1.
Transmission electron microscopy
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Transmission electron microscopy is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through it. TEMs are capable of imaging at a higher resolution than light microscopes. This enables the user to examine fine detail—even as small as a single column of atoms. TEM forms a major analysis method in a range of scientific fields, TEMs find application in cancer research, virology, materials science as well as pollution, nanotechnology and semiconductor research. At smaller magnifications TEM image contrast is due to absorption of electrons in the material, due to the thickness, at higher magnifications complex wave interactions modulate the intensity of the image, requiring expert analysis of observed images. The first TEM was built by Max Knoll and Ernst Ruska in 1931, with this group developing the first TEM with resolution greater than that of light in 1933, in 1986, Ruska was awarded the Nobel Prize in physics for the development of transmission electron microscopy. Developments into ultraviolet microscopes, led by Köhler and Rohr, allowed for an increase in resolving power of about a factor of two, however this required more expensive quartz optical components, due to the absorption of UV by glass. At this point it was believed that obtaining an image with sub-micrometer information was simply due to this wavelength constraint. It had earlier recognized by Plücker in 1858 that the deflection of cathode rays was possible by the use of magnetic fields. This effect had been utilized to build primitive cathode ray oscilloscopes as early as 1897 by Ferdinand Braun, indeed, in 1891 it was recognized by Riecke that the cathode rays could be focused by these magnetic fields, allowing for simple lens designs. Later this theory was extended by Hans Busch in his published in 1926. The team consisted of several PhD students including Ernst Ruska and Bodo von Borries, in 1931 the group successfully generated magnified images of mesh grids placed over the anode aperture. The device used two lenses to achieve higher magnifications, arguably the first electron microscope. In that same year, Reinhold Rudenberg, the director of the Siemens company, had patented an electrostatic lens electron microscope. At this time the nature of electrons, which were considered charged matter particles, had not been fully realized until the publication of the De Broglie hypothesis in 1927. In April 1932, Ruska suggested the construction of a new microscope for direct imaging of specimens inserted into the microscope. This goal was achieved in September 1933, using images of cotton fibers, research continued on the electron microscope at Siemens in 1936, the aim of the research was the development improvement of TEM imaging properties, particularly with regard to biological specimens. At this time electron microscopes were being fabricated for specific groups, in 1939 the first commercial electron microscope, pictured, was installed in the Physics department of IG Farben-Werke
2.
Carbon peapod
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Carbon peapod is a hybrid nanomaterial consisting of spheroidal fullerenes encapsulated within a carbon nanotube. It is named due to their resemblance to the seedpod of the pea plant, since the properties of carbon peapods differ from those of nanotubes and fullerenes, the carbon peapod can be recognized as a new type of a self-assembled graphitic structure. Single-walled nanotubes were first seen in 1993 as cylinders rolled from a single graphene sheet, in 1998, the first peapod was observed by Brian Smith, Marc Monthioux and David Luzzi. The idea of peapods came from the structure that was produced inside an electron microscope in 2000. They were first recognized in fragments obtained by a pulsed-laser vaporization synthesis followed by treatment with an acid, carbon peapods can be naturally produced during carbon nanotube synthesis by pulsed laser vaporization. C60 fullerene impurities are formed during the treatment and acid purification. The encapsulated fullerenes have diameters close to that of C60 and form a chain inside the tube, controlled production of carbon peapods allow for greater variety in both the nanotube structure and the fullerene composition. Varying elements can be incorporated into a carbon peapod through doping, the existence of carbon peapods demonstrates further properties of carbon nanotubes, such as potential to be a stringently controlled environment for reactions. Owing to the ease with which fullerenes can encapsulate or be doped with other molecules, additionally, due to the enclosed fullerenes being limited to only a one-dimensional degree of mobility, phenomena such as diffusion or phase transformations can easily be studied. The diameter of carbon peapods range from ca.1 to 50 nanometers, various combinations of fullerene C60 sizes and nanotube structures can lead to various electric conductivity property of carbon peapods due to orientation of rotations. For example, the C60 @ is a superconductor and the C60 @ peapod is a semiconductor. The calculated band gap of C60 @ equals 0.1 eV, research into their potential as semiconductors is still ongoing. Although both the doped fullerides and ropes of SWNTs are superconductors, unfortunately, the temperatures for the superconducting phase transition in these materials are low. There are hopes that carbon nano-peapods could be superconducting at room temperature, with chemical doping, the electronic characteristics of peapods can be further adjusted. When carbon peapod is doped with metal atoms like potassium. It forms a negatively charged C606− covalently bound, one-dimensional polymer chain with metallic conductivity, overall, the doping of SWNTs and peapods by alkali metal atoms actively enhances the conductivity of the molecule since the charge is relocated from the metal ions to the nanotubes. Doping carbon nanotubes with oxidized metal is another way to adjust conductivity and it creates a very interesting high temperature superconducting state as the Fermi level is significantly reduced. A good application would be the introduction of silicon dioxide to carbon nanotubes and it constructs memory effect as some research group has invented ways to create memory devices based on carbon peapods grown on Si/SiO2 surfaces
3.
Nanotechnology
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Nanotechnology is manipulation of matter on an atomic, molecular, and supramolecular scale. It is therefore common to see the plural form nanotechnologies as well as nanoscale technologies to refer to the range of research. Because of the variety of applications, governments have invested billions of dollars in nanotechnology research. Until 2012, through its National Nanotechnology Initiative, the USA has invested 3.7 billion dollars, scientists currently debate the future implications of nanotechnology. Nanotechnology may be able to create new materials and devices with a vast range of applications, such as in nanomedicine, nanoelectronics, biomaterials energy production. These concerns have led to a debate among advocacy groups and governments on whether regulation of nanotechnology is warranted. The term nano-technology was first used by Norio Taniguchi in 1974, also in 1986, Drexler co-founded The Foresight Institute to help increase public awareness and understanding of nanotechnology concepts and implications. In the 1980s, two major breakthroughs sparked the growth of nanotechnology in modern era, the microscopes developers Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratory received a Nobel Prize in Physics in 1986. Binnig, Quate and Gerber also invented the atomic force microscope that year. Second, Fullerenes were discovered in 1985 by Harry Kroto, Richard Smalley, and Robert Curl, in the early 2000s, the field garnered increased scientific, political, and commercial attention that led to both controversy and progress. Controversies emerged regarding the definitions and potential implications of nanotechnologies, exemplified by the Royal Societys report on nanotechnology, challenges were raised regarding the feasibility of applications envisioned by advocates of molecular nanotechnology, which culminated in a public debate between Drexler and Smalley in 2001 and 2003. Meanwhile, commercialization of products based on advancements in nanoscale technologies began emerging and these products are limited to bulk applications of nanomaterials and do not involve atomic control of matter. Governments moved to promote and fund research into nanotechnology, such as in the U. S, by the mid-2000s new and serious scientific attention began to flourish. Projects emerged to produce nanotechnology roadmaps which center on atomically precise manipulation of matter and discuss existing and projected capabilities, goals, Nanotechnology is the engineering of functional systems at the molecular scale. This covers both current work and concepts that are more advanced, one nanometer is one billionth, or 10−9, of a meter. By comparison, typical carbon-carbon bond lengths, or the spacing between atoms in a molecule, are in the range 0. 12–0.15 nm. On the other hand, the smallest cellular life-forms, the bacteria of the genus Mycoplasma, are around 200 nm in length, by convention, nanotechnology is taken as the scale range 1 to 100 nm following the definition used by the National Nanotechnology Initiative in the US. The lower limit is set by the size of atoms since nanotechnology must build its devices from atoms and molecules
4.
Carbon
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Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds, three isotopes occur naturally, 12C and 13C being stable, while 14C is a radioactive isotope, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity, Carbon is the 15th most abundant element in the Earths crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is the second most abundant element in the body by mass after oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon, the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form, for example, graphite is opaque and black while diamond is highly transparent. Graphite is soft enough to form a streak on paper, while diamond is the hardest naturally occurring material known, graphite is a good electrical conductor while diamond has a low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials, all carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen, the most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and transition metal carbonyl complexes. The largest sources of carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil. For this reason, carbon has often referred to as the king of the elements. The allotropes of carbon graphite, one of the softest known substances, and diamond. It bonds readily with other small atoms including other carbon atoms, Carbon is known to form almost ten million different compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements, although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon is the element, with a ground-state electron configuration of 1s22s22p2. Its first four ionisation energies,1086.5,2352.6,4620.5 and 6222.7 kJ/mol, are higher than those of the heavier group 14 elements. Carbons covalent radii are normally taken as 77.2 pm,66.7 pm and 60.3 pm, although these may vary depending on coordination number, in general, covalent radius decreases with lower coordination number and higher bond order. Carbon compounds form the basis of all life on Earth
5.
Carbon nanotube
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Carbon nanotubes are allotropes of carbon with a cylindrical nanostructure. These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology. Owing to the exceptional strength and stiffness, nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000,1. In addition, owing to their thermal conductivity, mechanical. For instance, nanotubes form a portion of the material in some baseball bats, golf clubs. Nanotubes are members of the fullerene structural family and their name is derived from their long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene. Nanotubes are categorized as single-walled nanotubes and multi-walled nanotubes, individual nanotubes naturally align themselves into ropes held together by van der Waals forces, more specifically, pi-stacking. Applied quantum chemistry, specifically, orbital hybridization best describes chemical bonding in nanotubes, the chemical bonding of nanotubes is composed entirely of sp2 bonds, similar to those of graphite. These bonds, which are stronger than the sp3 bonds found in alkanes and diamond, most single-walled nanotubes have a diameter of close to 1 nanometer, and can be many millions of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder, the way the graphene sheet is wrapped is represented by a pair of indices. The integers n and m denote the number of unit vectors along two directions in the crystal lattice of graphene. If m =0, the nanotubes are called zigzag nanotubes, and if n = m, the diameter of an ideal nanotube can be calculated from its indices as follows d = a π =78.3 p m, where a =0.246 nm. SWNTs are an important variety of carbon nanotube because most of their properties change significantly with the values, in particular, their band gap can vary from zero to about 2 eV and their electrical conductivity can show metallic or semiconducting behavior. Single-walled nanotubes are likely candidates for miniaturizing electronics, the most basic building block of these systems is the electric wire, and SWNTs with diameters of an order of a nanometer can be excellent conductors. One useful application of SWNTs is in the development of the first intermolecular field-effect transistors, the first intermolecular logic gate using SWCNT FETs was made in 2001. A logic gate requires both a p-FET and an n-FET, because SWNTs are p-FETs when exposed to oxygen and n-FETs otherwise, it is possible to expose half of an SWNT to oxygen and protect the other half from it. The resulting SWNT acts as a not logic gate with both p and n-type FETs in the same molecule, prices for single-walled nanotubes declined from around $1500 per gram as of 2000 to retail prices of around $50 per gram of as-produced 40–60% by weight SWNTs as of March 2010. As of 2016 the retail price of as-produced 75% by weight SWNTs were $2 per gram, SWNTs are forecast to make a large impact in electronics applications by 2020 according to the The Global Market for Carbon Nanotubes report
6.
Fullerene
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A fullerene is a molecule of carbon in the form of a hollow sphere, ellipsoid, tube, and many other shapes. Spherical fullerenes, also referred to as Buckminsterfullerenes, resemble the balls used in football, cylindrical fullerenes are also called carbon nanotubes. Fullerenes are similar in structure to graphite, which is composed of stacked sheets of linked hexagonal rings. The name was an homage to Buckminster Fuller, whose geodesic domes it resembles, the structure was also identified some five years earlier by Sumio Iijima, from an electron microscope image, where it formed the core of a bucky onion. Fullerenes have since found to occur in nature. More recently, fullerenes have been detected in outer space, according to astronomer Letizia Stanghellini, It’s possible that buckyballs from outer space provided seeds for life on Earth. The discovery of fullerenes greatly expanded the number of carbon allotropes, which until recently were limited to graphite, graphene, diamond. The icosahedral C60H60 cage was mentioned in 1965 as a topological structure. Eiji Osawa of Toyohashi University of Technology predicted the existence of C60 in 1970 and he noticed that the structure of a corannulene molecule was a subset of an Association football shape, and he 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 proposed the structure and made a model of C60. Unfortunately, the evidence for new form of carbon was very weak and was not accepted. The results were never published but were acknowledged in Carbon in 1999, in 1973 independently from Henson, a group of scientists from the USSR, directed by Prof. Bochvar, made a quantum-chemical analysis of the stability of C60 and calculated its electronic structure. As in the cases, the scientific community did not accept the theoretical prediction. The paper was published in 1973 in Proceedings of the USSR Academy of Sciences, in mass spectrometry discrete peaks appeared corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms. Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this class of molecules, C60 and other fullerenes were later noticed occurring outside the laboratory. By 1990 it was easy to produce gram-sized samples of fullerene powder using the techniques of Donald Huffman, Wolfgang Krätschmer, Lowell D. Lamb. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices, so-called endohedral fullerenes have ions or small molecules incorporated inside the cage atoms. Fullerene is a reactant in many organic reactions such as the Bingel reaction discovered in 1993
7.
Allotropes of carbon
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Carbon is capable of forming many allotropes due to its valency. Well-known forms of carbon include diamond and graphite, in recent decades many more allotropes and forms of carbon have been discovered and researched including ball shapes such as buckminsterfullerene and sheets such as graphene. Larger scale structures of carbon nanotubes, nanobuds and nanoribbons. Other unusual forms of carbon exist at high temperatures or extreme pressures. Around 350 hypotetical 3-periodic allotropes of carbon are known at the present time according to SACADA database, Diamond is a well known allotrope of carbon. The hardness and high dispersion of light of diamond make it useful for industrial applications and jewelry. Diamond is the hardest known natural mineral and this makes it an excellent abrasive and makes it hold polish and luster extremely well. No known naturally occurring substance can cut a diamond, except another diamond, the market for industrial-grade diamonds operates much differently from its gem-grade counterpart. Industrial diamonds are valued mostly for their hardness and heat conductivity, making many of the characteristics of diamond, including clarity and color. This helps explain why 80% of mined diamonds are unsuitable for use as gemstones, the dominant industrial use of diamond is in cutting, drilling, grinding, and polishing. Most uses of diamonds in these technologies do not require large diamonds, in fact, diamonds are embedded in drill tips or saw blades, or ground into a powder for use in grinding and polishing applications. Specialized applications include use in laboratories as containment for high pressure experiments, high-performance bearings, with the continuing advances being made in the production of synthetic diamond, future applications are beginning to become feasible. Garnering much excitement is the use of diamond as a semiconductor suitable to build microchips from. Each carbon atom in a diamond is covalently bonded to four other carbons in a tetrahedron and these tetrahedrons together form a 3-dimensional network of six-membered carbon rings, in the chair conformation, allowing for zero bond angle strain. This stable network of covalent bonds and hexagonal rings, is the reason that diamond is so strong.1 kJ/mol compared to graphite, graphite, named by Abraham Gottlob Werner in 1789, from the Greek γράφειν is one of the most common allotropes of carbon. Unlike diamond, graphite is an electrical conductor, thus, it can be used in, for instance, electrical arc lamp electrodes. Likewise, under conditions, graphite is the most stable form of carbon. Therefore, it is used in thermochemistry as the state for defining the heat of formation of carbon compounds
8.
Bud
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In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of a stem. Once formed, a bud may remain for some time in a dormant condition, buds may be specialized to develop flowers or short shoots, or may have the potential for general shoot development. The term bud is used in zoology, where it refers to an outgrowth from the body which can develop into a new individual. The buds of many plants, especially in temperate or cold climates, are protected by a covering of modified leaves called scales which tightly enclose the more delicate parts of the bud. Many bud scales are covered by a substance which serves as added protection. When the bud develops, the scales may enlarge somewhat but usually just drop off, leaving on the surface of the growing stem a series of horizontally-elongated scars. By means of these one can determine the age of any young branch, since each years growth ends in the formation of a bud. Continued growth of the branch causes these scars to be obliterated after a few years so that the age of older branches cannot be determined by this means. In many plants scales do not form over the bud, the minute underdeveloped leaves in such buds are often excessively hairy. Naked buds are found in shrubs, like some species of the Sumac and Viburnums. In many of the latter, buds are more reduced. A terminal bud occurs on the end of a stem and lateral buds are found on the side, a head of cabbage is an exceptionally large terminal bud, while Brussels sprouts are large lateral buds. Since buds are formed in the axils of leaves, their distribution on the stem is the same as that of leaves, there are alternate, opposite, and whorled buds, as well as the terminal bud at the tip of the stem. In many plants buds appear in unexpected places, these are known as adventitious buds, often it is possible to find a bud in a remarkable series of gradations of bud scales. Such a series suggests that the scales of the bud are in truth leaves, buds are often useful in the identification of plants, especially for woody plants in winter when leaves have fallen. Buds may be classified and described according to different criteria, location, status, morphology, buds The term bud is used by analogy within zoology as well, where it refers to an outgrowth from the body which develops into a new individual. It is a form of asexual reproduction limited to animals or plants of relatively simple structure, in this process a portion of the wall of the parent cell softens and pushes out. The protuberance thus formed enlarges rapidly while at time the nucleus of the parent cell divides
9.
Covalent bond
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A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, for many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration. Covalent bonding includes many kinds of interactions, including σ-bonding, π-bonding, metal-to-metal bonding, agostic interactions, bent bonds, the term covalent bond dates from 1939. In the molecule H2, the atoms share the two electrons via covalent bonding. Covalency is greatest between atoms of similar electronegativities, thus, covalent bonding does not necessarily require that the two atoms be of the same elements, only that they be of comparable electronegativity. Covalent bonding that entails sharing of electrons more than two atoms is said to be delocalized. The term covalence in regard to bonding was first used in 1919 by Irving Langmuir in a Journal of the American Chemical Society article entitled The Arrangement of Electrons in Atoms and Molecules. Langmuir wrote that we shall denote by the term covalence the number of pairs of electrons that an atom shares with its neighbors. The idea of covalent bonding can be traced several years before 1919 to Gilbert N. Lewis and he introduced the Lewis notation or electron dot notation or Lewis dot structure, in which valence electrons are represented as dots around the atomic symbols. Pairs of electrons located between atoms represent covalent bonds, multiple pairs represent multiple bonds, such as double bonds and triple bonds. An alternative form of representation, not shown here, has bond-forming electron pairs represented as solid lines, Lewis proposed that an atom forms enough covalent bonds to form a full outer electron shell. In the diagram of methane shown here, the atom has a valence of four and is, therefore, surrounded by eight electrons, four from the carbon itself. Each hydrogen has a valence of one and is surrounded by two electrons – its own one electron plus one from the carbon, walter Heitler and Fritz London are credited with the first successful quantum mechanical explanation of a chemical bond in 1927. Their work was based on the valence bond model, which assumes that a bond is formed when there is good overlap between the atomic orbitals of participating atoms. Atomic orbitals have specific directional properties leading to different types of covalent bonds, sigma bonds are the strongest covalent bonds and are due to head-on overlapping of orbitals on two different atoms. A single bond is usually a σ bond, pi bonds are weaker and are due to lateral overlap between p orbitals. A double bond between two given atoms consists of one σ and one π bond, and a bond is one σ. Covalent bonds are also affected by the electronegativity of the atoms which determines the chemical polarity of the bond
10.
Electrical resistivity and conductivity
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Electrical resistivity is an intrinsic property that quantifies how strongly a given material opposes the flow of electric current. A low resistivity indicates a material that allows the flow of electric current. Resistivity is commonly represented by the Greek letter ρ, the SI unit of electrical resistivity is the ohm-metre. Electrical conductivity or specific conductance is the reciprocal of electrical resistivity and it is commonly represented by the Greek letter σ, but κ or γ are also occasionally used. Its SI unit is siemens per metre and CGSE unit is reciprocal second, many resistors and conductors have a uniform cross section with a uniform flow of electric current, and are made of one material. All copper wires, irrespective of their shape and size, have approximately the same resistivity, every material has its own characteristic resistivity – for example, resistivity of rubber is far larger than coppers. If the pipes are the size and shape, the pipe full of sand has higher resistance to flow. Resistance, however, is not solely determined by the presence or absence of sand and it also depends on the length and width of the pipe, short or wide pipes have lower resistance than narrow or long pipes. The above equation can be transposed to get Pouillets law, R = ρ ℓ A, the resistance of a given material increases with length, but decreases with increasing cross-sectional area. From the above equations, resistivity has the SI unit ohm metre, the formula R = ρ ℓ / A can be used to intuitively understand the meaning of a resistivity value. For example, if A = 7000100000000000000♠1 m2 ℓ = 7000100000000000000♠1 m, Conductivity, σ, is defined as the inverse of resistivity, σ =1 ρ. Conductivity has SI units of siemens per metre, the above definition was specific to resistors or conductors with a uniform cross-section, where current flows uniformly through them. A more basic and general definition starts from the fact that a field inside a material makes electric current flow. Conductivity is the inverse, σ =1 ρ = J E, for example, rubber is a material with large ρ and small σ—because even a very large electric field in rubber makes almost no current flow through it. On the other hand, copper is a material with small ρ, according to elementary quantum mechanics, electrons in an atom do not take on arbitrary energy values. Rather, electrons only occupy discrete energy levels in an atom or crystal. When a large number of such allowed energy levels are spaced close together —i. e. have similar energies— we can talk about these energy levels together as an energy band. There can be many such bands in a material, depending on the atomic number
11.
Field electron emission
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Field electron emission is emission of electrons induced by an electrostatic field. The most common context is field emission from a surface into vacuum. However, field emission can take place from solid or liquid surfaces, into vacuum, air, the field-induced promotion of electrons from the valence to conduction band of semiconductors can also be regarded as a form of field emission. When field emission is used without qualifiers it typically means cold emission, Field emission in pure metals occurs in high electric fields, the gradients are typically higher than 1 gigavolt per metre and strongly dependent upon the work function. Examples of applications for surface field emission include construction of bright electron sources for electron microscopes or the discharge of induced charges from spacecraft. Devices which eliminate induced charges are termed charge-neutralizers, Field emission was explained by quantum tunneling of electrons in the late 1920s. This was one of the triumphs of the nascent quantum mechanics, the theory of field emission from bulk metals was proposed by Ralph H. Fowler and Lothar Wolfgang Nordheim. A family of approximate equations, Fowler–Nordheim equations, is named after them, in some respects, field electron emission is a paradigm example of what physicists mean by tunneling. Unfortunately, it is also an example of the intense mathematical difficulties that can arise. Simple solvable models of the tunneling barrier lead to equations that get predictions of current density too low by a factor of 100 or more. To get even an approximate solution, it is necessary to use special approximate methods known in physics as semi-classical or quasi-classical methods, a consequence of these difficulties has been a heritage of misunderstanding and disinformation that still persists in some current field emission research literature. This article tries to present an account of field emission for the 21st century. Field electron emission, field-induced electron emission, field emission and electron field emission are general names for this experimental phenomenon, the first name is used here. Fowler–Nordheim tunneling is the tunneling of electrons through a rounded triangular barrier created at the surface of an electron conductor by applying a very high electric field. Individual electrons can escape by Fowler-Nordheim tunneling from many materials in different circumstances. Many solid and liquid materials can emit electrons in a CFE regime if a field of an appropriate size is applied. Fowler–Nordheim-type equations are a family of approximate equations derived to describe CFE from the electron states in bulk metals. The different members of the family represent different degrees of approximation to reality, there is no theoretical reason to believe that Fowler-Nordheim-type equations validly describe field emission from materials other than bulk crystalline solids
12.
Work function
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In solid-state physics, the work function is the minimum thermodynamic work needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here immediately means that the final position is far from the surface on the atomic scale. The work function is not a characteristic of a bulk material, the term −eϕ is the energy of an electron at rest in the vacuum nearby the surface. In words, the function is thus defined as the thermodynamic work required to remove an electron from the material to a state at rest in the vacuum nearby the surface. In practice, one directly controls EF by the applied to the material through electrodes. Rearranging the above equation, one has ϕ = V − W e where V = −EF/e is the voltage of the material, relative to an electrical ground that is defined as having zero Fermi level. The fact that ϕ depends on material means that the space between two dissimilar conductors will have a built-in electric field, when those conductors are in total equilibrium with each other. An example of this situation is depicted in the adjacent figure, as described in the next section, these built-in vacuum electric fields can have important consequences in some cases. Thermionic emission In thermionic electron guns, the function and temperature of the hot cathode are critical parameters in determining the amount of current that can be emitted. Tungsten, the choice for vacuum tube filaments, can survive to high temperatures. By coating the tungsten with a substance of lower work function and this prolongs the lifetime of the filament by allowing operation at lower temperatures. In reality these work function heuristics are inaccurate due to their neglect of numerous microscopic effects and they provide a convenient estimate until the true value can be determined by experiment. Equilibrium electric fields in vacuum chambers Variation in work function between different surfaces causes a non-uniform electrostatic potential in the vacuum, even on an ostensibly uniform surface, variations in W known as patch potentials are always present due to microscopic inhomogeneities. Patch potentials have disrupted sensitive apparatus that rely on a perfectly uniform vacuum, such as Casimir force experiments, critical apparatus may have surfaces covered with molybdenum, which shows low variations in work function between different crystal faces. Contact electrification If two conducting surfaces are moved relative to other, and there is potential difference in the space between them, then an electric current will be driven. This is because the charge on a conductor depends on the magnitude of the electric field. The externally observed electrical effects are largest when the conductors are separated by the smallest distance without touching and these contact currents can damage sensitive microelectronic circuitry and occur even when the conductors would be grounded in the absence of motion. Certain physical phenomena are highly sensitive to the value of the work function, the observed data from these effects can be fitted to simplified theoretical models, allowing one to extract a value of the work function
13.
Intellectual property
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Intellectual property refers to creations of the intellect for which a monopoly is assigned to designated owners by law. Intellectual property rights are the protections granted to the creators of IP, and include trademarks, copyright, patents, industrial design rights, and in some jurisdictions trade secrets. Artistic works including music and literature, as well as discoveries, inventions, words, phrases, symbols, the Statute of Monopolies and the British Statute of Anne are seen as the origins of patent law and copyright respectively, firmly establishing the concept of intellectual property. The first known use of the intellectual property dates to 1769. The first clear example of modern usage goes back as early as 1808, the German equivalent was used with the founding of the North German Confederation whose constitution granted legislative power over the protection of intellectual property to the confederation. According to Lemley, it was only at point that the term really began to be used in the United States. The history of patents does not begin with inventions, but rather with royal grants by Queen Elizabeth I for monopoly privileges, the evolution of patents from royal prerogative to common-law doctrine. The term can be used in an October 1845 Massachusetts Circuit Court ruling in the patent case Davoll et al. v. Brown. The statement that discoveries are. property goes back earlier, in Europe, French author A. Nion mentioned propriété intellectuelle in his Droits civils des auteurs, artistes et inventeurs, published in 1846. Until recently, the purpose of property law was to give as little protection as possible in order to encourage innovation. Historically, therefore, they were granted only when they were necessary to encourage invention, limited in time, the concepts origins can potentially be traced back further. In 500 BCE, the government of the Greek state of Sybaris offered one years patent to all who should discover any new refinement in luxury. Intellectual property rights include patents, copyright, industrial design rights, trademarks, plant variety rights, trade dress, geographical indications, a copyright gives the creator of an original work exclusive rights to it, usually for a limited time. Copyright may apply to a range of creative, intellectual, or artistic forms. Copyright does not cover ideas and information themselves, only the form or manner in which they are expressed, an industrial design right protects the visual design of objects that are not purely utilitarian. An industrial design consists of the creation of a shape, configuration or composition of pattern or color, or combination of pattern, an industrial design can be a two- or three-dimensional pattern used to produce a product, industrial commodity or handicraft. Plant breeders rights or plant variety rights are the rights to use a new variety of a plant. The variety must amongst others be novel and distinct and for registration the evaluation of propagating material of the variety is examined, a trademark is a recognizable sign, design or expression which distinguishes products or services of a particular trader from the similar products or services of other traders
14.
Electric field
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An electric field is a vector field that associates to each point in space the Coulomb force that would be experienced per unit of electric charge, by an infinitesimal test charge at that point. Electric fields are created by electric charges and can be induced by time-varying magnetic fields, the electric field combines with the magnetic field to form the electromagnetic field. The electric field, E, at a point is defined as the force, F. A particle of charge q would be subject to a force F = q E and its SI units are newtons per coulomb or, equivalently, volts per metre, which in terms of SI base units are kg⋅m⋅s−3⋅A−1. Electric fields are caused by electric charges or varying magnetic fields, in the special case of a steady state, the Maxwell-Faraday inductive effect disappears. The resulting two equations, taken together, are equivalent to Coulombs law, written as E =14 π ε0 ∫ d r ′ ρ r − r ′ | r − r ′ |3 for a charge density ρ. Notice that ε0, the permittivity of vacuum, must be substituted if charges are considered in non-empty media, the equations of electromagnetism are best described in a continuous description. A charge q located at r 0 can be described mathematically as a charge density ρ = q δ, conversely, a charge distribution can be approximated by many small point charges. Electric fields satisfy the principle, because Maxwells equations are linear. This principle is useful to calculate the field created by point charges. Q n are stationary in space at r 1, r 2, in that case, Coulombs law fully describes the field. If a system is static, such that magnetic fields are not time-varying, then by Faradays law, in this case, one can define an electric potential, that is, a function Φ such that E = − ∇ Φ. This is analogous to the gravitational potential, Coulombs law, which describes the interaction of electric charges, F = q = q E is similar to Newtons law of universal gravitation, F = m = m g. This suggests similarities between the electric field E and the gravitational field g, or their associated potentials, mass is sometimes called gravitational charge because of that similarity. Electrostatic and gravitational forces both are central, conservative and obey an inverse-square law, a uniform field is one in which the electric field is constant at every point. It can be approximated by placing two conducting plates parallel to other and maintaining a voltage between them, it is only an approximation because of boundary effects. Assuming infinite planes, the magnitude of the electric field E is, electrodynamic fields are E-fields which do change with time, for instance when charges are in motion. The electric field cannot be described independently of the field in that case
15.
Metal
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A metal is a material that is typically hard, opaque, shiny, and has good electrical and thermal conductivity. Metals are generally malleable—that is, they can be hammered or pressed permanently out of shape without breaking or cracking—as well as fusible and ductile, about 91 of the 118 elements in the periodic table are metals, the others are nonmetals or metalloids. Some elements appear in both metallic and non-metallic forms, astrophysicists use the term metal to collectively describe all elements other than hydrogen and helium, the simplest two, in a star. The star fuses smaller atoms, mostly hydrogen and helium, to larger ones over its lifetime. In that sense, the metallicity of an object is the proportion of its matter made up of all chemical elements. Many elements and compounds that are not normally classified as metals become metallic under high pressures, the atoms of metallic substances are typically arranged in one of three common crystal structures, namely body-centered cubic, face-centered cubic, and hexagonal close-packed. In bcc, each atom is positioned at the center of a cube of eight others, in fcc and hcp, each atom is surrounded by twelve others, but the stacking of the layers differs. Some metals adopt different structures depending on the temperature, atoms of metals readily lose their outer shell electrons, resulting in a free flowing cloud of electrons within their otherwise solid arrangement. This provides the ability of metallic substances to easily transmit heat, while this flow of electrons occurs, the solid characteristic of the metal is produced by electrostatic interactions between each atom and the electron cloud. This type of bond is called a metallic bond, Metals are usually inclined to form cations through electron loss, reacting with oxygen in the air to form oxides over various timescales. Examples,4 Na + O2 →2 Na2O2 Ca + O2 →2 CaO4 Al +3 O2 →2 Al2O3, the transition metals are slower to oxidize because they form a passivating layer of oxide that protects the interior. Others, like palladium, platinum and gold, do not react with the atmosphere at all, some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades. The oxides of metals are generally basic, as opposed to those of nonmetals, exceptions are largely oxides with very high oxidation states such as CrO3, Mn2O7, and OsO4, which have strictly acidic reactions. Painting, anodizing or plating metals are good ways to prevent their corrosion, however, a more reactive metal in the electrochemical series must be chosen for coating, especially when chipping of the coating is expected. Water and the two form an electrochemical cell, and if the coating is less reactive than the coatee. Metals in general have high conductivity, high thermal conductivity. Typically they are malleable and ductile, deforming under stress without cleaving, in terms of optical properties, metals are shiny and lustrous. Sheets of metal beyond a few micrometres in thickness appear opaque, although most metals have higher densities than most nonmetals, there is wide variation in their densities, lithium being the least dense solid element and osmium the densest
16.
Semiconductor
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Semiconductors are crystalline or amorphous solids with distinct electrical characteristics. They are of high electrical resistance — higher than typical resistance materials and their resistance decreases as their temperature increases, which is behavior opposite to that of a metal. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Semiconductor devices can display a range of properties such as passing current more easily in one direction than the other, showing variable resistance. The modern understanding of the properties of a semiconductor relies on quantum physics to explain the movement of carriers in a crystal lattice. Doping greatly increases the number of carriers within the crystal. When a doped semiconductor contains mostly free holes it is called p-type, the semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of p- and n-type dopants. A single semiconductor crystal can have many p- and n-type regions, although some pure elements and many compounds display semiconductor properties, silicon, germanium, and compounds of gallium are the most widely used in electronic devices. Elements near the so-called metalloid staircase, where the metalloids are located on the table, are usually used as semiconductors. Some of the properties of materials were observed throughout the mid 19th. The first practical application of semiconductors in electronics was the 1904 development of the Cats-whisker detector, developments in quantum physics in turn allowed the development of the transistor in 1947 and the integrated circuit in 1958. There are several developed techniques that allow semiconducting materials to behave like conducting materials and these modifications have two outcomes, n-type and p-type. These refer to the excess or shortage of electrons, respectively, an unbalanced number of electrons would cause a current to flow through the material. Heterojunctions Heterojunctions occur when two differently doped semiconducting materials are joined together, for example, a configuration could consist of p-doped and n-doped germanium. This results in an exchange of electrons and holes between the differently doped semiconducting materials, the n-doped germanium would have an excess of electrons, and the p-doped germanium would have an excess of holes. The transfer occurs until equilibrium is reached by a process called recombination, a product of this process is charged ions, which result in an electric field. Excited Electrons A difference in electric potential on a material would cause it to leave thermal equilibrium. This introduces electrons and holes to the system, which interact via a process called ambipolar diffusion, whenever thermal equilibrium is disturbed in a semiconducting material, the amount of holes and electrons changes
17.
Orbital hybridisation
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In chemistry, hybridisation is the concept of mixing atomic orbitals into new hybrid orbitals suitable for the pairing of electrons to form chemical bonds in valence bond theory. Hybrid orbitals are very useful in the explanation of molecular geometry, although sometimes taught together with the valence shell electron-pair repulsion theory, valence bond and hybridisation are in fact not related to the VSEPR model. Chemist Linus Pauling first developed the theory in 1931 in order to explain the structure of simple molecules such as methane using atomic orbitals. In reality however, methane has four bonds of equivalent strength separated by the bond angle of 109. 5°. It gives a simple orbital picture equivalent to Lewis structures, hybridisation theory finds its use mainly in organic chemistry. Orbitals are a representation of the behaviour of electrons within molecules. In heavier atoms, such as carbon, nitrogen, and oxygen, hybrid orbitals are assumed to be mixtures of atomic orbitals, superimposed on each other in various proportions. For example, in methane, the C hybrid orbital which forms each carbon–hydrogen bond consists of 25% s character and 75% p character and is thus described as sp3 hybridised. Quantum mechanics describes this hybrid as an sp3 wavefunction of the form N, the ratio of coefficients is √3 in this example. Since the electron density associated with an orbital is proportional to the square of the wavefunction, the p character or the weight of the p component is N2λ2 = 3/4. The amount of p character or s character, which is decided mainly by orbital hybridisation, molecules with multiple bonds or multiple lone pairs can have orbitals represented in terms of sigma and pi symmetry or equivalent orbitals. The sigma and pi representation of Erich Hückel is the common one compared to the equivalent orbital representation of Linus Pauling. The two have mathematically equivalent total many-electron wave functions, and are related by a transformation of the set of occupied molecular orbitals. Hybridisation describes the bonding atoms from a point of view. For a tetrahedrally coordinated carbon, the carbon should have 4 orbitals with the symmetry to bond to the 4 hydrogen atoms. The carbon atom can also bond to four hydrogen atoms by an excitation of an electron from the doubly occupied 2s orbital to the empty 2p orbital, producing four singly occupied orbitals. The energy released by formation of two additional bonds more than compensates for the energy required, energetically favouring the formation of four C-H bonds. Quantum mechanically, the lowest energy is obtained if the four bonds are equivalent, a set of four equivalent orbitals can be obtained that are linear combinations of the valence-shell s and p wave functions, which are the four sp3 hybrids
18.
Scanning tunneling microscope
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A scanning tunneling microscope is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer, for a STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. With this resolution, individual atoms within materials are routinely imaged and manipulated, the STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to over 1000°C. STM is based on the concept of quantum tunneling, when a conducting tip is brought very near to the surface to be examined, a bias applied between the two can allow electrons to tunnel through the vacuum between them. The resulting tunneling current is a function of tip position, applied voltage, information is acquired by monitoring the current as the tips position scans across the surface, and is usually displayed in image form. First, a bias is applied and the tip is brought close to the sample by coarse sample-to-tip control. In this situation, the bias will cause electrons to tunnel between the tip and sample, creating a current that can be measured. Once tunneling is established, the bias and position with respect to the sample can be varied. If the tip is moved across the sample in the x-y plane and these changes are mapped in images. This change in current with respect to position can be measured itself, or the height, z and these two modes are called constant height mode and constant current mode, respectively. In constant current mode, feedback electronics adjust the height by a voltage to the piezoelectric height control mechanism, all images produced by STM are grayscale, with color optionally added in post-processing in order to visually emphasize important features. In addition to scanning across the sample, information on the structure at a given location in the sample can be obtained by sweeping voltage. This type of measurement is called scanning tunneling spectroscopy and typically results in a plot of the density of states as a function of energy within the sample. Framerates of at least 25 Hz enable so called video-rate STM, framerates up to 80 Hz are possible with fully working feedback that adjusts the height of the tip. Video-rate STM can be used to surface diffusion. The components of an STM include scanning tip, piezoelectric controlled height and x, y scanner, coarse sample-to-tip control, vibration isolation system, the resolution of an image is limited by the radius of curvature of the scanning tip of the STM. Additionally, image artifacts can occur if the tip has two tips at the end rather than an atom, this leads to “double-tip imaging. Therefore, it has been essential to develop processes for consistently obtaining sharp, recently, carbon nanotubes have been used in this instance
19.
Toluene
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Toluene /ˈtɒljuːiːn/, also known as toluol /ˈtɒljuːɒl/, is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a benzene derivative, consisting of a CH3 group attached to a phenyl group. As such, its IUPAC systematic name is methylbenzene, Toluene is widely used as an industrial feedstock and a solvent. In 2013, worldwide sales of toluene amounted to about 24.5 billion US-dollars, as the solvent in some types of paint thinner, contact cement and model airplane glue, toluene is sometimes used as a recreational inhalant and has the potential of causing severe neurological harm. The compound was first isolated in 1837 through a distillation of oil by a Polish chemist named Filip Walter. In 1843, Jöns Jacob Berzelius recommended the name toluin, in 1850, French chemist Auguste Cahours isolated from a distillate of wood a hydrocarbon which he recognized as similar to Devilles benzoène and which Cahours named toluène. Toluene reacts as an aromatic hydrocarbon in electrophilic aromatic substitution. Because the methyl group has greater electron-releasing properties than an atom in the same position. It undergoes sulfonation to give p-toluenesulfonic acid, and chlorination by Cl2 in the presence of FeCl3 to give ortho, importantly, the methyl side chain in toluene is susceptible to oxidation. Toluene reacts with Potassium permanganate to yield benzoic acid, and with chromyl chloride to yield benzaldehyde, the methyl group undergoes halogenation under free radical conditions. For example, N-bromosuccinimide heated with toluene in the presence of AIBN leads to benzyl bromide, the same conversion can be effected with elemental bromine in the presence of UV light or even sunlight. Toluene may also be brominated by treating it with HBr and H2O2 in the presence of light. C6H5CH3 + Br2 → C6H5CH2Br + HBr C6H5CH2Br + Br2 → C6H5CHBr2 + HBr The methyl group in toluene undergoes deprotonation only with strong bases. Catalytic hydrogenation of toluene gives methylcyclohexane, the reaction requires a high pressure of hydrogen and a catalyst. Final separation and purification is done by any of the distillation or solvent extraction processes used for BTX aromatics, Toluene is so inexpensively produced industrially that it is not prepared in the laboratory. In principle it could be prepared by a variety of methods, Toluene is mainly used as a precursor to benzene via hydrodealkylation, C6H5CH3 + H2 → C6H6 + CH4 The second ranked application involves its disproportionation to a mixture of benzene and xylene. When oxidized it yields benzaldehyde and benzoic acid, two important intermediates in chemistry, in addition to the synthesis of benzene and xylene, toluene is a feedstock for toluene diisocyanate, trinitrotoluene, and a number of synthetic drugs. Toluene is a solvent, e. g. for paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives, lacquers, leather tanners
20.
PubMed Identifier
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PubMed is a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine at the National Institutes of Health maintains the database as part of the Entrez system of information retrieval, from 1971 to 1997, MEDLINE online access to the MEDLARS Online computerized database primarily had been through institutional facilities, such as university libraries. PubMed, first released in January 1996, ushered in the era of private, free, home-, the PubMed system was offered free to the public in June 1997, when MEDLINE searches via the Web were demonstrated, in a ceremony, by Vice President Al Gore. Information about the journals indexed in MEDLINE, and available through PubMed, is found in the NLM Catalog. As of 5 January 2017, PubMed has more than 26.8 million records going back to 1966, selectively to the year 1865, and very selectively to 1809, about 500,000 new records are added each year. As of the date,13.1 million of PubMeds records are listed with their abstracts. In 2016, NLM changed the system so that publishers will be able to directly correct typos. Simple searches on PubMed can be carried out by entering key aspects of a subject into PubMeds search window, when a journal article is indexed, numerous article parameters are extracted and stored as structured information. Such parameters are, Article Type, Secondary identifiers, Language, publication type parameter enables many special features. As these clinical girish can generate small sets of robust studies with considerable precision, since July 2005, the MEDLINE article indexing process extracts important identifiers from the article abstract and puts those in a field called Secondary Identifier. The secondary identifier field is to store numbers to various databases of molecular sequence data, gene expression or chemical compounds. For clinical trials, PubMed extracts trial IDs for the two largest trial registries, ClinicalTrials. gov and the International Standard Randomized Controlled Trial Number Register, a reference which is judged particularly relevant can be marked and related articles can be identified. If relevant, several studies can be selected and related articles to all of them can be generated using the Find related data option, the related articles are then listed in order of relatedness. To create these lists of related articles, PubMed compares words from the title and abstract of each citation, as well as the MeSH headings assigned, using a powerful word-weighted algorithm. The related articles function has been judged to be so precise that some researchers suggest it can be used instead of a full search, a strong feature of PubMed is its ability to automatically link to MeSH terms and subheadings. Examples would be, bad breath links to halitosis, heart attack to myocardial infarction, where appropriate, these MeSH terms are automatically expanded, that is, include more specific terms. Terms like nursing are automatically linked to Nursing or Nursing and this important feature makes PubMed searches automatically more sensitive and avoids false-negative hits by compensating for the diversity of medical terminology. The My NCBI area can be accessed from any computer with web-access, an earlier version of My NCBI was called PubMed Cubby
21.
Physical Review B
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Physical Review B, Condensed Matter and Materials Physics is a peer-reviewed, scientific journal, published by the American Physical Society. The Editor of PRB is Laurens W. Molenkamp and it is part of the Physical Review family of journals. The current acting Editor in Chief is Dan T. Kulp, PRB currently publishes over 4500 papers a year, making it one of the largest physics journals in the world. According to the Journal Citation Reports, PRBs most recent impact factors have been 3.664 for 2013,3.736 for 2014 and 3.718 for 2015. PRB was created in 1970 by the split of the original Physical Review into four parts, peter D. Adams was the Editor from inception until 2012 when Laurens W. Molenkamp took over. Anthony M. Begley is currently the Managing Editor, PRB has a reputation among professional physicists for publishing useful, comprehensive long papers in physics. It also contains short papers in its Rapid Communications section, designed for research important enough to deserve special handling, the journal can be searched for free via PROLA. Titles and abstracts can be viewed for free but a subscription is needed to read the full text of papers. PRB and the other APS journals are available entirely for free at many US public libraries, PRB is rare among physics journals in that it has a staff of 12 full-time professional editors and does not employ the more common model of using part-time editors who are active researchers. The journal is available in print format but the version is the online one. Authors can pay extra charges to make their papers open access, a small percentage of the PRB papers published are chosen by the PRB editors to be Editors Suggestions, as seen at http, //prb. aps. org. Artistic images from papers in the journal are published as a feature named Kaleidoscope at http, Physical Review B is indexed in the following bibliographic databases, American Physical Society#APS journals PRB home page
22.
Diamond
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Diamond is a metastable allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the rate from diamond to graphite is negligible at standard conditions. Diamond is renowned as a material with superlative physical qualities, most of which originate from the covalent bonding between its atoms. In particular, diamond has the highest hardness and thermal conductivity of any bulk material and those properties determine the major industrial application of diamond in cutting and polishing tools and the scientific applications in diamond knives and diamond anvil cells. Because of its extremely rigid lattice, it can be contaminated by very few types of impurities, such as boron, small amounts of defects or impurities color diamond blue, yellow, brown, green, purple, pink, orange or red. Diamond also has relatively high optical dispersion, most natural diamonds are formed at high temperature and pressure at depths of 140 to 190 kilometers in the Earths mantle. Carbon-containing minerals provide the source, and the growth occurs over periods from 1 billion to 3.3 billion years. Diamonds are brought close to the Earths surface through deep volcanic eruptions by magma, Diamonds can also be produced synthetically in a HPHT method which approximately simulates the conditions in the Earths mantle. An alternative, and completely different growth technique is chemical vapor deposition, several non-diamond materials, which include cubic zirconia and silicon carbide and are often called diamond simulants, resemble diamond in appearance and many properties. Special gemological techniques have developed to distinguish natural diamonds, synthetic diamonds. The word is from the ancient Greek ἀδάμας – adámas unbreakable, the name diamond is derived from the ancient Greek αδάμας, proper, unalterable, unbreakable, untamed, from ἀ-, un- + δαμάω, I overpower, I tame. Diamonds have been known in India for at least 3,000 years, Diamonds have been treasured as gemstones since their use as religious icons in ancient India. Their usage in engraving tools also dates to early human history, later in 1797, the English chemist Smithson Tennant repeated and expanded that experiment. By demonstrating that burning diamond and graphite releases the same amount of gas, the most familiar uses of diamonds today are as gemstones used for adornment, a use which dates back into antiquity, and as industrial abrasives for cutting hard materials. The dispersion of light into spectral colors is the primary gemological characteristic of gem diamonds. In the 20th century, experts in gemology developed methods of grading diamonds, four characteristics, known informally as the four Cs, are now commonly used as the basic descriptors of diamonds, these are carat, cut, color, and clarity. A large, flawless diamond is known as a paragon and these conditions are met in two places on Earth, in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike. The conditions for diamond formation to happen in the mantle occur at considerable depth corresponding to the requirements of temperature and pressure
23.
Lonsdaleite
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Lonsdaleite, also called hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice. In nature, it forms when meteorites containing graphite strike the Earth, the great heat and stress of the impact transforms the graphite into diamond, but retains graphites hexagonal crystal lattice. Lonsdaleite was first identified in 1967 from the Canyon Diablo meteorite, hexagonal diamond has also been synthesized in the laboratory by compressing and heating graphite either in a static press or using explosives. It has also produced by chemical vapor deposition, and also by the thermal decomposition of a polymer, poly, at atmospheric pressure, under argon atmosphere. It is translucent, brownish-yellow, and has an index of refraction of 2.40 to 2.41 and its hardness is theoretically superior to that of cubic diamond according to computational simulations but natural specimens exhibited somewhat lower hardness through a large range of values. The cause is speculated as being due to the samples having been riddled with lattice defects, a quantitative analysis of the X-ray diffraction data of lonsdaleite has shown that about equal amounts of hexagonal and cubic stacking sequences are present. Consequently, it has suggested that stacking disordered diamond is the most accurate structural description of lonsdaleite. According to the picture, Lonsdaleite has a hexagonal unit cell, related to the diamond unit cell in the same way that the hexagonal. The diamond structure can be considered to be made up of interlocking rings of six carbon atoms, in lonsdaleite, some rings are in the boat conformation instead. Lonsdaleite is simulated to be 58% harder than diamond on the face and to resist indentation pressures of 152 GPa. This is yet exceeded by IIa diamonds <111> tip hardness of 162 GPa, Lonsdaleite occurs as microscopic crystals associated with diamond in several meteorites, Canyon Diablo, Kenna, and Allan Hills 77283. It is also occurring in non-bolide diamond placer deposits in the Sakha Republic. Its presence in local deposits is claimed as evidence for the Tunguska event being caused by a meteor rather than by a cometary fragment. Aggregated diamond nanorod Glossary of meteoritics List of minerals List of minerals named after people Anthony, materials Science and Technology Division, Naval Research Laboratory website accessed 14 May 2006. Diamond no longer natures hardest material lonsdaleite 3D animation
24.
Graphite
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Graphite, archaically referred to as plumbago, is a crystalline form of carbon, a semimetal, a native element mineral, and one of the allotropes of carbon. Graphite is the most stable form of carbon under standard conditions, therefore, it is used in thermochemistry as the standard state for defining the heat of formation of carbon compounds. Highly ordered pyrolytic graphite or more correctly highly oriented pyrolytic graphite refers to graphite with a spread between the graphite sheets of less than 1°. The name graphite fiber is sometimes used to refer to carbon fibers or carbon fiber-reinforced polymer. Graphite occurs in rocks as a result of the reduction of sedimentary carbon compounds during metamorphism. It also occurs in rocks and in meteorites. Minerals associated with graphite include quartz, calcite, micas and tourmaline, in meteorites it occurs with troilite and silicate minerals. Small graphitic crystals in meteoritic iron are called cliftonite, Graphite is not mined in the United States, but U. S. production of synthetic graphite in 2010 was 134 kt valued at $1.07 billion. Graphite has a layered, planar structure, the individual layers are called graphene. In each layer, the atoms are arranged in a honeycomb lattice with separation of 0.142 nm. Atoms in the plane are bonded covalently, with three of the four potential bonding sites satisfied. The fourth electron is free to migrate in the plane, making graphite electrically conductive, however, it does not conduct in a direction at right angles to the plane. Bonding between layers is via weak van der Waals bonds, which allows layers of graphite to be easily separated, the two known forms of graphite, alpha and beta, have very similar physical properties, except for that the graphene layers stack slightly differently. The alpha graphite may be flat or buckled. The alpha form can be converted to the form through mechanical treatment. The acoustic and thermal properties of graphite are highly anisotropic, since phonons propagate quickly along the tightly-bound planes, graphites high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen containing atmospheres graphite readily oxidizes to form CO2 at temperatures of 700 °C, Graphite is an electric conductor, consequently, useful in such applications as arc lamp electrodes. It can conduct electricity due to the vast electron delocalization within the carbon layers and these valence electrons are free to move, so are able to conduct electricity
25.
Graphene
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Graphene is an allotrope of carbon in the form of a two-dimensional, atomic-scale, hexagonal lattice in which one atom forms each vertex. It is the structural element of other allotropes, including graphite, charcoal. It can be considered as a large aromatic molecule, the ultimate case of the family of flat polycyclic aromatic hydrocarbons. It is about 200 times stronger than the strongest steel and it efficiently conducts heat and electricity and is nearly transparent. Graphene shows a large and nonlinear diamagnetism, greater than graphite, scientists have theorized about graphene for years. It has unintentionally been produced in small quantities for centuries, through the use of pencils and it was originally observed in electron microscopes in 1962, but it was studied only while supported on metal surfaces. The material was later rediscovered, isolated, and characterized in 2004 by Andre Geim, research was informed by existing theoretical descriptions of its composition, structure, and properties. This work resulted in the two winning the Nobel Prize in Physics in 2010 for groundbreaking experiments regarding the material graphene. The global market for graphene reached $9 million by 2012 with most sales in the semiconductor, electronics, battery energy, Graphene is a combination of graphite and the suffix -ene, named by Hanns-Peter Boehm, who described single-layer carbon foils in 1962. The term was used in early descriptions of carbon nanotubes, as well as for epitaxial graphene. Graphene can be considered an infinite alternant polycyclic aromatic hydrocarbon, the IUPAC compendium of technology states, previously, descriptions such as graphite layers, carbon layers, or carbon sheets have been used for the term graphene. It is incorrect to use for a single layer a term includes the term graphite. The term graphene should be used only when the reactions, structural relations or other properties of individual layers are discussed and this definition is narrower than the IUPAC definition and refers to cloven, transferred and suspended graphene. Other forms such as graphene grown on various metals, can become free-standing if, for example, in 1859 Benjamin Collins Brodie became aware of the highly lamellar structure of thermally reduced graphite oxide. The structure of graphite was identified in 1916 by the method of powder diffraction. It was studied in detail by Kohlschütter and Haenni in 1918 and its structure was determined from single-crystal diffraction in 1924. The theory of graphene was first explored by Wallace in 1947 as a point for understanding the electronic properties of 3D graphite. The emergent massless Dirac equation was first pointed out by Semenoff, DiVincenzo, Semenoff emphasized the occurrence in a magnetic field of an electronic Landau level precisely at the Dirac point
26.
Buckminsterfullerene
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Buckminsterfullerene is a spherical fullerene molecule with the formula C60. It was first generated in 1985 by Harold Kroto, James R. Heath, Sean OBrien, Robert Curl, Kroto, Curl and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of buckminsterfullerene and the related class of molecules, the fullerenes. The name is a reference to Buckminster Fuller, as C60 resembles his trademark geodesic domes, Buckminsterfullerene is the most common naturally occurring fullerene molecule, as it can be found in small quantities in soot. Solid and gaseous forms of the molecule have been detected in deep space, Buckminsterfullerene is one of the largest objects to have been shown to exhibit wave–particle duality, as stated in the theory every object exhibits this behavior. Its discovery led to the exploration of a new field of chemistry, Buckminsterfullerene derives from the name of the noted futurist and inventor Buckminster Fuller. One of his designs of a dome structure bears great resemblance to C60, as a result. The general public, however, sometimes refers to buckminsterfullerene, and even Fullers dome structure, the structure associated with fullerenes was described by Leonardo da Vinci. Albrecht Dürer also reproduced a similar icosahedron containing 12 pentagonal and 20 hexagonal faces, theoretical predictions of buckyball molecules appeared in the late 1960s – early 1970s, but they went largely unnoticed. In the early 1970s, the chemistry of unsaturated carbon configurations was studied by a group at the University of Sussex, led by Harry Kroto, in the 1980s a technique was developed by Richard Smalley and Robert Curl at Rice University, Texas to isolate these substances. They used laser vaporization of a target to produce clusters of atoms. Kroto realized that by using a target, any carbon chains formed could be studied. Another interesting fact is that, at the time, astrophysicists were working along with spectroscopists to study infrared emissions from giant red carbon stars. Smalley and team were able to use a laser vaporization technique to create carbon clusters which could potentially emit infrared at the wavelength as had been emitted by the red carbon star. Hence, the inspiration came to Smalley and team to use the technique on graphite to create the first fullerene molecule. C60 was discovered in 1985 by Robert Curl, Harold Kroto, using laser evaporation of graphite they found Cn clusters of which the most common were C60 and C70. A solid rotating graphite disk was used as the surface from which carbon was vaporized using a laser beam creating hot plasma that was passed through a stream of high-density helium gas. The carbon species were subsequently cooled and ionized resulting in the formation of clusters, clusters ranged in molecular masses but Kroto and Smalley found predominance in a C60 cluster that could be enhanced further by letting the plasma react longer. They also discovered that the C60 molecule formed a cage-like structure, for this discovery they were awarded the 1996 Nobel Prize in Chemistry
27.
C70 fullerene
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C70 fullerene is the fullerene molecule consisting of 70 carbon atoms. It is a cage-like fused-ring structure which resembles a ball, made of 25 hexagons and 12 pentagons, with a carbon atom at the vertices of each polygon. A related fullerene molecule, named buckminsterfullerene, consists of 60 carbon atoms and it was first intentionally prepared in 1985 by Harold Kroto, James R. Heath, Sean OBrien, Robert Curl and Richard Smalley at Rice University. Kroto, Curl and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of cage-like fullerenes, the name is a homage to Buckminster Fuller, whose geodesic domes these molecules resemble. Theoretical predictions of buckyball molecules appeared in the late 1960s – early 1970s, in the early 1970s, the chemistry of unsaturated carbon configurations was studied by a group at the University of Sussex, led by Harry Kroto and David Walton. In the 1980s a technique was developed by Richard Smalley and Bob Curl at Rice University and they used laser vaporization of a suitable target to produce clusters of atoms. Kroto realized that by using a graphite target, C70 was discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley. Using laser evaporation of graphite they found Cn clusters of which the most common were C60, for this discovery they were awarded the 1996 Nobel Prize in Chemistry. The discovery of buckyballs was serendipitous, as the scientists were aiming to produce carbon plasmas to replicate, mass spectrometry analysis of the product indicated the formation of spheroidal carbon molecules. In 1990, K. Fostiropoulos, W. Krätchmer and D. R. Huffman developed a simple and efficient method of producing fullerenes in gram and even kilogram amounts which boosted fullerene research. In this technique, carbon soot is produced from two high-purity graphite electrodes by igniting an arc discharge between them in an inert atmosphere, alternatively, soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Fullerenes are extracted from the using a multistep procedure. First, the soot is dissolved in organic solvents. This step yields a solution containing up to 70% of C60 and 15% of C70 and these fractions are separated using chromatography. The C70 molecule has a D5h symmetry and contains 37 faces with an atom at the vertices of each polygon. Its structure is similar to that of C60 molecule, but has a belt of 5 hexagons inserted at the equator, the molecule has eight bond lengths ranging between 0.137 and 0.146 nm. Each carbon atom in the structure is bonded covalently with 3 others, C70 can undergo six reversible, one-electron reductions to C6−70, whereas oxidation is irreversible. The first reduction requires ~1.0 V, indicating that C70 is an electron acceptor, Fullerenes are sparingly soluble in many aromatic solvents such as toluene and others like carbon disulfide, but not in water
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Glassy carbon
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Glass-like carbon, often called glassy carbon or vitreous carbon, is a non-graphitizing, or nongraphitizable, carbon which combines glassy and ceramic properties with those of graphite. The names glassy carbon and vitreous carbon have been introduced as trademarks, therefore, Vitreous carbon can also be produced as a foam. It is then called reticulated vitreous carbon and this foam was first developed in the mid to late 1960s as a thermally insulating, microporous glassy carbon electrode material. RVC foam is a strong, inert, electrically and thermally conductive, due to these characteristics, the most widespread scientific use of RVC is in electrochemistry. Glassy carbon was first observed in the laboratories of The Carborundum Company, Manchester, UK, in the mid-1950s by Bernard Redfern and he noticed that Sellotape he used to hold ceramic samples in a furnace maintained a sort of structural identity after firing in an inert atmosphere. He searched for a matrix to mirror a diamond structure and discovered a resole resin that would, with special preparation. Using this phenolic resin, crucibles were produced, crucibles were distributed to organisations such as UKAEA Harwell. Bernard Redfern left The Carborundum Co. which officially wrote off all interests in the glassy carbon invention, while working at the Plessey Company laboratory in Towcester, UK, Redfern received a glassy carbon crucible for duplication from UKAEA. He identified it as one he had made from markings he had engraved into the uncured precursor prior to carbonisation. The Plessey Company set up a laboratory first in a previously used to make briar pipes, in Litchborough, UK. Caswell became the Plessey Research Centre and then the Allen Clark Research Centre, Glassy carbon arrived at the Plessey Company Limited as a fait accompli. Redfern was assigned J. C. Lewis, as a laboratory assistant, F. C. Cowlard was assigned to Redferns department later, as a laboratory administrator. Cowlard was an administrator who previously had some association with Silane, neither he nor Lewis had any previous connection with glassy carbon. The contribution of Bernard Redfern to the invention and production of glassy / Vitreous carbon is acknowledged by his co-authorship of early articles, but references to Redfern were not obvious in subsequent publications by Cowlard and Lewis. Original boat crucibles, thick section rods and precursor samples exist and this came after the rescinded British patent. This prior art is not referenced in US patent 4,668,496,26 May 1987 for Vitreous Carbon, patents were filed Bodies and shapes of carbonaceous materials and processes for their production and the name Vitreous Carbon presented to the product by the son of Redfern. Glassy/Vitreous Carbon was under investigation used for components for thermonuclear detonation systems, large sections of the precursor material were produced as castings, moldings or machined into a predetermined shape. Large crucibles and other forms were manufactured, carbonisation took place in two stages
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Carbide-derived carbon
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Carbide-derived carbon, also known as tunable nanoporous carbon, is the common term for carbon materials derived from carbide precursors, such as binary, or ternary carbides, also known as MAX phases. CDCs have also derived from polymer-derived ceramics such as Si-O-C or Ti-C. CDCs can occur in structures, ranging from amorphous to crystalline carbon, from sp2- to sp3-bonded. Among carbon materials, microporous CDCs exhibit some of the highest reported specific surface areas, by varying the type of the precursor and the CDC synthesis conditions, microporous and mesoporous structures with controllable average pore size and pore size distributions can be produced. Depending on the precursor and the conditions, the average pore size control can be applied at sub-Angstrom accuracy. The production of SiCl4 by high temperature reaction of Chlorine gas with Silicon Carbide was first patented in 1918 by Otis Hutchins, the solid porous carbon product was initially regarded as a waste byproduct until its properties and potential applications were investigated in more detail in 1959 by Walter Mohun. Most recently, research activities have centered on optimized CDC synthesis, historically, various terms have been used for CDC, such as mineral carbon or nanoporous carbon. Later, a more adequate nomenclature introduced by Yury Gogotsi was adopted that clearly denotes the precursor, for example, CDC derived from silicon carbide has been referred to as SiC-CDC, Si-CDC, or SiCDC. Recently, it was recommended to adhere to a unified precursor-CDC-nomenclature to reflect the composition of the precursor. CDCs have been synthesized using several chemical and physical synthesis methods, most commonly, dry chlorine treatment is used to selectively etch metal or metalloid atoms from the carbide precursor lattice. The term chlorine treatment is to be preferred over chlorination as the product, metal chloride, is the discarded byproduct. This method is implemented for commercial production of CDC by Skeleton in Estonia, hydrothermal etching has also been used for synthesis of SiC-CDC which yielded a route for porous carbon films and nanodiamond synthesis. The most common method for producing porous carbide-derived carbons involves high-temperature etching with halogens, temperatures above 1000 °C result in predominantly graphitic carbon and an observed shrinkage of the material due to graphitization. The linear growth rate of the carbon product phase suggests a reaction-driven kinetic mechanism. A high mass transport condition facilitates the removal of the chloride, most produced CDCs exhibit a prevalence of micropores and mesopores, with specific distributions affected by carbide precursor and synthesis conditions. Hierarchic porosity can be achieved by using polymer-derived ceramics with or without utilizing a templating method, templating yields an ordered array of mesopores in addition to the disordered network of micropores. It has been shown that the crystal structure of the carbide is the primary factor affecting the CDC porosity. In general, a spacing between carbon atoms in the lattice correlates with an increase in the average pore diameter
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Atomic carbon
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Atomic carbon, also called monocarbon, is an inorganic chemical with the chemical formula C. It is a gas that exists above 3,642 °C. The trivial name monocarbon is the preferred IUPAC name, the systematic names, methanediylidene and carbon, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively. Methanediylidene is viewed as methane with all four hydrogen atoms removed, by default, this name pays no regard to the radicality of the atomic carbon. Although, in more specific context, it can also name the non-radical excited states. Many of atomic carbons electronic states lie relatively close to each other, the ground state is a triplet radical with two unpaired electrons, and the first two excited states are a singlet non-radical and a singlet radical with two unpaired electrons. A sample of atomic carbon exists as a mixture of electronic states even at room temperature, Atomic carbon can accept two such ligands. The methylylidyne group can also donate an electron-pair to a centre by adduction, M + → Because of this donation of the electron-pair. Since a Lewis base is also Brønsted base, atomic carbon can in theory be protonated to form a conjugate acid, Atomic carbon is not stable in aqueous solution, as it is rapidly oxidized to form carbon monoxide or formaldehyde. Atomic carbon is reactive, most reactions are very exothermic. They are generally carried out in the gas phase at liquid nitrogen temperatures, insertion into carbon -carbon double bonds to form a cyclopropylidene which undergoes ring-opening, a simple example being insertion into an alkene to form a cumulene. With water insertion into the O-H bond forms the carbene, H-C-OH that rearranges to formaldehyde, normally, a sample of atomic carbon exists as a mixture of excited states in addition to the ground-state in thermodynamic equilibrium. Each state contributes differently to the mechanisms that can take place. The diradical ground-state normally undergoes abstraction reactions, Atomic carbon has been used to generate true carbenes by the abstraction of oxygen atoms from carbonyl groups, R2C=O +, C, → R2C, + CO Carbenes formed in this way will exhibit true carbenic behaviour. Carbenes prepared by methods such as diazo compounds, might exhibit properties better attributed to the diazo compound used to make the carbene. This is important from an understanding of true carbene behaviour perspective. This very short lived species is created by passing a current through two adjacent carbon rods, generating an electric arc. Atomic carbon is generated in the process, professor Phil Shevlin has done the principal work in the field based at Auburn University in the USA
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Diatomic carbon
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Diatomic carbon, also called dicarbon, is an inorganic chemical with the chemical formula C=C. It is a gas that exists above 3,642 °C ). It occurs in carbon vapor, for example in electric arcs, in comets, stellar atmospheres and the interstellar medium, dicarbon is the preferred IUPAC name. The systematic names ethenediylidene and dicarbon, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, ethenediylidene is viewed as ethylene with four hydrogen atoms removed. By default, this name pays no regard to the state of the diatomic carbon. Molecular orbital theory shows that there are two sets of paired electrons in a degenerate pi bonding set of orbitals and this gives a bond order of 2, meaning that there should exist a double bond between the two carbons in a C2 molecule. However, a recent paper by S. Shaik has suggested that a quadruple bond exists in diatomic carbon, M. Zhang has proved from CASSCF that the quadruple bond based Molecular orbital theory is also reasonable. Bond dissociation energies of B2, C2, and N2 show increasing BDE, indicating single, double, C2 is a component of carbon vapor. One paper estimates that carbon vapor is around 28% diatomic, the light of fainter comets mainly originates from the emission of diatomic carbon. An example is C/2014 Q2, where there are lines of C2 light, mostly in the visible spectrum. The triplet state has a bond length than the singlet state. Diatomic carbon will react with acetone and acetaldehyde to produce acetylene by two different pathways, triplet C2 molecules will react through an intermolecular pathway, which is shown to exhibit radical character. The intermediate for this pathway is the ethylene radical and its abstraction is correlated with bond energies. Singlet C2 molecules will react through an intramolecular, nonradical pathway in two hydrogen atoms will be taken away from one molecule. The intermediate for this pathway is singlet vinylidene, the singlet reaction can happen through a 1, 1-diabstraction or a 1, 2-diabstraction. This reaction is insensitive to isotope substitution, the different abstractions are possibly due to the spatial orientations of the collisions rather than the bond energies. Singlet C2 will also react with alkenes, acetylene is a major product, however, it appears C2 will insert into carbon-hydrogen bonds. C2 is 2.5 times more likely to insert into a group as into methylene groups
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Tricarbon
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Tricarbon is an inorganic compound with the chemical formula C2. It is a gas that only persists in dilution or solution as an adduct. It is one of the simplest unsaturated carbenes and it is a small carbon cluster first spectroscopically observed in the beginning 20th century in the tail of a comet by William Huggins and subsequently identified in stellar atmospheres. Tricarbon can be found in space and can be produced in the laboratory by a process called laser ablation. Small carbon clusters like tricarbon and dicarbon are regarded as soot precursors and are implicated in the formation of industrial diamonds. The ionization potential is determined experimentally at 11 to 13.5 electronvolts, in contrast to the linear tricarbon molecule the C3+ cation is bent. C3 has also identified as a transient species in various combustion reactions. The generation of C3 was investigated by Professor Emeritus Philip S. Skell of Pennsylvania State University in the 1960s, the systematic names 1λ2, 3λ2-propadiene, and μ-carbidodicarbon, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively. By default, these names pay no regard to the radicality of the tricarbon molecule, hydrocarbons Alkenes List of molecules in interstellar space Cyclopropatriene Gaydon, Alfred G. Wolfhard, Hans G. Flames, their structure, radiation and temperature. Hinkle, Kenneth W. Keady, John J. Bernath, Peter F. Detection of C3 in the Circumstellar Shell of IRC+10216
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Penta-graphene
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Penta-graphene is a carbon allotrope composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling. Penta-graphene was proposed in 2014 on the basis of analyses and simulations, further calculations showed that it is unstable in its pure form, but can be stabilized by hydrogenation. Owing to its configuration, penta-graphene has an unusually negative Poisson’s ratio and very high ideal strength believed to exceed that of a similar material. Penta-graphene contains both sp2 and sp3 hybridized carbon atoms, contrary to graphene, which is a good conductor of electricity, penta-graphene is an insulator with an indirect band gap of 4. 1–4.3 eV. Its hydrogenated form is called penta-graphane and it has a diamond-like structure with sp3 and no sp2 bonds, and therefore a wider band gap than penta-graphene. Chiral penta-graphene nanotubes have also studied as metastable allotropes of carbon
