A bed of amorphous hydrogenated silicon wires that were prepared in the pores of optical fibers. The wires have been chemically etched out of the optical fiber to reveal them. Scale bar is 100 µm. Inset: An array of amorphous hydrogenated silicon tubes deposited in an optical fiber. The optical fiber has been cleaved in half to reveal the array of tubes. The very thin glass walls of the fiber surrounding each tube are largely obscured. Scale bar is 5 µm. Image: John Badding laboratory, Penn State University |
A
new chemical technique for depositing a non-crystalline form of silicon
into the long, ultra-thin pores of optical fibers has been developed by
an international team of scientists in the United States and the United
Kingdom. The technique, which is the first of its kind to use
high-pressure chemistry for making well-developed films and wires of
this particular kind of silicon semiconductor, will help scientists to
make more efficient and more flexible optical fibers. The findings, by
an international team led by John Badding, a professor of chemistry at
Penn State University, will be published in a future print edition of
the Journal of the American Chemical Society.
Badding
explained that hydrogenated amorphous silicon—a noncrystalline form of
silicon—is ideal for applications such as solar cells. Hydrogenated
amorphous silicon also would be useful for the light-guiding cores of
optical fibers; however, depositing the silicon compound into an optical
fiber—which is thinner than the width of a human hair—presents a
challenge. “Traditionally, hydrogenated amorphous silicon is created
using an expensive laboratory device known as a plasma reactor,” Badding
explains. “Such a reactor begins with a precursor called silane—a
silicon-hydrogen compound. Our goal was not only to find a simpler way
to create hydrogenated amorphous silicon using silane, but also to use
it in the development of an optical fiber.”
Because
traditional, low-pressure chemistry techniques cannot be used for
depositing hydrogenated amorphous silicon into a fiber, the team had to
find another approach. “While the low-pressure plasma reactor technique
works well enough for depositing hydrogenated amorphous silicon onto a
surface to make solar cells, it does not allow the silane precursor
molecules to be pushed into the long, thin holes in an optical fiber,”
says Pier J. A. Sazio of the University of Southampton in the United
Kingdom and one of the team’s leaders. “The trick was to develop a
high-pressure technique that could force the molecules of silane all the
way down into the fiber and then also convert them to amorphous
hydrogenated silicon. The high-pressure chemistry technique is unique in
allowing the silane to decompose into the useful hydrogenated form of
amorphous silicon, rather than the much less-useful non-hydrogenated
form that otherwise would form without a plasma reactor. Using pressure
in this way is very practical because the optical fibers are so small.”
Optical
fibers with a non-crystalline form of silicon have many applications.
For example, such fibers could be used in telecommunications devices, or
even to change laser light into different infrared wavelengths.
Infrared light could be used to improve surgical techniques, military
countermeasure devices, or chemical-sensing tools, such as those that
detect pollutants or environmental toxins. The team members also hope
that their research will be used to improve existing solar-cell
technology. “What’s most exciting about our research is that, for the
first time, optical fibers with hydrogenated amorphous silicon are
possible; however, our technique also reduces certain production costs,
so there’s no reason it could not help in the manufacture of
less-expensive solar cells, as well,” Badding says.
In
addition to Badding and Sazio, other members of the research team
include Neil F. Baril, Rongrui He, Todd D. Day, Justin R. Sparks,
Banafsheh Keshavarzi, Mahesh Krishna-murthi, Ali Borhan, and Venkatraman
Gopalan of Penn State; and Anna C. Peacock and Noel Healy of the
University of Southampton in the United Kingdom.
