Carbon nanotube field-effect transistor

A carbon nanotube field-effect transistor (CNTFET) is a field-effect transistor that utilizes a single carbon nanotube or an array of carbon nanotubes as the channel material instead of bulk silicon in the traditional MOSFET structure. First demonstrated in 1998, there have been major developments in CNTFETs since.
Introduction and background.
According to Moore's law, the dimensions of individual devices in an integrated circuit have been decreased by a factor of approximately two every two years. This scaling down of devices has been the driving force in technological advances since the late 20th century. However, as noted by ITRS 2009 edition, further scaling down has faced serious limits related to fabrication technology and device performances as the critical dimension shrunk down to sub-22 nm range. The limits involve electron tunneling through short channels and thin insulator films, the associated leakage currents, passive power dissipation, short channel effects, and variations in device structure and doping. These limits can be overcome to some extent and facilitate further scaling down of device dimensions by modifying the channel material in the traditional bulk MOSFET structure with a single carbon nanotube or an array of carbon nanotubes.
Electronic structure of carbon nanotubes.
To a first approximation, the exceptional electrical properties of carbon nanotubes can be viewed as inherited from the unique electronic structure of graphene, provided the carbon nanotube is thought of as graphene rolled up along one of its Bravais lattice vectors Ĉh to form a hollow cylinder. In this construction, periodic boundary conditions are imposed over Ĉh to yield a lattice of seamlessly bonded carbon atoms on the cylinder surface.
Thus, the circumference of such a carbon nanotube can be expressed in terms of its rollup vector:
Ĉh=nâ1+mâ2
that connects two crystallographically equivalent sites of the two-dimensional graphene sheet. Here formula_1 and formula_2 are integers and â1 and â2 are primitive lattice vectors of the hexagonal lattice. Therefore, the structure of any carbon nanotube can be described by an index with a pair of integers formula_3 that define its rollup vector. In terms of the integers formula_3, the nanotube diameter formula_5 and the chiral angle formula_6 are given by: formula_7; and, formula_8, where formula_9 is the C—C bond distance.
Differences in the chiral angle and the diameter cause the differences in the properties of the various carbon nanotubes. For example, it can be shown that an formula_3 carbon nanotube is metallic when formula_11, is a small band gap semiconductor when formula_12 and formula_13, and is a moderate band gap semiconductor when formula_14, where formula_15 is an integer.
These results can be motivated by noting that periodic boundary conditions for 1D carbon nanotubes permit only a few wave vectors to exist around their circumferences. Metallic conduction could be expected to occur when one of these wave vectors passes through the K-point of graphene’s 2D hexagonal Brillouin zone, where the valence and conduction bands are degenerate.
This analysis, however, neglects the effects of curvature caused by rolling up the graphene sheet that converts all nanotubes with formula_16 to small band gap semiconductors, with the exception of the armchair tubes (formula_11) that remain metallic. Although the band gaps of carbon nanotubes with formula_12 and formula_13 are relatively small, some can still easily exceed room temperature, if the nanotube diameter is about a nanometer.
The band gaps formula_20 of semiconducting carbon nanotubes with formula_14 depend predominately on their diameters. In fact, according to a single-particle tight-binding description of the electronic structure of these nanotubes
formula_22 where formula_23 is the nearest-neighbor hopping matrix element. That this result is an excellent approximation so long as formula_24 is a lot less than one has been verified both by all-electron first principles local density functional calculations and experiment.
Scatter plots of the band gaps of carbon nanotubes with diameters up to three nanometers calculated using an all valence tight binding model that includes curvature effects appeared early in carbon nanotube research and were reprinted in a review.
Motivations for transistor applications.
A carbon nanotube’s bandgap is directly affected by its chiral angle and diameter. If those properties can be controlled, CNTs would be a promising candidate for future nano-scale transistor devices. Moreover, because of the lack of boundaries in the perfect and hollow cylinder structure of CNTs, there is no boundary scattering. CNTs are also quasi-1D materials in which only forward scattering and back scattering are allowed, and elastic scattering means that free paths in carbon nanotubes are long, typically on the order of micrometers. As a result, quasi-ballistic transport can be observed in nanotubes at relatively long lengths and low fields.
Because of the strong covalent carbon–carbon bonding in the sp2 configuration, carbon nanotubes are chemically inert and are able to transport large electric currents. In theory, carbon nanotubes are also able to conduct heat nearly as well as diamond or sapphire, and because of their miniaturized dimensions, the CNTFET should switch reliably using much less power than a silicon-based device.
Device fabrication.
There are many types of CNTFET devices; a general survey of the most common geometries are covered below.
Back-gated CNTFETs.
The earliest techniques for fabricating carbon nanotube (CNT) field-effect transistors involved pre-patterning parallel strips of metal across a silicon dioxide substrate, and then depositing the CNTs on top in a random pattern. The semiconducting CNTs that happened to fall across two metal strips meet all the requirements necessary for a rudimentary field-effect transistor. One metal strip is the "source" contact while the other is the "drain" contact. The silicon oxide substrate can be used as the gate oxide and adding a metal contact on the back makes the semiconducting CNT gateable.
This technique suffered from several drawbacks, which made for non-optimized transistors. The first was the metal contact, which actually had very little contact to the CNT, since the nanotube just lay on top of it and the contact area was therefore very small. Also, due to the semiconducting nature of the CNT, a Schottky barrier forms at the metal–semiconductor interface, increasing the contact resistance. The second drawback was due to the back-gate device geometry. Its thickness made it difficult to switch the devices on and off using low voltages, and the fabrication process led to poor contact between the gate dielectric and CNT.
Top-gated CNTFETs.
Eventually, researchers migrated from the back-gate approach to a more advanced top-gate fabrication process. In the first step, single-walled carbon nanotubes are solution deposited onto a silicon oxide substrate. Individual nanotubes are then located via atomic force microscope or scanning electron microscope. After an individual tube is isolated, source and drain contacts are defined and patterned using high resolution electron beam lithography. A high temperature anneal step reduces the contact resistance by improving adhesion between the contacts and CNT. A thin top-gate dielectric is then deposited on top of the nanotube, either via evaporation or atomic layer deposition. Finally, the top gate contact is deposited on the gate dielectric, completing the process.
Arrays of top-gated CNTFETs can be fabricated on the same wafer, since the gate contacts are electrically isolated from each other, unlike in the back-gated case. Also, due to the thinness of the gate dielectric, a larger electric field can be generated with respect to the nanotube using a lower gate voltage. These advantages mean top-gated devices are generally preferred over back-gated CNTFETs, despite their more complex fabrication process.
Wrap-around gate CNTFETs.
Wrap-around gate CNTFETs, also known as gate-all-around CNTFETs were developed in 2008, and are a further improvement upon the top-gate device geometry. In this device, instead of gating just the part of the CNT that is closer to the metal gate contact, the entire circumference of the nanotube is gated. This should ideally improve the electrical performance of the CNTFET, reducing leakage current and improving the device on/off ratio.
Device fabrication begins by first wrapping CNTs in a gate dielectric and gate contact via atomic layer deposition. These wrapped nanotubes are then solution-deposited on an insulating substrate, where the wrappings are partially etched off, exposing the ends of the nanotube. The source, drain, and gate contacts are then deposited onto the CNT ends and the metallic outer gate wrapping.
Suspended CNTFETs.
Yet another CNTFET device geometry involves suspending the nanotube over a trench to reduce contact with the substrate and gate oxide. This technique has the advantage of reduced scattering at the CNT-substrate interface, improving device performance. There are many methods used to fabricate suspended CNTFETs, ranging from growing them over trenches using catalyst particles, transferring them onto a substrate and then under-etching the dielectric beneath, and transfer-printing onto a trenched substrate.
The main problem suffered by suspended CNTFETs is that they have very limited material options for use as a gate dielectric (generally air or vacuum), and applying a gate bias has the effect of pulling the nanotube closer to the gate, which places an upper limit on how much the nanotube can be gated. This technique will also only work for shorter nanotubes, as longer tubes will flex in the middle and droop towards the gate, possibly touching the metal contact and shorting the device. In general, suspended CNTFETs are not practical for commercial applications, but they can be useful for studying the intrinsic properties of clean nanotubes.
CNTFET material considerations.
There are general decisions one must make when considering what materials to use when fabricating a CNTFET. Semiconducting single-walled carbon nanotubes are preferred over metallic single-walled and metallic multi-walled tubes since they are able to be fully switched off, at least for low source/drain biases. A lot of work has been put into finding a suitable contact material for semiconducting CNTs; the best material to date is Palladium, because its work function closely matches that of nanotubes and it adheres to the CNTs quite well.
I–V characteristics.
In CNT–metal contacts, the different work functions of the metal and the CNT result in a Schottky barrier at the source and drain, which are made of metals like silver, titanium, palladium and aluminum. Even though like Schottky barrier diodes, the barriers would have made this FET to transport only one type of carrier, the carrier transport through the metal-CNT interface is dominated by quantum mechanical tunneling through the Schottky barrier. CNTFETs can easily be thinned by the gate field such that tunneling through them results in a substantial current contribution. CNTFETs are ambipolar; either electrons or holes, or both electrons and holes can be injected simultaneously. This makes the thickness of the Schottky barrier a critical factor.
CNTFETs conduct electrons when a positive bias is applied to the gate and holes when a negative bias is applied, and drain current increases with increasing a magnitude of an applied gate voltage. Around Vg = Vds/2, the current gets the minimum due to the same amount of the electron and hole contributions to the current.
Like other FETs, the drain current increases with an increasing drain bias unless the applied gate voltage is below the threshold voltage. For planar CNTFETs with different design parameters, the FET with a shorter channel length produces a higher saturation current, and the saturation drain current also becomes higher for the FET consisting of smaller diameter keeping the length constant. For cylindrical CNTFETs, it is clear that a higher drain current is driven than that of planar CNTFETs since a CNT is surrounded by an oxide layer which is finally surrounded by a metal contact serving as the gate terminal.
Theoretical derivation of drain current.
Theoretical investigation on drain current of the top-gate CNT transistor has been done by Kazierski and colleagues. When an electric field is applied to a CNT transistor, a mobile charge is induced in the tube from the source and drain. These charges are from the density of positive velocity states filled by the source NS and that of negative velocity states filled by the drain ND, and these densities are determined by the Fermi-Dirac probability distributions.
and the equilibrium electron density is
where the density of states at the channel D(E), USF, and UDF are defined as
The term, formula_31 is 1 when the value inside the bracket is positive and 0 when negative. VSC is the self-consistent voltage that illustrates that the CNT energy is affected by external terminal voltages and is implicitly related to the device terminal voltages and charges at terminal capacitances by the following nonlinear equation:
where Qt represents the charge stored in terminal capacitances, and the total terminal capacitance CΣ is the sum of the gate, drain, source, and substrate capacitances shown in the figure above. The standard approach to the solution to the self-consistent voltage equation is to use the Newton-Raphson iterative method. According to the CNT ballistic transport theory, the drain current caused by the transport of the nonequilibrium charge across the nanotube can be calculated using the Fermi–Dirac statistics.
Here "F"0 represents the Fermi–Dirac integral of order 0, "k" is the Boltzmann’s constant, "T" is the temperature, and ℏ the reduced Planck’s constant. This equation can be solved easily as long as the self-consistent voltage is known. However the calculation could be time-consuming when it needs to solve the self-consistent voltage with the iterative method, and this is the main drawback of this calculation.
Comparison to MOSFETs.
CNTFETs show different characteristics compared to MOSFETs in their performances. In a planar gate structure, the p-CNTFET produces ~1500 A/m of the on-current per unit width at a gate overdrive of 0.6 V while p-MOSFET produces ~500 A/m at the same gate voltage. This on-current advantage comes from the high gate capacitance and improved channel transport. Since an effective gate capacitance per unit width of CNTFET is about double that of p-MOSFET, the compatibility with high-k gate dielectrics becomes a definite advantage for CNTFETs. About twice higher carrier velocity of CNTFETs than MOSFETs comes from the increased mobility and the band structure. CNTFETs, in addition, have about four times higher transconductance.
The first sub-10-nanometer CNT transistor was made which outperformed the best competing silicon devices with more than four times the diameter-normalized current density (2.41 mA/μm) at an operating voltage of 0.5 V. The inverse subthreshold slope of the CNTFET was 94 mV/decade.
Heat dissipation.
The decrease of the current and burning of the CNT can occur due to the temperature raised by several hundreds of kelvins. Generally, the self-heating effect is much less severe in a semiconducting CNTFET than in a metallic one due to different heat dissipation mechanisms. A small fraction of the heat generated in the CNTFET is dissipated through the channel. The heat is non-uniformly distributed, and the highest values appear at the source and drain sides of the channel. Therefore, the temperature significantly gets lowered near the source and drain regions. For semiconducting CNT, the temperature rise has a relatively small effect on the I-V characteristics compared to silicon.
Disadvantages.
Lifetime (degradation).
Carbon nanotubes degrade in a few days when exposed to oxygen. There have been several works done on passivating the nanotubes with different polymers and increasing their lifetime.
Carbon nanotubes have recently been shown to be stable in air for many months and likely more, even when under continual operation. While gate voltages are being applied, the device current can experience some undesirable drift/settling, but changes in gating quickly reset this behavior with little change in threshold voltage.
Reliability.
Carbon nanotubes have shown reliability issues when operated under high electric field or temperature gradients. Avalanche breakdown occurs in semiconducting CNT and joule breakdown in metallic CNT. Unlike avalanche behavior in silicon, avalanche in CNTs is negligibly temperature-dependent. Applying high voltages beyond avalanche point results in Joule heating and eventual breakdown in CNTs.
This reliability issue has been studied, and it is noticed that the multi-channeled structure can improve the reliability of the CNTFET. The multi-channeled CNTFETs can keep a stable performance after several months, while the single-channeled CNTFETs are usually wear out after a few weeks in the ambient atmosphere. The multi-channeled CNTFETs keep operating when some channels break down, with a small change in electrical properties.
Difficulties in mass production, production cost.
Although CNTs have unique properties such as stiffness, strength, and tenacity compared to other materials especially to silicon, there is currently no technology for their mass production and high production cost. To overcome the fabrication difficulties, several methods have been studied such as direct growth, solution dropping, and various transfer printing techniques. The most promising methods for mass production involve some degree of self-assembly of pre-produced nanotubes into the desired positions. Individually manipulating many tubes is impractical at a large scale and growing them in their final positions presents many challenges.
Future work.
The most desirable future work involved in CNTFETs will be the transistor with higher reliability, cheap production cost, or the one with more enhanced performances. For example, such efforts could be made: adding effects external to the inner CNT transistor like the Schottky barrier between the CNT and metal contacts, multiple CNTs at a single gate, channel fringe capacitances, parasitic source/drain resistance, and series resistance due to the scattering effects.


Lithium nitride

Lithium nitride is a compound with the formula Li3N. It is the only stable alkali metal nitride. The solid has a reddish-pink color and high melting point.
Preparation and handling.
Lithium nitride is prepared by direct combination of elemental lithium with nitrogen gas:
Instead of burning lithium metal in an atmosphere of nitrogen, a solution of lithium in liquid sodium metal can be treated with N2.
Lithium nitride reacts violently with water to produce ammonia:
Structure and properties.
"alpha"-Li3N (stable at room temperature and pressure) has an unusual crystal structure that consists of two types of layers, one sheet has the composition Li2N− contains 6-coordinate N centers and the other sheet consists only of lithium cations. Two other forms are known: "beta"-Lithium nitride, formed from the alpha phase at has the sodium arsenide (Na3As) structure; "gamma"-Lithium nitride (same structure as Li3Bi) forms from the beta form at .
Lithium nitride shows ionic conductivity for Li+, with a value of c. 2×10−4Ω−1cm−1, and an (intracrystal) activation energy of c. 0.26eV (c. 24 kJ/mol). Hydrogen doping increases conductivity, whilst doping with metal ions (Al, Cu, Mg) reduces it. The activation energy for lithium transfer across lithium nitride crystals (intercrystalline) has been determined to be higher at c. 68.5 kJ/mol. The alpha form is a semiconductor with band gap of c. 2.1 eV.
Reaction with hydrogen at under 300 °C (0.5 MPa pressure) produces lithium hydride and lithium amide.
Lithium nitride has been investigated as a storage medium for hydrogen gas, as the reaction is reversible at 270 °C. Up to 11.5% by weight absorption of hydrogen has been achieved.
Reacting lithium nitride with carbon dioxide results in amorphous carbon nitride (C3N4), a semiconductor, and lithium cyanamide (Li2CN2), a precursor to fertilizers, in an exothermic reaction.


Tetrabenzylzirconium

Tetrabenzylzirconium is an organozirconium compound with the formula Zr(CH2C6H5)4. The molecule features diamagnetic Zr(IV) bonded to four benzyl ligands. It is an orange air- and photo-sensitive solid, which is soluble in hydrocarbon solvents. The compound is a precursor to catalysts for the polymerization of olefins.
Structure, synthesis, reactions.
X-ray crystallography demonstrates that the benzyl ligands are highly flexible: one polymorph features four η2-ligands, whereas another has two η1- and two η2-benzyl ligands.
The compound is prepared by combining benzylmagnesium chloride and zirconium tetrachloride in diethyl ether. It readily undergoes protonolysis, e.g. with hydrogen chloride:


Lucchesiite

Lucchesiite is a new member of tourmaline-group of minerals. Lucchesiite has the formula CaFe3Al6(Si6O18)(BO3)3(OH)3O. It is the calcium and oxygen-analogue of schorl. It has two co-type localizations, one in Czech Republic and the other in Sri Lanka. As the other members of the tourmaline group, it is trigonal.
Notes on chemistry.
Impurites in lucchesiite, depending on the provenience, are sodium, magnesium, aluminium, titanium, trivalent iron, and minor vanadium, potassium, manganese and zinc.


Nickel silicide

Nickel silicides include several intermetallic compounds of nickel and silicon. Nickel silicides are important in microelectronics as they form at junctions of nickel and silicon. Additionally thin layers of nickel silicides may have application in imparting surface resistance to nickel alloys.
Compounds.
Nickel silicides include Ni3Si, Ni31Si12, Ni2Si, Ni3Si2, NiSi and NiSi2. Ni31Si12, Ni2Si, and NiSi have congruent melting points; the others form via a peritectic transformation.
The silicides can be made via fusion or solid state reaction between the elements, diffusion at a junction of the two elements, and other methods including ion beam mixing.
Properties.
Nickel silicides are generally chemically and thermally stable. They have low electrical resistivity; with NiSi 10.5–18 μΩ·cm, Ni2Si 24–30 μΩ·cm, NiSi2 34–50 μΩ·cm; nickel-rich silicides have higher resistivity rising to 90–150 μΩ·cm in Ni31Si12.
Uses.
Microelectronics.
Nickel silicides are important in microelectronic devices – specific silicides are good conductors, with NiSi having a conductivity approaching that of elemental nickel.
With silicon carbide as the semiconductor nickel reacts at elevated temperatures to form nickel silicides and carbon.
Other.
Nickel silicides have potential as coatings for nickel-based superalloys and stainless steel, due to their corrosion, oxidation, and wear resistance.
NiSi has been investigated as a hydrogenation catalyst for unsaturated hydrocarbons. Nickel silicide nanoparticles supported on silica support have been suggested as an alternative catalyst to widely used pyrophoric Raney nickel.


Telluroxide

A telluroxide is a type of organotellurium compound with the formula R2TeO. These compounds are analogous to sulfoxides in some respects. Reflecting the decreased tendency of Te to form multiple bonds, telluroxides exist both the monomer and the polymer, which are favored in solution and the solid state, respectively. They are prepared from the telluroethers by halogenation followed by base hydrolysis:


Silver sulfite

Silver sulfite is the chemical compound with the formula Ag2SO3. This unstable silver compound when heated and/or in light it decomposes to silver dithionate and silver sulfate.
Preparation.
Silver sulfite can be prepared by dissolving silver nitrate with the stoichiometric quantity of sodium sulfite solution, yielding a precipitation of silver sulfite by the following reaction:
After precipitation then filtering silver sulfite, washing it using well-boiled water, and drying it in vacuum.


Trichloroethylsilane

Trichloroethylsilane is a compound with formula Si(C2H5)Cl3.


Ytterbium(II) chloride

Ytterbium(II) chloride (YbCl2) is an inorganic chemical compound. It was first prepared in 1929 by W. K. Klemm and W. Schuth, by reduction of ytterbium(III) chloride, YbCl3, using hydrogen.
Like other Yb(II) compounds and other low-valence rare earth compounds, it is a strong reducing agent. It is unstable in aqueous solution, reducing water to hydrogen gas.


Mesaite

Mesaite is a very rare mineral with formula CaMn2+5(V2O7)3•12H2O. It is monoclinic (space group "P"2/"n"). It is related to fianelite, another manganese-rich divanadate. Examples of other divanadate minerals are volborthite, engelhauptite, karpenkoite, and martyite.


Methyltrichlorosilane

Methyltrichlorosilane, also known as trichloromethylsilane, is an organosilicon compound with the formula CH3SiCl3. It is a colorless liquid with a sharp odor similar to that of hydrochloric acid. As methyltrichlorosilane is a reactive compound, it is mainly used a precursor for forming various cross-linked siloxane polymers.
Preparation.
Methyltrichlorosilane results from the direct process of chloromethane with elemental silicon in the presence of a copper catalyst, usually at a temperature of at least 250°C.
While this reaction is the standard in industrial silicone production and is nearly identical to the first direct synthesis of methyltrichlorosilane, the overall process is inefficient with respect to methyltrichlorosilane. Even though dimethyldichlorosilane is usually the major product, if methyltrichlorosilane is needed, the amount of metal catalyst is reduced.
Reactions.
Hydrolysis and alcoholysis.
Methyltrichlorosilane undergoes hydrolysis, shown in idealized form here:
The silanol is unstable and will eventually condense to give a polymer network:
Methyltrichlorosilane undergoes alcoholysis (reaction with alcohol) to give alkoxysilanes. Methanol converts it to trimethoxymethylsilane:
Reduction.
Reduction of methyltrichlorosilane with alkali metals forms a highly crosslinked material called poly(methylsilyne):
The reaction illustrates the susceptibility of silicon halides to reductive coupling. Poly(methylsilyne) is soluble in organic solvents, and can be applied to surfaces before being pyrolyzed to give the ceramic material, silicon carbide.
Applications.
Conversion to polymers and resins.
One use for methyltrichlorosilane is in the production of methyl silicone resins (highly crosslinked polymers). Because of the stability of the cross-linked polymers resulting from condensation, the resin is stable to 550°C in a vacuum, making it an ideal material for electrical insulation at high temperatures. These resins can be used to coat computer chips or other electronic parts since they both repel water and provide thermal isolation.
Surface treatments.
Methyltrichlorosilane vapor reacts with water on surfaces to give a thin layer of methylpolysiloxane, which changes the contact angle of the surface to water. This effect arises because of the oriented layer of methyl groups, making a water-repellent film. Filter paper treated with methyltrichlorosilane allows organic solvents to pass through, but not water. Another benefit of such water-repellent films is that the polymers formed are stable: one of the only ways to remove the siloxane film is by acid strong enough to dissolve silicone.
Reagent in organic synthesis.
A combination of methyltrichlorosilane and sodium iodide can be used to cleave carbon-oxygen bonds such as methyl ethers.
Esters and lactones can also be cleaved with methyltrichlorosilane and sodium iodide to give the corresponding carboxylic acids. Acetals convert to carbonyl compounds. Thus, methyltrichlorosilane can be used to remove acetal protecting groups from carbonyl compounds under mild conditions.
Methyltrichlorosilane and sodium iodide can be used as a means of converting alcohols to their corresponding iodides; however, this reaction does not work as well with primary alcohols.
Silicon carbide epitaxy.
Methyltrichlorosilane is used as a reagent in silicon carbide epitaxy to introduce chloride in the gas phase. Chloride is used to reduce the tendency of silicon to react in the gas phase and thus to increase the growth rate of the process. Methyltrichlorosilane is an alternative to HCl gas or to trichlorosilane


Peroxynitric acid

Peroxynitric acid or peroxonitric acid is a chemical compound with the formula . It is an oxyacid of nitrogen, after peroxynitrous acid.
Preparation.
Peroxynitrate, the conjugate base of peroxynitric acid, is formed rapidly during decomposition of peroxynitrite in neutral conditions.
Atmospheric chemistry.
Peroxynitric acid is formed in the atmosphere, although it is unstable, it is important as a reservoir for NO2 through the reversible radical reaction:


Organoantimony chemistry

Organoantimony chemistry is the chemistry of compounds containing a carbon to antimony (Sb) chemical bond. Relevant oxidation states are Sb(V) and Sb(III). The toxicity of antimony limits practical application in organic chemistry.
Organoantimony(V) chemistry.
Antimony compounds of the type R5Sb (stiboranes) can be synthesised from trivalent Sb precursors:
Asymmetric compounds can also be obtained through the stibonium ion:
Just as in the related organobismuth compounds (same group 15), organoantimony(V) compounds form onium compounds and ate complexes.
Pentaphenylantimony decomposes at 200 °C to triphenylstibine and biphenyl. It forms a trigonal bipyramidal molecular geometry. In the related Me5Sb all methyl protons are equivalent at -100 °C in proton NMR. Compounds of the type R4SbX tend to form dimers.
Organoantimony(III) chemistry.
Compounds of the type R3Sb (stibines) can be accessed by reaction of antimony trichloride with organolithium or Grignard reagents.
Typical reactions are:
Stibanes are weak Lewis acids and therefore ate complexes are not encountered. On the other hand, they have good donor properties and are therefore widely used in coordination chemistry. R3Sb compounds are more air-sensitive than the R5Sb counterparts.
Antimony Metallocenes are known as well:
The Cp*-Sb-Cp* angle is 154°.
The cyclic compound stibole, a structural analog of pyrrole, has not been isolated, but substituted derivatives known as stiboles are known.
Organoantimony(II) chemistry.
Distibines have a Sb-Sb single bond and are of some interest as thermochromic materials. For example, tetramethyldistibine is a colorless as a gas, yellow as a liquid, red as solid just below the melting point of 18.5 °C and again yellow well below the melting point.


Tetrafluoride

A tetrafluoride is a chemical compound with four fluorines in its formula.
Ions.
Some atoms can form a complex ion with four fluorine atoms which may form compounds containing the term tetrafluoride. Examples include


Fullerene

A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ellipsoid, tube, or many other shapes and sizes. Graphene (isolated atomic layers of graphite), which is a flat mesh of regular hexagonal rings, can be seen as an extreme member of the family.
Fullerenes with a closed mesh topology are informally denoted by their empirical formula C"n", often written C"n", where "n" is the number of carbon atoms. However, for some values of "n" there may be more than one isomer.
The family is named after buckminsterfullerene (C60), the most famous member, which in turn is named after Buckminster Fuller. The closed fullerenes, especially C60, are also informally called buckyballs for their resemblance to the standard ball of association football ("soccer"). Nested closed fullerenes have been named bucky onions. Cylindrical fullerenes are also called carbon nanotubes or buckytubes. The bulk solid form of pure or mixed fullerenes is called fullerite.
Fullerenes had been predicted for some time, but only after their accidental synthesis in 1985 were they detected in nature and outer space. The discovery of fullerenes greatly expanded the number of known allotropes of carbon, which had previously been limited to graphite, diamond, and amorphous carbon such as soot and charcoal. They have been the subject of intense research, both for their chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology.
History.
Predictions and limited observations.
The icosahedral cage was mentioned in 1965 as a possible topological structure. Eiji Osawa predicted the existence of in 1970. He noticed that the structure of a corannulene molecule was a subset of the shape of a football, and hypothesised that a full ball shape could also exist. Japanese scientific journals reported his idea, but neither it nor any translations of it reached Europe or the Americas.
Also in 1970, R. W. Henson (then of the UK Atomic Energy Research Establishment) proposed the structure and made a model of it. Unfortunately, the evidence for that new form of carbon was very weak at the time, so the proposal was met with skepticism, and was never published. It was acknowledged only in 1999.
In 1973, independently from Henson, a group of scientists from the USSR made a quantum-chemical analysis of the stability of and calculated its electronic structure. The paper was published in 1973, but the scientific community did not give much importance to this theoretical prediction.
Around 1980, Sumio Iijima identified the molecule of from an electron microscope image of carbon black, where it formed the core of a particle with the structure of a "bucky onion".
Discovery of.
In 1985 Harold Kroto of the University of Sussex, working with James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley from Rice University, discovered fullerenes in the sooty residue created by vaporising carbon in a helium atmosphere. In the mass spectrum of the product, discrete peaks appeared corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms, namely and . The team identified their structure as the now familiar "buckyballs".
The name "buckminsterfullerene" was eventually chosen for by the discoverers as an homage to American architect Buckminster Fuller for the vague similarity of the structure to the geodesic domes which he popularized; which, if they were extended to a full sphere, would also have the icosahedral symmetry group. The "ene" ending was chosen to indicate that the carbons are unsaturated, being connected to only three other atoms instead of the normal four. The shortened name "fullerene" eventually came to be applied to the whole family.
Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this class of molecules.
Further developments.
Kroto and the Rice team already discovered other fullerenes besides C60, and the list was much expanded in the following years. Carbon nanotubes were first discovered and synthesized in 1991.
After their discovery, minute quantities of fullerenes were found to be produced in sooty flames, and by lightning discharges in the atmosphere. In 1992, fullerenes were found in a family of mineraloids known as shungites in Karelia, Russia.
The production techniques were improved by many scientists, including Donald Huffman, Wolfgang Krätschmer, Lowell D. Lamb, and Konstantinos Fostiropoulos. Thanks to their efforts, by 1990 it was relatively easy to produce gram-sized samples of fullerene powder. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices.
In 2010, the spectral signatures of C60 and C70 were observed by NASA's Spitzer infrared telescope in a cloud of cosmic dust surrounding a star 6500 light years away. Kroto commented: "This most exciting breakthrough provides convincing evidence that the buckyball has, as I long suspected, existed since time immemorial in the dark recesses of our galaxy." According to astronomer Letizia Stanghellini, "It’s possible that buckyballs from outer space provided seeds for life on Earth." In 2019, ionized C60 molecules were detected with the Hubble Space Telescope in the space between those stars.
Types.
There are two major families of fullerenes, with fairly distinct properties and applications: the closed buckyballs and the open-ended cylindrical carbon nanotubes. However, hybrid structures exist between those two classes, such as carbon nanobuds — nanotubes capped by hemispherical meshes or larger "buckybuds".
Buckyballs.
Buckminsterfullerene.
Buckminsterfullerene is the smallest fullerene molecule containing pentagonal and hexagonal rings in which no two pentagons share an edge (which can be destabilizing, as in pentalene). It is also most common in terms of natural occurrence, as it can often be found in soot.
The empirical formula of buckminsterfullerene is and its structure is a truncated icosahedron, which resembles an association football ball of the type made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.
The van der Waals diameter of a buckminsterfullerene molecule is about 1.1 nanometers (nm). The nucleus to nucleus diameter of a buckminsterfullerene molecule is about 0.71 nm.
The buckminsterfullerene molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter than the 6:5 bonds (between a hexagon and a pentagon). Its average bond length is 1.4 Å.
Other fullerenes.
Another fairly common fullerene has empirical formula , but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.
The smallest possible fullerene is the dodecahedral . There are no fullerenes with 22 vertices. The number of different fullerenes C2n grows with increasing "n" = 12, 13, 14, ..., roughly in proportion to "n"9 . For instance, there are 1812 non-isomorphic fullerenes . Note that only one form of , buckminsterfullerene, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenes , 15,655,672 of which have no adjacent pentagons. Optimized structures of many fullerene isomers are published and listed on the web.
Heterofullerenes have heteroatoms substituting carbons in cage or tube-shaped structures. They were discovered in 1993 and greatly expand the overall fullerene class of compounds and can have dangling bonds on their surfaces. Notable examples include boron, nitrogen (azafullerene), oxygen, and phosphorus derivatives.
Trimetasphere carbon nanomaterials were discovered by researchers at Virginia Tech and licensed exclusively to Luna Innovations. This class of novel molecules contains 80 carbon atoms () forming a sphere which encloses a complex of three metal atoms and one nitrogen atom. These fullerenes encapsulate metals which puts them in the subset referred to as metallofullerenes. Trimetaspheres have the potential for use in diagnostics (as safe imaging agents), in therapeutics and in organic solar cells.
Carbon nanotubes.
Carbon nanotubes are cylindrical fullerenes. These tubes of carbon are usually only a few nanometres wide, but they can range from less than a micrometer to several millimeters in length. They often have closed ends, but can be open-ended as well. There are also cases in which the tube reduces in diameter before closing off. Their unique molecular structure results in extraordinary macroscopic properties, including high tensile strength, high electrical conductivity, high ductility, high heat conductivity, and relative chemical inactivity (as it is cylindrical and "planar" — that is, it has no "exposed" atoms that can be easily displaced). One proposed use of carbon nanotubes is in paper batteries, developed in 2007 by researchers at Rensselaer Polytechnic Institute. Another highly speculative proposed use in the field of space technologies is to produce high-tensile carbon cables required by a space elevator.
Derivatives.
Buckyballs and carbon nanotubes have been used as building blocks for a great variety of derivatives and larger structures, such as
Heterofullerenes and non-carbon fullerenes.
After the discovery of C60, many fullerenes have been synthesized (or studied theoretically by molecular modeling methods) in which some or all the carbon atoms are replaced by other elements. Inorganic nanotubes, in particular, have attracted much attention.
Silicon.
Silicon buckyballs have been created around metal ions.
Boron.
A type of buckyball which uses boron atoms, instead of the usual carbon, was predicted and described in 2007. The structure, with each atom forming 5 or 6 bonds, is predicted to be more stable than the buckyball. One reason for this given by the researchers is that is actually more like the original geodesic dome structure popularized by Buckminster Fuller, which uses triangles rather than hexagons. However, this work has been subject to much criticism by quantum chemists as it was concluded that the predicted Ih symmetric structure was vibrationally unstable and the resulting cage undergoes a spontaneous symmetry break, yielding a puckered cage with rare Th symmetry (symmetry of a volleyball). The number of six-member rings in this molecule is 20 and number of five-member rings is 12. There is an additional atom in the center of each six-member ring, bonded to each atom surrounding it. By employing a systematic global search algorithm, it was later found that the previously proposed B80 fullerene is not a global maximum for 80-atom boron clusters and hence can not be found in nature. In the same paper by Sandip De et al., it was concluded that boron's energy landscape is significantly different from other fullerenes already found in nature, hence pure boron fullerenes are unlikely to exist in nature.
Other elements.
Inorganic (carbon-free) fullerene-type structures have been built with the disulfides of molybdenum (MoS2), long used as a graphite-like lubricant, tungsten (WS2), titanium (TiS2) and niobium (NbS2). These materials were found to be stable up to at least 350 tons/cm2 (34.3 GPa).
Main fullerenes.
Below is a table of main closed carbon fullerenes synthesized and characterized so far, with their CAS number when known. Fullerenes with fewer than 60 carbon atoms have been called "lower fullerenes", and those with more than 70 atoms "higher fullerenes".
In the table, "Num.Isom." is the number of possible isomers within the "isolated pentagon rule", which states that two pentagons in a fullerene should not share edges. "Mol.Symm." is the symmetry of the molecule, whereas "Cryst.Symm." is that of the crystalline framework in the solid state. Both are specified for the most experimentally abundant form(s). The asterisk * marks symmetries with more than one chiral form.
When or crystals are grown from toluene solution they have a monoclinic symmetry. The crystal structure contains toluene molecules packed between the spheres of the fullerene. However, evaporation of the solvent from transforms it into a face-centered cubic form. Both monoclinic and face-centered cubic (fcc) phases are known for better-characterized and fullerenes.
Properties.
Topology.
Schlegel diagrams are often used to clarify the 3D structure of closed-shell fullerenes, as 2D projections are often not ideal in this sense.
In mathematical terms, the combinatorial topology (that is, the carbon atoms and the bonds between them, ignoring their positions and distances) of a closed-shell fullerene with a simple sphere-like mean surface (orientable, genus zero) can be represented as a convex polyhedron; more precisely, its one-dimensional skeleton, consisting of its vertices and edges. The Schlegel diagram is a projection of that skeleton onto one of the faces of the polyhedron, through a point just outside that face; so that all other vertices project inside that face.
The Schlegel diagram of a closed fullerene is a graph that is planar and 3-regular (or "cubic"; meaning that all vertices have degree 3).
A closed fullerene with sphere-like shell must have at least some cycles that are pentagons or heptagons. More precisely, if all the faces have 5 or 6 sides, it follows from Euler's polyhedron formula, "V"−"E"+"F"=2 (where "V", "E", "F" are the numbers of vertices, edges, and faces), that "V" must be even, and that there must be exactly 12 pentagons and "V"/2−10 hexagons. Similar constraints exist if the fullerene has heptagonal (seven-atom) cycles.
Open fullerenes, like carbon nanotubes and graphene, can consist entirely of hexagonal rings. In theory, a long nanotube with ends joined to form a closed torus-like sheet could also consist entirely of hexagons.
Bonding.
Since each carbon atom is connected to only three neighbors, instead of the usual four, it is customary to describe those bonds as being a mixture of single and double covalent bonds.
Encapsulation.
So-called endohedral fullerenes have ions or small molecules incorporated inside the cage atoms.
Research.
In the early 2000s, the chemical and physical properties of fullerenes were a hot topic in the field of research and development. "Popular Science" discussed possible uses of fullerenes (graphene) in armor.
In the field of nanotechnology, heat resistance and superconductivity are some of the more heavily studied properties.
There are many calculations that have been done using ab-initio quantum methods applied to fullerenes. By DFT and TD-DFT methods one can obtain IR, Raman and UV spectra. Results of such calculations can be compared with experimental results.
Fullerene is an unusual reactant in many organic reactions such as the Bingel reaction discovered in 1993.
Aromaticity.
Researchers have been able to increase the reactivity of fullerenes by attaching active groups to their surfaces. Buckminsterfullerene does not exhibit "superaromaticity": that is, the electrons in the hexagonal rings do not delocalize over the whole molecule.
A spherical fullerene of "n" carbon atoms has "n" pi-bonding electrons, free to delocalize. These should try to delocalize over the whole molecule. The quantum mechanics of such an arrangement should be like only one shell of the well-known quantum mechanical structure of a single atom, with a stable filled shell for "n" = 2, 8, 18, 32, 50, 72, 98, 128, etc. (i.e., twice a perfect square number), but this series does not include 60. This 2("N" + 1)2 rule (with "N" integer) for spherical aromaticity is the three-dimensional analogue of Hückel's rule. The 10+ cation would satisfy this rule, and should be aromatic. This has been shown to be the case using quantum chemical modelling, which showed the existence of strong diamagnetic sphere currents in the cation.
As a result, in water tends to pick up two more electrons and become an anion. The "n" described below may be the result of trying to form a loose metallic bond.
Reactions.
Polymerization.
Under high pressure and temperature, buckyballs collapse to form various one-, two-, or three-dimensional carbon frameworks. Single-strand polymers are formed using the Atom Transfer Radical Addition Polymerization (ATRAP) route
"Ultrahard fullerite" is a coined term frequently used to describe material produced by high-pressure high-temperature (HPHT) processing of fullerite. Such treatment converts fullerite into a nanocrystalline form of diamond which has been reported to exhibit remarkable mechanical properties.
Chemistry.
Fullerenes are stable, but not totally unreactive. The sp2-hybridized carbon atoms, which are at their energy minimum in planar graphite, must be bent to form the closed sphere or tube, which produces angle strain. The characteristic reaction of fullerenes is electrophilic addition at 6,6-double bonds, which reduces angle strain by changing sp2-hybridized carbons into sp3-hybridized ones. The change in hybridized orbitals causes the bond angles to decrease from about 120° in the sp2 orbitals to about 109.5° in the sp3 orbitals. This decrease in bond angles allows for the bonds to bend less when closing the sphere or tube, and thus, the molecule becomes more stable.
Other atoms can be trapped inside fullerenes to form inclusion compounds known as endohedral fullerenes. An unusual example is the egg-shaped fullerene Tb3N@, which violates the isolated pentagon rule. Recent evidence for a meteor impact at the end of the Permian period was found by analyzing noble gases so preserved. Metallofullerene-based inoculates using the rhonditic steel process are beginning production as one of the first commercially viable uses of buckyballs.
Solubility.
Fullerenes are soluble in many organic solvents, such as toluene, chlorobenzene, and 1,2,3-trichloropropane. Solubilities are generally rather low, such as 8 g/L for C60 in carbon disulfide. Still, fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature. Among the best solvents is 1-chloronaphthalene, which will dissolve 51 g/L of C60.
Solutions of pure buckminsterfullerene have a deep purple color. Solutions of are a reddish brown. The higher fullerenes to have a variety of colors.
Millimeter-sized crystals of and , both pure and solvated, can be grown from benzene solution. Crystallization of from benzene solution below 30 °C (when solubility is maximum) yields a triclinic solid solvate ·4. Above 30 °C one obtains solvate-free fcc .
Quantum mechanics.
In 1999, researchers from the University of Vienna demonstrated that wave-particle duality applied to molecules such as fullerene.
Superconductivity.
Fullerenes are normally electrical insulators, but when crystallized with alkali metals, the resultant compound can be conducting or even superconducting.
Chirality.
Some fullerenes (e.g. , , , and ) are inherently chiral because they are D2-symmetric, and have been successfully resolved. Research efforts are ongoing to develop specific sensors for their enantiomers.
Stability.
Two theories have been proposed to describe the molecular mechanisms that make fullerenes. The older, “bottom-up” theory proposes that they are built atom-by-atom. The alternative “top-down” approach claims that fullerenes form when much larger structures break into constituent parts.
In 2013 researchers discovered that asymmetrical fullerenes formed from larger structures settle into stable fullerenes. The synthesized substance was a particular metallofullerene consisting of 84 carbon atoms with two additional carbon atoms and two yttrium atoms inside the cage. The process produced approximately 100 micrograms.
However, they found that the asymmetrical molecule could theoretically collapse to form nearly every known fullerene and metallofullerene. Minor perturbations involving the breaking of a few molecular bonds cause the cage to become highly symmetrical and stable. This insight supports the theory that fullerenes can be formed from graphene when the appropriate molecular bonds are severed.
Systematic naming.
According to the IUPAC, to name a fullerene, one must cite the number of member atoms for the rings which comprise the fullerene, its symmetry point group in the Schoenflies notation, and the total number of atoms. For example, buckminsterfullerene C60 is systematically named (-"I"h)[5,6]fullerene. The name of the point group should be retained in any derivative of said fullerene, even if that symmetry is lost by the derivation.
To indicate the position of substituted or attached elements, the fullerene atoms are usually numbered in a spiral path, usually starting with the ring on one of the main axes. If the structure of the fullerene does not allow such numbering, another starting atom was chosen to still achieve a spiral path sequence.
The latter is the case for C70, which is (-"D"5h(6))[5,6]fullerene in IUPAC notation. The symmetry "D"5h(6) means that this is the isomer where the "C"5 axis goes through a pentagon surrounded by hexagons rather than pentagons.
In IUPAC's nomenclature, fully saturated analogues of fullerenes are called "fulleranes". If the mesh has other element(s) substituted for one or more carbons, the compound is named a "heterofullerene". If a double bond is replaced by a methylene bridge , the resulting structure is a "homofullerene". If an atom is fully deleted and missing valences saturated with hydrogen atoms, it is a "norfullerene". When bonds are removed (both sigma and pi), the compound becomes "secofullerene"; if some new bonds are added in an unconventional order, it is a "cyclofullerene".
Production.
Fullerene production generally starts by producing fullerene-rich soot. The original (and still current) method was to send a large electric current between two nearby graphite electrodes in an inert atmosphere. The resulting electric arc vaporizes the carbon into a plasma that then cools into sooty residue. Alternatively, soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Combustion is the most efficient process, developed at MIT.
These processes yield a mixture of various fullerenes and other forms of carbon. The fullerenes are then extracted from the soot using appropriate organic solvents and separated by chromatography. One can obtain milligram quantities of fullerenes with 80 atoms or more.
C76, C78 and C84 are available commercially.
Applications.
Fullerenes have been extensively used for several biomedical applications including the design of high-performance MRI contrast agents, X-ray imaging contrast agents, photodynamic therapy and drug and gene delivery, summarized in several comprehensive reviews.
Medical research.
In April 2003, fullerenes were under study for potential medicinal use: binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma. The October 2005 issue of "Chemistry & Biology" contained an article describing the use of fullerenes as light-activated antimicrobial agents.
Tumor research.
While past cancer research has involved radiation therapy, photodynamic therapy is important to study because breakthroughs in treatments for tumor cells will give more options to patients with different conditions. Recent experiments using HeLa cells in cancer research involves the development of new photosensitizers with increased ability to be absorbed by cancer cells and still trigger cell death. It is also important that a new photosensitizer does not stay in the body for a long time to prevent unwanted cell damage.
Fullerenes can be made to be absorbed by HeLa cells. The derivatives can be delivered to the cells by using the functional groups L-phenylalanine, folic acid, and L-arginine among others.
Functionalizing the fullerenes aims to increase the solubility of the molecule by the cancer cells. Cancer cells take up these molecules at an increased rate because of an upregulation of transporters in the cancer cell, in this case amino acid transporters will bring in the L-arginine and L-phenylalanine functional groups of the fullerenes.
Once absorbed by the cells, the derivatives would react to light radiation by turning molecular oxygen into reactive oxygen which triggers apoptosis in the HeLa cells and other cancer cells that can absorb the fullerene molecule. This research shows that a reactive substance can target cancer cells and then be triggered by light radiation, minimizing damage to surrounding tissues while undergoing treatment.
When absorbed by cancer cells and exposed to light radiation, the reaction that creates reactive oxygen damages the DNA, proteins, and lipids that make up the cancer cell. This cellular damage forces the cancerous cell to go through apoptosis, which can lead to the reduction in size of a tumor. Once the light radiation treatment is finished the fullerene will reabsorb the free radicals to prevent damage of other tissues. Since this treatment focuses on cancer cells, it is a good option for patients whose cancer cells are within reach of light radiation. As this research continues, the treatment may penetrate deeper into the body and be absorbed by cancer cells more effectively.
Safety and toxicity.
Lalwani "et al." published a comprehensive review on fullerene toxicity in 2013. These authors review the works on fullerene toxicity beginning in the early 1990s to present, and conclude that very little evidence gathered since the discovery of fullerenes indicate that is toxic. The toxicity of these carbon nanoparticles is not only dose- and time-dependent, but also depends on a number of other factors such as:
The authors therefore recommend assessing the pharmacology of every new fullerene- or metallofullerene-based complex individually as a different compound.
Popular culture.
Examples of fullerenes appear frequently in popular culture. Fullerenes appeared in fiction well before scientists took serious interest in them. In a humorously speculative 1966 column for "New Scientist", David Jones suggested the possibility of making giant hollow carbon molecules by distorting a plane hexagonal net with the addition of impurity atoms.


Weston cell

The Weston cell is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters. Invented by Edward Weston in 1893, it was adopted as the International Standard for EMF from 1911 until superseded by the Josephson voltage standard in 1990.
Chemistry.
The anode is an amalgam of cadmium with mercury with a cathode of pure mercury over which a paste of mercurous sulfate and mercury is placed. The electrolyte is a saturated solution of cadmium sulfate, and the depolarizer is a paste of mercurous sulfate.
As shown in the illustration, the cell is set up in an H-shaped glass vessel with the cadmium amalgam in one leg and the pure mercury in the other. Electrical connections to the cadmium amalgam and the mercury are made by platinum wires fused through the lower ends of the legs.
Reference cells must be applied in such a way that no current is drawn from them.
Characteristics.
The original design was a saturated cadmium cell producing a reference and had the advantage of having a lower temperature coefficient than the previously used Clark cell.
One of the great advantages of the Weston normal cell is its small change of electromotive force with change of temperature. At any temperature between and , . This temperature formula was adopted by the London conference of 1908
The temperature coefficient can be reduced by shifting to an unsaturated design, the predominant type today. However, an unsaturated cell's output decreases by some 80 microvolts per year, which is compensated by periodic calibration against a saturated cell.


Unpentennium


Strontium vapor laser

A strontium vapor laser is a laser that produces at its output, high-intensity pulsed light at a wavelength of 430.5 nm in the blue-violet region of the visible spectrum via vaporized strontium metal gas contained within a glass tube.
History.
Laser action on two of the infra-red transitions in Sr+ was first discovered in the Clarendon Laboratory, Oxford by Deech and Sanders as early as 1968. Gain was measured over a 9 cm length of strontium vapor present in a 3 torr buffer gas of helium or neon and maintained at the correct temperature by an externally heated furnace. Three years later, twelve further infra-red laser transitions in neutral strontium were reported by Cahuzac. Again the heat needed to provide sufficient vapor pressure was produced by external means. The tubes used here were 5–10 mm in diameter and 75 cm in length. A 1.25 m cavity was used with mirrors of approximately 98% reflectivity.
In 1973, Latush and Sém from Rostov-on-Don State University, Russia, observed visible laser action from the strontium vapor laser for the first time, at wavelengths of 430.5 nm and 416.2 nm. The active volume was contained in a ceramic tube 8 mm in diameter and 60 cm long. Small pieces of strontium were placed inside the tube at equally spaced intervals and the necessary vapor pressure was produced by externally heating the assembly. Helium was used as the buffer gas, at pressures ranging from 2.5–35 torr. Output power was found to increase with increasing buffer gas pressure.
Population Inversion Mechanism.
The strontium laser is excited by a high current, pulsed electrical discharge. The gain medium consists of a small amount of strontium vapor held in a relatively high pressure buffer gas of helium. Average gas temperatures are in the region of 800°C.
A capacitor, charged to several tens of kilovolts, is repetitively discharged through the gas mixture. During each discharge pulse through the laser medium the neutral strontium vapor is ionized to Sr2+ as the electrons in the outer shell are removed, while only a small fraction of the helium buffer gas is ionized due to its greater ionization potential. On termination of the current pulse, rapid cooling of the electrons occurs, permitting three-body electron-electron-Sr2+ collisions to occur to form the most
highly excited states of Sr+, as shown:
The excess kinetic energy evolved in this process is carried away by the third body, an electron. De-excitation of the high-lying energy levels of Sr+ then occurs, due to collisions with the remaining free electrons in the plasma. This cascade of recombined electrons, down the Sr+ energy levels, continues freely until the 62S level is reached. The downward transition across the relatively large energy gap, 62S-52P, acts as a bottleneck for the electron de-excitation process, which generally proceeds faster for closely spaced levels. A population inversion therefore builds up in the 62S1/2 upper laser level. Inversion occurs between this and the 52P3/2 lower laser level, which is cleared to the metastable and ground levels also by collisions with electrons.


Sodium pyrosilicate

Sodium pyrosilicate is the chemical compound . It is one of the sodium silicates, specifically a pyrosilicate, formally a salt of the unstable pyrosilicic acid .
Structure.
The anhydrous solid has the triclinic crystal structure, with space group (a = 5.8007(8) Å, b = 11.5811(15) Å, c = 23.157(3) Å, α = 89.709(10)°, β = 88.915(11)°, γ = 89.004(11)°, V = 1555.1(4) Å3, Z = 8, Dx = 2.615 g·cm−3, μ(Mo‐Kα) = 7.94 cm−1). The anions are arranged in layers parallel to the (100) plane, with the sodium cations distributed in 24 distinct crystallographic positions, coordinated by 4 to 6 near oxygen atoms. Some of the 4-coordinated sodium atoms can be interpreted as parallel columns of edge-sharing tetrahedra. The columnar arrangement forms tunnels that house the remaining sodium cations. Twinning at a microscopic scale simulates a much larger monoclinic C centered lattice (V′ = 6220 Å3, Z = 32).


Zigrasite

Zigrasite is a phosphate mineral with the chemical formula of MgZr(PO4)2(H2O)4. Zigrasite was discovered and is only known to occur in the Dunton Quarry at Oxford County, Maine. Zigrasite was specifically found in the giant 1972 gem tourmaline-bearing pocket at the Dunton Quarry. Zigrasite is named after James Zigras who originally discovered and brought the mineral to attention.
Occurrence.
Zigrasite is found in association with tourmaline, microcline, quartz, albite, beryl, amblygonite-montebrasite, childrenite-eosphorite, and apatite. It is crystallized as one of the last minerals during pocket formation. The crystals themselves were found perched on a crystal of tourmaline from the giant 1972 pocket at the Dunton Quarry. The quarry itself is located in a complex rare-element granitic pegmatite that has produced large quantities of gem tourmaline as well as many other rare phosphate species.
Physical Properties.
Zigrasite is an offwhite to pale yellow or light tan in color. Zigrasite exhibits a vitreous luster, and a white streak. The Mohs hardness is 3.The measured( Berman Balance) and calculated densities are 2.76(4) and 2.66 g/cm3 respectively. Zigrasite exhibits imperfect cleavage in two directions, parallel to (010) and (001), it shows no parting, is brittle and has hackly fracture. In transmitted light zigrasite is colorless and non-pleochroic, biaxial negative with indices of refraction α 1.597(1) β 1.622 (1), γ 1.635 (1). Zigrasite is translucent and shows a light blue to pale yellow cathodoluminescence.
Crystal Structure.
Zigrasite's atomic coordination consists of two phosphorus sites which are solely occupied by phosphorus and tetrahedrally coordinated by four oxygen anions with distances of 1.532 and 1.533 Å. There are also two magnesium, both of which are occupied solely by Mg and are octahedrally coordinated by two oxygen anions and four (H2O) groups with <Mg-O> distances of 2.064 and 2.075 Å. There is a single zircon site solely occupied by zircon and octahedrally coorinated by six oxygen anions with a <Zr-O> distance of 2.065 Å.
The zircon octahedron shares corners with six (PO4) tetrehedra to form a pinwheel cluster. This cluster forms a fragment of a Zr(PO4)2 sheet which is parallel to the (001) direction. Alternating tetrahedra point up and down relative to the plane of the sheet. Isolated Mg(H2O)6) octahedrea are arranged in layers parallel to (001). These layers are intercalated between the [Zr(PO4)2] sheets linking through the apical vertices of the (PO4) tetrahedra. This structure is formally a heteropolyhedral framework with linkage weaker in the c direction accounting for cleavage.
Chemical Properties.
Zigrasite is one of 3 compositional variations in zircon phosphate grains from the giant 1972 tourmaline pocket, representing the magnesium phase with malhmoodite representing the iron phase and an as of yet unnamed calcium-analogue. Zigrasite can be considered to be the magnesium end member for the zircon phosphate grains found in the giant 1972 pocket. Zigrasite exhibits hydrogen bonding which increases linkage within the layer of magnesium octahedra.
X-ray crystallography.
The powder-diffraction pattern was recorded with Cu-Ka X-radiation on a DebyeScherrer camera with a diameter of114.6 mm and a Gandolfi attachment. Refinement of the unit-cell parameters gave the following values: a = 5.321(7) Å , b = 9.360(10) Å , c = 9.660(8)Å , a = 97.38(10)º, b = 91.29(9)º, g = 90.58(9)º, V = 477.0(5) Å 3 . Unit-cell dimensions were also determined on a Bruker single-crystal diffractometer using graphite-monochromated Mo-Ka Xradiation and the resulting values (a = 5.3049(2) Å , b = 9.3372(4) Å, c = 9.6282(5) Å, a = 97.348(1)º, b = 91.534(1)º, g = 90.512(4)º) are in close agreement with those determined by powder diffraction.
See also.
List of MineralsList of minerals named after people


Sodium oxalate

Sodium oxalate, or disodium oxalate, is the sodium salt of oxalic acid with the formula Na2C2O4. It is a white, crystalline, odorless solid, that decomposes above 290 °C.
Disodium oxalate can act as a reducing agent, and it may be used as a primary standard for standardizing potassium permanganate (KMnO4) solutions.
The mineral form of sodium oxalate is natroxalate. It is only very rarely found and restricted to extremely sodic conditions of ultra-alkaline pegmatites.
Preparation.
Sodium oxalate can be prepared through the neutralization of oxalic acid with sodium hydroxide (NaOH) in a 1:2 acid-to-base molar ratio. Evaporation yields the anhydrous oxalate that can be thoroughly dried by heating to between 200 and 250 °C.
Half-neutralization can be accomplished with NaOH in a 1:1 ratio which produces NaHC2O4, monobasic sodium oxalate or sodium hydrogenoxalate.
Alternatively, it can be produced by decomposing sodium formate by heating it at a temperature exceeding 360 °C.
Reactions.
Sodium oxalate starts to decompose above 290 °C into sodium carbonate and carbon monoxide:
When heated at between 200 and 525°C with vanadium pentoxide in a 1:2 molar ratio, the above reaction is suppressed, yielding instead a sodium vanadium oxibronze with release of carbon dioxide
with "x" increasing up to 1 as the temperature increases.
Sodium oxalate is used to standardize potassium permanganate solutions. It is desirable that the temperature of the titration mixture is greater than 60 °C to ensure that all the permanganate added reacts quickly. The kinetics of the reaction is complex, and the manganese(II) ions formed catalyze the further reaction between permanganate and oxalic acid (formed "in situ" by the addition of excess sulfuric acid). The final equation is as follows:
Biological activity.
Like several other oxalates, sodium oxalate is toxic to humans. It can cause burning pain in the mouth, throat and stomach, bloody vomiting, headache, muscle cramps, cramps and convulsions, drop in blood pressure, heart failure, shock, coma, and possible death. Mean lethal dose by ingestion of oxalates is 10-15 grams/kilogram of body weight (per MSDS).
Sodium oxalate, like citrates, can also be used to remove calcium ions (Ca2+) from blood plasma. It also prevents blood from clotting. Note that by removing calcium ions from the blood, sodium oxalate can impair brain function, and deposit calcium oxalate in the kidneys.


Tungstate

In chemistry, a tungstate is a compound that contains an oxoanion of tungsten or is a mixed oxide containing tungsten. The simplest tungstate ion is , "orthotungstate". Many other tungstates belong to a large group of polyatomic ions that are termed polyoxometalates, ("POMs"), and specifically termed isopolyoxometalates as they contain, along with oxygen and maybe hydrogen, only one other element. Almost all useful tungsten ores are tungstates.
Structures.
Orthotungstates feature tetrahedral W(VI) centres with short W–O distances of 1.79 Å. Structurally, they resemble sulfates. Six-coordinate, octahedral tungsten dominates in the polyoxotungstates. In these compounds, the W–O distances are elongated.
Some examples of tungstate ions:
See the for a list of tungstates.
Occurrence.
Tungstates occur naturally with molybdates. Scheelite, the mineral calcium tungstate, often contains a small amount of molybdate. Wolframite is manganese and iron tungstate, and all these are valuable sources of tungsten. Powellite is a mineral form of calcium molybdate containing a small amount of tungstate.
Reactions.
Solutions of tungstates, like those of , give intensely blue solutions of complex tungstate(V,VI) analogous to the molybdenum blues when reduced by most organic materials.
Unlike chromate, tungstate is not a good oxidizer, but like chromate, solutions of tungstate condense to give the isopolytungstates upon acidification.


Ammonia kinase

In enzymology, an ammonia kinase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are ATP and NH3, whereas its two products are ADP and phosphoramide.
This enzyme belongs to the family of transferases, specifically those transferring phosphorus-containing groups (phosphotransferases) with a nitrogenous group as acceptor. The systematic name of this enzyme class is ATP:ammonia phosphotransferase. Other names in common use include phosphoramidate-adenosine diphosphate phosphotransferase, and phosphoramidate-ADP-phosphotransferase.


Edwin Larwill

Edwin Larwill (c. 1806 – December 29, 1876) was a tinsmith and political figure in Canada West.
He came to Chatham in 1841. He helped form a "Free and Easy Club" there, a counter-attack against the Temperance movements in the region. Larwill served as school commissioner. He was opposed to black settlement in the area and debated the issue with William King at a public debate held in 1849. In 1854, he was elected to the Legislative Assembly of the Province of Canada for Kent as a Conservative. Larwill voted for Quebec City over Kingston as seat of government for the Canadas. This unpopular position and organized opposition by black residents of the county led to his defeat in 1857; Archibald McKellar was elected in Kent. Larwill died in Chatham in 1876.


Aragonite

Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including precipitation from marine and freshwater environments.
The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal. Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching helictitic forms called "flos-ferri" ("flowers of iron") from their association with the ores at the Carinthian iron mines.
Occurrence.
The type location for aragonite is Molina de Aragón in the Province of Guadalajara in Castilla-La Mancha, Spain, for which it was named in 1797. Aragonite is found in this locality as cyclic twins inside gypsum and marls of the Keuper facies of the Triassic. This type of aragonite deposit is very common in Spain, and there are also some in France and Morocco.
An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia. In the US, aragonite in the form of stalactites and "cave flowers" (anthodite) is known from Carlsbad Caverns and other caves. Massive deposits of oolitic aragonite sand are found on the seabed in the Bahamas.
Aragonite is the high pressure polymorph of calcium carbonate. As such, it occurs in high pressure metamorphic rocks such as those formed at subduction zones.
Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water corals (Scleractinia). Several serpulids have aragonitic tubes. Because the mineral deposition in mollusk shells is strongly biologically controlled, some crystal forms are distinctively different from those of inorganic aragonite. In some mollusks, the entire shell is aragonite; in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite). The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite.
Aragonite also forms naturally in the endocarp of Celtis occidentalis.
Aragonite also forms in the ocean and in caves as inorganic precipitates called marine cements and speleothems, respectively. Aragonite is not uncommon in serpentinites where high Mg in pore solutions apparently inhibits calcite growth and promotes aragonite precipitation.
Aragonite is metastable at the low pressures near the Earth's surface and is thus commonly replaced by calcite in fossils. Aragonite older than the Carboniferous is essentially unknown. It can also be synthesized by adding a calcium chloride solution to a sodium carbonate solution at temperatures above or in water-ethanol mixtures at ambient temperatures.
Physical properties.
Aragonite is thermodynamically unstable at standard temperature and pressure, and tends to alter to calcite on scales of 107 to 108 years.
The mineral vaterite, also known as μ-CaCO3, is another phase of calcium carbonate that is metastable at ambient conditions typical of Earth's surface, and decomposes even more readily than aragonite.
Uses.
In aquaria, aragonite is considered essential for the replication of reef conditions. Aragonite provides the materials necessary for much sea life and also keeps the pH of the water close to its natural level, to prevent the dissolution of biogenic calcium carbonate.
Aragonite has been successfully tested for the removal of pollutants like zinc, cobalt and lead from contaminated wastewaters.
It is claimed by some water conditioner manufacturers that their technology converts calcite to aragonite in order to reduce limescale.


California Fuel Cell Partnership

The California Fuel Cell Partnership (CaFCP) is a public-private partnership to promote hydrogen vehicles (including cars and buses) in California. It is notable as one of the first initiatives for that purpose undertaken in the United States. The challenge is which come first, hydrogen cars or filling stations.
In January 1995, three state government agencies—California Air Resources Board, South Coast Air Quality Management District and California Energy Commission joined with six private sector companies—Ballard Power Systems, DaimlerChrysler, Ford Motor Company, BP, Shell Hydrogen and ChevronTexaco—to form the California Fuel Cell Partnership. The goal was to demonstrate and promote the potential for fuel cell vehicles (FCV) as a clean, safe, and practical alternative to vehicles powered by internal combustion engines.
In November 2000, the West Sacramento headquarters opened. The building includes a public gallery, offices, a hydrogen fueling station and indoor service bays for vehicle maintenance. At first, the automakers had just handful of cars all stationed in Sacramento. The goal was to see if these vehicles and fuel could be technically viable. If the answer was no, then CaFCP would close its doors in 2004.
Before the first phase was finished, CaFCP members knew that the technology could succeed. The number of members grew to 33 and set a new set of goals for the next phase of operation, from 2004-2007. During this period, CaFCP members worked on project to prove or disprove the commercially viability—would the vehicles, fuels and public policies meet consumer expectations?
In mid-2006, the members agreed that FCVs could be a commercial success. They agreed to a third phase through the end of 2012 to lay the foundations for a commercial market, which includes developing early retail stations, policies to enable the sale of retail hydrogen and necessary steps to building a supplier base for large-scale production in the future.
CaFCP completed its first phase of operation in December, 2003. The second phase will conclude in December, 2007. In mid-2006, the members agreed to enter a third phase to encourage fuel cell vehicle commercialization that will continue through 2012.
Mission.
CaFCP's mission is to promote the benefits of hydrogen fuel cells. Hydrogen fuel cells is a zero emission technology - a technology that would eliminate air toxics, criteria pollutants that create urban smog, and global gas pollutants such as carbon dioxide. Hydrogen can be created from multiple sources - from a range of fossil fuels and from cleaner 'electrolytic process'. In the initial demonstration stage, hydrogen will be made from natural gas, a fossil fuel. That's because over 95% of industrial hydrogen is currently made very efficiently from "natural gas". (Industrial hydrogen production makes up only a small fraction (6 percent) of natural gas consumption. Natural gas is used extensively around the globe for primarily residential, commercial heating, and in making electricity).
Natural gas will be the cheapest option for producing hydrogen, but it emits carbon dioxide. However, the whole aim of hydrogen fuel sector is to move towards splitting sea-water by electrolysis into pure hydrogen (and oxygen). Electrolysis ("the electrolytic process") uses electricity from renewable clean resources, such as solar, and wind. Such electrolysis eliminates any pollutants to the atmosphere. In mid 90s, the South Coast Air Quality Management (SCAQMD) initiated and funded several pilot hydrogen projects at Sunline Transit, in Riverside County, near Palm Springs, California. The SCAQMD team included several technology companies (such as Ballard, Air Products, Electrolyzer, Schwatz Energy Center) assembled together to demonstrate hydrogen in general transportation use, both in light duty vehicles and transit buses. The project used hydrogen initially from steam reforming, and later from solar panels based electrolysis of water. Hydrogen produced was stored on site, and dispensed as fuel. These projects were first of its kind in California, and proved to be highly successful. The projects clearly demonstrated the safety of hydrogen for general public use. The Hindenburg curse that caused experts to villify hydrogen was lifted. Success of these pilot projects convinced CAFCP to identify hydrogen as the "fuel for the future". As of early 2021, Japan, South Korea, Germany and Saudi Arabia have announced massive programs into producing hydrogen via solar/wind/renewable resources, and deploying it in fuel cell vehicles. Other Western European countries are following suit. China reportedly is gearing up to spent billions as well in advancing hydrogen fuel cells.
Hydrogen has been criticized by some as a fossil fuel based fuel. This is only true in initial stages. The hydrogen currently dispensed in hydrogen vehicles in California is already exceeding the 30 percent mandate of renewable hydrogen to fossil fuel mix. The aim is to achieve 100 percent in the not too distant future. Manufacturers, such as Toyota, Honda and Hyundai, are offering generous incentives to buyers of their hydrogen cars to offset the cost of the fuel.
In summary, hydrogen fuel cell technology (at around 60 percent efficiency) is twice more efficient than the regular gasoline cars. Current gasoline car wastes 70 percent in heat, using only 30 percent of the valuable energy to drive the vehicle. SUVs are the current craze among consumers. But SUVs waste even more valuable energy - in excess of 75 percent. In addition, whereas hydrogen fuel cells emit only water vapor, gasoline/ diesel vehicles emit a range of air toxics, urban smog pollutants and global warming emissions. In view of this, California has announced a target of zero sales of new gasoline combustion automobiles by year 2035, with all of its sales coming from zero emission battery and hydrogen fuel cell cars.
Membership.
The California Fuel Cell Partnership is a unique collaborative of auto manufacturers, fossil fuel companies, fuel cell technology companies and government agencies. The members collaborate on activities that advance the technology, such as first responder training, community outreach and agreeing on protocols while standards are being developed.
Automotive members provide fuel cell passenger vehicles that are placed in demonstration programs, where they are tested in real-world driving conditions.
Fossil Fuel members work to build hydrogen stations within an infrastructure that is safe, convenient and fits into the community.
Fuel cell technology members provide fuel cells for passenger vehicles and transit buses.
Government members lay the groundwork for demonstration programs by facilitating steps to creating a hydrogen fueling infrastructure.
Full members.
Automobile makers
Hydrogen Fuel Cells
Fossil Fuel Companies
Government Agencies
Associate Members.
Associate members assist the full members meet the partnership's objectives by helping in specific areas of expertise.
Transit Agencies
Fueling Providers
Construction
Others


Acanthite

Acanthite is a form of silver sulfide with the chemical formula Ag2S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below . Argentite is the stable form above that temperature. As argentite cools below that temperature its cubic form is distorted to the monoclinic form of acanthite. Below 173 °C acanthite forms directly. Acanthite is the only stable form in normal air temperature.
Occurrence.
Acanthite is a common silver mineral in moderately low-temperature hydrothermal
veins and in zones of supergene enrichment. It occurs in association with native silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena,
chalcopyrite, sphalerite, calcite and quartz.
Acanthite was first described in 1855 for an occurrence in the Jáchymov (St Joachimsthal) District, Krušné Hory Mts (Erzgebirge), Karlovy Vary Region, Bohemia, Czech Republic. The name is from the Greek "akantha" meaning thorn or arrow, in reference to its crystal shape.