Porous silicon
Encyclopedia
Porous silicon is a form of the chemical element silicon
which has introduced nanoporous
holes in its microstructure
, rendering a large surface to volume ratio in the order of 500 m2/cm3.
in the U.S. At the time, the Ulhirs were in the process of developing a technique for polishing and shaping the surfaces of silicon and germanium
. However, it was found that under several conditions a crude product in the form of thick black, red or brown film were formed on the surface of the material. At the time, the findings were not taken further and were only mentioned in Bell Lab's technical notes.
Despite the discovery of porous silicon in the 1950s, the scientific community was not interested in porous silicon until the late 1980s. At the time, Leigh Canham
– whilst working at the Defence Research Agency
in England – reasoned that the porous silicon may display quantum confinement effects. The intuition was followed by successful experimental results published in the 1990. In the published experiment, it was revealed that silicon wafers can emit light if subjected to electrochemical and chemical dissolution.
The published result stimulated the interest of the scientific community in its non-linear optical
and electrical properties. The growing interest was evidenced in the number of published work concerning the properties and potential applications of porous silicon. In an article published in 2000, it was found that the number of published work grew exponentially in between 1991 and 1995.
In 2001, a team of scientists at the Technical University of Munich
inadvertently discovered that hydrogenated
porous silicon reacts explosively with oxygen
at cryogenic temperatures, releasing several times as much energy as an equivalent 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 satellite
s.
or anodization cell, drying and storage of porous silicon and surface modification needed.
(HF) electrolyte. Corrosion
of the anode
is produced by running electrical current through the cell. It is noted that the running of constant DC
is usually implemented to ensure steady tip-concentration of HF resulting in a more homogeneous
porosity layer although pulsed current is more appropriate for the formation of thick silicon wafers bigger than 50 µm.
It was noted by Halimaoui that hydrogen
evolution occurs during the formation of porous silicon.
in concentration exceeding 15%. It was found that the introduction of ethanol eliminates hydrogen and ensures complete infiltration of HF solution within the pores. Subsequently, uniform distribution of porosity and thickness is improved.
, nitric acid
and water
. A publication in 1957 revealed that stain films can be grown in dilute solutions of nitric acid in concentrated hydrofluoric acid. Porous silicon formation by stain-etching is particularly attractive because of its simplicity and the presence of readily available corrosive reagents; namely nitric acid (HNO3) and hydrogen fluoride (HF). Furthermore, stain-etching is useful if one needs to produce a very thin porous Si films. A publication in 1960 by R.J. Archer revealed that it is possible to create stain films as thin as 25Å through stain-etching with HF-HNO3 solution.
stress due to the minute size of the pores. In particular, it has been known that cracks will appear for porous silicon samples with thickness larger than a certain critical value. Bellet concluded that it was impossible to avoid cracking in thick porous silicon layers under normal evaporating conditions. Hence, several appropriate techniques have been developed to minimize the risk of cracks formed during drying.
Supercritical drying
Supercritical drying
is reputed to be the most efficient drying technique but is rather expensive and difficult to implement. It was first implemented by Canham in 1994 and involves superheating
the liquid pore above the critical point to avoid interfacial tension.
Freeze drying
Freeze drying
procedure was first documented around 1996. After the formation of porous silicon, the sample is frozen at a temperature of about 200 K and sublimed under vacuum.
Pentane drying
The technique uses pentane
as the drying liquid instead of water. In doing so the capillary stress is reduced because pentane has a lower surface tension than water.
Slow evaporation
Slow evaporating
technique can be implemented following the water or ethanol rinsing. It was found that slow evaporation decreased the trap density
bonded hydrogen. Although the hydrogen coated surface is sufficiently stable when exposed to inert atmosphere for a short period of time, prolonged exposure render the surface prone to oxidation by atmospheric oxygen. The oxidation promotes instability in the surface and is undesirable for many applications. Thus, several methods were developed to promote the surface stability of porous silicon.
An approach that can be taken is through thermal oxidation
. The process involves heating the silicon to a temperature above 1000 C to promote full oxidation of silicon. The method reportedly produced samples with good stability to aging and electronic surface passivation
.
Porous silicon exhibits a high degree of biocompatibility
. The large surface area enables 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
.
. One particular research in 2005 studied the mammalian cell adhesion on the modified surfaces of porous silicon. The research used rat PC12 cells
and Human Lens Epithelial (HLE)
cells cultured for four hours on the surface modified porous silicon. Cells were then stained with vital dye FDA and observed under fluorescence microscopy. The research concluded that ‘amino silanisation and coating the pSi surface with collagen enhanced cell attachment and spreading’.
, mesoporous, and microporous
.
and suggest that Si itself should be seriously considered for development as a material for widespread in vivo
applications. Another paper published the finding that porous silicon may be used a substrate for hydroxyapatite growth either by simple soaking process or laser-liquid-solid interaction process.
Since then, in-vitro studies have been conducted to evaluate the interaction of cells with porous silicon. A 1995 study of the interaction of B50 rat hippocampal cells
with porous silicon found that B50 cells have clear preference for adhesion to porous silicon over untreated surface. The study indicated that porous silicon can be suitable for cell culturing
purposes and can be used to control cell growth pattern.
ic silicic acid (SiOH4). Silicic acid is reputed to be the most natural form of element in the environment and is readily removed by kidney
s.
The human blood plasma
contains monomeric silicic acid at levels of less than 1 mg Si/l, corresponding to the average dietary intake of 20–50 mg/day. It was proposed that the small thickness of silicon coatings presents minimal risk to a toxic concentration being reached. The proposal was supported by an experiment involving volunteers and silicic-acid drinks. It was found that concentration of the acid rose only briefly above the normal 1 mg Si/l level and was efficiently expelled by urine excretion.
, microfluidic devices for the improved surface-based bioanalysis.
based on porosity and the medium inside the pores. The effective refractive index
of pSi is determined by the porosity and refractive index of the medium inside the pores. If the refractive index of the medium inside pores is high, the effective refractive index of pSi will be high as well. This phenomenon causes the spectrum
to shift towards longer wavelength
.
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
which has introduced nanoporous
Nanoporous
Nanoporous materials consist of a regular organic or inorganic framework supporting a regular, porous structure. Pores are by definition roughly in the nanometre range, that is between 1x10−7 and 0.2x10−9 m.Subdivisions:...
holes in its microstructure
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
, rendering a large surface to volume ratio in the order of 500 m2/cm3.
History
Porous silicon was discovered by accident in 1956 by Arthur Uhlir Jr. and Ingeborg Uhlir at the Bell LabsBell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
in the U.S. At the time, the Ulhirs were in the process of developing a technique for polishing and shaping the surfaces of silicon and germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
. However, it was found that under several conditions a crude product in the form of thick black, red or brown film were formed on the surface of the material. At the time, the findings were not taken further and were only mentioned in Bell Lab's technical notes.
Despite the discovery of porous silicon in the 1950s, the scientific community was not interested in porous silicon until the late 1980s. At the time, Leigh Canham
Leigh Canham
Leigh Canham is a British scientist who has pioneered the optoelectronic and biomedical applications of porous silicon.Leigh Canham graduated from University College London in 1979 with a BSc in Physics and completed his PhD at King's College London in 1983....
– whilst working at the Defence Research Agency
Defence Research Agency
The Defence Research Agency , was an executive agency of the UK Ministry of Defence from April 1991 until April 1995. At the time the DRA was Britain's largest science and technology organisation...
in England – reasoned that the porous silicon may display quantum confinement effects. The intuition was followed by successful experimental results published in the 1990. In the published experiment, it was revealed that silicon wafers can emit light if subjected to electrochemical and chemical dissolution.
The published result stimulated the interest of the scientific community in its non-linear optical
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
and electrical properties. The growing interest was evidenced in the number of published work concerning the properties and potential applications of porous silicon. In an article published in 2000, it was found that the number of published work grew exponentially in between 1991 and 1995.
In 2001, a team of scientists at the Technical University of Munich
Technical University of Munich
The Technische Universität München is a research university with campuses in Munich, Garching, and Weihenstephan...
inadvertently discovered that hydrogenated
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
porous silicon reacts explosively with oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
at cryogenic temperatures, releasing several times as much energy as an equivalent 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
Detonation
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...
. 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 satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s.
Fabrication of porous silicon
Fabrication of porous silicon may range from its initial formation through stain-etchingEtching (microfabrication)
Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....
or anodization cell, drying and storage of porous silicon and surface modification needed.
Anodization
One method of introducing pores in silicon is through the use of an anodization cell. A possible anodization cell employs platinum cathode and silicon wafer anode immersed in Hydrogen FluorideHydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
(HF) electrolyte. Corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
of the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
is produced by running electrical current through the cell. It is noted that the running of constant DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
is usually implemented to ensure steady tip-concentration of HF resulting in a more homogeneous
Homogeneous (chemistry)
A substance that is uniform in composition is a definition of homogeneous. This is in contrast to a substance that is heterogeneous.The definition of homogeneous strongly depends on the context used. In Chemistry, a homogeneous suspension of material means that when dividing the volume in half, the...
porosity layer although pulsed current is more appropriate for the formation of thick silicon wafers bigger than 50 µm.
It was noted by Halimaoui that hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
evolution occurs during the formation of porous silicon.
The hydrogen evolution is normally treated with absolute ethanol
“When purely aqueous HF solutions are used for the PS formation, the hydrogen bubbles stick to the surface and induce lateral and in-depth inhomogeneity”
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
in concentration exceeding 15%. It was found that the introduction of ethanol eliminates hydrogen and ensures complete infiltration of HF solution within the pores. Subsequently, uniform distribution of porosity and thickness is improved.
Stain etching
It is possible to obtain porous silicon through stain-etching with hydrofluoric acidHydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
, nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. A publication in 1957 revealed that stain films can be grown in dilute solutions of nitric acid in concentrated hydrofluoric acid. Porous silicon formation by stain-etching is particularly attractive because of its simplicity and the presence of readily available corrosive reagents; namely nitric acid (HNO3) and hydrogen fluoride (HF). Furthermore, stain-etching is useful if one needs to produce a very thin porous Si films. A publication in 1960 by R.J. Archer revealed that it is possible to create stain films as thin as 25Å through stain-etching with HF-HNO3 solution.
Drying of porous silicon
Porous silicon is systematically prone to presence of cracks when the water is evaporated. The cracks are particularly evident in thick or highly porous silicon layers. The origin of the cracks has been attributed to the large capillaryCapillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
stress due to the minute size of the pores. In particular, it has been known that cracks will appear for porous silicon samples with thickness larger than a certain critical value. Bellet concluded that it was impossible to avoid cracking in thick porous silicon layers under normal evaporating conditions. Hence, several appropriate techniques have been developed to minimize the risk of cracks formed during drying.
Supercritical drying
Supercritical drying
Supercritical drying
Supercritical drying is a process to remove liquid in a precisely controlled way. It is useful in the production of microelectromechanical systems , the drying of spices, the production of aerogel, and in the preparation of biological specimens for scanning electron microscopy.As the substance in...
is reputed to be the most efficient drying technique but is rather expensive and difficult to implement. It was first implemented by Canham in 1994 and involves superheating
Superheating
In physics, superheating is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling...
the liquid pore above the critical point to avoid interfacial tension.
Freeze drying
Freeze drying
Freeze drying
Freeze-drying is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport...
procedure was first documented around 1996. After the formation of porous silicon, the sample is frozen at a temperature of about 200 K and sublimed under vacuum.
Pentane drying
The technique uses pentane
Pentane
Pentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...
as the drying liquid instead of water. In doing so the capillary stress is reduced because pentane has a lower surface tension than water.
Slow evaporation
Slow evaporating
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
technique can be implemented following the water or ethanol rinsing. It was found that slow evaporation decreased the trap density
Surface modification of porous silicon
The surface of porous silicon may be modified to exhibit different properties. Often, freshly etched porous silicon may be unstable due to the rate of its oxidation by the atmosphere or unsuitable for cell attachment purposes. Therefore, it can be surface modified to improve stability and cell attachmentSurface modification improving stability
Following the formation of porous silicon, its surface is covered with covalentlyCovalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
bonded hydrogen. Although the hydrogen coated surface is sufficiently stable when exposed to inert atmosphere for a short period of time, prolonged exposure render the surface prone to oxidation by atmospheric oxygen. The oxidation promotes instability in the surface and is undesirable for many applications. Thus, several methods were developed to promote the surface stability of porous silicon.
An approach that can be taken is through thermal oxidation
Thermal oxidation
In microfabrication, thermal oxidation is a way to produce a thin layer of oxide on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The rate of oxide growth is often predicted by the Deal-Grove model...
. The process involves heating the silicon to a temperature above 1000 C to promote full oxidation of silicon. The method reportedly produced samples with good stability to aging and electronic surface passivation
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
.
Porous silicon exhibits a high degree of biocompatibility
Biocompatibility
Biocompatibility is related to the behavior of biomaterials in various contexts. The term may refer to specific properties of a material without specifying where or how the material is used , or to more empirical clinical success of a whole device in...
. The large surface area enables organic
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
molecules to adhere well. It degrades to silicic acid
Silicic acid
Silicic acid is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiOx4-2x]n...
, which causes no harm to the body. This has opened potential applications in medicine such as a framework of the growth of bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
.
Surface modification improving cell adhesion
Surface modification can also affect properties that promote cell adhesionCell adhesion
Cellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as selectins, integrins, and cadherins. Correct cellular adhesion is essential in maintaining multicellular structure...
. One particular research in 2005 studied the mammalian cell adhesion on the modified surfaces of porous silicon. The research used rat PC12 cells
PC12 cells
PC12 is a cell line derived from a pheochromocytoma of the rat adrenal medulla. PC12 cells stop dividing and terminally differentiate when treated with nerve growth factor...
and Human Lens Epithelial (HLE)
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
cells cultured for four hours on the surface modified porous silicon. Cells were then stained with vital dye FDA and observed under fluorescence microscopy. The research concluded that ‘amino silanisation and coating the pSi surface with collagen enhanced cell attachment and spreading’.
Porosity
Porosity is defined as the fraction of void within the pSi layer and can be determined easily by weight measurement. During formation of porous silicon layer through anodization, the porosity of a wafer can be increased through increasing current density, decreasing HF concentration and thicker silicon layer. The porosity of porous silicon may range from 4% for macroporous layers to 95% for mesoporous layers. A study by Canham in 1995 found that ‘a 1 µm thick layer of high porosity silicon completely dissolved within a day of in-vitro exposure to a simulated body fluid.’ It was also found that a silicon wafer with medium to low porosity displayed more stability. Hence, the porosity of porous silicon is varied depending on its potential application areas.Pore size
The porosity value of silicon is a macroscopic parameter and doesn’t yield any information regarding the microstructure of the layer. It is proposed that the properties of a sample are more accurately predicted if the pore size and its distribution within the sample can be obtained. Therefore, porous silicon has been divided into three categories based on the size of its pores; macroporousMacropore
In soil, macropores are defined as cavities that are larger than 75 μm. Functionally, pores of this size host preferential soil solution flow and rapid transport of solutes and colloids. Macropores increase the hydraulic conductivity of soil, allowing water to infiltrate and drain quickly,...
, mesoporous, and microporous
Microporous material
A microporous material is a material containing pores with diameters less than 2 nm.Porous materials are classified into several kinds by their size. According to IUPAC notation A microporous material is a material containing pores with diameters less than 2 nm.Porous materials are...
.
| Type | Microporous | Mesoporous | Macroporous |
|---|---|---|---|
| Pore width (Nanometer) | less than 2 | Between 2 and 50 | Larger than 50 |
Highly controllable properties
Porous silicon studies conducted in 1995 showed that the behavior of porous silicon can be altered in between ‘bio-inert’, ‘bioactive’ and ‘resorbable’ by varying the porosity of the silicon sample. The in-vitro study used simulated body fluid containing ion concentration similar to the human blood and tested the activities of porous silicon sample when exposed to the fluids for prolonged period of time. It was found that high porosity mesoporous layers were completely removed by the simulated body fluids within a day. In contrast, low to medium porosity microporous layers displayed more stable configurations and induced hydroxyapatite growth.Bioactive
The first sign of porous silicon as a bioactive material was found in 1995. In the conducted study, it was found that hydroxyapatite growth was occurring on porous silicon areas. It was then suggested that ‘hydrated microporous Si could be a bioactive form of the semiconductorSemiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
and suggest that Si itself should be seriously considered for development as a material for widespread in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
applications. Another paper published the finding that porous silicon may be used a substrate for hydroxyapatite growth either by simple soaking process or laser-liquid-solid interaction process.
Since then, in-vitro studies have been conducted to evaluate the interaction of cells with porous silicon. A 1995 study of the interaction of B50 rat hippocampal cells
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
with porous silicon found that B50 cells have clear preference for adhesion to porous silicon over untreated surface. The study indicated that porous silicon can be suitable for cell culturing
Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...
purposes and can be used to control cell growth pattern.
Non-toxic waste product
Another positive attribute of porous silicon is the degradation of porous silicon into monomerMonomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
ic silicic acid (SiOH4). Silicic acid is reputed to be the most natural form of element in the environment and is readily removed by kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s.
The human blood plasma
Blood plasma
Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
contains monomeric silicic acid at levels of less than 1 mg Si/l, corresponding to the average dietary intake of 20–50 mg/day. It was proposed that the small thickness of silicon coatings presents minimal risk to a toxic concentration being reached. The proposal was supported by an experiment involving volunteers and silicic-acid drinks. It was found that concentration of the acid rose only briefly above the normal 1 mg Si/l level and was efficiently expelled by urine excretion.
Superhydrophobicity
The simple adjustment of pore morphology and geometry of porous silicon also offers a convenient way to control its wetting behavior. Stable ultra- and superhydrophobic states on porous silicon can be fabricated and used in lab-on-a-chipLab-on-a-chip
A lab-on-a-chip is a device that integrates one or several laboratory functions on a single chip of only millimeters to a few square centimeters in size. LOCs deal with the handling of extremely small fluid volumes down to less than pico liters. Lab-on-a-chip devices are a subset of MEMS devices...
, microfluidic devices for the improved surface-based bioanalysis.
Optical properties
pSi demonstrates optical propertiesOptics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
based on porosity and the medium inside the pores. The effective refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of pSi is determined by the porosity and refractive index of the medium inside the pores. If the refractive index of the medium inside pores is high, the effective refractive index of pSi will be high as well. This phenomenon causes the spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
to shift towards longer wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
.
See also
- Nanocrystalline siliconNanocrystalline siliconNanocrystalline silicon , sometimes also known as microcrystalline silicon , is a form of porous silicon. It is an allotropic form of silicon with paracrystalline structure—is similar to amorphous silicon , in that it has an amorphous phase...
- SiliconSiliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
- PorosityPorosityPorosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
- Quantum wireQuantum wireIn condensed matter physics, a quantum wire is an electrically conducting wire, in which quantum effects are affecting transport properties. Due to the quantum confinement of conduction electrons in the transverse direction of the wire, their transverse energy is quantized into a series of...
- Etching (microfabrication)Etching (microfabrication)Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....