone in our lab put a thin film of zinc oxide onto a bit of silicon and passed it to me. Without thinking anything of it, I put it under the microscope, and it was really colorful.
Were you surprised by what you saw?
I was taken aback. It took me a minute to sort out what I was looking at. I wasn’t expecting to see colors because the material is intrinsically white or sometimes a faint yellow color.
What happened to make it so colorful?
That’s a bit of a puzzle. But the most likely explanation is a well-known effect in thin films. Zinc oxide, silicon, and air all have different refractive indices. You know how if you stick a pencil into water, the pencil appears to be bent? That’s because the refractive index of the water is different from that of the air. At the interface where the refractive index changes very sharply, you tend to get a reflection. If you have two such interfaces and the distance between them is about the same wavelength as light, then you can get either constructive or deconstructive interference. If it’s deconstructive interference, then the two reflections cancel each other out and you see no light. Now the trick is this: when that particular length falls into the length scale of visible light, some colors are erased. That converts white light into colored light. In this image, the film of organic material is at the right thickness to display this effect. When the thickness changes, it changes what color gets blocked out. The effect is that you get continuously changing colors as the thickness of the film changes. It’s the same effect that causes oil slicks in parking lots to appear colorful.
The clusters of zinc oxide nanoparticles formed more slowly in the lower left corner of the image because a big drop of the deposition solution got stuck at that spot and took longer to dry. That’s why the clusters are large and spindly.
This work is to improve solar cells. Why do solar cells need improvement?
The ultimate goal for most solar researchers is to bring solar power to the point where it can effectively displace fossil fuels. To do this, you have to improve the price of power, in terms of the watts per dollar. Two major strategies for improving the amount of power you get per dollar are to make the solar cells more efficient or to make them less expensive. We’re trying to demonstrate that these materials we’re using could be used to make a novel sort of solar cell—the kind that could lend itself to the inexpensive manufacturing that the plastic industry uses. Right now we’re starting with expensive materials, but by studying and tuning the way they are assembled, we hope the end result could be manufactured inexpensively.
You mentioned that the image looked best in reflective dark-field mode. What is reflective dark-field mode?
It’s just a different way to look at samples under the microscope. In the imaging mode I used, light is shined from the top, and you view the sample from the top. So you’re looking at light that’s reflected, like in the oil slick example. But I then used a dark-field aperture, so it blocked out the light that was directly reflected and only caught light that was scattered by the material itself. In the ordinary bright field mode, the colors were fainter. When I caught only scattered light, it was striking.
Why are you interested in solar research?
Actually as a kid, I was really interested in geology. Through high school I moved toward chemisty, and eventually I worked my way into engineering with the intention of using chemistry and physics to solve human problems. As I see it, the single most urgent global problem is the energy crisis. Among depleting fossil fuels, the rising standard of living around the world, and the threat of global warming, we need big steps in energy research just to preserve our way of life as far as the use of energy is concerned.
Why solar as opposed to other alternate forms of energy?
I like the idea of making a solar cell that you can just stick out there, and it generates useful power with nothing other than sunlight. You don’t have to burn a fuel or generate radioactive waste. You don’t have to disrupt a river, like you might with hydroelectric energy. Solar power seems like the least intrusive way to make energy. It’s the most conservative way to harvest energy because it doesn’t disrupt anything, use any fuel, or emit any byproducts. On top of that, solar energy is a largely untapped resource that far exceeds the amount of energy that humanity uses. And unlike fossil fuels, we don’t expect the sun to go out anytime soon.