| 1. |
- Aleksandrovskii, A. N., et al.
(författare)
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Thermal expansion of single-crystal fullerite C60 at helium temperatures
- 2000
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Ingår i: Low temperature physics (Woodbury, N.Y., Print). - : AIP Publishing. - 1063-777X .- 1090-6517. ; 26:1, s. 75-80
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Tidskriftsartikel (refereegranskat)abstract
- The thermal expansion of single-crystal fullerite C60 has been studied in the range of liquid-helium temperatures (2–10 K). At temperatures below ~4.5 K the thermal expansion of fullerite C60 becomes negative, in agreement with the previous results on polycrystalline materials. A qualitative explanation of the results is proposed.
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| 2. |
- McRae, Edward, et al.
(författare)
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Raman light scattering and c-axis resistivity evidence for a pressure-induced stage transformation in PdAl2Cl8 intercalated graphite.
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 62:20, s. 13757-13766
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Tidskriftsartikel (refereegranskat)abstract
- We have examined several samples of first- to third-stage PdAl2Cl8-intercalated graphite under hydrostatic pressures up to 1 GPa. In stage-1 highly oriented pyrolytic graphite–(HOPG) and single-crystal-graphite-based materials, the c-axis resistivity decreases sharply above a few kilobars; pressure release induces a reversible return to the initial value only in the case of the latter sample. Raman spectra taken in situ under pressure on a HOPG-based material show similarly irreversible effects. Analysis of the spectra taken on higher-stage samples leads to the conclusion that hydrostatic pressure beyond a few kilobars increases the density of the intercalate within the graphitic galleries, transforming the initial sample to a higher-stage material. Since there is no loss of intercalate, the overall intercalate-to-host charge transfer remains constant so that the Raman frequency is approximately the same for both first- and second-stage products. This is an unusual situation in which there is thus an apparent lack of Raman signature in spite of the stage change.
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| 3. |
- Liu, Bingbing, et al.
(författare)
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Synthesis and characterization of single-walled nanotubes produced with Ce/Ni as catalysts.
- 2000
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Ingår i: Chemical Physics Letters. - : Elsevier B.V.. - 0009-2614 .- 1873-4448. ; 320:3-4, s. 365-372
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Tidskriftsartikel (refereegranskat)abstract
- SWNTs were synthesized in high yield in the presence of Ce/Ni as catalyst. Raman and SEM results show that a fraction of the tubes are chiral and have small diameters (the smallest one is 0.79 nm), indicating that Ce plays an important role in the formation. Acid treatment is effective in eliminating impurities but changes the morphology and the electric resistance of mats with increasing treatment time. The resistance of as-grown and short time acid-treated mats follow a 2D VRH behaviour while mats treated for a long time follow a 3D model. The results indicate that hopping conduction occurs on bundles or on tubes and that there are intrinsic links between tubes in as-grown mats.
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| 4. |
- Moret, Roger, et al.
(författare)
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High-pressure synthesis, structural and Raman studies of a two-dimensional polymer crystal of C60.
- 2000
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Ingår i: European Physical Journal B. - : Springer. - 1434-6028 .- 1434-6036. ; 15:2, s. 253-263
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional polymerisation of a C60 single crystal has been obtained under high-pressure high temperature conditions (700 K - 2 GPa). Crystalline order is preserved but the crystal splits into variants (orientational domains). The analysis of X-ray diffraction and Raman spectroscopy data reveals that the polymer crystal is primarily tetragonal with some admixture of rhombohedral phase. Furthermore, Raman spectroscopy gives evidence for additional C60-C60 dimers, which are probably disordered. For the tetragonal phase, it is shown that successive polymer layers are rotated by about the stacking axis, according to the P42/mmc space group symmetry. The structure of the rhombohedral phase is also clarified. The role of the interlayer interactions in stabilising the two-dimensional polymer phases of C60 is discussed.
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| 5. |
- Persson, Per-Axel, et al.
(författare)
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Enhanced thermal dissociation of optically excited C60 chains.
- 2000
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Ingår i: Europhysics letters. - : EDP Sciences. - 0295-5075 .- 1286-4854. ; 49:5, s. 631-636
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Tidskriftsartikel (refereegranskat)abstract
- The thermal dissociation of C60 polymers has been studied using Raman scattering. The measured dissociation rate depends on the intensity of the 1064 nm NIR excitation, showing that i) the band gap is smaller than 1.17 eV and ii) a radiation-enhanced thermal breakdown path exists in addition to the "normal'' thermal breakdown mechanism. Quantum chemical calculations show that the energy barrier Ea for thermal breakdown is about 40% lower in the first (optically) excited state than in the ground state. This agrees well with the ratio between our radiation-enhanced value $E_{a} = (1.1 \pm 0.02)\,eV$ and values near 1.9 eV measured by purely thermal methods.
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