Researchers at Purdue University (West Lafayette, IN) have discovered how to harness the light-emitting properties of porous silicon to stabilize the material's surface and direct it to respond to specific chemical environments or cues. The development may allow scientists to tap the unique photoemissive qualities of porous silicon to create new types of drug-delivery systems, or biological and chemical sensors capable of performing real-time measurements in medicine and manufacturing, says Jillian Buriak, associate professor in Purdue's Department of Chemistry.
“We've shown, for example, that we can tailor the surface of a porous silicon wafer to survive within simulated in-vivo conditions, such as those found in blood plasma,” she says. “Untreated porous silicon dissolves too quickly to be useful in such environments. By functionalizing the surface, we may be able to develop sensors for use in diagnosing and treating disease.” Buriak and colleagues also believe the procedure can be used for applications in optoelectronics or integrating light-emitting devices with silicon chips.
The study, published in the 32nd issue of the Journal of the American Chemical Society, details how Buriak, working with doctoral student Michael P. Stewart, used the light-emitting properties of porous silicon to carry out an unprecedented chemical reaction on material's surface. The study also is the first to illustrate how nanocrystalline silicon – a form of porous silicon made up of crystals measuring just billionths of a meter in diameter – works to emit light. Nanocrystalline silicon gets its name from nanometer, which is one-billionth of a meter, or about 100,000 times smaller than the width of a human hair.
“This reaction only works on nanocrystalline silicon,” Buriak says. “It belongs to this whole field of nanotechnology where properties, such as color and reactivity, can depend on size. If you keep cutting a material down to the nanoscale, you may reach a point where the size will change the properties of the material. It gets really wacky.”
Though porous silicon is identical in makeup to the silicon used in many microelectronic and computing applications today, its surface contains tiny openings, or pores. In 1990 scientists discovered that some forms of porous silicon can absorb and emit light, properties that offered promises of powerful new technologies. To date, no commercial devices using the light-emitting powers of porous silicon are available, in part due to stability problems and insufficient output.
“Left untreated, porous silicon is too fragile to hold up to these applications,” Buriak says. “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.”
In 1998, the Purdue research group developed two new routes to protecting the surface from oxidation. The first involved treatment with a Lewis acid and a class of organic molecules. The second outlined preliminary results concerning this white light promoted reaction. “We had just published that second paper and then found that the reactions only work with porous silicon samples that emit light,” Buriak says. “Because only nanocrystalline silicon emits light, we hypothesized that a common mechanism linked the chemistry and light emission, which turned out to be correct.”
Buriak's latest study details a chain of events that occurs when photons of light interact with nanocrystalline silicon, causing electrons to jump to a higher energy level. Energy is then emitted in the form of light as the particles move back to their former state. In the process of moving to a higher energy level, the electrons leave a positively charged hole where a second electron might react, creating highly reactive molecules called excitons. This type of reaction, well understood in physics, also occurs in nanocrystalline particles of some other materials such as titanium dioxide, which is used in solar cell applications.
Using white light of moderate intensity from a tungsten source, Buriak and her team created excitons in the laboratory by exposing wafers made of nanocrystalline silicon to the light for 30 to 60 minutes in the presence of alkenes or alkynes, chemicals compounds that contain hydrogen and carbon. “In this highly reactive state, the nanocrystalline silicon reacts with the compounds to create a carbon-silicon bond that produces a stabilizing coat,” she adds. “This is a very clean, very practical reaction that allows us to stabilize the surface without the need for special equipment.” An additional benefit of the reaction is that by using a mask between the porous silicon and the light, the reaction can be photopatterned, since it only takes place where the surface is illuminated. This step allowed the researchers to tailor the material in specific areas, directing the light-emitting properties of porous silicon to respond with the chemicals where it was required.
“This method can be exploited to develop new types of sensing devices for use in medicine or industry,” Buriak says. “In drug delivery, for example, you need a device that releases a drug molecule in a very specific way. By functionalizing the surface of silicon in this way, we can develop a sensing device capable of bonding to specific sites or detecting specific molecules.”
The study also demonstrates how the material can be divided into defined arrays to prompt different chemical reactions on different parts of the material. Buriak says this feature can be useful for applications in optoelectronics, bioanalysis or integrating light-emitting devices with silicon chips.
“The beauty of porous silicon is that there are so many different ways that you can get a readout,” she says. “You can use the light emission as a readout, or watch it shift around or go up or down. You also can manipulate the material to get a readout that changes color depending upon the chemical composition of its environment.” Buriak says such applications may be in place within three to five years. Her research group is now looking at ways to use the light-promoted reaction to stabilize and tailor nanocrystalline silicon to develop nano-scale structures such as wires and flat sheets. Such structures could be used to combine light and electronics to build new types of computers and other optical devices. The Purdue scientists' research was funded by the National Science Foundation.