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Glowing press releases and news articles in 2015 proclaimed a new allotrope of carbon. However, even the journal article is light on chemical detail (e.g. no structural formula).

  • What is its bond structure?
  • Why is it ferromagnetic? fluorescent? harder than diamond? etc?
  • Or is it an erroneous result?

Update: as of late 2020, Bhaumik & Narayan have published further papers about Q-carbon, but I still have not seen any reports of replication by others.


Ref: Jagdish Narayan and Anagh Bhaumik J. Appl. Phys. 2015, 118, 215303.

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    $\begingroup$ At this point, it is my personal belief that the experimenters at NCSU are either self-deluded or outright lying. $\endgroup$
    – Foo Bar
    Commented Dec 31, 2017 at 13:54

2 Answers 2

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A study by Nayna Khosla and Jagdish Narayan from 2022 does address the fabrication of these Q-carbon nano-structures. The research describes a method to manufacture various carbon nanostructures including Q-carbon nanolayers, Q-carbon nanoballs, nanodiamonds, microdiamonds, and their composites. They achieve this by controlling laser and substrate variables guided by computer modeling. Interestingly, by changing the laser annealing energy density, they can transform thin film Q-carbon into Q-carbon nanoballs which can self-assemble into rings containing nanodiamonds. Furthermore, they can use higher laser energy density to grow high quality diamond films. The study proposes a model that pressure caused by quenching rate and undercooling may be the key factor in converting amorphous carbon into Q-carbon, diamond or their composites. This ability to control the resulting material is valuable for applications such as protective coatings, nano-sensors, field emission devices and targeted drug delivery. Finally, the research also demonstrates that Q-carbon nanoballs and nanodiamonds can be used as seeds to grow high quality microdiamond films. It seems to answer most of your questions.


Bond Structure

enter image description here

Figure 1: SEM images at (a) 0.60 J cm−2 showing 2D Q-carbon thin film, (b) 0.70 J cm−2 showing Q-carbon nanoballs with nanodiamonds embedded, (c) 0.80 J cm−2 showing nanodiamonds film, and (d) 0.85 J cm−2 showing microdiamonds on r-sapphire.

The bond structure of Q-carbon is characterized by a combination of diamond tetrahedra with sp3 bonding and regions with sp2 bonding. Specifically, Q-carbon consists of diamond tetrahedra that have center atoms arranged randomly. The bonding within these tetrahedra is of the sp3 type, typical of diamond, which provides high hardness and strength. However, between these tetrahedra, the bonding shifts to the sp2 type, more characteristic of graphite, adding a layer of flexibility and unique structural properties.

During the formation of Q-carbon, the sp3 to sp2 ratio is influenced by the laser energy density used in the annealing process. For instance, at laser energy densities of 0.80 J cm−2 and 0.85 J cm−2, the regrowth rates range from 5.8 ms−1 to ~6.5 ms−1, favoring diamond growth. In contrast, at energy densities between 0.60 J cm−2 to 0.70 J cm−2, the higher undercooling and rapid quenching result in regrowth velocities of ~10 ms−1, leading to the formation of Q-carbon and Q-carbon nanoballs.

The distinct bonding structure of Q-carbon is evident in the high-resolution scanning electron microscope (SEM) images, which show different phases depending on the annealing energy density. For example, at 0.6 J cm−2 (Figure 1a), a Q-carbon layer forms near the substrate interface with an α-carbon overlayer, where cracks appear due to thermal expansion differences. At 0.7 J cm−2, Q-carbon nanoballs form (Figure 1b), characterized by a uniform size of about 50 nm, with embedded nanodiamonds that nucleate and grow from these Q-carbon structures.

Thus, the bond structure of Q-carbon is a hybrid of sp3 and sp2 bonding, governed by the laser energy density during its formation, leading to various carbon phases such as Q-carbon, nanodiamonds, and microdiamonds with unique structural and functional properties.

In addition to this, a better visualisation can be seen in Image 1.1. This was take from another study, which you can find here , by Ruqian Lian et.al. The partial density of states (PDOS) and partial charge density for Q-carbon were analyzed in this study and compared with those of sp2 graphite and sp3 diamond. In graphite, the states of the 2px and 2pz orbitals are identical, indicating sp2 hybridization. In diamond, the states of the 2px, 2py, and 2pz orbitals overlap, corresponding to sp3 hybridization. However, in Q-carbon, there is no obvious degeneracy between the DOS of different 2p orbitals. The states of the 2s and 2pz orbitals in Q-carbon have similar energy levels, especially in the low-energy region, suggesting that the orbital hybridization in Q-carbon is unique and not one of the three basic hybridization types.

Further analysis of the orbital shapes and partial charge density at selected energy levels showed that in the low-energy range, the 2s orbital evenly distributes in three directions in graphite and four directions in diamond, indicating sp2 and sp3 hybridization, respectively. In contrast, Q-carbon shows almost invisible separation of the 2s orbital, indicating a low degree of s-p hybridization. Additionally, while the 2p orbitals in graphite and diamond overlap with their 2s orbitals, the four orbitals in Q-carbon have different distributions, further demonstrating the independence of each orbital in Q-carbon. This highlights the distinct orbital hybridization and electronic structure of Q-carbon compared to graphite and diamond.

enter image description here

Image 1.1 Schematic diagram of hybridization mechanism and typical materials for these hybridization types.


Hardnesss

The information on the hardness of the Q-carbon, as compared to diamonds, was limited in Nayna Khosla's work. However another study by Jagdish Narayan and Anagh Bhaumik(as aforementioned by you), does provide an explanation for it. Q-carbon is over 40% harder than diamond due to its unique formation process and structural properties. This phase is formed by nanosecond laser melting of amorphous carbon followed by rapid quenching from a super-undercooled state. During this process, closely packed atoms in the molten metallic carbon are rapidly quenched into Q-carbon, resulting in a structure with 80-85% sp3 bonding and the remainder sp2 bonding.

The increased hardness of Q-carbon compared to diamond is primarily due to its higher density of atoms and more efficient packing of tetrahedra. The number density of atoms in Q-carbon can be 40% to 60% higher than in the diamond cubic lattice, as the packing efficiency of tetrahedra increases from 70% in diamond to 80% in Q-carbon. This denser atomic arrangement contributes significantly to the material's overall hardness.

Additionally, the dominance of sp3 bonding in Q-carbon, which accounts for 80-85% of its structure, plays a crucial role in enhancing its hardness. Sp3 bonds are strong covalent bonds that provide significant rigidity and resistance to deformation. This combination of high-density atomic packing and a high percentage of sp3 bonding results in a material that is not only comparable to but surpasses diamond in hardness.


Ferromagnetism

enter image description here

Figure 2: M-H curve showing room temperature ferromagnetism in Q-carbon composites with finite coercivity. The inset shows the coercivity of the intercepts with field at 300 K.

Q-carbon exhibits robust coercivity and associated ferromagnetism at room temperature due to the presence of dangling bonds with unpaired spins at the sp3/sp2 interface. These unpaired spins at the interface contribute to the material's ferromagnetic properties. The formation of Q-carbon and its ferromagnetic behavior are further confirmed by magnetic results.

Figure 2 shows the magnetization vs. magnetic field plot for a Q-carbon sample, where a typical ferromagnetic loop with finite coercivity is observed. This behavior is noted across different temperatures, specifically at 10 K, 100 K, and 300 K. The room temperature ferromagnetism in Q-carbon is evidenced by a coercivity of 57.5 Oe. At 10 K, the coercivity increases to 76 Oe but decreases as the temperature rises. Controlled samples containing only diamond, diamond-like carbon (DLC), and sapphire substrate exhibit only diamagnetic behavior, which was subtracted in the represented magnetic plots. This indicates that the ferromagnetic properties are intrinsic to the Q-carbon structure and not due to the other phases or substrates.

Before laser annealing, the DLC film shows diamagnetic behavior, further highlighting that the ferromagnetism observed is a result of the unique bond structure and formation of Q-carbon.


I hope this is able to answer most your questions. As for "fluorescent", I could not find much about it either. I also want to clarify that Images 1 and 2, where taken from "the study" by Nayna Khosla and Jagdish Narayan. The link to that is available in my first sentence, and I highly suggest you to check it out.

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    $\begingroup$ As this "Q-carbon" is contentions, you'd do better to include verifying sources, not some wild claims. $\endgroup$
    – Mithoron
    Commented Jun 5 at 12:45
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There's no structural formula because it's what we call an amorphous solid. That is, it's like glass. It has no clearly defined crystal structure, and is likely to be extremely hard, but also extremely brittle.

As to why it's ferromagnetic or fluorescent, I'm unsure. It's likely they don't quite know, either. I suspect it has something to do with the curious bond structure of Q-carbon, which is a mishmash of both 3-way and 4-way bonds. In comparison, graphene is uniformly 3-way bonds, while diamond is a fairly uniform composition of 4-way bonds.

I speculate (don't quote me on this, I can't find a reliable source) that the way the electrons interact across the carbon atoms has something to do with why it illuminates (as photon emission is not an uncommon effect of electrons changing energy levels) and with why it's magnetic (as ferromagnetism explicitly refers to the quantum mechanical properties of electrons).

Either way. Please take this explanation with a grain of salt, but, if you're able to find more reliable sources, feel free to tack it on.

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  • $\begingroup$ Umm . . . I'm skeptical of this answer. Sources: google.com/search?q=amorphous-carbon and google.com/search?q=diamond-cubic $\endgroup$
    – Foo Bar
    Commented Jun 28, 2016 at 14:42
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    $\begingroup$ @FooBar I too think much of this answer is kind of weak. But, giving google search terms as sources for why you disagree with it doesn't really give any info as to why you are skeptical of the answer. For one thing, the same search terms will give different people different results. Then it's hard to say which result was of interest to you. Anyway, this answer is old and the poster doesn't seem to be around any more so it probably doesn't matter anyway ;) $\endgroup$
    – airhuff
    Commented Oct 16, 2017 at 20:02
  • $\begingroup$ @airhuff the original answer said that diamond is amorphous (see the edit history). $\endgroup$
    – Foo Bar
    Commented Dec 18, 2017 at 18:30

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