| 321. |
- Winter, Johannes, et al.
(författare)
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Charge transfer in alkali-metal-doped polymeric fullerenes
- 1996
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 54, s. 17486-17492
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Tidskriftsartikel (refereegranskat)abstract
- We present room-temperature Raman measurements of pressure-polymerized C60 and compare them with the spectra of RbC60 in the orthorhombic phase. Although both materials were prepared according to two completely different routes the spectra show a surprising similarity with respect to mode positions and line splitting. We concluded from this that both materials, the uncharged pressure-polymerized C60 and the rubidium-doped orthorhombic compound, have the same overall structure and the AC60 compounds can be considered as the doped species of the C60, polymerized using moderate low pressure and high temperatures. From a detailed comparison between both spectra mode shifting and line broadening as a consequence of the charge transfer was determined and electron-phonon coupling constants were estimated for the high-frequency Hg(7) and Hg(8) modes. The low values for the coupling constants compared to the ones in the K3C60 can explain the lack of superconductivity in the AC60 compounds.
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| 322. |
- Wågberg, Thomas, et al.
(författare)
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13C NMR on intercalated 2D-polymerised C60 and modified peapods.
- 2004
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Ingår i: Electronic Properties of Synthetic Nanostructures. - Melville, NY : American Institute of Physics (AIP). - 0735402043 ; , s. 238-41
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Konferensbidrag (refereegranskat)abstract
- We present 13C NMR results on the intercalated 2D C60-polymers Li4-xNaxC60 (x=0-4 with half integer steps) showing that while Li4C60 forms a tetragonal polymer with double polymer bonds between the molecules, Na4C60 form a monoclinic polymer with single polymer bonds. The chemical shift is much larger for Li-rich samples than for Na-rich samples, indicating perhaps a different type of interaction with the C60 molecules. The crossover between the two structures appears to be at the composition Li2.5Na1.5C60 . We also show NMR data on SWNT filled with C60 molecules, so called peapods, showing that the NMR line of C60 can not be observed when the C60 is inside the tubes. Weinterpret this as resulting from a very long relaxation time, indicating that the dynamics of the C60 molecule is hindered by the nanotube walls.
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| 323. |
- Wågberg, Thomas, et al.
(författare)
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2D polymerization and doping of fullerenes under pressure
- 2000
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Ingår i: High Pressure Research vol. 18. - : Gordon & Breach. ; 18:1-6, s. 139-143
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Konferensbidrag (refereegranskat)abstract
- Tetragonal polymeric C60 has been studied by Raman spectroscopy and other methods. Attempts have been made to transform samples from the tetragonal to the orthorhom-bic phase and vice versa. The results suggest that the transformation is direct with no intermediate stage with free molecules. Tetragonal C60 has also been intercalated by potassium metal.
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| 324. |
- Wågberg, Thomas, et al.
(författare)
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A Raman study of polymerised C60
- 1997
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Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 64:3, s. 223-226
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Tidskriftsartikel (refereegranskat)abstract
- Results of Raman scattering studies on high pressure polymerized and photo polymerized C60 are reported. Although prepared according to different routes the Raman spectra of the two polymeric phases of C60 show a quantitative agreement with respect to mode positions and intensity. We conclude from this that both materials have the same structure at least in the short range order, i.e. the same type of bonding and co-ordination between neighbouring C60 molecules. An investigation of the time dependence of the thermal decomposition of high pressure polymerised C60 is also presented. The rate of decomposition of the polymeric phase is found to be multi-exponential at all temperatures investigated. From an Arrhenius-type analysis of the short time data and the long time data, respectively, the activation energy for thermal dissociation of polymeric bonds was found to increase with time.
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| 325. |
- Wågberg, Thomas, et al.
(författare)
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Can two-dimensional fullerene polymers be intercalated?
- 2000
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Ingår i: Molecular Crystals and Liquid Crystals vol. 340. - : Overseas Publishing Association, B.V.. ; , s. 677-682
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Konferensbidrag (refereegranskat)abstract
- Two-dimensional C60 polymers can be produced by treatment at high T and high p. Attempts have previously been made to intercalate these layered materials by alkali metals using the gas phase method but the polymers have always decomposed. We investigate here whether intercalated 2D polymers can be formed by other methods, such as by polymerization in the presence of alkali metals. Preliminary x-ray and Raman data can be interpreted to indicate the presence of an alkali metal intercalated tetragonal fullerene polymeric structure.
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| 326. |
- Wågberg, Thomas, et al.
(författare)
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Comparative Raman study of photopolymerised and pressure-polymerised C60 films
- 1999
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 60:7, s. 4535-4538
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Tidskriftsartikel (refereegranskat)abstract
- We have made a Raman study of low-pressure polymerized and photopolymerized C60 films, polymerized in different ways, but otherwise initially identical. Although the structures developing in the films during polymerization are initially very similar, an analysis of characteristic features in the Raman spectra shows that photopolymerized material forms a more complex polymeric structure when the polymerization approaches saturation. We suggest that this complex structure contains branched polymeric chains and in particular that a mode near 1454 cm-1 in photopolymerized C60 is characteristic for this type of structure.
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| 327. |
- Wågberg, Thomas, et al.
(författare)
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Polymeric phases of C60
- 1997
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Ingår i: Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials, vol. 5. - Pennington, NJ : The Electrochemical Society. - 1566771927 ; , s. 674-679
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Konferensbidrag (refereegranskat)abstract
- We have investigated the "low" pressure region where C60 can be transformed into polymeric chains or clusters. To learn more about the polymerization process we have treated pristine C60 at several different temperatures under hydrostatic pressures in silicone oil. We found that the reaction rate varied with temperature. Above 520 K at 1.0 GPa similar and high polymer fractions were obtained in all samples, but at 497 K the polymer fraction was only 10% after three hours, as indicated by the shift of the intensity of the pentagonal pinch mode in the Raman spectrum. Also, samples treated at the same pressure and temperature but with different thermal pre-history showed different degrees of polymerization. We have also found a correlation between the shift of the pentagonal pinch mode in the Raman spectrum to 1463 /cm and the intermolecular vibrational mode at 97 /cm. We therefore make the conclusion that the shift of the pentagonal pinch mode to 1463 /cm corresponds to the presence of dimers in the sample.
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| 328. |
- Wågberg, Thomas, et al.
(författare)
-
Raman study of the two-dimensional polymers Na4C60 and tetragonal C60.
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 65:15, s. 155421-
-
Tidskriftsartikel (refereegranskat)abstract
- We present a Raman investigation of the monoclinic two-dimensionally polymerized phase Na4C60 and a comparison with the pure tetragonal C60 polymer and the doped K3C60 structure. The Raman spectrum of Na4C60 depends on the probing laser wavelength, and high wave-number modes are hard to observe with a low-energy laser excitation. The spectrum for Na4C60 is very rich with a pronounced splitting of the original Hg modes and a large number of new modes. Several modes at low wave numbers support the presence of intermolecular bonds and the line broadening observed for low-frequency Hg modes is typical for electron-phonon coupling in metallic fullerene systems. From the shift of the pentagonal pinch mode we deduce an unexpectedly low charge transfer of approximately three electrons per C60 molecule. The presence of a distinct mode around 980 cm-1 throws doubts on earlier assumptions that modes in this range are connected to vibrations in the intermolecular cyclobutane rings found in C60 polymers. No superconductivity is observed in Na4C60, although the electron-phonon interaction derived from the Raman spectrum is similar to that in K3C60.
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| 329. |
- Wågberg, Thomas, et al.
(författare)
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Spectroscopic study of phase transformations between orthorhombic and tetragonal C60 polymers
- 2006
-
Ingår i: The European Physical Journal B: Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 49:1, s. 59-65
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed study of transformations between the orthorhombic and tetragonal polymeric states of C60. The transformations are characterised by Raman spectroscopy and X-ray diffraction. We show that the transformation from the orthorhombic (O) phase to the tetragonal (T) phase is very fast and our results indicate that the transformation goes via an intermediate dimer (D) state in a two-stage process, O↦D and, D↦T transformations, where the second process is slower than the first. On the other hand, the transformation from the tetragonal to the orthorhombic phase is significantly slower, indicating a high-energy threshold to break the polymer bonds at the temperatures used. The results also support earlier suggestions that the tetragonal phase contains disordered dimers that can be viewed as lattice defects in the formation of higher polymers.
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| 330. |
- Wågberg, Thomas, et al.
(författare)
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Structural aspects of two-dimensional polymers: Li4C60, Na4C60 and tetragonal C60. Raman spectroscopy and X-ray diffraction.
- 2004
-
Ingår i: Journal of Physics and Chemistry of Solids vol. 65, issue no. 2-3. - Amsterdam : Elsevier B.V.. ; , s. 317-320
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Konferensbidrag (refereegranskat)abstract
- We present an analysis of three different two-dimensional polymers, tetragonal C60, Li4C60, and Na4C60. Based on X-ray diffraction and Raman spectroscopy, we conclude that Li4C60 forms a tetragonal structure with intermolecular bonds formed by 2 plus 2 cycloaddition, in the same way as for tetragonal C60. Na4C60, on the other hand, forms a monoclinic structure with single C–C bonds between the molecules. Our Raman spectroscopy results can be interpreted in two ways: either the charge transfer to the C60 molecules is the same in both doped compounds with four electrons/molecule or the electron charge transfer is smaller from the Li ions than from the Na ions.
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