Page 5 - UD Research Magazine Vol5-No2

Changing the color of light
UD researchers have received a $1 mil- lion grant from the W.M. Keck Foundation of Los Angeles to explore a
new idea that could improve solar cells, medical imaging and even cancer treatments. Simply put, they want to change the color of light.
They won’t be tinkering with what you see out your window: no purple days or chartreuse nights, no edits to rainbows and blazing sunsets. Their goal is to turn low-energy colors of light, such as red and orange, into higher-energy colors, like blue or green.
Changing the color of light would give solar technology a considerable boost. A traditional solar cell can only absorb light with energy above a certain threshold. Infrared light passes right through, its energy untapped.
However, if that low-energy light could be transformed into higher-energy light, a solar cell could absorb much more of the sun’s clean, free, abundant energy. The team predicts that their approach could increase the efficiency of commer- cial solar cells by 25 to 30 percent.
The research team, based in UD’s College of Engineering, is led by Matthew Doty, associate professor of materials science and engineering and co-director of UD’s Nanofabrication Facility. Doty’s co-investigators include Joshua Zide, Diane Sellers and Chris Kloxin, all in the Department of Materials Science and Engineering; and Emily Day and John Slater, both in the Department of Bio- medical Engineering.
A ray of light contains millions and millions of individual units of light called photons. The energy of each photon is di- rectly related to the color of the light—a photon of red light has less energy than a photon of blue light. You can’t simply turn
a red photon into a blue one, but you can combine the energy from two or more red photons to make one blue photon.
This process, called “photon upconver- sion,” isn’t new. However, the UD team’s approach to it is.
They want to design a new kind of semiconductor nanostructure that will act like a ratchet. It will absorb two red photons, one after the other, to push an electron into an excited state when it can emit a single high-energy (blue) photon.
These nanostructures will be so teeny they can only be viewed when magnified a million times under a high-powered electron microscope.
“Think of the electrons in this structure as if they were at a water park,” Doty says. “The first red photon has only enough energy to push an electron half- way up the ladder of the water slide. The second red photon pushes it the rest of the way up. Then the electron goes down the slide, releasing all of that energy in a single process, with the emission of the blue photon. The trick is to make sure the electron doesn’t slip down the ladder before the second photon arrives. The semiconductor ratchet structure is how we trap the electron in the middle of the ladder until the second photon arrives to push it the rest of the way up.”
The UD team has shown theoretically that their semiconductors could reach
an upconversion efficiency of 86 percent, which would be a vast improvement over the 36 percent efficiency demonstrated by today’s best materials. What’s more, Doty says, the amount of light absorbed and energy emitted by the structures could be customized for a variety of applications, from lightbulbs to laser-guided surgery.
As part of the project, the team aims to develop an upconversion nanoparticle that can be triggered by light to release its pay- load. The goal is to achieve the controlled release of drug therapies even deep within diseased human tissue while reducing the peripheral damage to normal tissue by minimizing the laser power required.
“This is high-risk, high-reward research,” Doty says. “High-risk because we don’t yet have proof-of-concept data. High-reward because it has such a huge potential impact in fields from renewable energy to medicine. It’s amazing to think that this same technology could be used to harvest more solar energy and to treat cancer.” —Tracey Bryant
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