Amorphous carbon
Encyclopedia
Amorphous carbon or free, reactive carbon, is an allotrope of carbon
that does not have any crystal
line structure. As with all glassy
materials, some short-range order can be observed. Amorphous carbon is often abbreviated to aC for general amorphous carbon, aC:H for hydrogenated amorphous carbon, or to ta-C for tetrahedral amorphous carbon (also called diamond-like carbon).
In mineralogy, amorphous carbon is the name used for coal
, soot
and other impure forms of the element, carbon that are neither graphite nor diamond. In a crystallographic sense, however, these materials are not truly amorphous, but are polycrystalline or nanocrystalline materials of graphite or diamond within an amorphous carbon matrix.
or diamond
technically polycrystalline or nanocrystalline materials. Commercial carbon also usually contains significant quantities of other elements, which may form crystalline impurities.
Coal
and soot
are both informally called amorphous carbon. However, both are products of pyrolysis
, which does not produce true amorphous carbon under normal conditions. The coal industry divides coal up into various grades depending on the amount of carbon present in the sample compared to the amount of impurities. The highest grade, anthracite
, is about 90 percent carbon and 10% other elements. Bituminous coal
is about 75-90 percent carbon, and lignite
is the name for coal that is around 55 percent carbon.
, and cathodic arc deposition
, it became possible to fabricate truly amorphous carbon materials.
In technical terms, true amorphous carbon has localized π electrons (as opposed to the aromatic
π bonds in graphite), and its bonds form with lengths and distances that are inconsistent with any other allotrope
of carbon
. It also contains a high concentration of dangling bonds, which cause deviations in interatomic spacing (as measured using diffraction
) of more than 5%, and noticeable variation in bond angle.
The properties of amorphous carbon films vary depending on the parameters used during deposition. One of the most common ways to characterize amorphous carbon is through the ratio of sp2 to sp3 hybridized bonds present in the material. Graphite consists purely of sp2 hybridized bonds, whereas diamond consists purely of sp3 hybridized bonds. Materials that are high in sp3 hybridized bonds are referred to as tetrahedral amorphous carbon (owing to the tetrahedral shape formed by sp3 hybridized bonds) or as diamond-like carbon
(owing to the similarity of many physical properties to those of diamond).
Experimentally, sp2 to sp3 ratios can be determined by comparing the relative intensities of various spectroscopic peaks (including EELS
, XPS
, and Raman Spectroscopy
) to those expected for graphite or diamond. In theoretical works, the sp2 to sp3 ratios are often obtained by counting the number of carbon atoms with three bonded neighbors versus those with four bonded neighbors. (Note that this relies heavily on deciding on a 'cutoff' distance that determines whether neighbouring atoms are bonded or not, and is therefore merely used as an indication of the relative sp2-sp3 ratio.)
Although the characterization of amorphous carbon materials by the sp2-sp3 ratio may seem to indicate a one-dimensional range of properties between graphite and diamond, this is most definitely not the case. Research is currently ongoing into ways to characterize and expand on the range of properties offered by amorphous carbon materials.
Amorphous carbon materials may also be stabilized by terminating dangling-π bonds with hydrogen
. These materials are then called hydrogenated amorphous carbon.
All practical forms of hydrogenated carbon—including cigarette smoke, wood fire smoke, chimney soot, mined coal such as bitumen and anthracite—contain large amounts of polycyclic aromatic hydrocarbon tars, and are therefore almost certainly carcinogenic.
Allotropes of carbon
This is a list of the allotropes of carbon.-Diamond:Diamond is one of the most well known allotropes of carbon. The hardness and high dispersion of light of diamond make it useful for both industrial applications and jewellery. Diamond is the hardest known natural mineral. This makes it an...
that does not have any crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line structure. As with all glassy
Amorphous solid
In condensed matter physics, an amorphous or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal....
materials, some short-range order can be observed. Amorphous carbon is often abbreviated to aC for general amorphous carbon, aC:H for hydrogenated amorphous carbon, or to ta-C for tetrahedral amorphous carbon (also called diamond-like carbon).
In mineralogy, amorphous carbon is the name used for coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
, soot
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
and other impure forms of the element, carbon that are neither graphite nor diamond. In a crystallographic sense, however, these materials are not truly amorphous, but are polycrystalline or nanocrystalline materials of graphite or diamond within an amorphous carbon matrix.
In mineralogy
Historically, the term "amorphous carbon" was used to describe carbonaceous materials found in soot and coal that could not be categorized as either diamond or graphite. However, these materials are not truly amorphous, but consist of crystallites of graphiteGraphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
or diamond
Diamond
In mineralogy, diamond is an 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 conversion rate from diamond to graphite is negligible at ambient conditions...
technically polycrystalline or nanocrystalline materials. Commercial carbon also usually contains significant quantities of other elements, which may form crystalline impurities.
Coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
and soot
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...
are both informally called amorphous carbon. However, both are products of pyrolysis
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
, which does not produce true amorphous carbon under normal conditions. The coal industry divides coal up into various grades depending on the amount of carbon present in the sample compared to the amount of impurities. The highest grade, anthracite
Anthracite coal
Anthracite is a hard, compact variety of mineral coal that has a high luster...
, is about 90 percent carbon and 10% other elements. Bituminous coal
Bituminous coal
Bituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than Anthracite...
is about 75-90 percent carbon, and lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
is the name for coal that is around 55 percent carbon.
In modern science
With the development of modern thin film deposition and growth techniques in the latter half of the 20th century, such as chemical vapour deposition, sputter depositionSputter deposition
Sputter deposition is a physical vapor deposition method of depositing thin films by sputtering, that is ejecting, material from a "target," that is source, which then deposits onto a "substrate," such as a silicon wafer...
, and cathodic arc deposition
Ion beam deposition
Ion Beam Deposition is a process of applying materials to a target through the application of an ion beam.thumb|Ion beam deposition setup with mass separator...
, it became possible to fabricate truly amorphous carbon materials.
In technical terms, true amorphous carbon has localized π electrons (as opposed to the aromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
π bonds in graphite), and its bonds form with lengths and distances that are inconsistent with any other allotrope
Allotropy
Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, known as allotropes of these elements...
of carbon
Allotropes of carbon
This is a list of the allotropes of carbon.-Diamond:Diamond is one of the most well known allotropes of carbon. The hardness and high dispersion of light of diamond make it useful for both industrial applications and jewellery. Diamond is the hardest known natural mineral. This makes it an...
. It also contains a high concentration of dangling bonds, which cause deviations in interatomic spacing (as measured using diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
) of more than 5%, and noticeable variation in bond angle.
The properties of amorphous carbon films vary depending on the parameters used during deposition. One of the most common ways to characterize amorphous carbon is through the ratio of sp2 to sp3 hybridized bonds present in the material. Graphite consists purely of sp2 hybridized bonds, whereas diamond consists purely of sp3 hybridized bonds. Materials that are high in sp3 hybridized bonds are referred to as tetrahedral amorphous carbon (owing to the tetrahedral shape formed by sp3 hybridized bonds) or as diamond-like carbon
Diamond-like carbon
Diamond-like carbon exists in seven different forms of amorphous carbon materials that display some of the typical properties of diamond. They are usually applied as coatings to other materials that could benefit from some of those properties. All seven contain significant amounts of sp3...
(owing to the similarity of many physical properties to those of diamond).
Experimentally, sp2 to sp3 ratios can be determined by comparing the relative intensities of various spectroscopic peaks (including EELS
Electron energy loss spectroscopy
In electron energy loss spectroscopy a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected...
, XPS
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...
, and Raman Spectroscopy
Raman spectroscopy
Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...
) to those expected for graphite or diamond. In theoretical works, the sp2 to sp3 ratios are often obtained by counting the number of carbon atoms with three bonded neighbors versus those with four bonded neighbors. (Note that this relies heavily on deciding on a 'cutoff' distance that determines whether neighbouring atoms are bonded or not, and is therefore merely used as an indication of the relative sp2-sp3 ratio.)
Although the characterization of amorphous carbon materials by the sp2-sp3 ratio may seem to indicate a one-dimensional range of properties between graphite and diamond, this is most definitely not the case. Research is currently ongoing into ways to characterize and expand on the range of properties offered by amorphous carbon materials.
Amorphous carbon materials may also be stabilized by terminating dangling-π bonds with hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. These materials are then called hydrogenated amorphous carbon.
All practical forms of hydrogenated carbon—including cigarette smoke, wood fire smoke, chimney soot, mined coal such as bitumen and anthracite—contain large amounts of polycyclic aromatic hydrocarbon tars, and are therefore almost certainly carcinogenic.