Feb. 9, 1998 WEST LAFAYETTE, Ind. -- A bright but frail member of the silicon family has found new vigor through a process developed at Purdue University, lighting the path to faster, smaller computers and new types of sensing devices.
Jillian Buriak, assistant professor in Purdue's Department of Chemistry, has developed a way to stabilize the surface of porous silicon, a light-emitting material that offers great promise for combining light and electronics to build new types of computers and other optical devices.
The method also may lead to fine-tuned sensors that can be used to perform real-time measurements in manufacturing and medicine.
"This is the most stable porous silicon surface to date," Buriak says. "Using our treatment, we can produce an incredibly stable surface that should stand up to the rigors of use."
Purdue is pursuing a patent on the method. Details of the discovery will be published in the Feb. 17 issue of the Journal of the American Chemical Society.
Porous silicon is identical in makeup to the silicon used in many technological applications today, but its surface contains tiny openings -- or pores. The pores contain microscopic structures made of silicon that emit light when ultraviolet light is applied. This type of silicon has been known to scientists since the 1950s, when they discovered that silicon could not always be polished smooth during manufacturing.
It wasn't until 1990 that this "rough" or porous silicon was found to have photoluminescent properties. In 1992, scientists discovered that it also emits light when electric current is applied, a finding that opened the door to coupling light and electronics to build computers and other devices.
"Because most of our current technology is based on silicon, it may be relatively easy to develop the optical applications and combine them with current technologies, as the manufacturing processes are already in place," Buriak says.
For example, porous silicon could serve as a flat, millimeter-thick display area for computer screens, replacing large, bulky computer screens that depend on cathode-ray tubes.
The properties of porous silicon also make it an ideal material to develop computers based on light signals instead of electrical signals. Such computers would be faster, as beams of light can transmit information much more quickly than electrons making their way through a solid material.
Using light to transmit data also would eliminate heat buildup in computers, allowing scientists to design smaller computers by stacking multiple layers of chips made of porous silicon.
Though the properties of porous silicon offer promises of powerful new technologies, Buriak says that until now, the untreated material was too fragile to hold up to these applications. Oxygen and water molecules in the air interact with the surface of porous silicon to create a glass-like coating that disrupts its photoluminescent properties.
"Within a few weeks, the material will oxidize, or 'rust,'" Buriak says. "But in this case, instead of leaving a brownish rough coating, the oxidation process produces a smoother, glass-like surface that limits the function of the material."
Buriak, working with undergraduate researcher Matthew Allen of Swartz Creek, Mich. , found a way to prevent this oxidation using a chemical process that works in liquids.
"A lot of reactions involve chemical bonds similar to the ones that develop on the surface of porous silicon. So I listed these reactions and came up with one that I thought had the best chance of working without damaging the surface," Buriak says.
"What we came up with is a very clean, very easy, room-temperature, one-hour reaction that allows us to stabilize the surface."
Buriak coats the porous surface of the silicon with Lewis acid, a solution that brings about a reaction that produces a greasy coating that protects the surface while allowing the porous silicon to maintain its photoluminescent properties.
To test how well the treatment stands up to environmental stresses, Buriak boiled samples of treated and untreated porous silicon in a highly basic solution of potassium hydroxide for an hour.
"Silicon and silica compounds generally dissolve in a solution with a pH greater than 7," she says. "By boiling it, we are accelerating the aging process to test how well this stabilizing method will stand up to rigorous conditions over a period of time."
The treated surfaces showed no oxidation and only minor changes in photoluminescent properties, while the surfaces of the untreated samples dissolved.
"This indicates that, once it is treated, the surface will remain stable for long periods of time," she says.
The new treatment also will allow scientists to add other compounds to the surface, so that the light-emitting properties of porous silicon can be manipulated to respond to certain chemicals or conditions.
This feature can be exploited to develop new types of medical or industrial sensing devices, Buriak says.
"When UV light strikes the surface of porous silicon, it reradiates back in the red wavelength, producing a bright orange color," Buriak says. "But if we add, for example, a chemical that binds to sodium ions, when sodium is present it will cause the reradiated wavelength to shift, producing a different color such as yellow or red. So you could look at the color difference and see whether sodium is present, and at what concentration it's present."
Using this knowledge, scientists could design sensing devices that could be used in a doctor's office, eliminating the need to send blood and other tissue samples to a laboratory for testing.
The same techniques could be applied to develop sensors that respond immediately to chemical changes in the environment. Such sensors could be used in factories to perform real-time, on-line quality control measurements.
"Currently, if you want to check a chemical mixture during the manufacturing process, you have to go through a time-consuming process of taking a sample and sending it to a quality control lab where it is tested," Buriak says. "The ideal situation would be to have a sensor placed in the vat where the chemical mixtures are prepared, so that the mixture is continuously monitored during the process."
Buriak says that with the development of a stable form of porous silicon, such applications may be in place within three to five years. Her research was funded by Purdue.
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