| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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