December 09, 1999
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Black Silicon, A New Way To Trap Light

By William J. Cromie
Gazette Staff

Eric Mazur and Claudia Wu hope to make solar cells (background) much more efficient and affordable. Photo by Justin Ide.
Visit the Mazur Group web page
Learn more about black silicon

A new type of material, accidentally discovered in a Harvard physics laboratory, could lead to more efficient ways of converting sunlight to electricity, communicating by light, and monitoring the environment for evidence of global warming.

The material came to light when some graduate students and their adviser, Eric Mazur, decided to treat silicon with a high-intensity laser. Silicon is the substance of which virtually all computer chips are made; without it, there would be no Internet, no cellular telephones, no electronics.

Mazur, Harvard College Professor and Gordon McKay Professor of Applied Physics, and his students were studying what kinds of new chemistry can occur when lasers shine on metals, like platinum. One day, they decided to put a chip of gray silicon into a vacuum chamber, add some halogen gas, and scan it with ultrashort, ultra-intense laser pulses.

Each pulse lasted a mere 100 millionths of a billionth of a second. However, the energy in a single pulse approximates focusing all the sunlight hitting Earth at one time onto a space the size of a fingernail.

After more than 500 pulses, the silicon turned black. It wasn’t burned; rather, its surface had been etched by the heat and gas into a dazzling forest of billions of minute needlelike spikes. If a light is shone on such a surface, it repeatedly bounces back and forth between the spikes in a way that most of it never comes back out again.

Mazur and his team quickly realized that anything that absorbs light this well would make an excellent solar cell, converting much more sunlight into electricity than any device now on the market.

The spiky surface also absorbs infrared radiation (heat), making it an excellent detector of clouds, pollution, water vapor, and specks of dirt and liquid that change the quality of our air and influence global climate.

There’s also the possibility that black silicon can be used for extremely fine computer and other electronic displays and for deli very of various drugs through the skin.

"Apart from serendipitously making something beautiful and unexpected, we’re getting bombarded with queries from industry about its many tantalizing practical applications," says Mazur.

A Magic Moment

At the time of the silicon experiment, Mazur and graduate students Claudia Wu and Tsing-Hua Her were trying to capture what is called "a magic moment" in science. They wanted to observe the birth of a new molecule on the surface of a metal hit with an intense burst of light. Many such reactions are important in industry, technology, and environmental monitoring.

"When, instead, we created a silicon chip as black as soot, we asked ourselves, ‘What is that?’ " Mazur recalls. "Looking at its surface under an electron microscope, we saw a neat forest of spikes, regular in size and spacing."

The spikes rose about two-thousandths of an inch high, with extremely sharp tips, about one-hundredth the diameter of a human hair.

"To get such a pattern, it’s essential that the laser pulses be very short and intense," Mazur explains. "Also the type of gas bathing the silicon is critical." Halogen gases like chlorine or sulfur hexafluoride work fine, but no spikes form with nitrogen or helium. This suggests that the spiky pattern is etched by chemical reactions – activated by the light – between the gas and the silicon.

Normal silicon, gray and translucent, absorbs only about 60 percent of sunlight striking its surface, reflecting the rest back into the air. Treated correctly, the black variety absorbs 96 to 98 percent of the light that hits it.

Commercial solar cells, usually made from silicon, are only about 40 to 50 percent efficient at converting sunlight directly into electricity. That level of inefficiency makes them expensive. A solar-power installation for a typical house in the United States costs $15,000 to $20,000. T he price would need to drop to about $6,000 to be competitive with oil or natural gas. Black silicon could help close the gap.

And the sooner the better, because air pollutants emitted by burning oil, natural gas, gasoline, and coal apparently are adding to, or even causing, global warming. Looking further ahead, such fuels are bound to run out at the rate we are using them.

"These fuels probably will no longer satisfy our needs by the end of the next century or sooner," Mazur believes.

Communication by Light

Another potential application involves telecommunication by light along fiber-optic cables, which is much faster than via copper wires. Ordinary silicon does not absorb light at wavelengths critical for such optical communications (1-2 microns); black silicon does.

"Being able to use a material that is cheap and abundant and that industry has learned how to process offers a tremendous opportunity in a multibillion-dollar market," Mazur comments.

Black silicon also absorbs infrared frequencies that are transparent to ordinary silicon but important to probing the environment. The National Aeronautics and Space Administration (NASA) is especially interested in materials that can be used to detect the amount of heat absorbed by everything in the atmosphere from clouds to tiny particles of soot and dust.

"Silicon could be an ideal material from which to make such detectors: it is cheap, and manufacturers have extensive experience in working with it," Mazur notes. The black variety now appears to make this prospect a realistic expectation, and NASA is providing funds for further experiments.

Mazur says that he is getting "lots of calls from industry" about practical uses of black silicon, and Harvard has filed a patent on the technology his group has developed to make it. The calls mostly concern solar cells and detectors for telecommunications and environmental monitoring, but other, more s peculative applications exist.

Besides absorbing light, black silicon can be made to emit light – to luminesce. Its spiky tips could create extremely fine lines and sharply defined shapes in electronic displays that would be exceptionally energy-efficient.

Also, these silicon tips, which are much thinner than the sharpest needles, might provide a better way to pierce the tough outer skin of humans and deliver continuous small doses of drugs. The body adjusts better to such doses than to a hypodermic needle full of a drug delivered all at once. Nitroglycerin is now given this way to relieve chest pain due to inadequate blood flow to heart muscles.

"A company in Germany is experimenting with black silicon for drug delivery, and we have started a collaboration with scientists at Harvard-affiliated Massachusetts General Hospital, in Boston," Mazur notes.

Summing up the situation, Mazur refers to the discovery of black silicon as "one of those times in research when your work takes a sharp, unpredictable turn. You try something on the side that yields unanticipated results. That’s why basic research, in addition to increasing knowledge, is so much fun."


Copyright 1999 President and Fellows of Harvard College