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[diamond graphene.doc](https://mdr.nims.go.jp/filesets/a2f8216c-afc6-4f8e-932a-8562c853fc12/download)

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[Xiaolu Yuan](https://orcid.org/0000-0002-8755-1539), Chunxia Chi, Feitong Ren, Jinlong Liu, Junjun Wei, Liangxian Chen, Jiangwei Liu, Wenrui Wang, Xiao Dong, Haitao Ye, Jincheng Tong, Chengming Li

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[Diamond-to-graphene by nickel-catalyzed solid-state transformation](https://mdr.nims.go.jp/datasets/8befd954-af02-4ffa-b5f1-d38bcfc1328c)

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Dynamic behavior of Ni-catalyzed diamond graphitization Xiaolu Yuan Jinlong Liu Jiangwei Liu Chengming Li1) Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China; 2) Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan; * Correspondence: Chengmli@mater.ustb.edu.cnAbstract: Diamond-to-graphite (or graphene) transformation was directly observed using in-situ transmission electron microscopy with Ni catalyst. The phase transition mechanism is further proved to be a metal-induced solid-state transformation at high temperature. The discrepancy in annealing condition will seriously affect the position and number of layers of transformed graphene. During the slow heating process, the transformed graphene or graphite is located on the Ni surface. And after the rapid annealing treatment, the transformed graphene is between the interface of diamond and Ni. In addition, the thermodynamic and kinetic analysis simulated by first-principle calculation both prove that the possibility of Ni-catalyzed transformation from diamond to graphene, which are consistent with experimental results. The graphene-on-diamond (GOD) structures, acquired under rapid annealing treatment with good surface conductivity, would be promoting the development of all-carbon electronic devices.Keywords：diamond graphitization; in-situ TEM; Ni catalyst; graphene; solid-state transformation.1. IntroductionDiamond graphitization has been extensively studied and applied to some fields for decades ADDIN EN.CITE 1-3. And the GOD structures with its outstanding current-carrying capacity also gain great attention on diamond electronic devices ADDIN EN.CITE 4,5. Understanding the transition behavior of diamond, graphite and graphene in different regimes has important applications for all carbon sp3-sp2 technology ADDIN EN.CITE 6,7. Actually, diamond under sufficiently high temperature can directly transform into graphene or graphite ADDIN EN.CITE 8,9. And the addition of transition metal catalyst (Fe, Co, Ni) could decrease significantly the transformation temperature10,11. Presumably, the thermal annealing condition (such as time, rate, etc.) plays a key role in the transformation of graphene on metal surface ADDIN EN.CITE 12-14. However, the related mechanisms responsible for this metal-induced transformation are still controversial ADDIN EN.CITE 13-15. Unlike the direct graphitization without catalyst caused by thermal excitation of carbon atoms, the behaviors of metal-catalyzed graphitization are more complicated. It includes the migration and ripening of metals, the dissolution of carbon atoms and the formation of graphene in the whole annealing period16. Previous studies believe that the diamond graphitization occurs in the rapid cooling stage, during which a sharp drop in temperature causes a decrease in the solubility of carbon in metal, and then leads to phase transition induced by precipitation ADDIN EN.CITE 13,14. On the other hand, some researchers think the formation of graphene occurs in the high temperature, regarding the metal-catalyzed graphitization as the transformation mechanism ADDIN EN.CITE 10,15. Therefore, the transformation mechanism of diamond graphitization needs to be clarified further.Ni is one of a favorable catalyst which shares the fcc structure with little difference in lattice parameter and has good solubility with carbon atoms at elevated temperature17. Here, we observed directly the graphitization behavior of diamond with Ni catalyst using in-situ transmission electron microscopy (TEM). Tow different thermal annealing treatments and four phases (diamond, Ni, graphene, graphite) were analyzed down to the atomic scale. The experimental and calculated results provide strong evidence for the transformation mechanism which was dominated by metal-induced solid state transformation. 2. ExperimentalA thin polycrystalline Ni film (about 40 nm) was deposited on a polished (100) facet single crystalline diamond substrate. Then the in-situ TEM sample was prepared by focused ion beam (FIB, FEI Helios nanolab 600i) onto a MEMS-based heating chip which was then mounted into an heating holder. And the heating rate was set to about 10 ℃/min with the maximum temperature up to 820 ℃. High resolution (HR)TEM and high-angle annular dark field-scanning TEM (HAADF-STEM) observations were performed via JEOL ARM-300F TEM system and FEI Titan 80-300 TEM system (operated at 300 kV). The electron energy-loss spectroscopy (EELS) measurement was carried out by an aberration-corrected JEOL ARM-300F TEM (equipped with a Gatan GIF Quantum ER Dual EELS system) after an in-situ experiment under a similar experimental condition. In the present work, all movies of in-situ TEM experiment were recorded using a Gatan OneView camera. Another thin section sample was prepared by FIB after rapid annealing treatment ( heating and cooling rate 30 ℃/s, maximum temperature 800 ℃ for 1 min) used for the observation of graphene on diamond structure. Raman spectrum was performed with 532 nm laser beam. Confocal laser scanning microscopy (CLSM, OLYMPUS) investigations were carried out and electrical measurements were performed by conductive atomic force microscopy (cAFM, OXFORD Instruments Cypher VRS 1250). First-principles calculations were carried out using density functional theory with generalized gradient approximation of Perdew-Burke-Ernzerhof implemented in the Vienna Ab-Initio Simulation Package18,19. The valence electronic states were expanded on the basis of plane waves with the core-valence interaction represented using the projector augmented plane wave approach and a cutoff of 450 eV20. A Γ-centered k-mesh of 4 × 4 × 4 was used for bulk calculations. Convergence is achieved when the forces acting on ions become smaller than 0.02 eV/Å.3. Results and discussionsFigures 1a-d show a few snapshots extracted from Movie 1, which is provided as Supporting Information. One can see how the recrystallization of the Ni grains carries on during the slow heat treatment. Two large crystals in the polycrystalline Ni layer are firmly attached to the diamond at some t s in Figure 1a. After 33 s (Figure 1b), another small crystal grain with a different plane orientation appears, marked with a white dotted line in the lower left corner. And then small grains grow up gradually over time as shown in Figure 1c and 1d. This is a direct observation about the recrystallization and growth of polycrystalline Ni. Another set of HRTEM images (Figures 1e-h) extracted from Movie 2 reveals that the graphitization starts with the formation of a few layer graphene at the surface of Ni. The side of Ni layer away from the diamond was deposited with a protective layer that Pt particles were wrapped in amorphous carbon for FIB. No layered graphene is produced at t s in Figure 1e. After about 35 s (Figure 1f), the layered graphene shows up indistinctly at the interface between the Ni layer and the protective layer. And more obvious multilayer graphenes are present in the Figures 1g and 1h. Meanwhile, the layer of Ni catalyst becomes thinner as the phase transition and diffusion proceed. Actually, before the in-situ HRTEM observation, in-situ STEM experiment was performed at room temperature followed by a stepwise increase of the temperature to 800 °C and rapid cooling to 25 °C (Figures S1a-c). The grow up and recrystallization of polycrystalline Ni are clearly seen in the temperature regime between 300 °C and 500 °C (Figures S1d and S1e). With the temperature increase continuously, contrast variations due to grains reconstruction or rearrangement are observed until suddenly the onset of diamond graphitization is observed at around 800 °C (Figure S1f). And a large amount of large-grained, carbon-rich nickel is embedded in the diamond. Figures 2a-d show the screenshots of diamond graphitization taken from Moives 3 and 4. Figure 2a displays an interface of diamond, Ni(C) (carbon-rich nickel layer) and graphite at the 800 °C. The number of transformed graphite layers keep increasing (Figures 2b and 2c), accompanied by the migration of Ni grains and diffusion of carbon atoms from diamond to Ni. It can be seen that the orientation arrangement among multilayer graphene tends to be consistent at high temperature. Additionally, in-situ TEM observation also uncovers the split in graphite because of the migration of different orientations Ni crystals (Figure 2d). In view of TEM observations, we can infer that a Ni-catalyzed solid-state transition is the mechanism of diamond graphitization. And the graphitization occurs in the high temperature rather than the cooling stage after the traces behind by the migrating Ni islands. After a long time of heat treatment, graphite (graphene) exists on the surface of the Ni layer in the end. However, after a rapid annealing treatment, transformed graphene presents in another way. Figure 2e shows the initial interface of diamond/Ni before rapid annealing. Due to the fast heating rate of temperature, it is hard to capture the dynamic behavior of graphene transformation using in-situ TEM. Therefore, the ex-situ HRTEM images after rapid annealing are captured. In Figures 2f and 2g, we can clearly see that the transformed graphene is located at the interface of diamond/Ni. This kind of graphene, existed on the diamond surface, possess turbostratic morphology caused by rapid cooling treatment21. After removing the nickel layer with dilute hydrochloric acid, part of graphene on diamond could curl up as shown in Figure S2a. Three main peaks identifying the vibrational modes of graphene are notated as the D (1350 cm-1), G (1580 cm-1) and 2D (2700 cm-1) peaks in Fig. S2c ADDIN EN.CITE 22,23. Apparently, no any signal intensity of D peak representing defects was detected. Raman spectrum and cAFM image in Figure S2d illustrate that the transformed graphene on diamond has good crystal quality and surface conductivity, respectively. Therefore, under rapid annealing treatment, different phase transition route emerged (Figure 3). Layered graphene appeared at the interface of diamond/Ni rather than on the Ni surface. Similarly, three parts could be used to describe this phase transition based on thermal treatment process. At fast heating stage, those Ni crystals are too late to rearrange and grow up while most crystals still keep its original grain size and orientation, which maybe contribute to C atoms diffuse easily into the Ni. In the short-term high temperature stage, the solid-solution reaction occurs sharply and quickly. The interdiffusion between C atoms and Ni atoms provide more opportunities for sp3-C transformed to sp2-C. Then the graphene was formed on the diamond surface behind the ripening and migration of Ni grains. At fast cooling stage, those reactions (migration or diffusion) were interrupted suddenly as the temperature decrease sharply. The diffused carbon atoms stuck in the interface of diamond/Ni and did not enough time to diffuse to the surface of Ni. The uppermost layer of nickel failed to participate in the reaction. In order to understand deeply the process and mechanism of Ni-catalyzed conversion of diamond to graphene, First-principles calculations are conducted in view of thermodynamic and kinetic, respectively (Figure 4). The very close lattice constants of Ni and diamond are beneficial to the formation of a two-phase interface with low lattice mismatch. In Figure 4a, we found that the process of C atoms in diamond entering the Ni lattice (Ni atoms entering the diamond lattice) requires only a very low energy of 0.11 eV to form a solid solution such as Ni3C. Then the Ni atoms gradually separated from the solid solution so that the remaining carbon atoms formed a more stable graphene. The calculated enthalpy change of the entire reaction is -2.09 eV, which means the whole process is easy to happen. From kinetic analysis, the calculations of diffusion energy barrier of C atoms in solid solution have been performed (Figure 4b). We found that the diffusion energy barriers of C atoms in solid solution are all below 1 eV. This shows that the diffusion rate of carbon in nickel is very fast. Therefore, when the rapid annealing treatment is performed, as the nickel crystal grains ripening and migrating, the carbon atoms in the solid solution are easily transformed into graphene.On the basis of above observations and analysis, we can make a few statements about the transformation of diamond to graphite. A schematic model of diamond graphitization could be divided into four stages (Figure 3). At stage I, the polycrystalline Ni presented clearly at the top of single crystalline diamond substrate. As the increase slowly of temperature (below 500 ℃), the recrystallization process is in a dominant position. Those small Ni grains have enough time to grow up and rearrangement gradually in stage Ⅱ. When the temperature continues rise to 800 ℃ (stage Ⅲ), the solid solution reaction occurs violently. The Ni atoms dissolve into the diamond which exactly destroys the C-C bond with sp3 hybrid. Conversely, the carbon atoms can quickly pass through the nickel grain boundaries. And then few layer graphene appears on the surface in the traces behind by the migrating Ni crystals. With the extension of heating time, more sp3-C atoms transform into sp2-C on the Ni surface, the transformed graphene gradually becomes graphite. However, the graphitization is not endless. The annealing time and the catalyst thickness determine the number of graphene layers. At stage Ⅳ, the graphitization behavior ends due to the catalyst Ni losing their active, leaving lots of NiCx carbides at the interface of diamond/Ni. The entire reaction process can be summarized as:To figure out the chemical stoichiometric ratio of NiCx solid solution at interface, a quantitative analysis of EELS spectrum imaging acquired in dual range mode was presented in Figure S3. According to the ADF BF image of interface (Figure S3a), EELS line scanning and mapping images were exhibited in Figure S3b and Figure S3c. Figures S3d and S3e were elements mapping images of carbon and nickel, respectively. A small amount of oxygen element as an impurity was also detected at the entire interface from two samples (Figures S3b, S3f and S3h). The stoichiometric ratio of NiCx was deduced in the range of 2.5~3 from fitting the signal peak intensity of sp3-carbon, sp2-carbon, oxygen and nickel (Figure S3f). Similarly, the Ni/C atomic ratio is gradually changed at the interface of another sample that has undergone rapid annealing treatment (Figures S3g and S3h). 4. ConclusionDirect observation of diamond graphitization with Ni catalyst was performed using in-situ TEM technology. The recrystallization of polycrystalline metal layers occurs firstly in the early stage of heating. Then at higher temperatures, the diffusion of carbon and migration of metal crystals caused a transformation of the underlying diffused carbon atoms to single- or multilayer graphene. Therefore, the mechanism of diamond graphitization is metal-catalyzed solid state transformation. Additionally, the diffusion path of carbon atoms will be greatly affected by annealing condition, which determines the location of transformed graphene. High-quality graphene on diamond structure with good surface conductivity was obtained after rapid thermal annealing treatment. This would be open a new path to the application of diamond in electronic devices or all carbon sp2-sp3 technologies.Acknowledgment I would like to thank qingsong Deng from Beijing University of Technology and Jing Xia from Technical Institute of Physics and Chemistry, CAS. This article was Supported by . References1Graphitization and plastic deformation of diamond. nature (1958).2Konov, V. I. Laser in micro and nanoprocessing of diamond materials. Laser & Photonics Reviews 6, 739-766, doi:10.1002/lpor.201100030 (2012).3Wang, J., Zhang, G., Chen, N., Zhou, M. & Chen, Y. A review of tool wear mechanism and suppression method in diamond turning of ferrous materials. The International Journal of Advanced Manufacturing Technology 113, 3027-3055, doi:10.1007/s00170-021-06700-8 (2021).4Yu, J., Liu, G., Sumant, A. V., Goyal, V. & Balandin, A. A. Graphene-on-diamond devices with increased current-carrying capacity: carbon sp2-on-sp3 technology. 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(2011).17Li, S. et al. Enhanced electron field emission properties of diamond/microcrystalline graphite composite films synthesized by thermal catalytic etching. Applied Surface Science 367, 473-479, doi:10.1016/j.apsusc.2016.01.195 (2016).18Furthmuller, K. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set.  Physical review. B, Condensed matter 54, 11169-11186. (1996).19Perdew JP, B. K., Ernzerhof M. Generalized Gradient Approximation Made Simple. Physical Review Letters 77, 3865-3868 (1998).20Blochl. Projector augmented-wave method. Physical review. B, Condensed matter 50, 17953-17979. (1994).21Ralchenko, V. et al. Structure and properties of high-temperature annealed CVD diamond. Diamond and Related Materials 12, 1964-1970, doi:10.1016/s0925-9635(03)00214-0 (2003).22Malard, L. M., Pimenta, M. A., Dresselhaus, G. & Dresselhaus, M. S. Raman spectroscopy in graphene. Physics Reports 473, 51-87, doi:10.1016/j.physrep.2009.02.003 (2009).23Cancado, L. G. et al. Quantifying defects in graphene via Raman spectroscopy at different excitation energies. Nano Lett 11, 3190-3196, doi:10.1021/nl201432g (2011).Figure 1 Video montage of diamond/Ni interface taken from the in-situ TEM during heating process. (a)-(d) the recrystallization of Ni (e)-(h) the appearance of graphene. The heating rate is 10 ℃/min and the highest temperature is up to 820 ℃.Figure 2 Video montage of diamond graphitization taken from the in-situ heating TEM and the GOD structure after rapid annealing treatment. (a)-(d) the process of diamond graphitization. (e) the initial interface of diamond/Ni and (f) the GOD structure at the interface. (g) zoom-in image of white dotted rectangle in (f). The heating and cooling rates are 30 ℃/s and the maximum temperature is up to 800 ℃ for 1 min.Figure 3 Schematic depiction of two different thermal treatment for diamond graphitization and the formation of GOD structure, respectively. Figure 4 Thermodynamic and kinetic analysis of the Ni-catalyzed transformation from diamond to graphene. (a) calculation of Enthalpy change. (b) the diffusion energy barrier of carbon atoms in solid solution. IS=Initial State, MS=Middle state, TS=Transition State, FS=Final State. (∆E= Enthalpy change, grey = C, blue = Ni, orange = mobile C atoms).