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Tailoring Building Blocks and Their Boundary Interaction for the Creation of New, Potentially Superhard, Carbon Materials Mingguang Yao, Wen Cui, Mingrun Du, Junping Xiao, Xigui Yang, Shijie Liu, Ran Liu, Fei Wang, Tian Cui, Bertil Sundqvist, and Bingbing Liu* The synthesis of new incompressible, superhard materials with desirable mechanical properties and structures is always an important topic in both materials science and condensed matter physics because of their wide applications in various fields. Several recent breakthroughs in this research field include the synthesis of superhard polycrystalline materials composed of nanostructured building blocks, in some cases even harder than diamond.[1–7] Recent developments include the synthesis of polycrystalline diamond consisting of diamond nanocrystals, aggregate diamond nanorods, and boron nitride nanocomposites. For C-BN superhard structures it has been found that the size of nanocrystals and the strong covalent bonding at the nanocrystal grain boundaries determine the properties of the bulk samples synthesized.[5–7] A polycrystalline material containing 3.8 nm twin nanocrystals has been reported to have a hardness even higher than that of diamond and a high thermal stability.[7] All studies show that the characteristics of the building blocks and their grain boundary interactions are both critical factors for the properties of the superhard materials synthesized. Furthermore, amorphous superhard materials have attracted intense research interest due to their many advantages compared to crystalline hard materials, such as excellent isotropic hardness, elasticity, lubricity, etc. Previous literature shows that some amorphous carbon-based phases might be superhard,[8–13] thanks to the strong covalent bonding ability of carbon. In particular, nanostructured fullerene-like fragments have been suggested to be important components on the construction of amorphous superhard materials. For example, high pressure and high temperature treated C60 solids are spectroscopically amorphous, show a high hardness and can scratch the diamond surface.[8] However, it is difficult to obtain clear information about the microstructures and the formation mechanism of these amorphous hard/superhard phases, and thus the synthesis in a controllable way remains challenging. Prof. M. Yao, Dr. W. Cui, Dr. M. Du, J. Xiao, Dr. X. Yang, Dr. S. Liu, R. Liu, F. Wang, Prof. T. Cui, Prof. B. Sundqvist, Prof. B. Liu State Key Laboratory of Superhard Materials Jilin University Changchun 130012, China E-mail: [email protected] Prof. B. Sundqvist Department of Physics Umeå University S-901 87 Umeå, Sweden
DOI: 10.1002/adma.201500188
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Besides these crystalline and amorphous structures, a recent striking report shows that when compressed m-xylene intercalated C60 crystals C60 molecules collapse and transform into amorphous carbon clusters but still stay in their positions in the lattice and preserve the long range order (i.e., ordered amorphous carbon cluster, OACC).[14] The resulting structure is incompressible and hard enough to indent diamond anvils at high pressure. However, such transformation has only been reported in m-xylene intercalated fullerenes, and it is unknown whether other dopants can promote analogous structure transitions or not and what are the important features of dopants for driving such transformations. This is a very important and fundamental factor for exploring possible OACC family. On the other hand, from this study we propose a new strategy for constructing superhard hybrid structure, i.e., amorphous carbon clusters could be potential hard building blocks (HBBs), play roles similar to hard nanocrystals in polycrystalline nanodiamond/c-BN. In this case the boundary interactions of the amorphous HBBs on the structures and properties of the formed hybrid structures are crucial. Such amorphous HBBs would be beneficial for the creation of new hard/superhard structures with additional advantages compared to those with a crystalline basis, expanding the possible applications. In this work, we report the synthesis of hybrid carbon structures by compressing a series of intercalated fullerides, predesigned by selecting various dopants with special features. In the formed hybrid structures, both collapsed C60 and C70 clusters can act as HBBs, suggesting that the HBBs size might be controlled by using different sized fullerenes in the starting materials. More importantly, the dopants tune the boundary interactions of HBBs and affect the structures and compressive behaviors of the formed hybrid structures. Irrespective of the arrangement of HBBs, the phases synthesized can be ultraimcompressible and hard enough to create indentations in the diamond anvils, when the dopants have a π-aromatic carbon ring configuration that can promote π-electron rehybridization between HBBs and dopants. This provides a way for the creation of new hard/superhard materials with desirable properties. The dopants studied include three aromatic solvents with different radicals on the hexagonal carbon ring, ferrocene (Fc, having no π-aromatic carbon ring but showing charge transfer to the fullerene molecules) and NiOEP (large size).[15–19] The five as-prepared intercalated C60 crystals (see the Experimental Section) have been compressed up to 45(±5) GPa to synthesize new hybrid structures. Their Raman spectra at ambient conditions are shown in Figure 1. The spectra of the m-dichlorobenzene (m-Cl)/C60, m-xylene/C60, and 1,2,4-trimethylbenzene
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(TMB)/C60 samples show Raman signals only from C60 molecules, while the spectra of Fc/C60 and NiOEP/C60 contain Raman signals from both the intercalant molecules and C60. In situ high pressure Raman spectra have been recorded for all five samples to study the transformations of the materials under pressure. The results show that all Raman peaks originating from C60 molecules in all samples become broad and diffuse at 15–18 GPa, above which pressure only two broad bands at 600–800 and 1000–1700 cm−1 survive, suggesting that C60s lose their molecular features and undergo amorphization. To understand the interaction between the dopant and C60 molecules, we plotted the frequencies for all the Raman modes of C60s in the five samples as functions of pressure below the amorphization transition. We found that each mode showed a quite similar pressure evolution in all samples but with different pressure coefficients depending on the intercalated dopants. As we know, the Raman modes of C60, especially the high frequency Ag and Hg modes, are sensitive to the charge transfer from dopant to C60 and to the covalent bonds formed between C60s.[19,20] Figure 1d shows the frequency of the most characteristic Ag2 mode as a function of pressure. For the three samples intercalated with aromatic molecules, the pressure coefficients for each corresponding Raman mode are similar and very close to that of pristine C60, indicating little or no charge transfer or polymerization below the transition pressure. For the other two samples, the pressure coefficients are larger. The difference in the pressure coefficients of the Raman
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modes suggests different interactions between C60s and the dopants under pressure, which can be related to the difference in the size and properties of the dopants. For example, in Fc/ C60 crystal, Fc has a relatively strong electron donating ability and can transfer an increasing amount of charge to C60 molecules as pressure increases, favoring the polymerization of C60 and thus resulting in larger pressure coefficients of the high frequency Ag and Hg modes.[19] Upon decompression, indentations were observed in the diamond anvils after compressing m-Cl/C60, m-xylene/C60, and TMB/C60 to pressures of around 42 GPa. Figure 2 shows typical optical images of diamond anvils with ring-like and arc-like crack (marked with arrows) indentations after compression. Our atomic force microscope (AFM) measurements (Figure S2, Supporting Information) show that the ring indentation exhibits a clear step of 7–11 nm in depth at the indentation edge, while arc-like cracks follow the original boundary of the sample loaded in the gasket and have caniniform-like features. As far as we know, ring cracks have only been observed when a diamond anvil is indented by another superhard material, such as an opposing beveled diamond anvil. Similar ring crack indentations have also been observed in the diamond anvils after compressing graphite (15–20 GPa) due to the formation of superhard carbon phases,[21] which has been recently confirmed by theoretical simulations[22] to be a superhard phase. Other indentations such as plough-like indentations/ cracks have also been observed on the diamond anvils when
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Figure 2. Typical optical images of diamond anvils with ring indentations created by the high pressure phases of the solvated fullerenes. A) Full image of the anvil culet and B) amplified image of the culet taken with light from backside.
a superhard phase was formed after the application of a shear deformation on mixed CS2/C60s.[23] In addition, it should be noted that such regular indentations are different from the damage created on diamond anvils due to the cumulative effect[24] and the effect of steel gasket and asymmetrical stress distribution.[25,26] These features strongly suggest the formation of superhard phases in the whole samples under high pressure, which can be further supported by their ultra-incompressibility (see below). In contrast, no indentations were created in the anvils in the case of compressing pure C60, Fc/C60 and NiOEP/ C60 up to similar or even higher pressure (for example, 55GPa for NiOEP/C60), suggesting a much smaller hardening of these materials upon compression. The broad Raman bands of the high pressure phases are preserved in the spectra of the samples released from high pressure (see Figure 3), indicating that the high pressure phases of the samples are quenchable to ambient conditions. Except for the spectra of recovered NiOEP/C60, all the Raman spectra show the typical features of “amorphous carbon”, with one broad and asymmetric band ranging from 1000 to 1750 cm−1 and another weak and broad one at 600–750 cm−1. It is noted that the most intense peak of the high frequency band occurs at 1470 and 1545 cm−1 when the excitation laser wavelength is 830 and 514 nm, respectively. These Raman bands have been assigned to the vibration modes of the C C/C C bonds of fullerene/ fullerene fragments (containing hexagonal and pentagonal carbon rings) in the materials, characteristic for mixed sp2 and sp3 bonded carbon material.[27–29] This suggests that sp3 carbon bonds were created in the released materials during compression. The band at 600–750 also originates from vibrations in fullerene or fullerene-like fragments.[14,29] These results suggest that although C60 molecules are amorphized, fullerenelike fragments are preserved. Note that the “amorphized C60s” here refers to deformed/collapsed C60s which have lost the molecular symmetry but are mediated by the surrounding solvent molecules. Unlike 3D polymerized C60s, our laser heating or heat treatment at above 500 °C does not recover intact C60 molecules in the released samples. It should be noted that due to the H/Cl-terminated edge on the aromatic molecules intercalated in the C60 crystals, the highly compressed or collapsed fullerene molecules should be positionally constrained by the
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surrounding dopants, which affects the bonding and polymerization of the compressed/collapsed fullerene molecules. This is different from that of compressing pure C60, in which highly compressed or collapsed molecule can form intermolecular
Figure 3. I) Raman spectrum of Fc/C60 decompressed from 46 GPa; II,III) representative spectra of m-xylene/C60, mCl/C60, and TMB/C60 decompressed from above 41 GPa, measured using a 830 nm laser and a 514 nm laser, respectively. Only one representative spectrum measured at each laser wavelength is shown for these samples because of the very similar Raman features. Bottom two curves show Raman spectra of NiOEP/C60 decompressed from IV) 44 GPa and V) 55 GPa.
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covalent bonds with its neighbors resulting in the formation of large dense carbon clusters that may reconstruct and transform into heterogeneous mixtures of carbon polytypes.[30] In the spectra from the recovered NiOEP/C60 the observed Raman peaks are mainly from the vibrations of intact NiOEP molecules while the broadbands at 1500 and 600–800 cm−1 are from amorphized C60s. The Raman vibrations from NiOEP can still be detected in samples decompressed from high pressure of 55 GPa, suggesting the high stability of the intercalated molecules under pressure. The results clearly show that all the released samples consisted of compressed/collapsed fullerene units. The IR spectra of the released samples were recorded to study whether the intercalated molecules were preserved or not after the compression (Figure 4). It is clear that IR signals from the dopants are present in the spectra, while a broadband with frequencies in the range of 1000–1750 cm−1 in the spectra is from the amorphized C60. As was also found for decompressed m-xylene/C60,[14] the IR signals of TMB and m-Cl were clearly observed in the released samples, which suggests the preservation of the dopants. The preservation of NiOEP in the decompressed sample has been confirmed by Raman measurements shown above. In the released Fc/C60 sample only IR signals of C H and very weak and broad signals from Fc were observed, which might indicate that Fc was partially decomposed. The results show that most intercalated species are preserved while the C60 molecules are amorphized. Thus, it is reasonable that the dopants still act as “spacers” in the samples to preserve the highly compressed and collapsed C60 molecules as building blocks. To further investigate the arrangement of the “amorphized” C60 units (considered as building blocks) in the released samples, we used transmission electron microscopy (TEM) in combination with selected area electron diffraction (SAED) to study the quenched samples. TEM observations show that the three samples which left indentation in the diamond anvils contain a number of disordered phases and a small amount of ordered phases. Figure 5a shows a TEM image of a released sample which contains ordered structures and the inset shows a small area in greater magnification. Besides the ordered building
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Figure 4. IR spectra of decompressed samples with different dopants. I) m-Cl/C60 from 43 GPa; II) TMB/C60 from 40.7 GPa; III) m-xylene/C60 from ref.[14]; and IV) Fc/C60 from 46 GPa.
blocks, randomly distributed building blocks can also be seen in the disordered area (Figure 5c). It should be noted that no graphite-like structures have been observed in the samples, in contrast to results for pure C60 quenched from even lower pressure,[30] further supporting that the collapsed C60s are stabilized by the dopants. A detailed TEM analysis of the released m-xylene/C60, m-Cl/C60, and TMB/C60 samples showed that the ordered phase in these samples can be indexed by the structures of the corresponding pristine materials, which has also been confirmed by SAED results obtained from the ordered phase of the samples (for example, Figure 5b). Furthermore, for the ordered phases observed in this study we can also find some areas that exhibit clear lattice fringes while the profile of the cell units cannot be clearly distinguishable, as shown in Figure 5d. These inhomogeneous cell units might contain both highly compressed C60 cages and collapsed C60 clusters (simulated image shown in the inset), which may cause the broad diffraction rings observed in the corresponding diffraction patterns.[14] It should also be mentioned that the samples were compressed in the experiments without using a pressure medium and the possible pressure distribution in the chamber might affect the arrangement of the amorphous HBBs in the sample. Irrespective of the arrangement of the HBBs in the formed phases, the samples always transformed into phases that were hard enough to create indentation in the anvils in all experiments on aromatic solvent intercalated fullerenes compressed to above ≈42 GPa. We also decompressed a sample of m-Cl/C60 from a pressure (35.4 GPa) slightly lower than the transition pressure into the phase that was able to create indentation in the diamond anvils and studied the structures of the quenched sample (Figure 6). It was found that a large amount of well crystallized structure containing clear and ordered building blocks was preserved in the released sample and that most of the building blocks are well separated. The recorded Raman spectrum from the sample shows that both amorphized and unchanged C60 molecules exist in the material, but that the retained C60s may still be highly strained since a large downshift of the Ag2 modes (from starting 1469 to 1429 and 1456 cm−1) and significant weakening/disappearance of other modes were observed. The results further support the idea that the intercalated dopant can enhance the stability of the lattice arrangement of the highly compressed and collapsed fullerenes, which has also been observed in the in situ X-ray diffraction (XRD) studies (see Figure S3, Supporting Information, and Ref. [14]). For comparison, we also performed high pressure experiments on m-xylene solvated C70 crystals. In situ measurements suggested that C70 molecules undergo amorphization above 18 GPa, lose their molecular features and transform into carbon clusters. Furthermore, m-xylene/C70 was also transformed into a phase that is hard enough to leave ring indentation in the anvils at above 42 GPa,[31] which was similar to that of m-xylene/C60. This indicates that highly compressed and collapsed C70s behave in a way similar to that of amorphous C60 hard clusters. Both fullerenes act as hard building blocks for superhard phases in spite of the significant differences between the C60 and C70 cages; it is well known that C70 is reluctant to form polymerized structures under high pressure,[20] and that part of the amorhization is reversible such that molecules can
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Figure 5. High-resolution TEM images of the decompressed samples. Aromatic solvents/C60 samples of m-xylene/C60 and m-Cl/C60 decompressed from 42 GPa, showing that the visibly resolved HBBs are organized in ordered (a,b) and disordered structures (c). The insets in (a) and (c) depict a structure simulation output as colored carbon clusters superimposed upon the scaled image of the observed HBBs arranged in the structure of the samples quenched from high pressure; intermolecular bonding and solvent molecules have been omitted. b) The electron diffraction pattern of (a) shows diffraction spots from a crystalline structure and also weak diffraction rings, which can be assigned to the starting hexagonal structure. d) TEM image from the ordered areas of the quenched structure shows that the profile of the contained HBBs are not distinguished due to the collapse/ deformation of C60 (inset simulates a sketch map for the HBB arrangement in a hexagonal structure).
Figure 6. An m-Cl/C60 sample decompressed from 35.4 GPa. a) Raman spectrum and b) high-resolution TEM image.
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the rigid C60 molecules by π electron rehybridization. Such a covalently bonded 3D network can exhibit excellent mechanical properties. In this case, the dopant molecules with π-aromatic rings act not only as a spacer to stabilize the C60 units but also as a linker between the collapsed C60 units to construct strong HBB boundaries and make the structures superhard. Our XRD measurements on the samples that left indentations in the diamond anvils show that after the transformations all their high pressure phases become highly incompressible, evidenced by the nearly constant lattice parameters as pressure increases (Figure S3, Supporting Information). This indicates that such carbon phases have extremely low compressibilities. In contrast, Fc and NiOEP both contain ionic carbon–metal (Fe or Ni in the molecules) bonding and such ionic bonds usually result in low bond-bending force constants (for example, the Fc molecules also partially break down under pressure). When they act as a unit boundary mediator of HBBs this leads to a low shear modulus and hardness. Moreover, the large size of NiOEP may prevent the intermolecular bonding of highly compressed/collapsed C60s in NiOEP/C60 due to spatial separation, and thus fewer covalent bonds are formed in the high pressure phases. Our results further confirmed that the covalent bonding between hard C60 units and the dopants significantly improve the hardness of the phases formed under pressure. On the other hand, such dopants containing metal atoms can stabilize the ordered arrangements of amorphous fullerene clusters after compression, indicating that it is possible to select a proper dopant for the creation of new materials with excellent mechanical and electronic properties.[33] In summary, our studies show that amorphous carbon clusters can act as hard building blocks for constructing new hybrid structures with potential superhardness. Besides the formation of an ordered structure, the amorphous building blocks can also be arranged in a disordered structure, similar to the random stacking of nanocrystals (HBBs) in polycrystalline superhard material. More importantly, the boundary interactions of the formed carbon clusters are mediated by the dopants intercalated. We can control the size of the HBBs by using different sized fullerenes (C60, C70, and probably other fullerenes with different size) and tune the boundary interaction of the building blocks by using different dopants in the starting crystals, respectively. Irrespective of the arrangements of HBBs, the phases synthesized can be ultra-incompressible and hard enough to create indentations in the diamond anvils, when the intercalated dopants have π-aromatic rings which are able to promote π-electron rehybridization between HBBs and dopants under pressure. This new strategy for the creation of hybrid structures enables the building blocks and their boundaries to be tuned for various properties. Thus it becomes possible to synthesize superhard materials with desirable structural, electronic, and optical properties when suitable components of starting materials are selected.
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be recovered even after high pressure compression to 50 GPa. In addition, due to the good separation of C70 molecules from m-xylene molecules and the unique molecular morphology of C70s, we found that the quenched m-xylene/C70 sample preserved a high percentage of ordered phases, as confirmed by XRD and TEM measurements. It is also interesting to mention that TEM observations on the released Fc/C60 sample (Figure S4, Supporting Information) show that no visible lattice fringe can be found, while in some areas clear though weak electron diffraction spots from the material can be collected. The d-spacings calculated from the SAED patterns also fit well with those of the starting triclinic structure of pristine Fc/C60 crystals.[19] This suggests the preservation of a periodic arrangement of cell units in the released sample, while the lack of distinguishable lattice fringes in the TEM images indicates that the units in the lattice might be amorphous. This observation is also consistent with the amorphous features of the Raman spectra of the released samples. In contrast, TEM observations and SAED show that the NiOEP/C60 sample decompressed from 48 GPa only contains amorphous phase despite the preservation of NiOEP molecules. Our results clearly show that the highly compressed or collapsed C60s/C70s act as building blocks while their boundaries were defined by the intercalated dopants. The latter factor plays an important role for the mechanical properties of the structures formed under pressure. In fact, amorphous fullerene-like fragments have also been regarded as important components for the formation of hard/superhard structures in glassy carbon and amorphous thin carbon films.[13,32] For example, the transparent superhard carbon phase with a strength comparable to that of diamond is formed in glassy carbon,[12,13] which is promoted by the bonding of the fullerene-like fragments. However, in those materials the boundaries of the building blocks are complicated and difficult to tune. In our case, the role of the dopants on the formation mechanism of the superhard phases under pressure can be uncovered. As we know, the carbon atoms in a C60 molecule have their valences satisfied by two single bonds and one double bond, leading to an electron configuration where the outer surface is covered with π electrons and appears to be aromatic. Thus, the dopant with π-aromatic rings has a high chemical affinity to the fullerene presumably because of the strong interactions between their π-aromatic rings. The intermolecular C C distances decrease rapidly with increasing pressures according to the literature.[14] When the nearest-neighbor C C distances in C60 C60 or C60–aromatic dopant at some particular pressure become approximately the same as in an sp3hybridized C C bond (≈1.5 Å), intermolecular covalent bonds may form between C60–solvent and C60–C60. (When the size of the dopant is small, bonding between C60s is allowed in certain directions due to the incomplete separation of C60 molecules by the dopant molecules in the crystals.) Our Raman results show that C60 molecules were highly deformed, lost the cage symmetry and started to collapse at above ≈15–18 GPa, independent of the dopant used. Inelastic X-ray scattering measurements on m-xylene/C60 solvates show that almost all carbon gradually transforms into sp3 bonded C above this transition pressure,[14] confirming that C60 units can form intra/intermolecular sp3 bonding, becoming rigid building blocks. The C atoms in the dopant molecules (≈10%) may also form covalent bonds with
Experimental Section Material Fabrication and Characterization Experiment: We selected five dopants with different characteristics with which to intercalate fullerenes to synthesize different fullerides. The dopants include three aromatic
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www.MaterialsViews.com solvents with different radicals on the hexagonal carbon ring, ferrocene and Ni(II) octaethylporphine (NiOEP). The synthesis and the sample characterization have been reported previously.[15–19] In brief, saturated solutions of C60 either in m-xylene, m-dichlorobenzene, or TMB were evaporated at room temperature on a glass substrate for growing solvated C60 single crystals. Fc/C60 and NiOEP/C60 single crystals were prepared by introducing Fc or NiOEP into saturated C60/toluene solution, respectively, then adding isopropyl alcohol as precipitation agent into the solution at 10 °C and leaving for 24 h for crystals to grow. The as-prepared crystals contain solvents and crystallize in a hexagonal structure for m-xylene/C60 (a =b = 2.37 nm and c = 1.012 nm) and m-Cl/C60 (a = b = 2.37 nm and c = 1.06 nm; 90.00, 90.00, 120.00); an orthorhombic structure for TMB/C60 (a = 1.02, b = 2.05 and c = 2.56 nm); and a triclinic structure for Fc/C60 (a = 0.98, b = 1.04 and c = 1.12 nm; α = 94.9, β = 93.8, γ = 117.9) and NiOEP/C60 (a = 1.33, b = 1.11 and c = 1.08 nm; α = 106.4, β = 104.9, γ = 98.4). Crystal structures are shown in Figure S1 (Supporting Information). High Pressure Experiment: High pressure was generated by a diamond-anvil cell with culet size of 300 or 400 µm. The solvated samples were loaded into a chamber 100–120 µm in diameter drilled in a T301 stainless-steel gasket preindented to a thickness of ≈50 µm. No pressure transmitting medium was used in the experiments. Highpressure and room-temperature Raman spectra were recorded using a Renishaw 1000 notch filter spectrometer. Two exciting laser wavelengths (514.5 nm Ar+ laser and 830 nm diode laser) were used for the measurements. The R1 fluorescence emission of a ruby sphere placed in the gasket hole was used for pressure calibration. Upon decompression from around 42 GPa, the sample thickness was usually reduced to 20–25 µm. The released samples were studied by TEM equipped with electron energy loss spectroscopy (JEOL 2200FS, Japan). The infrared absorption spectrum for recovered sample was measured using a Bruker Vertex80V FTIR spectrometer. The background was carefully subtracted by taking a background at the area without sample. In situ highpressure synchrotron XRD measurements were performed at Shanghai Synchrotron Radiation Facility at room temperature.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported financially by the National Basic Research Program of China (2011CB808200), the National Natural Science Foundation of China (No. 11474121, 51320105007), the Cheung Kong Scholars Programme of China, and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1132). Received: January 13, 2015 Revised: April 5, 2015 Published online: June 2, 2015
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[3] H. Sumiya, K. Harano, Diamond Relat. Mater. 2012, 24, 44. [4] K. Tanigaki, H. Ogi, H. Sumiya, K. Kusakabe, N. Nakamura, M. Hirao, H. Ledbetter, Nat. Commun. 2013, 4, 2343. [5] V. L. Solozhenko, O. O. Kurakevych, Y. Le Godec, Adv. Mater. 2012, 24, 1540. [6] N. Dubrovinskaia, V. L. Solozhenko, N. Miyajima, V. Dmitriev, O. O. Kurakevych, L. Dubrovinsky, Appl. Phys. Lett. 2007, 90, 101912. [7] Y. J. Tian, B. Xu, D. L. Yu, Y. M. Ma, Y. B. Wang, Y. B. Jiang, W. T. Hu, C. C. Tang, Y. F. Gao, K. Luo, Z. S. Zhao, L. M. Wang, B. Wen, J. L. He, Z. Y. Liu, Nature 2013, 493, 385. [8] V. D. Blank, S. G. Buga, N. R. Serebryanaya, V. N. Denisov, G. A. Dubitsky, A. N. Ivlev, B. N. Marvrin, M. Yu. Popov, Phys. Lett. A 1995, 205, 208. [9] H. Hirai, Y. Tabira, K. Kondo, T. Oikawa, N. Ishizawa, Phys. Rev. B 1995, 52, 6162. [10] J. Robertson, Mater. Sci. Eng. R. 2002, 37, 129. [11] R. Lossy, D. L. Pappas, R. A. Roy, J. J. Cuomo, V. M. Sura, Appl. Phys. Lett. 1992, 61, 171. [12] Y. Lin, L. Zhang, H. K. Mao, P. Chow, Y. M. Xiao, M. Baldini, J. F. Shu, W. L. Mao, Phys. Rev. Lett. 2011, 107, 175504. [13] M. G. Yao, J. P. Xiao, X. H. Fan, R. Liu, B. B. Liu, Appl. Phys. Lett. 2014, 104, 021916. [14] L. Wang, B. B. Liu, H. Li, W. G. Yang, Y. Ding, S. V. Sinogeikin, Y. Meng, Z. X. Liu, X. C. Zeng, W. L. Mao, Science 2012, 337, 825. [15] L. Wang, B. B. Liu, D. D. Liu, M. G. Yao, Y. Y. Hou, S. D. Yu, T. Cui, D. M. Li, G. T. Zou, A. Iwasiewicz, B. Sundqvist, Adv. Mater. 2006, 18, 1883. [16] M. G. Yao, B. M. Andersson, P. Stenmark, B. Sundqvist, B. B. Liu, T. Wågberg, Carbon 2009, 47, 1181. [17] J. F. Geng, W. Z. Zhou, P. Skelton, W. B. Yue, I. A. Kinloch, A. H. Windle, B. F. G. JohnsonJ. Am. Chem. Soc. 2008, 130, 2527. [18] T. Ishii, N. Aizawa, R. Kanehama, M. Yamashita, H. Matsuzaka, T. Kodama, K. Kikuchi, I. Ikemoto, Inorg. Chim. Acta 2001, 317, 81. [19] W. Cui, M. G. Yao, D. D. Liu, Q. J. Li, R. Liu, B. Zou, T. Cui, B. B. Liu, J. Phys. Chem. B 2012, 116, 2643. [20] B. Sundqvist, Adv. Phys. 1999, 48, 1. [21] W. L. Mao, H. K. Mao, P. J. Eng, T. P. Trainor, M. Newville, C.-C. Kao, D. L. Heinz, J. Shu, Y. Meng, R. J. Hemley, Science 2003, 302, 425. [22] Q. Li, Y. M. Ma, A. R. Oganov, H. B. Wang, H. Wang, Y. Xu, T. Cui, H. K. Mao, G. T. Zou, Phys. Rev. Lett. 2009, 102, 175506. [23] M. Popov, V. Mordkovich, S. Perfilov, A. Kirichenko, B. Kulnitskiy, I. Perezhogin, V. Blank, Carbon 2014, 76, 250. [24] C. A. Brookes, L. Y. Zhang, Diamond Relat. Mater. 1999, 8, 1515. [25] H. K. Mao, P. M. Bell, K. J. Dunn, R. M. Chrenko, R. C. DeVries, Rev. Sci. Instrum. 1979, 50, 1002. [26] M. Popov, High Pressure Res. 2010, 30, 670. [27] A. V. Talyzin, L. S. Dubrovinsky, Phys. Rev. B 2003, 68, 233207. [28] M. G. Yao, V. Pischedda, B. Sundqvist, T. Wågberg, M. Mezouar, R. Debord, A. San Miguel, Phys. Rev. B 2011, 84, 144106. [29] M. G. Yao, W. Cui, J. P. Xiao, S. L. Chen, J. X. Cui, R. Liu, T. Cui, B. Zou, B. B. Liu, B. Sundqvist, Appl. Phys. Lett. 2013, 103, 071913. [30] M. Álvarez-Murga, P. Bleuet, G. Garbarino, A. Salamat, M. Mezauar, J. L. Hodeau, Phys. Rev. Lett. 2012, 109, 025502. [31] W. Cui, M. G. Yao, S. J. Liu, F. X. Ma, Q. J. Li, R. Liu, B. Liu, B. Zou, T. Cui, B. B. Liu, Adv. Mater. 2014, 26, 7257. [32] Q. Wang, C. B. Wang, Z. Wang, J. Y. Zhang, D. Y. He, Appl. Phys. Lett. 2007, 91, 141902. [33] D. B. Wang, A. Fernandez-Martinez, Science 2012, 337, 812.
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Tailoring Building Blocks and Their Boundary Interaction for the Creation of New, Potentially Superhard, Carbon Materials Mingguang Yao, Wen Cui, Mingrun Du, Junping Xiao, Xigui Yang, Shijie Liu, Ran Liu, Fei Wang, Tian Cui, Bertil Sundqvist, and Bingbing Liu* The synthesis of new incompressible, superhard materials with desirable mechanical properties and structures is always an important topic in both materials science and condensed matter physics because of their wide applications in various fields. Several recent breakthroughs in this research field include the synthesis of superhard polycrystalline materials composed of nanostructured building blocks, in some cases even harder than diamond.[1–7] Recent developments include the synthesis of polycrystalline diamond consisting of diamond nanocrystals, aggregate diamond nanorods, and boron nitride nanocomposites. For C-BN superhard structures it has been found that the size of nanocrystals and the strong covalent bonding at the nanocrystal grain boundaries determine the properties of the bulk samples synthesized.[5–7] A polycrystalline material containing 3.8 nm twin nanocrystals has been reported to have a hardness even higher than that of diamond and a high thermal stability.[7] All studies show that the characteristics of the building blocks and their grain boundary interactions are both critical factors for the properties of the superhard materials synthesized. Furthermore, amorphous superhard materials have attracted intense research interest due to their many advantages compared to crystalline hard materials, such as excellent isotropic hardness, elasticity, lubricity, etc. Previous literature shows that some amorphous carbon-based phases might be superhard,[8–13] thanks to the strong covalent bonding ability of carbon. In particular, nanostructured fullerene-like fragments have been suggested to be important components on the construction of amorphous superhard materials. For example, high pressure and high temperature treated C60 solids are spectroscopically amorphous, show a high hardness and can scratch the diamond surface.[8] However, it is difficult to obtain clear information about the microstructures and the formation mechanism of these amorphous hard/superhard phases, and thus the synthesis in a controllable way remains challenging. Prof. M. Yao, Dr. W. Cui, Dr. M. Du, J. Xiao, Dr. X. Yang, Dr. S. Liu, R. Liu, F. Wang, Prof. T. Cui, Prof. B. Sundqvist, Prof. B. Liu State Key Laboratory of Superhard Materials Jilin University Changchun 130012, China E-mail: [email protected] Prof. B. Sundqvist Department of Physics Umeå University S-901 87 Umeå, Sweden
DOI: 10.1002/adma.201500188
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Besides these crystalline and amorphous structures, a recent striking report shows that when compressed m-xylene intercalated C60 crystals C60 molecules collapse and transform into amorphous carbon clusters but still stay in their positions in the lattice and preserve the long range order (i.e., ordered amorphous carbon cluster, OACC).[14] The resulting structure is incompressible and hard enough to indent diamond anvils at high pressure. However, such transformation has only been reported in m-xylene intercalated fullerenes, and it is unknown whether other dopants can promote analogous structure transitions or not and what are the important features of dopants for driving such transformations. This is a very important and fundamental factor for exploring possible OACC family. On the other hand, from this study we propose a new strategy for constructing superhard hybrid structure, i.e., amorphous carbon clusters could be potential hard building blocks (HBBs), play roles similar to hard nanocrystals in polycrystalline nanodiamond/c-BN. In this case the boundary interactions of the amorphous HBBs on the structures and properties of the formed hybrid structures are crucial. Such amorphous HBBs would be beneficial for the creation of new hard/superhard structures with additional advantages compared to those with a crystalline basis, expanding the possible applications. In this work, we report the synthesis of hybrid carbon structures by compressing a series of intercalated fullerides, predesigned by selecting various dopants with special features. In the formed hybrid structures, both collapsed C60 and C70 clusters can act as HBBs, suggesting that the HBBs size might be controlled by using different sized fullerenes in the starting materials. More importantly, the dopants tune the boundary interactions of HBBs and affect the structures and compressive behaviors of the formed hybrid structures. Irrespective of the arrangement of HBBs, the phases synthesized can be ultraimcompressible and hard enough to create indentations in the diamond anvils, when the dopants have a π-aromatic carbon ring configuration that can promote π-electron rehybridization between HBBs and dopants. This provides a way for the creation of new hard/superhard materials with desirable properties. The dopants studied include three aromatic solvents with different radicals on the hexagonal carbon ring, ferrocene (Fc, having no π-aromatic carbon ring but showing charge transfer to the fullerene molecules) and NiOEP (large size).[15–19] The five as-prepared intercalated C60 crystals (see the Experimental Section) have been compressed up to 45(±5) GPa to synthesize new hybrid structures. Their Raman spectra at ambient conditions are shown in Figure 1. The spectra of the m-dichlorobenzene (m-Cl)/C60, m-xylene/C60, and 1,2,4-trimethylbenzene
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(TMB)/C60 samples show Raman signals only from C60 molecules, while the spectra of Fc/C60 and NiOEP/C60 contain Raman signals from both the intercalant molecules and C60. In situ high pressure Raman spectra have been recorded for all five samples to study the transformations of the materials under pressure. The results show that all Raman peaks originating from C60 molecules in all samples become broad and diffuse at 15–18 GPa, above which pressure only two broad bands at 600–800 and 1000–1700 cm−1 survive, suggesting that C60s lose their molecular features and undergo amorphization. To understand the interaction between the dopant and C60 molecules, we plotted the frequencies for all the Raman modes of C60s in the five samples as functions of pressure below the amorphization transition. We found that each mode showed a quite similar pressure evolution in all samples but with different pressure coefficients depending on the intercalated dopants. As we know, the Raman modes of C60, especially the high frequency Ag and Hg modes, are sensitive to the charge transfer from dopant to C60 and to the covalent bonds formed between C60s.[19,20] Figure 1d shows the frequency of the most characteristic Ag2 mode as a function of pressure. For the three samples intercalated with aromatic molecules, the pressure coefficients for each corresponding Raman mode are similar and very close to that of pristine C60, indicating little or no charge transfer or polymerization below the transition pressure. For the other two samples, the pressure coefficients are larger. The difference in the pressure coefficients of the Raman
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modes suggests different interactions between C60s and the dopants under pressure, which can be related to the difference in the size and properties of the dopants. For example, in Fc/ C60 crystal, Fc has a relatively strong electron donating ability and can transfer an increasing amount of charge to C60 molecules as pressure increases, favoring the polymerization of C60 and thus resulting in larger pressure coefficients of the high frequency Ag and Hg modes.[19] Upon decompression, indentations were observed in the diamond anvils after compressing m-Cl/C60, m-xylene/C60, and TMB/C60 to pressures of around 42 GPa. Figure 2 shows typical optical images of diamond anvils with ring-like and arc-like crack (marked with arrows) indentations after compression. Our atomic force microscope (AFM) measurements (Figure S2, Supporting Information) show that the ring indentation exhibits a clear step of 7–11 nm in depth at the indentation edge, while arc-like cracks follow the original boundary of the sample loaded in the gasket and have caniniform-like features. As far as we know, ring cracks have only been observed when a diamond anvil is indented by another superhard material, such as an opposing beveled diamond anvil. Similar ring crack indentations have also been observed in the diamond anvils after compressing graphite (15–20 GPa) due to the formation of superhard carbon phases,[21] which has been recently confirmed by theoretical simulations[22] to be a superhard phase. Other indentations such as plough-like indentations/ cracks have also been observed on the diamond anvils when
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Figure 2. Typical optical images of diamond anvils with ring indentations created by the high pressure phases of the solvated fullerenes. A) Full image of the anvil culet and B) amplified image of the culet taken with light from backside.
a superhard phase was formed after the application of a shear deformation on mixed CS2/C60s.[23] In addition, it should be noted that such regular indentations are different from the damage created on diamond anvils due to the cumulative effect[24] and the effect of steel gasket and asymmetrical stress distribution.[25,26] These features strongly suggest the formation of superhard phases in the whole samples under high pressure, which can be further supported by their ultra-incompressibility (see below). In contrast, no indentations were created in the anvils in the case of compressing pure C60, Fc/C60 and NiOEP/ C60 up to similar or even higher pressure (for example, 55GPa for NiOEP/C60), suggesting a much smaller hardening of these materials upon compression. The broad Raman bands of the high pressure phases are preserved in the spectra of the samples released from high pressure (see Figure 3), indicating that the high pressure phases of the samples are quenchable to ambient conditions. Except for the spectra of recovered NiOEP/C60, all the Raman spectra show the typical features of “amorphous carbon”, with one broad and asymmetric band ranging from 1000 to 1750 cm−1 and another weak and broad one at 600–750 cm−1. It is noted that the most intense peak of the high frequency band occurs at 1470 and 1545 cm−1 when the excitation laser wavelength is 830 and 514 nm, respectively. These Raman bands have been assigned to the vibration modes of the C C/C C bonds of fullerene/ fullerene fragments (containing hexagonal and pentagonal carbon rings) in the materials, characteristic for mixed sp2 and sp3 bonded carbon material.[27–29] This suggests that sp3 carbon bonds were created in the released materials during compression. The band at 600–750 also originates from vibrations in fullerene or fullerene-like fragments.[14,29] These results suggest that although C60 molecules are amorphized, fullerenelike fragments are preserved. Note that the “amorphized C60s” here refers to deformed/collapsed C60s which have lost the molecular symmetry but are mediated by the surrounding solvent molecules. Unlike 3D polymerized C60s, our laser heating or heat treatment at above 500 °C does not recover intact C60 molecules in the released samples. It should be noted that due to the H/Cl-terminated edge on the aromatic molecules intercalated in the C60 crystals, the highly compressed or collapsed fullerene molecules should be positionally constrained by the
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surrounding dopants, which affects the bonding and polymerization of the compressed/collapsed fullerene molecules. This is different from that of compressing pure C60, in which highly compressed or collapsed molecule can form intermolecular
Figure 3. I) Raman spectrum of Fc/C60 decompressed from 46 GPa; II,III) representative spectra of m-xylene/C60, mCl/C60, and TMB/C60 decompressed from above 41 GPa, measured using a 830 nm laser and a 514 nm laser, respectively. Only one representative spectrum measured at each laser wavelength is shown for these samples because of the very similar Raman features. Bottom two curves show Raman spectra of NiOEP/C60 decompressed from IV) 44 GPa and V) 55 GPa.
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covalent bonds with its neighbors resulting in the formation of large dense carbon clusters that may reconstruct and transform into heterogeneous mixtures of carbon polytypes.[30] In the spectra from the recovered NiOEP/C60 the observed Raman peaks are mainly from the vibrations of intact NiOEP molecules while the broadbands at 1500 and 600–800 cm−1 are from amorphized C60s. The Raman vibrations from NiOEP can still be detected in samples decompressed from high pressure of 55 GPa, suggesting the high stability of the intercalated molecules under pressure. The results clearly show that all the released samples consisted of compressed/collapsed fullerene units. The IR spectra of the released samples were recorded to study whether the intercalated molecules were preserved or not after the compression (Figure 4). It is clear that IR signals from the dopants are present in the spectra, while a broadband with frequencies in the range of 1000–1750 cm−1 in the spectra is from the amorphized C60. As was also found for decompressed m-xylene/C60,[14] the IR signals of TMB and m-Cl were clearly observed in the released samples, which suggests the preservation of the dopants. The preservation of NiOEP in the decompressed sample has been confirmed by Raman measurements shown above. In the released Fc/C60 sample only IR signals of C H and very weak and broad signals from Fc were observed, which might indicate that Fc was partially decomposed. The results show that most intercalated species are preserved while the C60 molecules are amorphized. Thus, it is reasonable that the dopants still act as “spacers” in the samples to preserve the highly compressed and collapsed C60 molecules as building blocks. To further investigate the arrangement of the “amorphized” C60 units (considered as building blocks) in the released samples, we used transmission electron microscopy (TEM) in combination with selected area electron diffraction (SAED) to study the quenched samples. TEM observations show that the three samples which left indentation in the diamond anvils contain a number of disordered phases and a small amount of ordered phases. Figure 5a shows a TEM image of a released sample which contains ordered structures and the inset shows a small area in greater magnification. Besides the ordered building
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Figure 4. IR spectra of decompressed samples with different dopants. I) m-Cl/C60 from 43 GPa; II) TMB/C60 from 40.7 GPa; III) m-xylene/C60 from ref.[14]; and IV) Fc/C60 from 46 GPa.
blocks, randomly distributed building blocks can also be seen in the disordered area (Figure 5c). It should be noted that no graphite-like structures have been observed in the samples, in contrast to results for pure C60 quenched from even lower pressure,[30] further supporting that the collapsed C60s are stabilized by the dopants. A detailed TEM analysis of the released m-xylene/C60, m-Cl/C60, and TMB/C60 samples showed that the ordered phase in these samples can be indexed by the structures of the corresponding pristine materials, which has also been confirmed by SAED results obtained from the ordered phase of the samples (for example, Figure 5b). Furthermore, for the ordered phases observed in this study we can also find some areas that exhibit clear lattice fringes while the profile of the cell units cannot be clearly distinguishable, as shown in Figure 5d. These inhomogeneous cell units might contain both highly compressed C60 cages and collapsed C60 clusters (simulated image shown in the inset), which may cause the broad diffraction rings observed in the corresponding diffraction patterns.[14] It should also be mentioned that the samples were compressed in the experiments without using a pressure medium and the possible pressure distribution in the chamber might affect the arrangement of the amorphous HBBs in the sample. Irrespective of the arrangement of the HBBs in the formed phases, the samples always transformed into phases that were hard enough to create indentation in the anvils in all experiments on aromatic solvent intercalated fullerenes compressed to above ≈42 GPa. We also decompressed a sample of m-Cl/C60 from a pressure (35.4 GPa) slightly lower than the transition pressure into the phase that was able to create indentation in the diamond anvils and studied the structures of the quenched sample (Figure 6). It was found that a large amount of well crystallized structure containing clear and ordered building blocks was preserved in the released sample and that most of the building blocks are well separated. The recorded Raman spectrum from the sample shows that both amorphized and unchanged C60 molecules exist in the material, but that the retained C60s may still be highly strained since a large downshift of the Ag2 modes (from starting 1469 to 1429 and 1456 cm−1) and significant weakening/disappearance of other modes were observed. The results further support the idea that the intercalated dopant can enhance the stability of the lattice arrangement of the highly compressed and collapsed fullerenes, which has also been observed in the in situ X-ray diffraction (XRD) studies (see Figure S3, Supporting Information, and Ref. [14]). For comparison, we also performed high pressure experiments on m-xylene solvated C70 crystals. In situ measurements suggested that C70 molecules undergo amorphization above 18 GPa, lose their molecular features and transform into carbon clusters. Furthermore, m-xylene/C70 was also transformed into a phase that is hard enough to leave ring indentation in the anvils at above 42 GPa,[31] which was similar to that of m-xylene/C60. This indicates that highly compressed and collapsed C70s behave in a way similar to that of amorphous C60 hard clusters. Both fullerenes act as hard building blocks for superhard phases in spite of the significant differences between the C60 and C70 cages; it is well known that C70 is reluctant to form polymerized structures under high pressure,[20] and that part of the amorhization is reversible such that molecules can
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Figure 5. High-resolution TEM images of the decompressed samples. Aromatic solvents/C60 samples of m-xylene/C60 and m-Cl/C60 decompressed from 42 GPa, showing that the visibly resolved HBBs are organized in ordered (a,b) and disordered structures (c). The insets in (a) and (c) depict a structure simulation output as colored carbon clusters superimposed upon the scaled image of the observed HBBs arranged in the structure of the samples quenched from high pressure; intermolecular bonding and solvent molecules have been omitted. b) The electron diffraction pattern of (a) shows diffraction spots from a crystalline structure and also weak diffraction rings, which can be assigned to the starting hexagonal structure. d) TEM image from the ordered areas of the quenched structure shows that the profile of the contained HBBs are not distinguished due to the collapse/ deformation of C60 (inset simulates a sketch map for the HBB arrangement in a hexagonal structure).
Figure 6. An m-Cl/C60 sample decompressed from 35.4 GPa. a) Raman spectrum and b) high-resolution TEM image.
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the rigid C60 molecules by π electron rehybridization. Such a covalently bonded 3D network can exhibit excellent mechanical properties. In this case, the dopant molecules with π-aromatic rings act not only as a spacer to stabilize the C60 units but also as a linker between the collapsed C60 units to construct strong HBB boundaries and make the structures superhard. Our XRD measurements on the samples that left indentations in the diamond anvils show that after the transformations all their high pressure phases become highly incompressible, evidenced by the nearly constant lattice parameters as pressure increases (Figure S3, Supporting Information). This indicates that such carbon phases have extremely low compressibilities. In contrast, Fc and NiOEP both contain ionic carbon–metal (Fe or Ni in the molecules) bonding and such ionic bonds usually result in low bond-bending force constants (for example, the Fc molecules also partially break down under pressure). When they act as a unit boundary mediator of HBBs this leads to a low shear modulus and hardness. Moreover, the large size of NiOEP may prevent the intermolecular bonding of highly compressed/collapsed C60s in NiOEP/C60 due to spatial separation, and thus fewer covalent bonds are formed in the high pressure phases. Our results further confirmed that the covalent bonding between hard C60 units and the dopants significantly improve the hardness of the phases formed under pressure. On the other hand, such dopants containing metal atoms can stabilize the ordered arrangements of amorphous fullerene clusters after compression, indicating that it is possible to select a proper dopant for the creation of new materials with excellent mechanical and electronic properties.[33] In summary, our studies show that amorphous carbon clusters can act as hard building blocks for constructing new hybrid structures with potential superhardness. Besides the formation of an ordered structure, the amorphous building blocks can also be arranged in a disordered structure, similar to the random stacking of nanocrystals (HBBs) in polycrystalline superhard material. More importantly, the boundary interactions of the formed carbon clusters are mediated by the dopants intercalated. We can control the size of the HBBs by using different sized fullerenes (C60, C70, and probably other fullerenes with different size) and tune the boundary interaction of the building blocks by using different dopants in the starting crystals, respectively. Irrespective of the arrangements of HBBs, the phases synthesized can be ultra-incompressible and hard enough to create indentations in the diamond anvils, when the intercalated dopants have π-aromatic rings which are able to promote π-electron rehybridization between HBBs and dopants under pressure. This new strategy for the creation of hybrid structures enables the building blocks and their boundaries to be tuned for various properties. Thus it becomes possible to synthesize superhard materials with desirable structural, electronic, and optical properties when suitable components of starting materials are selected.
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be recovered even after high pressure compression to 50 GPa. In addition, due to the good separation of C70 molecules from m-xylene molecules and the unique molecular morphology of C70s, we found that the quenched m-xylene/C70 sample preserved a high percentage of ordered phases, as confirmed by XRD and TEM measurements. It is also interesting to mention that TEM observations on the released Fc/C60 sample (Figure S4, Supporting Information) show that no visible lattice fringe can be found, while in some areas clear though weak electron diffraction spots from the material can be collected. The d-spacings calculated from the SAED patterns also fit well with those of the starting triclinic structure of pristine Fc/C60 crystals.[19] This suggests the preservation of a periodic arrangement of cell units in the released sample, while the lack of distinguishable lattice fringes in the TEM images indicates that the units in the lattice might be amorphous. This observation is also consistent with the amorphous features of the Raman spectra of the released samples. In contrast, TEM observations and SAED show that the NiOEP/C60 sample decompressed from 48 GPa only contains amorphous phase despite the preservation of NiOEP molecules. Our results clearly show that the highly compressed or collapsed C60s/C70s act as building blocks while their boundaries were defined by the intercalated dopants. The latter factor plays an important role for the mechanical properties of the structures formed under pressure. In fact, amorphous fullerene-like fragments have also been regarded as important components for the formation of hard/superhard structures in glassy carbon and amorphous thin carbon films.[13,32] For example, the transparent superhard carbon phase with a strength comparable to that of diamond is formed in glassy carbon,[12,13] which is promoted by the bonding of the fullerene-like fragments. However, in those materials the boundaries of the building blocks are complicated and difficult to tune. In our case, the role of the dopants on the formation mechanism of the superhard phases under pressure can be uncovered. As we know, the carbon atoms in a C60 molecule have their valences satisfied by two single bonds and one double bond, leading to an electron configuration where the outer surface is covered with π electrons and appears to be aromatic. Thus, the dopant with π-aromatic rings has a high chemical affinity to the fullerene presumably because of the strong interactions between their π-aromatic rings. The intermolecular C C distances decrease rapidly with increasing pressures according to the literature.[14] When the nearest-neighbor C C distances in C60 C60 or C60–aromatic dopant at some particular pressure become approximately the same as in an sp3hybridized C C bond (≈1.5 Å), intermolecular covalent bonds may form between C60–solvent and C60–C60. (When the size of the dopant is small, bonding between C60s is allowed in certain directions due to the incomplete separation of C60 molecules by the dopant molecules in the crystals.) Our Raman results show that C60 molecules were highly deformed, lost the cage symmetry and started to collapse at above ≈15–18 GPa, independent of the dopant used. Inelastic X-ray scattering measurements on m-xylene/C60 solvates show that almost all carbon gradually transforms into sp3 bonded C above this transition pressure,[14] confirming that C60 units can form intra/intermolecular sp3 bonding, becoming rigid building blocks. The C atoms in the dopant molecules (≈10%) may also form covalent bonds with
Experimental Section Material Fabrication and Characterization Experiment: We selected five dopants with different characteristics with which to intercalate fullerenes to synthesize different fullerides. The dopants include three aromatic
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www.MaterialsViews.com solvents with different radicals on the hexagonal carbon ring, ferrocene and Ni(II) octaethylporphine (NiOEP). The synthesis and the sample characterization have been reported previously.[15–19] In brief, saturated solutions of C60 either in m-xylene, m-dichlorobenzene, or TMB were evaporated at room temperature on a glass substrate for growing solvated C60 single crystals. Fc/C60 and NiOEP/C60 single crystals were prepared by introducing Fc or NiOEP into saturated C60/toluene solution, respectively, then adding isopropyl alcohol as precipitation agent into the solution at 10 °C and leaving for 24 h for crystals to grow. The as-prepared crystals contain solvents and crystallize in a hexagonal structure for m-xylene/C60 (a =b = 2.37 nm and c = 1.012 nm) and m-Cl/C60 (a = b = 2.37 nm and c = 1.06 nm; 90.00, 90.00, 120.00); an orthorhombic structure for TMB/C60 (a = 1.02, b = 2.05 and c = 2.56 nm); and a triclinic structure for Fc/C60 (a = 0.98, b = 1.04 and c = 1.12 nm; α = 94.9, β = 93.8, γ = 117.9) and NiOEP/C60 (a = 1.33, b = 1.11 and c = 1.08 nm; α = 106.4, β = 104.9, γ = 98.4). Crystal structures are shown in Figure S1 (Supporting Information). High Pressure Experiment: High pressure was generated by a diamond-anvil cell with culet size of 300 or 400 µm. The solvated samples were loaded into a chamber 100–120 µm in diameter drilled in a T301 stainless-steel gasket preindented to a thickness of ≈50 µm. No pressure transmitting medium was used in the experiments. Highpressure and room-temperature Raman spectra were recorded using a Renishaw 1000 notch filter spectrometer. Two exciting laser wavelengths (514.5 nm Ar+ laser and 830 nm diode laser) were used for the measurements. The R1 fluorescence emission of a ruby sphere placed in the gasket hole was used for pressure calibration. Upon decompression from around 42 GPa, the sample thickness was usually reduced to 20–25 µm. The released samples were studied by TEM equipped with electron energy loss spectroscopy (JEOL 2200FS, Japan). The infrared absorption spectrum for recovered sample was measured using a Bruker Vertex80V FTIR spectrometer. The background was carefully subtracted by taking a background at the area without sample. In situ highpressure synchrotron XRD measurements were performed at Shanghai Synchrotron Radiation Facility at room temperature.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported financially by the National Basic Research Program of China (2011CB808200), the National Natural Science Foundation of China (No. 11474121, 51320105007), the Cheung Kong Scholars Programme of China, and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1132). Received: January 13, 2015 Revised: April 5, 2015 Published online: June 2, 2015
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