Mysterious Black Silicon Could Have Bright Future in Many Fields
When researchers at Harvard University first looked at the strange silicon chip they had produced in their lab, they couldn’t believe their eyes. The chip, which had been bombarded by powerful laser bursts lasting less than a trillionth of a second, had turned as black as tar.
The chip appeared black because it absorbed virtually all the light that fell on its surface; but when the researchers looked at it with an electron microscope, the mystery only deepened.
The researchers thought they might have fried the surface, leaving it covered with soot, but the microscope revealed something quite different.
“We saw these incredible spikes,” physics professor Eric Mazur said. Billions of needlelike silicon spikes, about two-thousandths of an inch high with tips one-hundredth the diameter of a human hair, blanketed the surface like a forest of tiny needles.
The researchers had accidentally discovered black silicon, and if it lives up to its promise, it could revolutionize everything from solar cells to optical communications.
Light striking the surface of black silicon bounces back and forth between the spikes, and virtually none of it escapes. That absorbing power makes it an astonishingly effective candidate for a wide range of applications.
The researchers made their discovery more than a year ago, but “we tried to keep it low-key for a while just because we wanted to be 100% sure that this was something truly interesting,” Mazur said.
But last March, they presented their findings to a meeting of the American Physical Society in Atlanta.
Since then, Mazur has been bombarded with requests from various industries for samples, including manufacturers of photovoltaic solar cells. The National Aeronautics and Space Administration is also involved because of possible use of the material for ultra-sensitive atmospheric monitoring.
All of the attention is both “thrilling” and “bewildering” to Mazur for two reasons: “There’s no question that we can create a material that has absorption characteristics that are unlike anything that has ever been seen before,” Mazur said. “But it is totally mystifying and baffling as to why it happens.”
In short, there is no explanation for why the laser pulses etched such a dramatic pattern on the surface of the silicon.
Furthermore, researchers will need months of testing in various labs to see if the absorbed light produces enough of an electric current to make it commercially valuable.
Mazur said that every test conducted in his lab shows the material absorbs both visible and infrared light, which is critical for optical communications and atmospheric sensors, and that labs outside Harvard have had the same results.
He said that tests so far have shown that black silicon is far more efficient at producing a current than normal silicon, but that the mechanism remains unclear.
“It’s like stepping into unknown territory and trying to understand what is going on,” he said.
However it turns out, Mazur has reached a turning point in his career, all because a couple of graduate students were trying to do something quite different in the lab. Tsing-Hua Her, a doctoral candidate, was hoping to create a new molecule when he bombarded the surface of a metal with an intense burst of light.
He put a chip of gray silicon into a vacuum chamber, added some halogen gas and then scanned it with ultra-short, ultra-intense laser pulses. Each pulse lasted less than a trillionth of a second, but the energy is equivalent to concentrating all the sunlight hitting the Earth at one time onto a space the size of a fingernail.
After 500 pulses, the silicon turned black.
He received his doctorate and moved on, and the torch was passed to another graduate student, Claudia Wu. At that point, Mazur said, “it sort of exploded.”
The researchers published their findings in Applied Physics and Physical Review Letters, two peer-reviewed journals, and the lid was really “off the box.”
Harvard is negotiating with a number of manufacturers, including some of the leaders in photovoltaics, the most obvious application for the new technology. The current generation of commercial solar cells convert only about 40% to 50% of the light hitting their surface into electricity. That runs up the cost because most of the energy is just reflected back by the mirror-like silicon.
If black silicon increases that efficiency significantly, it could help bring the cost down for solar energy.
Mazur cites telecommunications as another possible application. A long-range goal of that industry is to communicate by light through fiber-optic cables. But ordinary silicon does not absorb light at the critical wavelengths of 1 micron to 2 microns, and black silicon does.
It also absorbs infrared radiation, so black silicon could detect the heat signature of airborne particles, enhancing environmental monitoring.
Black silicon can also be made to luminesce, or emit light, by exciting it with a laser. And because the needle tips are so fine, Mazur believes it might be useful for high-definition displays.
Those sharp needles also could be used to pierce human skin, and one company in Germany is experimenting with using black silicon to provide a continuous flow of pharmaceutical drugs.
These days Mazur sounds a little like a kid with a terrific new toy, unsure exactly why or how it came his way, and a little uneasy about what it all means.
“It’s so easy in science to get very excited about something, only to find out that there’s a problem somewhere,” he said.
But with all the attention flowing his way, chances are he’s on to something.
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Lee Dye can be reached at leedye@ptialaska.net.
