Journal Pre-proof Preparation of T-carbon by plasma enhanced chemical vapor deposition Kai Xu, Hao Liu, Yan-Chao Shi, Jing-Yang You, Xing-Yu Ma, Hui-Juan Cui, Qing-Bo Yan, Guang-Chao Chen, Gang Su PII:
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Received Date: 12 August 2019 Revised Date:
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Please cite this article as: K. Xu, H. Liu, Y.-C. Shi, J.-Y. You, X.-Y. Ma, H.-J. Cui, Q.-B. Yan, G.-C. Chen, G. Su, Preparation of T-carbon by plasma enhanced chemical vapor deposition, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.10.032. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
DC Arc CVD
Dual radio frequency inductively coupled plasma CVD
The experimental preparation of T-carbon by PECVD method indicates that the massive production of T-carbon may be feasible
Preparation of T-carbon by Plasma Enhanced Chemical Vapor Deposition Kai Xu1, Hao Liu1, Yan-Chao Shi1, Jing-Yang You2, Xing-Yu Ma2, Hui-Juan Cui2, Qing-Bo Yan1, Guang-Chao Chen1,*, and Gang Su2,∗ 1
College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China
School of Physical Sciences, Kavli Institute for Theoretical Sciences, CAS Center of Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract A novel carbon allotrope, T-carbon, is attempted to obtain by using plasma enhanced chemical vapor deposition (PECVD) on the substrates of polycrystalline diamond and single crystalline diamond, respectively. Our measured x-ray diffraction, Raman and infrared spectra of the new form of carbon are in good agreement with the calculated results of T-carbon, and the lattice parameter meets with the results of calculation as well as the high resolution electron microscopy of T-carbon nanowires synthesized by the pulsed laser irradiating modification of carbon multiwall nanotubes, revealing that the phase of T-carbon can be identified in our samples. The experimental preparation of T-carbon by PECVD method indicates that the massive production of T-carbon may be feasible, and extensive investigations with high-throughput experiments on this novel phase are highly expected.
*Corresponding authors: [email protected]
(Guang-Chao Chen); [email protected]
1. Introduction Carbon is one of the most basic elements in periodic table, which can form sp-, sp2- and sp3-hybridized bonds, revealing a strong binding ability to form numerous compounds in nature. The carbon element has various allotropes, including graphite, cubic and hexagonal diamond , amorphous carbon, fullerene , carbyne [3, 4], nanotube [5, 6], graphene , and graphdiyne , etc. The first three carbon allotropes have conventional three-dimensional (3D) structures that can form naturally, while the others, in most cases, were only synthesized in laboratory. These latter carbon allotropes were unveiled to possess unusual and intriguing properties. The discoveries of fullerene and graphene undoubtedly spur great interest worldwide in the pace of seeking for novel carbon allotropes. Graphite was found to undergo a transition at high pressure of 17 GPa at ambient temperature, which was identified as a new phase of carbon . However, the nature and structure of this new carbon still retain less understood, which leads to a number of theoretical proposals for its possible structure, such as M-carbon , body-centered tetragonal carbon (bct-C4) , and many other orthorhombic polymorphs. Recent experimental evidence supports that this high-pressure structure of graphite may be consistent with M-carbon, albeit the phase transition was shown quite sluggish . A few years ago, a new fluffy 3D allotrope of carbon, dubbed as T-carbon, was proposed by means of first-principles calculations . Such a structure is formed by substituting each atom in cubic diamond with a carbon tetrahedron, bearing the same
space group Fd 3m as diamond, and is a semiconductor with a direct band gap. It is thermodynamically and dynamically stable at ambient pressure. In contrast to the high-pressure phase of graphite, T-carbon is fluffy, and has an astonishingly low density around 1.5g/cm3, being 32% smaller than that of graphite. From the structural point of view, T-carbon has large interspaces between carbon atoms along certain directions, and the volume per atom is about 1.5 times larger than graphite, while its bulk modulus is 36% of cubic diamond and 57% of graphite. T-carbon is lighter than graphite in density. Owing to the large interspaces between atoms, the hydrogen storage in T-carbon was also possible. Furthermore, it was reported that in T-carbon the fractions of sp3-hybrized atoms in grains decrease with an increasing tensile load, especially after the tensile strain exceeds the initial fracture strain, and the reduced sp3-hybrized atoms are changed into the grain-boundary atoms . The tensile strain can induce the global graphitization of T-carbon nanowires . T-carbon is a natural semiconductor with a direct band gap, and its conduction band energy level matches well with that of perovskites . Besides, it is more important that T-carbon possesses a high electron mobility of 2.36×103 cm2 s–1 V–1, which is superior to TiO2, ZnO, SnO2, and even MAPbI3 perovskite . In light of these amazing properties, T-carbon has recently gained great interest and led to a number of subsequent derived proposals and wide potential applications . According to density functional theory (DFT) calculations, the enthalpy of T-carbon is lower than those of graphite and diamond at certain negative pressure, suggesting that T-carbon may be easily formed at the circumstances under negative pressure. This calculation also hints that T-carbon might exist in the interstellar
space, because the average pressure in our accelerated expanding universe is negative, which appears to be more in favor of the formation of T-carbon. Therefore, a possible way to search for T-carbon is to observe the optical absorption spectra with an astronomical telescope in interstellar medium, and to compare with the simulated optical absorption spectra of T-carbon. On the other hand, the conventional way to obtain T-carbon is to create an environment properly for the growth of T-carbon in laboratory. Fortunately, T-carbon nanowires have been recently successfully synthesized with a multi-walled carbon nanotube suspension in methanol by picosecond pulsed-laser irradiation, where the computed electronic band structures and projected density of states are in good agreement with the measured optical absorption and photoluminescence spectra of T-carbon nanowires . Here we report that T-carbon can also be obtained in laboratory by using the plasma enhanced chemical vapor deposition (PECVD) on the substrates of polycrystalline cubic diamond and high temperature high pressure (HTHP) single crystalline cubic diamond, respectively. The measured x-ray diffraction (XRD), Raman spectra and Fourier transform infrared spectroscopy (FT-IR) of the new form of carbon show the features that differ totally from the known carbon allotropes but are consistent with the simulated results of T-carbon, suggesting that the new form of carbon in our samples can be identified as T-carbon. 2. Experimental and Calculational Methods We utilize two kinds of PECVD methods at a proper environmental pressure. One is that the plasma feed gas is the mixture of Ar/H2/CH4 with 3 standard liters per minute
(slm) of Ar, 3slm of H2 and 30 standard cubic centimeters per minute (sccm) of CH4, respectively. The plasma is lighted and sustained by DC arc . The chamber pressure is kept at 8 KPa. The substrate is chosen as a polycrystalline cubic diamond, and is heated up to around 1000 °C. After the growth of 4 hours, the sample is obtained as the rod-like carbon sticking out from the cauliflower-like diamond crystals. The other one is that the plasma was lighted and sustained by the dual radio frequency inductively coupled plasma (RF ICP) [20, 21]. The plasma feed gas was the mixture of Ar/H2/CH4 with 4 slm of Ar, 1.2 slm of H2 and 60 sccm of CH4, respectively. The chamber pressure was kept at 8 KPa. The substrate was chosen as a HTHP single crystalline cubic diamond, and was heated up to around 890
. After the growth of 200
hours, the sample was obtained as the rod bunch binding together. The as-grown deposits were characterized by XRD (Rigaku Ultima IV), Raman spectra (532 nm, Renishaw inVia Raman Microscope), and FT-IR spectra (Bruker TENSOR2), respectively. The morphology of the samples is observed by scanning electron microscopy (SEM, KYKY-8000F). The sample was thinned by focus ion beam (FIB) technique (Helios G4CX), and characterized by selected area diffraction in transmittance electron microscope (TEM, JEM2010) and electron energy loss spectrum (EELS, Enfina 1000).
Besides the experimental characterization, the computation is also carried out. The calculation of Raman spectra  of T-carbon was performed with vasp_raman_py package∗. The FT-IR was simulated with package IR-JaGeo . 3. Results The morphology of the sample grown on the polycrystalline diamond by the DC Arc method is presented in Figs. 1(a) and 1(b), which shows that the length of the sticking carbon rod from the cauliflower-like diamond can reach several microns, and some rods could even connect with each other, forming kink joints [Fig. 1(b)]. These carbon rods look like neither any known morphologies of diamond including diamond rod, whisker, and tree, nor any known graphite morphologies, but is likely a new form of carbon. The morphology of the sample grown on the single crystal diamond by the dual RF ICP method is presented in Figs. 1(c) and 1(d). Fig. 1(c) reveals that the as-grown sample looks like the rod bunch binding together, of which the rod shape can be seen clearly by the red boundary lines. The side of the rod bunch contains many small faceted crystalline surfaces [shown in Fig. 1(d)], called as “pineapple morphology” .
A. Fonari, S. Stauffer, book, 2013, “https://github.com/raman-sc/VASP/”
Figure 1 SEM images of the new form of carbon. (a) and (b) SEM images of the new form of carbon that sticks out as rods or kink joints from cauliflower-like diamond, deposited on a polycrystalline diamond substrate by the DC Arc CVD. (c) and (d) SEM images of the new form of carbon that look like the rod bunch binding together, deposited on an HTHP single crystalline diamond substrate by the dual RF ICP CVD.
Figure 2 XRD patterns of the new form of carbon in two samples. (a) XRD pattern of the sample by the DC Arc CVD on a polycrystalline diamond substrate. (b) and (c) Enlarged parts of XRD pattern around the peaks of (a), where the sharp peak with a shoulder can be decomposed into multiple peaks with indicated positions. (d) XRD pattern of the sample by the dual RF ICP CVD on an HTHP single crystalline diamond. (e) and (f) Enlarged parts of XRD pattern around the peaks of (d), where the round peak can be decomposed into multiple peaks with indicated positions. To characterize the possible crystalline structure of the new form of carbon in these two samples, we carried out the x-ray diffraction (XRD) measurement with wavelength of 1.54 Å (Ultima IV, Rigaku, Japan). However, due to the tiny quantity of the as-grown rods of the new carbon, it is insufficient to collect the XRD data. Instead, we choose to make an XRD measurement on the new carbon phase together with cauliflower-like or pineapple morphology diamond crystals [as in Figs. 1(a) and 1(c)]. The XRD patterns of
the two samples are presented in Figs. 2(a) and 2(d), respectively, which look quite similar. In the patterns of Fig. 2(a), the two sharp peaks come from the cauliflower-like diamond crystal (cubic diamond), while the diffraction peak around 2θ ≈ 26° and the small peak around 40° appear to be new features different from that of diamond. By enlarging the part of XRD pattern at small angles, as given in Figs. 2(b)-2(c), we found the small peak locates at 2θ = 39.23° and there is another small peak at 31.34°, while the diffraction peak around 25.84° may come from graphite that may be contained in the cauliflower-like diamond crystal, the other peaks at 2θ ≈ 31.67° and 2θ ≈ 39.23° cannot be ascribed to the presently known carbon allotropes. If the contour of spectrum within 20-40° as the profile of multipeaks, then, it could be decomposed into multiple peaks at 2θ = 20.94°, 25.84°, 31.67°, respectively, in terms of the conventional quantitative analysis methods for XRD pattern. In other words, it indicates that this sharp peak with a shoulder maybe contains behind three small peaks. It is notable that these peaks, i.e., 2θ ≈ 20.94°, 31.67°, 39.23°, and 53.77°, however, are well consistent with the simulated XRD pattern of T-carbon where the diffraction peaks appear at 20.45°, 33.68°, 39.72°, 50.7° and 61.35°, as reported in Ref. . For the sample deposited on the HTHP single crystalline diamond by the dual RF ICP CVD, the XRD peaks appeared at 2θ ≈ 26.16°, 36.22°, 39.41°, 61.34° and 66.94°, as shown in Fig. 2(d). The possible peaks at 2θ ≈ 20.40° and 33.65° are also decomposed, shown in Figs. 2(e)-2(f). We note that the peaks at 20.40°, 33.65°, 39.41°, and 61.34° are quite consistent with the calculated XRD pattern of T-carbon , while the peak at 26.16° can be ascribed to graphite and the peak at 36.22° is to be identified.
According to Bragg’s law, i.e., 2dsinθ=λ (where d is the d-spacing, θ is the Bragg diffraction angle, and λ is the wavelength of X-ray ), the d-spacing values are deduced as 2.28 Å and 1.51 Å, respectively, corresponding to the obvious diffraction peaks at 2θ ≈ 39.41° and 61.34°. They are consistent with the crystalline planes of (222) and (115) of T-carbon, respectively. Then, the lattice parameter, a, is deduced as 7.89 Å and 7.85 Å, respectively, according to the formula of a = d√h + k + l (where h, k, l are the miller indices of crystallographic plane) adopted in cubic lattice. The values of lattice parameter, a, are comparable with the calculated value 7.5Å of T-carbon in Ref. , and is also consistent with 7.8Å reported in Ref. . It is known that the weight percentage of crystalline composition in a mixture sample could be calculated according to the intensity ratio of diffraction peaks . In this sample, it is deduced that T-carbon phase takes up about 1.46% of the whole sample weight, which also manifests that the feature peaks of T-carbon are much weaker than those of diamond and graphite.
Figure 3 SEM images and TEM diffraction patterns, and EELS of the new form of carbon. (a) The origin morphology and position of the sample on the as-grown deposit, and the sample thinned by FIB. (b) Kikuchi lines of the diffraction. (c) Selected area diffraction (SAD) pattern. (d) EELS result of the sample.
Selected area diffraction (SAD) in transmission electron microscope (TEM) was adopted to provide more information on crystalline structure of the new carbon phase. The sample grown by dual RF ICP was thinning by focus ion beam (FIB) technique. Fig. 3(a) is the morphology of the as-grown deposit with small magnification. The morphology and the position of the sample for FIB processing are also shown on the left side of the photo as the insets. It can be seen that the morphology is step-bunching one, and the left panel with after picking-up indicates that the sample was well prepared by FIB method. The fabricated sample by FIB thinning is also shown in the right corner as an inset. The thinnest place in the sample is about 80 nm. Fig. 3(b) is the image of Kikuchi lines in the diffraction pattern, which indicates that -zone is diffracted. Fig. 3(c) presents the SAD pattern. It is a square shape diffraction pattern. By measuring the distance between the diffraction spots on line passing through the center spot, we obtain 16.134 nm-1 and 22.694 nm-1, respectively. Therefore, the values of d-spacing are deduced as 0.124 nm and 0.088 nm, respectively. If we assume this diffraction spot ascribes to a crystalline surface of T-carbon, the Miller indices (hkl) can be deduced according to the formula, a = d√h + k + l
(as mentioned above), where the
lattice constant, a, can be replaced by the reported one of T-carbon as a=7.5 Å. Then, (hkl) can be deduced as (006). Using the same method, (066) can also be deduced for d=0.088 nm. The interplanar angle between (006) and (066) is 45° which consists well with the diffraction experimental result. Fig. 3(d) is the electron energy loss spectrum (EELS) result. We may see that there is only σ-bond, and π-bond is not found. This is consistent with the sp3 hybrid bond feature in T-carbon  as well as the previous
experimental result . Note that the small peak below 288 eV in Fig. 3(d) may be due to noises where the counts become negative.
Figure 4 Raman spectra of the new form of carbon in two samples. (a) Raman spectra of the sample on the substrate of polycrystalline diamond by the DC Arc CVD. (b) Enlarged parts of (a) for the wave number between 500-800 cm-1. (c) Raman spectra of the sample on the substrate of single crystalline diamond by the dual RF ICP CVD. (d) The simulated Raman spectra of T-carbon. The Raman spectra (inVia-Reflex, Renishaw, UK) with the laser beam of 532 nm wavelength were measured for the interested carbon material. For the same reason of tiny amount of the crystalline sample deposited by the two PECVD methods as in obtaining XRD spectra, we measured the Raman spectra on the two samples together with the diamond substrates. The results for the two samples are presented in Figs.
4(a)-4(c), respectively. In Fig. 4(a), the sharp peak at 1334 cm-1 characters the T2g active mode of cubic diamond, and the peak at 1562 cm-1 is the E2g mode of graphite, while the peak at 1504 cm-1 and the peaks below 1000 cm-1 and above 1800 cm-1 cannot be related to diamond and graphite but belong to the new form of carbon. In order to make it clearly, we enlarged the Raman spectra of the sample with polycrystalline diamond between 500-800 cm-1 [Fig. 4(b)], where we observed the characteristic peaks at 430, 569，585, 689 cm-1. By comparing with the simulated Raman spectra of T-carbon, which include Eg mode at 552 cm-1, two T2g modes at 689 and 1467 cm-1, and A1g mode at 1802 cm-1, as shown in Fig. 4(d), we disclose that the observed Raman peaks for the new carbon at 585, 689, 1504, 1802 cm-1 are indeed consistent with those simulated values of T-carbon. For the sample grown on the substrate of single crystalline diamond, we measured its Raman spectra with the facility the same as on the sample with polycrystalline diamond substrate. The results are given in Fig. 4(c), which look very similar to Fig. 4(a). The peaks at 1331 cm-1 and 1555 cm-1 character the modes of cubic diamond and graphite, respectively. The small peaks at 1413 cm-1 and 1800 cm-1 may be from T-carbon, which are very close to the simulated peaks at 1467 cm-1 and 1802 cm-1 of T-carbon [Fig. 4(d)]. The other peaks still remain a mystery to be further inspected. It should be remarked here that as the amount of new carbon contained in the samples that were used in measurements of XRD and Raman spectra is so small that the characteristic peaks of new carbon are much smaller than those of diamond and graphite.
Figure 5 Infrared spectra of the new form of carbon. (a) Infrared spectra of the sample grown on the substrate of HTHP single crystalline diamond. (b) The simulated FT-IR spectra of T-carbon. The FT-IR spectra (Bruker TENSOR2) were measured to detect the transmittance of the sample deposited by the dual RF ICP CVD. There exists an obvious absorption peak locating at 1085 cm-1 [Fig. 5(a)], which meets well with the absorption feature of the simulated spectra of T-carbon [Fig. 5(b)]. Several small absorption peaks also exist in the spectra of Fig. 5(a), locating near 471, 619 and 855 cm-1, respectively, which may be related to N, O and methane on cyclopentane from air leakage. 4. Discussion and Conclusion By utilizing the PECVD method at proper environmental pressure, rod-like (or rod bunch binding together) carbons were grown on the substrate of polycrystalline cubic diamond or the substrate of HTHP single crystalline cubic diamond. We performed the XRD and Raman studies on the as-grown samples. We found that the XRD peaks at angles 2θ ≈ 20.94°, 33.31°, 39.25° and 61.35° as well as the Raman active modes at wavenumber 1802, 1504, 689, and 585 cm-1 are well consistent with the simulated results of T-carbon. In particular, the result of FT-IR of the new form of carbon accords
well with the absorption feature of T-carbon. With these experimental evidences, we may conclude that the rod-like carbon grown on the substrate of polycrystalline cubic diamond and the rod bunch binding together carbon grown on the substrate of single crystalline cubic diamond may contain the phase of T-carbon. According to calculations, T-carbon could be easily formed under the condition of negative pressure environment [17,27]. In preparation of T-carbon by PECVD method on diamond substrate, carbon atoms in the environment of enhanced plasma attempt to deposit on the diamond substrate. Because the temperature of enhanced plasma gas is beyond 2000 ℃ according to the estimation in references [21,26], while the diamond substrate is about 1000℃, the hot carbon atoms in the plasma gas are inclined to move upward, and the cold carbon atoms move downward to deposit onto the substrate, which are subject to the upward impact from the moving hot carbon atoms in the deposition process, equivalently to experience a negative pressure circumstance, and meanwhile, due to the cubic structure of the diamond substrate and the sp3 hybridization characteristics, these combined factors together may lead to the formation of T-carbon. The PECVD method as a practical technique is applied to the growth of T-carbon, suggesting that T-carbon may also be possible to be obtained by high-throughput experimental technology, and its mass production should be feasible. The successful preparation of T-carbon (as in Ref.  and present work) not only adds a new member to carbon family but also helps better understanding the versatile characteristics of carbon, which would lead to wide applications in diverse areas . It is certain that
more attempts on experiments of this new phase of carbon are highly expected in near future.
Acknowledgements The authors would like to thank Wu Zhou and Jin-An Shi at UCAS for their kind help in FIB thinning the sample, and Xue-Dong Bai and Xiao-Ming Li at Institute of Physics, CAS for their generous help in SAD and EELS characterizations. This work is supported in part by the National Key R&D Program of China (Grant No. 2018FYA0305800), the Strategic Priority Research Program of CAS (Grant No. XDB28000000), NSFC (Grant No. 11834014 and 51272281), Beijing Municipal Science and Technology Commission (Grant No. Z118100004218001), Scientific Research Equipment Project of CAS (Grant No. yz201356), Hundred Talents Program of CAS, Hebei Science and Technology Plan (Grant No.14291110) and Beijing Science and Technology Project (Grant No. Z151100003315024).
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: