Q-carbon

Q-carbon is an allotrope of carbon. It is reported to be ferromagnetic, electrically conductive, and able to glow when exposed to low levels of energy.^[1] It is relatively inexpensive to make and some media reports claim that it has replaced diamond as the world's hardest substance. 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 bonds found in diamonds.^[2][3] Carbon is melted using nanosecond laser pulses then quenched rapidly to form Q-carbon or a mixture of Q-carbon and diamond or diamond in the form of micro- and nanoscale crystals and needles or large area single crystal films. Researchers are also able to create nitrogen-vacancy (NV) nanodiamonds in a controlled way and self-organize them for a variety of potential applications ranging from nanosensing and quantum computing to biomarkers. The transition from NV^o to NV- can be driven optically as well as electronically.

History

Late 2015 discoveries of Q-carbon and Q-BN and conversion of carbon into diamond and h-BN into c-BN at ambient temperatures and pressures in air represent a major breakthrough in science and technology of diamond and related materials. The discovery of Q-carbon was announced in 2015 by a research group headed by Materials Science & Engineering Professor Jagdish Narayan and graduate student Anagh Bhaumik at North Carolina State University.^{[4][5][6][7][8][9][10][11][12][13][14]}

The discovery of Q-phases and direct conversion of carbon into diamond and h-BN into c-BN started with Narayan's seminal papers on laser annealing published in Science (Science 204, 461 (1979) and Science 252, 416 (1991)) and culminated in 2015-16 with a series of papers in APL Materials 3, 100702 (2015); APL Materials 4, 202701 (2016); J. Appl. Phys. 118, 215303 (2015); J. Appl. Phys. 119, 185302 (2016); Materials Res. Letters 2016; doi:10.1080/21663931.2015.1126865; Advanced Materials and Processes 174, 24 (2016); and three US Patents Pending: 62/245,108 (2015); 62/202,202 (2015); and 62/331.217 (2016). These patents have been licensed by Q-Carbon, LLC to commercialize Q-carbon, diamond, Q-BN and c-BN based products.

Production

Typically, diamond is formed by heating at very high temperatures (>5,000 K) and pressures (>120,000 atmospheres). However, according to this invention, Narayan and his group have utilized kinetics and time control of pulsed nanosecond laser melting to overcome thermodynamic limitations and create a super-undercooled 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 undercooling below the melting temperature determines the new phase of carbon, whether Q-carbon or diamond. Higher degrees of undercooling result in Q-carbon whereas diamond tends to form when the free energy of diamond equals that of the carbon liquid. Using this technique, diamond can be doped with both n- and p-type dopants, which is critical for high-power solid state devices. During rapid crystal growth from the melt, dopant concentrations can far exceed the thermodynamic solubility limit through a solute trapping phenomenon, which is relevant for achieving sufficiently high free carrier concentrations since these dopants tend to be deep donors with high ionization energies. Narayan and his group are also able to create NV (nitrogen-vacancy) nanodiamonds in a controlled way and self-organize them for a variety of applications ranging from nanosensing and quantum computing to biomarkers.

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 research has 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. (APL Materials 4, 202701 (2016))

Properties

Q-carbon is amorphous (non-crystalline), and while it has mixed sp² and sp³ bonding, it is mostly sp³ which lead to its unique hardness, electrical, optical and magnetic properties. Q-carbon is harder than diamond by 10-20% because carbon is metallic in the molten state and it gets closely packed with C-C 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. Depending on the quenching rate from the super undercooled state, Q-carbon can be semiconductor or metallic. Q-carbon glows more than diamond when exposed to even low levels of energetic radiation, because of higher negative electron affinity than diamond. Similar results have been obtained for boron nitride, including discovery of Q-BN and direct conversion of h-BN into c-BN (cousin to diamond) at room-temperature and atmospheric pressure in air. This is even more significant for BN, as CVD and PVD processes do not yield phase-pure or doped BN.

Applications

Since the discovery of Q-carbon is relatively new and still in a development stage, practical applications are in their infancy. However, the research represents a major breakthrough for diamond and c-BN based technologies considering the novel properties that these materials exhibit and the multiple forms from nanoneedles to large area diamond films that it can be made. Potential applications based on the novel properties of Q-carbon, Q-BN, diamond and c-BN range from high power electronic and photonic devices to high speed machining, deep sea drilling and biomedical sensing. Its low work function and negative electron affinity make it an attractive alternative for efficient field emission displays. The ability to achieve controlled introduction of NV centers in a variety of nanostructures opens the feasibility of novel quantum nanotechnologies in the physical and biological sciences, for example, single photon sensors, nanoscale electronic and magnetic sensing, single spin magnetic resonance, fluorescent biomarkers and nanosensors. The ease of the manufacturing process with which Q-carbon can be made and its unique physical properties are what suggests a promising future for this technology.

See also

• Allotropes of carbon

References

Further reading

• Narayan, Jay; Shipman, Matt (November 30, 2015), "Researchers Find New Phase of Carbon, Make Diamond at Room Temperature", NC State News 
• Narayan, Jagdish; Bhaumik, Anagh (December 7, 2015), "Novel phase of carbon, ferromagnetism, and conversion into diamond", Journal of Applied Physics, 118 (21): 215303, doi:10.1063/1.4936595 
• Narayan, Jagdish; Bhaumik, Anagh (October 7, 2015), "Research Update: Direct conversion of amorphous carbon into diamond at ambient pressures and temperatures in air", APL Materials, 3 (100702): 100702, doi:10.1063/1.4932622 
• Ben Brumfield, CNN (2 December 2015). "Q-carbon is harder, brighter than diamonds - CNN.com". CNN. 
• Q-carbon website