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High-Pressure Tetrahedral Amorphous Carbon Synthesized By pubs.acs.org/JPCC Article High-Pressure Tetrahedral Amorphous Carbon Synthesized by Compressing Glassy Carbon at Room Temperature Lijie Tan, Hongwei Sheng, Hongbo Lou, Benyuan Cheng, Yuanyuan Xuan, Vitali B. Prakapenka, Eran Greenberg, Qiaoshi Zeng, Fang Peng, and Zhidan Zeng* Cite This: J. Phys. Chem. C 2020, 124, 5489−5494 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: Tetrahedral amorphous carbon(ta-C) thin films with high sp3 fraction have extraordinary mechanical properties and wide applications. Despite intensive effort to increase the thickness of ta-C thin films in the past decades, bulk ta-C has not been achieved until date. In this study, by compressing a sp2- bonded amorphous carbon (glassy carbon) up to 93 GPa, we demonstrate that the formation of bulk ta-C is possible at high pressures and room temperature. We studied the atomic structure, stability, and mechanical properties of the ta-C synthesized under high pressure using in situ high-pressure X-ray diffraction and large-scale first-principles calculations. The high-pressure ta-C is mainly tetrahedrally bonded, with relatively large distortions in the sp3 C−C bonds. It can be preserved to approximately 8.5 GPa during pressure release, below which it transforms to disordered glassy carbon, accompanied by sp3-to-sp2 transition. Moreover, both the experiment and simulation show that the high-pressure ta-C has a high bulk modulus (363 ± 29 GPa, experimental) even comparable to diamond. These results deepen our understanding of amorphous carbon and help guide the synthesis of novel carbon materials using high pressure. − 1. INTRODUCTION bonding transitions under pressure.10,14 16 Utilizing the effect 3 Carbon forms a great variety of crystalline and disordered of pressure, various sp -bonded carbon forms have been structures owing to its flexibility in bonding (sp1-, sp2-, and sp3- synthesized by high-pressure and high-temperature (HPHT) hybridized bonds). Among the numerous carbon allotropes, treatment of GC, for example, nanocrystalline diamond, Downloaded via SICHUAN UNIV on June 28, 2020 at 12:46:15 (UTC). 3 hexagonal diamond, and recently discovered amorphous those with a high fraction of sp bonds usually have 17−19 extraordinary mechanical properties. Therefore, carbon diamond. In addition to HPHT, pressure-induced materials with high sp3 fraction, not only in crystalline but transitions have also been observed in cold-compressed (at − 2 3 also in amorphous forms, have been extensively studied.1 3 room temperature) GC. The sp -to-sp bonding transition in See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 3 ∼ cold-compressed GC was observed in in situ high-pressure X- Amorphous carbon with very high sp fraction (up to 88%) 10 contains a high percentage of tetrahedral bonding and is ray Raman scattering experiments and in molecular dynamics fi 14,15,20 named tetrahedrally bonded amorphous carbon (ta-C).2 and rst-principles simulations. This transition was fi Currently, ta-C is only available in the form of thin films, con rmed in various experiments with dramatic changes in which are usually grown using various film deposition Raman spectrum, mechanical strength, optical properties, and 10,18,21,22 techniques involving energetic ions or plasma beams.2 The electrical properties of GC. In contrast to the typical thickness of ta-C films was reported to range from a few consistent results on the property transitions of GC mentioned to tens of nanometers.2,4,5 However, the ta-C thin films have above, the structural description of these transitions remains in extraordinary mechanical properties, chemical inertness, a state of flux. In situ high-pressure X-ray diffraction (XRD) thermal stability, and large band gap, hence wide applica- experiments, up to 47 or 51.4 GPa, revealed that GC went tions.2,4,5 Considerable efforts have been devoted to increasing through structural transition during compression without a its thickness.6,7 However, to the best of our knowledge, bulk (three dimensional) ta-C has not been achieved until date. Received: January 10, 2020 Pressure has been proved to be a powerful tool to induce Published: February 6, 2020 sp2-to-sp3 bonding transition in sp2-bonded carbon materi- − als.8 12 Glassy carbon (GC), a mostly sp2 bonded amorphous carbon allotrope,13 was also found to undergo structural and © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.jpcc.0c00247 5489 J. Phys. Chem. C 2020, 124, 5489−5494 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article pressure-transmitting medium.22,23 However, complete tran- simulation, the phase transition pressure in simulation is sition has not been observed in these experiments because of much higher than that in experiments. limited pressure ranges applied. Moreover, a recent high- pressure pair distribution function (PDF) study on the cold- 3. RESULTS AND DISCUSSION compressed GC reported that GC maintains its original Figure 1 shows representative XRD patterns of the GC sample 24 graphite-like structure up to 49.0 GPa. Therefore, a couple of during compression to 93 GPa and following decompression to intriguing questions are raised: Does a complete transition exist in cold compression? How high is the pressure required to synthesize a pure high-pressure phase of GC? How stable will the high-pressure phase be? These questions call for further experimental studies. In this work, by combining in situ high-pressure XRD up to 93 GPa and first-principles calculations up to ∼200 GPa, we observed a complete transformation of GC into high-pressure ta-C under cold compression. The atomic structure, stability, and compressibility of the ta-C formed under high pressure were thoroughly investigated. Our results demonstrate that ta- C can be synthesized at room temperature with the aid of high pressure and clarify the structure of the cold-compressed GC. 2. EXPERIMENTS AND SIMULATION In situ high-pressure XRD experiments were carried out at Figure 1. In situ high-pressure XRD of GC. XRD patterns of GC at beamline 13 ID-D of the advanced photon source (APS), representative pressures during compression up to 93 GPa and Argonne National Laboratory (ANL), USA. The X-ray has a decompression to 0 GPa. The patterns of the high-pressure phase are wavelength of 0.3220 Å and was focused down to ∼3 μm × 3 shown in blue. No pressure medium was used in the experiment in μm. A MAR165 charge-coupled device detector was used for order to generate high deviatoric stress. The X-ray wavelength is data collection, and the software DIOPTAS was used to 0.3220 Å. The dashed lines serve as a guide to the eye. The numbers integrate the two-dimensional diffraction images.25 A sym- on each pattern are the pressures where the patterns were collected. metric diamond anvil cell (DAC) with culet sizes of ∼200 μm was used. The GC sample (type I, from Alfa Aesar) with a 0 GPa. The XRD pattern of the GC shows two broad thickness of ∼40 μm was loaded into an 80 μm diameter hole characteristic peaks, the first diffraction peak (FDP) and the drilled in a Re gasket. To maximize the sample thickness for second diffraction peak (SDP), corresponding to the average better signal, no pressure medium was used in our experiments. interlayer and intralayer characteristic distances in GC, A membrane system was used to increase and decrease respectively. As shown in Figure 1, with increasing pressure, pressure. A tiny piece of Au foil was loaded along with the the FDP becomes extremely weak at 50 GPa and eventually sample as the pressure standard. The pressure was determined disappears at 81 GPa. Meanwhile, the SDP of GC seems to using the equation of state of Au.26 Background scattering become stronger during compression. These changes in the from the diamond anvils was collected before loading the XRD patterns indicate that a structural transition occurs in GC sample and subtracted to obtain XRD patterns of the sample. during compression, and the transition has completed at ∼81 First-principles calculations based on the Vienna Ab initio GPa. Therefore, at 81 GPa, we obtained the XRD pattern of Simulation Package (VASP) were performed to derive the the pure high-pressure phase for the first time, without detailed atomic structure of GC during compression to ∼200 contributions from the original GC phase. It should be noted GPa and decompression.27 The simulation method is similar to that because of the layered feature of the structure of GC, what we used to investigate the phase transition mechanism of development of a strong texture in GC could also result in GC under high pressure.19 To recapitulate, liquid carbon was intensity weakening or even disappearance of the FDP,14,23 quenched with a fixed density of 2.1 g/cm3 from 5000 to 1000 similar to the disappearance of (002) peak in uniaxially K at a cooling rate of 5 × 1013 K/s using ab initio molecular compressed graphite as a result of the strong texture. If the dynamics simulation to obtain GC. The radial distribution intensity change of the FDP was dominated by the texture, the function and the structure factor, S(q), were calculated based FDP could not re-appear because the texture once formed on a large structure model of GC with 1024 atoms. The during compression by plastic anisotropy will remain in simulations were carried out in a canonical ensemble (NVT) decompression mainly with elastic recovery. However, as with each time step representing 2 fs. The temperature was shown in Figure 1, the FDP reappears upon full decom- controlled with the Nose−́Hoover thermostat.27 The projec- pression. Therefore, we conclude that the disappearance of the tor-augmented wave potential with a valence configuration of FDP should be dominated by reversible phase transitions.
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