| 51. |
- Cheng, Benyuan, et al.
(författare)
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Pressure-induced metallization and amorphization in VO2(A) nanorods
- 2016
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 93:18
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Tidskriftsartikel (refereegranskat)abstract
- A metallic state enabled by the metal-insulator transition (MIT) in single crystal VO2(A) nanorods is demonstrated, which provides important physical foundation in experimental understanding of MIT in VO2. The observed tetragonal metallic state at ∼28 GPa should be interpreted as a distinct metastable state, while increasing pressure to ∼32 GPa, it transforms into a metallic amorphous state completely. The metallization is due to V 3d orbital electrons delocalization, and the amorphization is attributed to the unique variation of V-O-V bond angle. A metallic amorphous VO2 state is found under pressure, which is beneficial to explore the phase diagram of VO2. Furthermore, this work proves the occurrence of both the metallization and amorphization in octahedrally coordinated materials.
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| 52. |
- Chorro, Matthieu, et al.
(författare)
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Discriminated structural behaviour of C60 and C70 peapods under extreme conditions
- 2007
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Ingår i: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 79:5, s. 56003-
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Tidskriftsartikel (refereegranskat)abstract
- We studied structural changes in C60 and C70 peapods—i.e. C60 and C70 chains inserted inside single-walled carbon nanotubes—when submitted to high pressure and temperature conditions. X-ray diffraction experiments showed that while C60 molecules polymerize inside nanotubes at pressures and temperatures consistent with polymerization in bulk C60 phases, no polymerization is observed in C70 peapods, even at pressures and temperatures above the threshold for polymerization in bulk C70. Tubular confinement in the nanotube container prevents the monomers to rearrange in the way needed for polymerization. This result testifies for the strong influence of confinement on the behaviour of C70.
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| 53. |
- Cui, Jinxing, et al.
(författare)
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Structural Deformation of Sm@C88under High Pressure
- 2015
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 5
-
Tidskriftsartikel (refereegranskat)abstract
- We have studied the structural transformation of Sm@C88 under pressure up to 18 GPa by infraredspectroscopy combined with theoretical simulations. The infrared-active vibrational modes of Sm@C88 at ambient conditions have been assigned for the first time. Pressure-induced blue and red shiftsof the corresponding vibrational modes indicate an anisotropic deformation of the carbon cage uponcompression. We propose that the carbon cage changes from ellipsoidal to approximately sphericalaround 7 GPa. A smaller deformation of the carbon bonds in the area close to the Sm atom in thecage suggests that the trapped Sm atom plays a role in minimizing the compression of the adjacentbonds. Pressure induced a significant reduction of the band gap of the crystal. The HOMO-LUMOgap of the Sm@C88 molecule decreases remarkably at 7 GPa as the carbon cage is deformed. Also,compression enhances intermolecular interactions and causes a widening of the energy bands. Botheffects decrease the band gap of the sample. The carbon cage deforms significantly above 7 GPa,from spherical to a peanut-like shape and collapses at 18 GPa.
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| 54. |
- Cui, Wen, et al.
(författare)
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High pressure and high temperature induced polymerization of doped C60 materials
- 2016
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Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 109, s. 269-275
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Tidskriftsartikel (refereegranskat)abstract
- Several metastable doped C60 polymers are synthesized under high pressure and high temperature (1.5GPa, 573K and 2GPa, 700K, respectively), using C60/ferrocene (Fc, Fe(C5H5)2), C60/Ni(OEP) and C60/AgNO3 as starting materials. Raman and IR spectroscopy are used to study the polymerization of these samples after HPHT treatment. It is found that the polymerization degree is always lower than that of pure C60 treated at same conditions, which is attributed to the space limitation by the dopants. We also find that even at same conditions, the three doped materials form different polymeric phases of the doped materials. This is attributed to the unique initial lattice structures and the different degrees of spatial confinement provided by the dopants.
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| 55. |
- Cui, Wen, et al.
(författare)
-
Pressure induced metastable polymerization in doped C60 materials
- 2017
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Ingår i: Carbon. - : Elsevier. - 0008-6223 .- 1873-3891. ; 115, s. 740-745
-
Tidskriftsartikel (refereegranskat)abstract
- High pressure Raman studies have been carried out on C60/AgNO3 and C60/Ni(OEP) up to 30 GPa. In both these doped C60 materials, pressure-induced metastable ordered polymers can be observed after pressure release. The results show that both the quenched materials contain chainlike polymers and dimers. We also find that the degree of polymerization is higher in these doped C60 materials than in bulk C60 materials after similar high pressure treatment and that C60/AgNO3 contains a higher fraction of chainlike polymers than C60/Ni(OEP) after decompression from same pressure. The results can be understood by considering the different initial lattice structures of these materials and the confinement effects of the dopants.
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| 56. |
- Cui, Wen, et al.
(författare)
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Reversible pressure-induced polymerization of Fe(C5H5)(2) doped C-70
- 2013
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Ingår i: Carbon. - : Pergamon-Elsevier Science. - 0008-6223 .- 1873-3891. ; 62, s. 447-454
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Tidskriftsartikel (refereegranskat)abstract
- High pressure Raman, IR and X-ray diffraction (XRD) studies have been carried out on C-70(Fe(C5H5)(2))(2) (hereafter, "C-70(Fc)(2)") sheets. Theoretical calculation is further used to analyze the Electron Localization Function (ELF) and charge transfer in the crystal and thus to understand the transformation of C-70(Fc)(2) under pressure. Our results show that even at room temperature dimeric phase and one dimensional (1D) polymer phase of C-70 molecules can be formed at about 3 and 8 GPa, respectively. The polymerization is found to be reversible Upon decompression and the reversibility is related to the pressure-tuned charge transfer, as well as the overridden steric repulsion of counter ions. According to the layered structure of the intercalated ferrocene molecules formed in the crystal, we suggest that ferrocene acts as not only a spacer to restrict the polymerization of C-70 molecules within a layer, but also as charge reservoir to tune the polymerization process. This supplies a possible way for us to design the polymerization of fullerenes at suitable conditions.
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| 57. |
- Cui, Wen, et al.
(författare)
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Synthesis of alkali-metal-doped C60 nanotubes
- 2011
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Ingår i: Diamond and Related Materials. - : Elsevier BV. ; , s. 93-96
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Konferensbidrag (refereegranskat)abstract
- C60 nanotubes have been synthesized by a solution–solution method. After degassing in a dynamic vacuum, the C60 nanotubes were doped with alkali metals by means of vapor evaporation method. Different temperatures have been studied to evaporate the alkali metals for the doping experiments. Raman spectrum was further employed to analyze the doping concentration of the obtained samples. It was found that all three alkali metals (Li, Na and K) used can be efficiently doped into the C60 nanotubes, forming AxC60 nanotubes. The doping concentration of Li, Na changed from low to high level, depending on the experiment temperatures, while K doping always gave saturated doping. The melt points, the ionic sizes and vapor pressures of alkali metals were thought to affect the final doping results.
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| 58. |
- Danilchenko, Boris A., et al.
(författare)
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1/f noise and mechanisms of the conductivity in carbon nanotube bundles
- 2011
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Ingår i: Carbon. - : Elsevier Ltd. - 0008-6223 .- 1873-3891. ; 49:15, s. 5201-5206
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Tidskriftsartikel (refereegranskat)abstract
- Experimental results are reported of the investigation of conductivity mechanisms in metallic single-wall carbon nanotube (SWCNT) bundles in a wide temperature range from 4.2 K to 300 K. The temperature dependence of the resistance and noise parameters – the logarithmic slope of the current dependence of noise as well as the normalized current noise – are compared. Remarkable changes in noise characteristics are registered at temperatures typical of the transition from hopping conductivity to Luttinger liquid conductivity and the transition from Luttinger liquid conductivity to diffusion conductivity. In the first transition region, the slope of the normalized noise level of the current changes significantly as a function of temperature. In the region of diffusion conductivity, a stronger variation of the normalized noise level is revealed. These changes in noise properties are correlated with changes in the transport characteristics of SWCNT bundles that allow us to adequately explain the mechanisms of conductivity in the system.
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| 59. |
- Danilchenko, Boris A., et al.
(författare)
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High temperature Luttinger liquid conductivity in carbon nanotube bundles
- 2010
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Ingår i: Applied Physics Letters. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 97:7, s. 072106-
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Tidskriftsartikel (refereegranskat)abstract
- The conductance and the current-voltage characteristics of metallic single wall carbon nanotube bundles have been measured between 4.2 and 330 K using 10–30 ns electric pulses to avoid overheating. The current-voltage characteristics at different temperatures collapse to a single curve when plotted in the specific coordinates following from the Tomonaga–Luttinger (T–L) liquid concept. Direct evidence is obtained for the existence of a T–L liquid phase up to 190 K and the system shows a transition between the T–L liquid state and a Mott insulating phase below 25 K.
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| 60. |
- Danilchenko, Boris A, et al.
(författare)
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Self-heating of metallic carbon nanotube bundles in the regime of the Luttinger-liquid conductivity
- 2011
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 37:8, s. 710-717
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Tidskriftsartikel (refereegranskat)abstract
- The conductivity of bundles of carbon single-walled nanotubes with metallic conductivity (metallic nanotubes) is investigated over the wide temperature range 4.2-330 K and electrical fields up to 50 V. The usage of short electrical pulses of the duration of 10 ns allowed to avoid an influence of a self-heating of the investigated structures on current-voltage characteristics. It is shown that the temperature dependence of conductivity is described by the power function G proportional to T(alpha). At helium temperatures the asymptotic dependence of current on applied voltage is close to J proportional to V(1+alpha) with alpha = 0.45. From comparison of the obtained results of measurements with calculations, it is shown that the conductivity of nanotube bundles is well described within the theory of the Luttinger-liquid conductivity for one-dimensional conductors. The self-heating of the carbon nanotube bundles was observed in the case of measurements in the regime of dc current. A method for determination of the self-heating temperature of nanotube bundles as a function of an applied electrical field is proposed. The power dependence of the self-heating temperature on voltage T proportional to V(p) with the exponent p = 2.1 was observed above some threshold voltage in the temperature range 4.2-200 K. Above 200 K the exponent decreased down to p = 1.35. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3662331]
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