Amorphous carbon



Amorphous carbon is an allotrope of carbon that does not have any crystalline structure. As with all glassy materials, some short-range order can be observed, but there is no long-range pattern of atomic positions. 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.

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 graphite or diamond with varying amounts of amorphous carbon holding them together, making them 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.

All practical forms of hydrogenated carbon—including cigarette smoke, wood fire smoke, smoked sausages, chimney soot, mined coal such as bitumen and anthracite—contain large amounts of polycyclic aromatic hydrocarbon tars, and are therefore carcinogenic.

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 deposition, 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 terminare dangling-π bonds with hydrogen. These materials are then called hydrogenated amorphous carbon.