Amorphous carbon

Amorphous carbon is free, reactive carbon that does not have any crystalline structure. Amorphous carbon materials may be stabilized by terminating dangling-π bonds with hydrogen. As with other amorphous solids, some short-range order can be observed. Amorphous carbon is often abbreviated to aC for general amorphous carbon, aC:H or HAC for hydrogenated amorphous carbon, or to ta-C for tetrahedral amorphous carbon (also called diamond-like carbon).[1]

In mineralogy

In mineralogy, amorphous carbon is the name used for coal, soot, carbide-derived carbon, and other impure forms of carbon that are neither graphite nor diamond. In a crystallographic sense, however, the materials are not truly amorphous but rather polycrystalline materials of graphite or diamond[2] within an amorphous carbon matrix. Commercial carbon also usually contains significant quantities of other elements, which may also form crystalline impurities.

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.

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; these cause deviations in interatomic spacing (as measured using diffraction) 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 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. (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 of hydrogenated carbon (e.g. smoke, chimney soot, mined coal such as bitumen and anthracite) contain large amounts of polycyclic aromatic hydrocarbon tars, and are therefore almost certainly carcinogenic.

Q-carbon

Q-carbon, short for quenched carbon, is a type of amorphous carbon, which is reported by its discoverers to be ferromagnetic, electrically conductive, harder than diamond,[3] and able to exhibit high-temperature superconductivity.[4][5][6][7] The original discoverers have published scientific papers on the synthesis and characterization of Q-carbon, but as of 2019, there is not yet any independent experimental synthesis or confirmation of these reported properties.

According to 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.[8][9] Carbon is melted using nanosecond laser pulses, then quenched rapidly to form Q-carbon, or a mixture of Q-carbon and diamond. Q-carbon can be made to take multiple forms, from nanoneedles to large-area diamond films. Researchers also report the creation of nitrogen-vacancy nanodiamonds.[10]

Discovery

In 2015, a research group led by Jagdish Narayan, a professor of materials science and engineering at North Carolina State University, and graduate student Anagh Bhaumik announced the discovery of Q-carbon.[4][6][11][12][13][14][15] They also announced the discovery of Q-boron nitride (Q-BN), and the conversion of carbon into diamond and h-BN into c-BN[16] at ambient temperatures and air pressures.[17]

The process started with Narayan's papers on laser annealing, published in Science,[18][19] and culminated in 2015–16 with another series of papers[20][21][22][23][24][25] and three United States patent applications: 62/245,108 (2015); 62/202,202 (2015); and 62/331.217 (2016). These have been licensed by Q-Carbon Inc[26] to commercialize products based on Q-carbon,[27][28][29] diamond,[30] Q-BN and c-BN.[31][32][33][34]

Production

Typically, diamond is formed by heating carbon at very high temperatures (>5,000 K) and pressures (>120,000 atmospheres). However, Narayan and his group used kinetics and time control of pulsed nanosecond laser melting to overcome thermodynamic limitations and create a supercooled state that enables conversion of carbon into Q-carbon and diamond at ambient temperatures and pressures. The process uses a high-powered laser pulse, similar to that used in eye surgery, lasting approximately 200 nanoseconds. This raises the temperature of the carbon to approximately 4,000 K (3,700 °C; 6,700 °F) at atmospheric pressure. The resulting liquid is then quenched (rapidly cooled); it is this stage that is the source of the "Q" in the material's name. The degree of supercooling below the melting temperature determines the new phase of carbon, whether Q-carbon or diamond. Higher rates of cooling result in Q-carbon, whereas diamond tends to form when the free energy of the carbon liquid equals that of diamond.

Using this technique, diamond can be doped with both n- and p-type dopants, which is critical for high-power solid-state electronics. During rapid crystal growth from the melting, dopant concentrations can far exceed the thermodynamic solubility limit through a solute trapping phenomenon. This is necessary to achieve sufficiently high free carrier concentrations, since these dopants tend to be deep donors with high ionization energies.

It took researchers only 15 minutes to make one carat of Q-carbon. The initial research created Q-carbon from a thin plate of sapphire coated with amorphous (non-crystalline) carbon. Further studies have demonstrated that other substrates, such as glass or polymer, also work. This work was subsequently extended to convert h-BN into phase-pure c-BN.[35]

Properties

Q-carbon is non-crystalline, and while it has mixed sp2 and sp3 bonding, it is mostly sp3, which is offered as an explaination of its hardness[36] and its electrical, optical and magnetic properties. Q-carbon is harder than diamond by 48–70% because carbon is metallic in the molten state and gets closely packed, with a bond length smaller than that in diamond. Unlike all other known forms of carbon, Q-carbon is ferromagnetic, with a saturation magnetization of 20 emu/g and an estimated Curie temperature of approximately 500 K.[37][38]

Depending on the quenching rate from the supercooled state, Q-carbon can be a semiconductor or metallic. It glows more than diamond when exposed even to low levels of energetic radiation because of its stronger negative electron affinity.[39]

Boron-doped Q-Carbon exhibits BCS-type superconductivity at up to 57K.[40][41][42][43][29]

Some groups have provided theoretical explanations of the reported properties of Q-carbon, including the record high-temperature superconductivity, ferromagnetism and hardness.[44][45]

See also

References
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  25. Advanced Materials and Processes. 174: 24. 2016. Missing or empty |title= (help)
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