Porous silicon
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Porous silicon (pSi) is a form of the chemical element silicon which has an extremely large surface to volume ratio.
It is usually manufactured by etching away most of a layer of silicon using wet chemical etches. Many etches are based on hydrofluoric acid. Electrochemical etching is the most commonly used porous silicon fabrication methods. What is left is a fine connected network of sub micrometre silicon threads. These thin structures often have properties that are different to bulk crystalline silicon. Fine control of these etching processes enables the properties of the network to be repeatably controlled. For example, controlling the mean diameter of the threads or the pores.
One property changed is luminescence. Ordinary amorphous, solid Silicon does not exhibit electroluminescence. Porous silicon exhibits electroluminesce at a range of wavelengths from the near infrared to the visible (blue). The wavelength depends on the thickness of the threads and on the chemical state of their surfaces. Porous silicon also emits light under illumination (photoluminescence), first reported by Leigh Canham 1990.[1]
Like bulk silicon, porous silicon is a semiconductor.
Porous silicon exhibits a high degree of biocompatibility. The large surface area enables bio-organic molecules to adhere well. It degrades to silicic acid, which causes no harm to the body. This has opened potential applications in medicine such as a framework of the growth of bone.
In 2001, a team of scientists at the Technical University of Munich inadvertandly discovered that hydrogenated porous silicon reacts explosively with oxygen at cryogenic temperatures, releasing several times as much energy as an eqivalent amount of TNT, at a much greater speed (an abstract of the study can be found below). Explosion occurs because the oxygen, which is in a liquid state at the necessary temperatures, is able to oxidize through the porous molecular structure of the silicon extremely rapidly, causing a very quick and efficient detonation. Although hydrogenated porous silicon would probably not be effective as a weapon, due to its functioning only at low temperatures, other uses are being explored for its explosive properties, such as providing thrust for sattelites.
Other materials can be made porous in a similar way, such as the semiconductor gallium arsenide.
[edit] See also
[edit] External links
[edit] References
- Porous Silicon, edited by Z C Feng & R Tsu, World Scientific (Singapore), 1994 ISBN 981-02-1634-3
- Properties of Porous Silicon, edited by L.T Canham,Inspec, IEE, London, 1997
- ^ Canham L.T. Appl. Phys. Lett. vol. 57 (1990) page 1046
