This article was modified on 21/03/2011
8 March 2011—Materials scientists at MIT say they’ve found a way to build simple semiconductor devices inside a fiber. This new manufacturing process could create fibers with logic, image-processing, and photovoltaic capabilities that could one day lead to smart, self-powered fabrics, say researchers.
Nicholas Orf a post-doctoral researcher in Yoel Fink's laboratory at MIT managed to synthesize semiconductor materials inside to a fiber, so that as the fiber is drawn out, the semiconductor forms into simple diodes with electrical contacts.
"You could end up with a piece of material that’s a kilometer long," says Fink. "That’s something that really could only happen if you can draw the materials."
A report on their work appears in this week’s early edition of Proceedings of the National Academy of Sciences.
Optical fiber is made starting with a thick, cylindrical boule of material—generally silica, although Orf and his colleagues used a polymer instead. The material is heated until it’s soft enough to flow. Then it is drawn out into a fiber that’s often hundreds of meters long but just a few micrometers in diameter.
Engineers have been intrigued by the idea of incorporating semiconductors into the fiber, but many useful semiconductors have high melting points and won’t flow at the temperatures typically used in the drawing process. Fink’s team got around this problem by using a multistep process. They started by lowering the melting points of the components of the compound semiconductor zinc selenide—mixing the zinc with tin and the selenium with sulfur. They drilled slots along the surface of a solid boule of the polymer polyethylsulfone and inserted tin-zinc wires into the slots. They then used evaporation to deposit a thin layer of the selenium sulfide on top of the wires and finally wrapped the whole thing with more polymer. This "preform" was then heated in a vacuum to form it into a single, solid structure; it was then pulled into fiber.
As the tin zinc and the selenium sulfide were pulled close together during the drawing process, they underwent a chemical reaction, crystallizing into small chunks of the semiconductor zinc selenide, with tin wires providing electrical contact. The result was semiconductor diodes, the building blocks of many circuits, spaced at regular intervals along the length of the fiber.
"The fact that you can synthesize a high-temperature compound [the zinc selenide] inside a low-temperature matrix is very interesting and surprising, and as far as I know, this is the first time it has been demonstrated in the context of a fiber," Fink says.
The fiber drawing process essentially makes large structures much smaller and brings them closer together, so it should be possible to lay out fairly complex circuits in the preform to create nanometer-scale devices within the fiber. With the right reagents, Fink says, the process should work to produce a wide variety of semiconductor materials. That could mean future to flexible electronic textiles and fibers that can convert light into electricity; perhaps one day a T-shirt could power a cellphone. In 2009, Fink and his team developed fibers with embedded semiconducting glass capable of sensing the angle, intensity, phase, and wavelength of light. This material could be the basis for a fabric that acts as a lensless camera. The new work, he says, could lead to more efficient and responsive light-sensing fibers.
"These fibers open the door to intriguing possibilities," says John Ballato, an optical fiber researcher at Clemson University, in South Carolina. However, the use of polymers and soft semiconductors and metals may limit the amount of optical power such a fiber could handle, he says. But the drawing method "can be considered an important step to a ’fiber that does everything’—creates, propagates, senses, and manipulates photons, electrons, phonons, et cetera."
This article appeared in print as “How to Draw a Diode."