Porous silicon is a nanostructured material prepared by electrochemical or chemical etching of crystalline silicon. It displays tuneable structural properties: a large specific surface area, large free volume, and pore sizes that can be controlled from a few nanometers to several hundreds of nanometers depending on the preparation conditions.
The surface of freshly prepared porous Si is easily modified with a large range of inorganic, organic, or biological molecules. It is biocompatible and bioresorbable. In the body, the silicon nanostructure oxidizes and hydrolyzes to silicic acid, Si(OH)4, a water-soluble species that is naturally present in human tissues.
Like many Si-based materials, porous Si offers attractive morphological and chemical properties for various applications but it has one supplementary dimension: its optical properties. Porous Si displays photoluminescence and electroluminescence deriving from Si quantum dot structures that are produced during the etch, and it can be prepared in the form of photonic crystals, to display unique optical reflectivity spectra.
Both the active (photoluminescence) and passive (optical reflectance) features allow porous Si to exhibit a signal that is affected in a predictable way when exposed to environmental changes. This presents new possibilities for the development of more advanced functional systems that incorporate a sensor for either diagnostic or therapeutic functions. The ease with which porous Si can be integrated into well-established Si microelectronics fabrication techniques allows its use in sophisticated devices for medical, sensor, thermoeletric, photovoltaic, and energy storage applications.
Porous silicon was accidentally discovered by Uhlir at Bell Laboratories in the mid 1950s . He was trying to develop a means to electrochemically machine silicon wafers for use in microelectronic circuits. He found that under the appropriate conditions of applied current and solution composition, the silicon did not dissolve uniformly but instead fine holes were produced, which propagated primarily in the <100> direction in the wafer. Since this did not provide the smooth polish they desired, these curious results were reported in a Bell labs technical note  and the material was pretty much ignored. In the 1970s and 1980s a moderate level of interest arose because the high surface area of porous Si was found to be useful as a model of the crystalline Si surface in spectroscopic studies [2-5], as a precursor to generate thick oxide layers on Si, and as a dielectric layer in capacitance-based chemical sensors . Quantum confinement effects and in particular room-temperature photoluminescence was discovered from this material in the late 1980s by two research groups working independently: Leigh Canham at the Defense Research Agency in England , and Ulrich Goesele and Voelker Lehmann at Duke University in the United States . Both teams reasoned that the pore walls in the material could be made sufficiently thin to display quantum confinement effects. This intuition turned out to be correct. Using a chemical dissolution process, Canham thinned the pore walls of the electrochemically etched material to thicknesses of less than 5 nm. This resulted in a material that glowed with a bright red-orange color when it was illuminated with ultraviolet light . As expected from the quantum confinement relationship, this color is at an energy that is significantly larger than the bandgap energy for bulk Si, which occurs in the infrared region of the spectrum [7, 8].
With the discovery of efficient visible light emission from porous Si came an explosion of work focused on creating Si-based optoelectronic switches, displays, and lasers. Problems with the materials chemical and mechanical stability and its disappointingly low electroluminescence efficiency led to a waning of interest by the mid 1990s. In the same time period, the unique features of the material- its large surface area within a small volume, its controllable pore sizes, its convenient surface chemistry, and its compatibility with conventional silicon microfabrication technologies-inspired research into applications covering a vast range of disciplines--electronics, biomedicine, optics, sensors, solar cells, and batteries. Of particular relevance to our research are the biomedical [10-18] and sensor properties of the material .
Synthesis of Nanocrystalline Porous Silicon Layers. Porous silicon is most commonly generated by etching crystalline silicon in aqueous ethanolic hydrofluoric acid (HF) electrolytes. The open pore structure and large specific surface area (a few hundred m2 per cm3, corresponding to about a thousand times the surface of the polished silicon wafer) make porous Si a convenient material for sensitive detection of liquid and gaseous analytes. The ability to electrochemically tune the pore diameters  and to chemically modify the surface [21, 22] provides control over the size and type of molecules adsorbed.
Typical Preparation. This is the preparation used to prepare a Fabry-Perot layer or a photonic crystal. We usually use highly doped p-type, boron doped, polished (100) silicon wafers, with a resistivity of between 0.0005 and 0.001 Ohm-cm and 400 microns thick. We have found Siltronix to be a consistent, high quality manufacturer of these materials. A square approximately 1 cm on an edge is cleaved, and the chip is placed in a Teflon etch cell shown at left (for schematics click here), using a piece of aluminum foil as a back contact and a Viton O-ring to seal the cell. The cell is filled with a 3:1 (v/v) mixture of 48% aqueous HF and absolute ethanol (CAUTION: Hydrofluoric acid is highly toxic and corrosive and contact with skin should be avoided. Procedures involving HF should always be carried out in a fume hood configured to handle HF and the operator should wear appropriate protective gloves, gown, and face shield). A loop of platinum wire is immersed in the solution as the counter electrode. An anodic current of approx. 40 mA/cm2 is passed between the aluminum back contact and the platinum counter electrode for 5-15 minutes. For a photonic crystal such as a rugate filter, the current density is modulated between approx 10 and 100 mA/cm2 with a period of approx 10 sec. After completion of the etching, the cell and sample are washed several times with ethanol and dried under a stream of nitrogen. This produces a film that is a few microns thick with pores on the order of several nm in diameter.
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