| 41. |
- Yao, Mingguang, et al.
(author)
-
Laser-induced transformation of Li4C60 and Na4C60 polymers into metallic monomeric fulleride phases
- 2010
-
In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 489:1-3, s. 64-68
-
Journal article (peer-reviewed)abstract
- We demonstrate that laser irradiation on the narrow-gap insulator Li4C60 and Na4C60 polymers results in metallic monomeric structures, having the same Raman features as metallic fcc K3C60 and Rb3C60, with orientationally disordered C3-60 anions stable at ambient conditions. The Raman Ag(2) mode indicates that a single phase was obtained. Additionally, a monomeric A4C60-type phase (A = K, Rb) is observed in Na4C60 after irradiation. The electron–phonon coupling constants derived from Raman modes of C3-60 fullerides is found to be higher than that of C3-60 fullerides, agreeing with the trend found in heavy alkali fullerides. We propose possible structures of the metallic phases.
|
|
| 42. |
- Yao, Mingguang, et al.
(author)
-
Pressure-induced transformation and superhard phase in fullerenes : the effect of solvent intercalation
- 2013
-
In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:7, s. 071913-
-
Journal article (peer-reviewed)abstract
- We studied the behavior of solvated and desolvated C-60 crystals under pressure by in situ Raman spectroscopy. The pressure-induced bonding change and structural transformation of C60s are similar in the two samples, both undergoing deformation and amorphization. Nevertheless, the high pressure phases of solvated C-60 can indent diamond anvils while that of desolvated C(60)s cannot. Further experiments suggest that the solvents in the solvated C-60 act as both spacers and bridges by forming covalent bonds with neighbors in 3D network at high pressure, and thus, a fraction of fullerenes may preserve the periodic arrangement in spite of their amorphization.
|
|
| 43. |
- Yao, Mingguang, et al.
(author)
-
Pressure-induced transformation in Na4C60 polymer: X-ray diffraction and Raman scattering experiments
- 2011
-
In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:14, s. 144106-
-
Journal article (peer-reviewed)abstract
- In this article the alkali metal-intercalated two-dimensional (2D) polymer Na4C60 is studied under pressure up to 41 GPa at room temperature by Raman spectroscopy and x-ray diffraction (XRD) measurements. Two transitions are identified in the studied pressure range. The first one is observed at ∼3 GPa by both diffraction and Raman scattering. A kink in the pressure slope of the cell parameters (especially along the c axis) shows the appearance of a less compressible phase. The decrease in the C60-C60 distance and the Na-C distance, combined with the frequency softening of the Raman modes, leads to a picture of higher electron hopping. The second transition occurs at around 15 GPa, where the distinct Raman peaks of 2D-Na4C60 disappear and become very broad and diffuse. New bands at 200–800 cm−1, 1590 cm−1, and ∼1800 cm−1, exhibit similar features to those of a reported 3D-C60 polymeric structure. The XRD data show that the cell parameters a, b, and c deviate from their early pressure evolution and become almost pressure independent, accompanied by the formation of amorphous material. Both the evolution of the Raman features of Na4C60 at pressures above 15 GPa and the Raman measurements of the samples on decompression indicate that most C60 molecules in the material are preserved after such a high pressure cycle. Our findings are discussed in terms of the formation of the first high-pressure intercalated C60 3D-polymer structure through the random creation of new polymeric bonds between fullerene molecules.
|
|
| 44. |
- Yao, Mingguang, et al.
(author)
-
Raman signature to identify the structural transition of single-wall carbon nanotubes under high pressure
- 2008
-
In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 78:20
-
Journal article (peer-reviewed)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.
|
|
| 45. |
- Yao, Mingguang, et al.
(author)
-
Reversible nano-scale phase separation of Rb4C60 under pressure
- 2010
-
In: Journal of physics: conference series. - Bristol : Institute of Physics (IOP). ; , s. 012020-
-
Conference paper (peer-reviewed)abstract
- The alkali fulleride Rb4C60 has been investigated by studies of the resistance and by Raman spectroscopy under pressures up to 2 GPa and 13 GPa, respectively. Our data show a reversible phase separation into metallic Rb3C60 and Rb6C60 at pressures above 1 GPa. The reversibility indicates that the phase separation primarily occurs on the nanometer length scale. The data explain several puzzling results reported in the recent literature.
|
|
| 46. |
- Yao, Mingguang, et al.
(author)
-
Reversible pressure-driven nanoscale phase separation in Rb4C60
- 2009
-
In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 79:8, s. 081403-
-
Journal article (peer-reviewed)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.
|
|
| 47. |
- Yao, Mingguang, et al.
(author)
-
Synthesis and characterization of SWCNTs with Ho/Ni as catalyst
- 2006
-
In: New Carbon Materials. - 1007-8827. ; 21:1, s. 70-74
-
Journal article (peer-reviewed)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.
|
|
| 48. |
- Yao, Mingguang, et al.
(author)
-
Synthesis and growth mechanism of differently shaped C60 nano/microcrystals produced by evaporation of various aromatic C60 solutions
- 2009
-
In: Carbon. - Amsterdam : Elsevier. - 0008-6223 .- 1873-3891. ; 47:4, s. 1181-1188
-
Journal article (peer-reviewed)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.
|
|
| 49. |
- Yao, Mingguang, et al.
(author)
-
Synthesis of differently shaped C(70) nano/microcrystals by using various aromatic solvents and their crystallinity-dependent photoluminescence
- 2012
-
In: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 50:1, s. 209-215
-
Journal article (peer-reviewed)abstract
- We report a simple synthesis of Cm microcrystals by a fast solvent-assisted evaporation method by selectively using aromatic solvents with halogen radicals as a controller. In a detailed analysis we show that depending on solvent, C(70) concentration in the solution, and synthesis temperature both C(70) microrods and microcubes can be produced. The samples were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. From meta-type solvents microrods or microcrystal cubes grow with a single orthorhombic or a monoclinic structure or a mixture of both. These crystals show traces of solvents in the structure. From para-type solvents cubic (or rectangular) crystals grow with a hexagonal structure and with no trace of solvents. Room temperature photoluminescence (PL) of the C(70) crystal samples reveals that the PL efficiency of the crystals increases with crystalline order and that the best crystalline Cm cube crystals show 10 times higher PL efficiency than that of pristine C(70) polycrystalline samples. (C) 2011 Elsevier Ltd. All rights reserved.
|
|
| 50. |
- Yao, Mingguang, et al.
(author)
-
Synthesis of single-wall carbon nanotubes and long nanotube ribbons with Ho/Ni as catalyst by arc discharge
- 2005
-
In: Carbon. - : Elsevier BV. - 0008-6223. ; 43:14, s. 2894-2901
-
Journal article (peer-reviewed)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.
|
|
