Stable surface of porous silicon
Purdue University recently developed a method for stabilizing the surface of porous silicon. This new material can be used to combine light and electronics to build computers and optical devices. It can also be used to develop precise sensors for real-time measurements, for example in production and medicine. Porous silicon is identical in composition to the silicon used in many technological applications today; however, its surface contains tiny pores. These pores contain microscopic structures of silicon that emit light when exposed to ultraviolet light. Scientists have known about this type of silicon since the 1950's when they discovered that silicon could not always be polished smooth during manufacture.
It wasn't until the 1990s, however, that porous silicon was discovered to have photoluminescent properties. In 1992, scientists discovered that it also emits light when an electric current is applied. This realization opened up the possibility of coupling light and electronics to build computers and other devices.
Untreated porous silicon, however, oxidizes within a short time. The oxidation process creates a smooth, glass-like surface that limits the function of the material. Together with her assistant Matthew Allen, Jillian Buriak, an assistant professor of chemistry at Purdue University, has now discovered a way to prevent the surface from oxidizing using a chemical process. “Many reactions involve chemical bonds similar to those that form on the surface of porous silicon. So I listed these reactions and came across a reaction that I thought might prevent the surface from oxidizing,” says Buriak."The result was a very clean, very simple 1 hour reaction at room temperature that allowed us to stabilize the surface."
Buriak coats the porous silicon surface with a Lewis acid (EtAlCl2). The hydrosilylation of alkynes and alkenes using the Lewis acid resulted in vinyl and alkyl groups covalently bonded to the surface. The Lewis acid plays a dual role in this-it mediates the hydrosilylation event and acts as a reversible protecting group for Lewis bases in the unsaturated substrate. The resulting coating protects the surface while maintaining the photoluminescent properties of the porous silicon.
To test how stable the surface is to external influences, Buriak boiled treated and untreated samples in a strongly basic potassium hydroxide solution (KOH pH 10) for one hour. “Silicon compounds generally dissolve in basic solutions. We accelerated the aging process by boiling to test how well this stabilization method withstands extreme conditions over a period of time.” The treated surfaces showed no oxidation and their photoluminescent properties changed only slightly, whereas the porous layer of the untreated samples dissolved. "This indicates that the surface will remain stable over long periods of time once treated."
"To date, this is the most stable porous silicon surface," says Buriak. "With the help of our process, we can create a surface that should also meet the toughest requirements." Purdue is currently applying for a patent for the method. Details of the discovery are presented in the February 17 issue of the Journal of the American Chemical Society.
"Because much of our modern technology is based on silicon, optical applications could be developed relatively easily and combined with modern technologies since the production processes are already in place," says Buriak. For example, porous silicon could serve as a flat, millimeter-thick display area for computer screens, replacing today's bulky cathode-ray monitors.
The properties of porous silicon also make it an ideal material for the development of computers that operate on the basis of light signals instead of electrical signals. Such computers would be significantly faster because light can transmit information much faster than electrons. Using light to transmit data would also eliminate heat build-up in computers, allowing smaller computers to be built by stacking multiple porous silicon chips on top of each other.
According to Buriak, this property can be exploited for the development of new types of medical or industrial measuring devices. "When UV light hits the surface of porous silicon, it is reflected in the red wavelength and produces a bright, orange color," says Buriak.“However, if you add a chemical that reacts with sodium, for example, the reflected wavelength will change, creating a different color, such as yellow. So you can just look at the color difference to see if sodium is present and at what concentration.”
Using this knowledge, scientists could develop measuring devices that could be used in a doctor's office, so that blood and other tissue samples would no longer have to be sent to a laboratory for analysis. The same techniques could be used to develop sensors that respond instantly to chemical changes in the environment. Such gauges could be used in factories to perform real-time and on-line quality control measurements.
"Currently, if you want to check a chemical mixture during the manufacturing process, you lose a lot of time taking samples and sending the samples to a quality control laboratory," says Buriak."The ideal situation would be a sensor placed in the tub where the chemical mixtures are prepared so that the mixture is continuously monitored throughout the process."
Buriak believes that once a stable form of porous silicon has been developed, such applications could exist within three to five years.
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