Amorphous carbon is free, reactive carbon that has nocrystallinestructure. Amorphous carbon materials may be stabilized by terminatingdangling-π bonds withhydrogen. As with otheramorphous solids, some short-range order can be observed. Amorphous carbon is often abbreviated toaC for general amorphous carbon,aC:H orHAC for hydrogenated amorphous carbon, or tota-C fortetrahedral amorphous carbon (also calleddiamond-like carbon).[1]
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Inmineralogy, amorphous carbon is the name used forcoal,carbide-derived carbon, and other impure forms of carbon that are neither graphite nor diamond. In acrystallographic sense, however, the materials are not truly amorphous but ratherpolycrystalline materials ofgraphite ordiamond[2] within an amorphous carbonmatrix. Commercial carbon also usually contains significant quantities of other elements, which may also form crystalline impurities.
With the development of modern thin film deposition and growth techniques in the latter half of the 20th century, such aschemical vapour deposition,sputter deposition, andcathodic arc deposition, it became possible to fabricate truly amorphous carbon materials.
True amorphous carbon has localized π electrons (as opposed to thearomaticπ bonds in graphite), and its bonds form with lengths and distances that are inconsistent with any otherallotropeof carbon. It also contains a high concentration of dangling bonds; these cause deviations in interatomic spacing (as measured usingdiffraction) of more than 5% as well as noticeable variation in bond angle.[2]
The properties of amorphous carbon films vary depending on the parameters used during deposition. The primary method for characterizing amorphous carbon is through the ratio ofsp2 tosp3hybridized bonds present in the material. Graphite consists purely ofsp2 hybridized bonds, whereas diamond consists purely ofsp3 hybridized bonds. Materials that are high insp3 hybridized bonds are referred to as tetrahedral amorphous carbon, owing to the tetrahedral shape formed bysp3 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 (includingEELS,XPS, andRaman spectroscopy) to those expected for graphite or diamond. In theoretical works, thesp2 tosp3 ratios are often obtained by counting the number of carbon atoms with three bonded neighbors versus those with four bonded neighbors. (This technique requires deciding on a somewhat arbitrary metric for determining whether neighboring atoms are considered 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.
All practical forms ofhydrogenated carbon (e.g. smoke, chimney soot, mined coal such as bitumen and anthracite) contain large amounts ofpolycyclic aromatic hydrocarbon tars, and are therefore almost certainly carcinogenic.
Q-carbon, short for quenched carbon, is claimed to be a type of amorphous carbon that isferromagnetic,electrically conductive, harder thandiamond,[3] and able to exhibithigh-temperature superconductivity.[4] A research group led by ProfessorJagdish Narayan and graduate student Anagh Bhaumik atNorth Carolina State University announced the discovery of Q-carbon in 2015.[5] They have published numerous papers on the synthesis and characterization of Q-carbon,[6] but years later, there is no independent experimental confirmation of this substance and its properties.
According to the researchers, Q-carbon exhibits a random amorphous structure that is a mix of 3-way (sp2) and 4-way (sp3)bonding, rather than the uniform sp3 bonding found in diamonds.[7] Carbon is melted using nanosecond laser pulses, thenquenched rapidly to form Q-carbon, or a mixture of Q-carbon and diamond. Q-carbon can be made to take multiple forms, fromnanoneedles to large-area diamond films. The researchers also reported the creation ofnitrogen-vacancynanodiamonds[8] and Q-boron nitride (Q-BN), as well as the conversion of carbon into diamond and h-BN into c-BN[9] at ambient temperatures and air pressures.[10] The group obtainedpatents on q-materials and intended to commercialize them.[11]
In 2018, a team atUniversity of Texas at Austin used simulations to propose theoretical explanations of the reported properties of Q-carbon, including the record high-temperature superconductivity, ferromagnetism and hardness.[12][13] However, their simulations have not been verified by other researchers.