| 11. |
- Yao, Mingguang, et al.
(författare)
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Electrical transport properties of Na2C60 under high pressure
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 80:11, s. 115405-
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Tidskriftsartikel (refereegranskat)abstract
- We present direct resistance measurements on Na2C60 under high pressure up to 2 GPa in the temperature range ~90–500 K. We show that fresh, nominally pure Na2C60 exhibits a semiconductor-type conduction behavior at all experimental conditions, with a 1.2 eV band gap and a conduction strongly influenced by defect gap states. Contrary to Rb4C60 and Na4C60, both band gap and gap states have large pressure dependences. In contrast, a Na2C60 sample preserved in an oxygen-free environment for several months shows conduction by another mechanism, variable range 3D hopping. This is consistent with recent findings of nanoscale phase segregation in Na2C60 and can be understood in terms of conduction by intergrain hopping. Heating the sample at 2 GPa leads to a significant change in the resistivity, suggesting a structural transition. By characterizing the sample after the phase transformation with Raman spectroscopy and x-ray diffraction we identify a phase separation of the material into two main structures, linearly polymerized orthorhombic C60 and two-dimensionally polymerized Na4C60.
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| 12. |
- Yao, Mingguang, et al.
(författare)
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Raman signature to identify the structural transition of single-wall carbon nanotubes under high pressure
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 78:20
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Tidskriftsartikel (refereegranskat)abstract
- Raman spectra of single-walled carbon nanotubes (SWNTs) with diameters of 0.6–1.3 nm have been studied under high pressure. A “plateau” in the pressure dependence of the G-band frequencies was observed in all experiments, both with and without pressure transmission medium. Near the onset of the G-band plateau, the corresponding radial breathing mode (RBM) lines become very weak. A strong broadening of the full width at half maximum of the RBMs just before the onset of the G-band plateau suggests that a structural transition starts in the SWNTs. Raman spectra from SWNTs released from different pressures also indicate that a significant structural transition occurs during the G-band plateau process. Simulations of the structural changes and the corresponding Raman modes of a nanotube under compression show a behavior similar to the experimental observations. Based on the experimental results and the theoretical simulation, a detailed model is suggested for the structural transition of SWNTs, corresponding to the experimentally obtained Raman results in the high-pressure domain.
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| 13. |
- Yao, Mingguang, et al.
(författare)
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Reversible pressure-driven nanoscale phase separation in Rb4C60
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 79:8, s. 081403-
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Tidskriftsartikel (refereegranskat)abstract
- Wepresent Raman spectra for Rb4C60 at pressures up to 13GPa. Above 1 GPa we observe a phase separation ofRb4C60 into Rb6C60 and Rb3C60, identified by a splitting ofthe Ag(2) mode and the appearance of characteristic Raman modesof the daughter phases. Remarkably, the phase separation is completelyreversible and the sample returns to a homogeneous Rb4C60 phaseafter pressure release. The reversibility of the phase separation suggeststhat the structural change is associated with small displacements ofRb ions in the unit cell, giving local differences inthe charge state of the C60 molecules. The pressure dependenceof the Raman shifts agrees well with earlier measurements. Ourobservation of a reversible phase separation explains several peculiar resultsreported earlier for Rb4C60.
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| 14. |
- Yao, Mingguang, et al.
(författare)
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Synthesis and characterization of SWCNTs with Ho/Ni as catalyst
- 2006
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Ingår i: New Carbon Materials. - 1007-8827. ; 21:1, s. 70-74
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Tidskriftsartikel (refereegranskat)abstract
- SWCNTs were synthesized with high yield by DC arc discharge using Ho/Ni as catalysts. The morphologies, diameter distribution and the content of SWCNTs in the products were characterized by SEM, Raman scattering and TGA. Results indicate that SWCNTs can be efficiently synthesized with Ho/Ni as catalysts. The volume content of SWCNTs in a "web-like" form is as high as 80% in the product. Furthermore, the diameter distribution of SWCNTs, estimated by analyzing the resonance Raman scattering using excitation wavelengths of 632 and 488nm, varies from 1.35 to 1.69nm with a dominant diameter of 1.5nm, which is different from that of the SWCNTs synthesized with Ce/Ni as catalyst, proving that the diameter distribution of SWCNTs is dependent on the properties of the metal catalysts.
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| 15. |
- Yao, Mingguang, et al.
(författare)
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Synthesis and growth mechanism of differently shaped C60 nano/microcrystals produced by evaporation of various aromatic C60 solutions
- 2009
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Ingår i: Carbon. - Amsterdam : Elsevier. - 0008-6223 .- 1873-3891. ; 47:4, s. 1181-1188
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of the synthesis of C60 nano- and microrods as well as crystals with normal shapes by the evaporation of C60 solutions based on different aromatic solvents. C60 nano- and microrods are grown with high yield by vaporizing C60 solutions in meta-isomers of aromatic solvents on different substrates while para-isomers give a different type of growth leading to highly crystalline two- and three-dimensional nano- and microcrystals with fcc structure. The role of solvent properties was investigated by using positional isomers containing different halogen radicals. The as-grown crystal rods formhexagonal structures but transform into fcc structure on annealing in vacuum. IR and EDX analysis indicate that solvents remain in the hexagonal nano- and microrods, while it is hard to detect any trace of solvents in the two- or three-dimensional nano- and microcrystals. Furthermore, we present direct proof of the nucleation-growth mechanism for C60 rods. By the vaporization method, the solubility of C60 in the studied halogen aromatic solvents is found to correlate with the diameter distribution of the C60 rods and we suggest that the chemical affinity of the aromatic solvent molecules to C60 may determine its ability to force C60 to form rod-shaped crystals.
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| 16. |
- Yao, Mingguang, et al.
(författare)
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Synthesis of single-wall carbon nanotubes and long nanotube ribbons with Ho/Ni as catalyst by arc discharge
- 2005
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Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 43:14, s. 2894-2901
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Tidskriftsartikel (refereegranskat)abstract
- The effect of a new bimetallic catalyst Ho/Ni for synthesis of single-walled carbon nanotubes (SWNTs) by arc discharge has been studied. Long ribbons consisting of roughly-aligned SWNT bundles were obtained by a modified arc discharge apparatus. Ribbon lengths can reach as much as 20 cm. Both elements Ho and Ni play important roles in the synthesis of SWNTs with high yield and purity. Changes in the Ho and Ni concentration in the catalyst hardly affect the diameter distribution of SWNTs, but the yield and purity of SWNTs are very sensitive to the concentration. An optimal range of Ho/Ni compositions for synthesis of SWNTs with relatively high purity and yield is given.
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| 17. |
- Zou, Yonggang, et al.
(författare)
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Effective polymerization of C60 in SWNTs under high pressure and simultaneous UV light irradiation
- 2007
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Ingår i: Acta Physica Sinica (Wuli Xuebao). - 1000-3290. ; 56:9, s. 5172-5175
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Tidskriftsartikel (refereegranskat)abstract
- The C-60 @ SWNT ( peapod) samples were prepared by the vapor diffusion method. Polymerization of C60 molecules in single walled carbon nanotubes (SWNTs) under high pressure and simultaneous irradiation of UV laser (325 nm) has been carried out for the first time by using diamond anvil cell. Raman spectra of the peapod samples decompressed from high pressure indicated that C60 molecules form one-dimensional orthorhombic polymer in SWNTs under UV laser irradiation at a high pressure of 21.5 GPa, which is lower than that for the polymerization of samples induced by high pressure only. The polymerization is an irreversible phase transition in the peapod.
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| 18. |
- Zou, Yonggang, et al.
(författare)
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Raman spectroscopy study of carbon nanotube peapods excited by near-IR laser under high pressure
- 2007
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Ingår i: Physical Review B. - 0163-1829. ; 76:19, s. 195417-
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Tidskriftsartikel (refereegranskat)abstract
- A resonant Raman spectroscopy study has been carried out under high pressure, using a diamond anvil cell,on carbon nanotube peapods (C60@SWNT) synthesized in high yield in our laboratory. The Raman signal wasexcited by a near IR laser (830 nm) to avoid photopolymerization of C60 and thus obtain the intrinsic vibrationalinformation on the C60 molecules in the nanotubes. It is found that the surrounding tubes create aneffective pressure on the encapsulated C60 due to tube-fullerene interactions, resulting in a shift of the intrinsicAg(2) vibrational mode to 1474 cm−1 at ambient pressure. High pressure Raman spectroscopy indicates that(C60)2 dimers form near 1 GPa, and that a further polymerization of C60 occurs near 23 GPa, creating linearchains of covalently linked C60 molecules in the tubes.
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| 19. |
- Zou, Yonggang, et al.
(författare)
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Rotational dynamics of confined C60 from near-infrared Raman studies under high pressure
- 2009
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 106:52, s. 22135-22138
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Tidskriftsartikel (refereegranskat)abstract
- Peapods present a model system for studying the properties of dimensionally constrained crystal structures, whose dynamical properties are very important. We have recently studied the rotational dynamics of C60 molecules confined inside single walled carbon nanotube (SWNT) by analyzing the intermediate frequency mode lattice vibrations using near-infrared Raman spectroscopy. The rotation of C60 was tuned to a known state by applying high pressure, at which condition C60 first forms dimers at low pressure and then forms a single-chain, nonrotating, polymer structure at high pressure. In the latter state the molecules form chains with a 2-fold symmetry. We propose that the C60 molecules in SWNT exhibit an unusual type of ratcheted rotation due to the interaction between C60 and SWNT in the “hexagon orientation,” and the characteristic vibrations of ratcheted rotation becomes more obvious with decreasing temperature.
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