Properties of C60 polymerized under high pressure and temperature

60 polymers. Pure and mixed phase polymeric samples were synthesized by simultaneously subjecting microcrystalline C₆₀ powder or pellets to various pressures () and temperatures (). The optical spectra of the orthorhombic, tetragonal, and rhombohedral C₆₀ polymer phases are observed to be quite distinct and rich. These spectra exhibit numerous lines and an overall downshift in frequency relative to C₆₀ is observed, consistent with a loss of double bonds from the fullerene cage. The LDMS spectra of a sample synthesized at under hydrostatic conditions and , exhibited a succession of clear peaks at mass numbers corresponding to , similar to the LDMS data on the C₆₀ photopolymer. This is taken as further evidence for interfullerene bonds in these high-pressure polymers. The XRD pattern of this sample indicates the presence of a strong texture in the sample. Received: 14 November 1996/Accepted: 8 January 1997     

NMR study of the magnetic properties of the polymerized phase of Cs1C60

\chem{Cs_{1}C_{60}} . The NMR spectrum allows, when the Magic Angle Spinning (MAS) technique is used, to get information about the electronic structure of this phase, which points out that the spin density is not uniformly distributed on the ball. An estimation of the strength of the quadrupolar effects on is then given, which shows that it is possible to analyse the spin-lattice relaxation time in terms of magnetic effects. A comparison with the of is then justified and allows us to point out the importance of low dimensional antiferromagnetic fluctuations in this compound. We also investigate the nature of the magnetic ground state in : the broadening of the spectra states unambiguously its magnetic nature and a detailed analysis of their shape gives further indications on the antiferromagnetic nature of the magnetic order, which seems characterized by a large amount of disorder. Received: 19 November 1996/Accepted: 19 December 1996     

Investigation of structural transformations in nanophase titanium dioxide by Raman spectroscopy

2 ) prepared by a colloidal chemistry method. The evolution of the anatase phase upon annealing was characterized by frequency downshift, intensity increase, and linewidth sharpening of the lowest-frequency E_g mode. The rutile phase was shown to be stabilized at temperatures below 350 °C and was postulated to reside in the surface region of the nanophase. The beginning and ending temperatures for the anatase–rutile transformation in the nanophase were found to be lower than those in single crystals or polycrystalline powders. The microscopic mechanism of the phase transformation was analyzed and the surface effect was suggested. An anomalous phenomenon of amorphization was also detected at the end of the anatase–rutile transformation in the nanophase TiO₂. Received: 5 November 1997/Accepted: 6 November 1997     

Lattice dynamics of RbC60, RbC70 and phase transitions in A1C60: A neutron scattering study

1 C₆₀ (A = Rb, Cs, K) and RbC₇₀ have been investigated by neutron scattering. The studies include measurements in the high temperature rotator-, the polymer-, the dimer- and the low temperature isolator-phase and include investigations of the particular phase transitions. It is found that each of the phases can be characterized by specific features of its lattice dynamics which can be used for detailed studies of the dynamics of the rotator-polymer and the dimer-polymer transitions. Particular insight in the lattice dynamics is obtained from model calculations for the RbC₆₀ polymer and dimer phase. Received: 13 September 1996/Accepted: 11 November 1996     

The phase diagram of charge- and spin-density waves in polymeric C60

\chem{(C_{60})_{\chemindex{x}}} at interfullerene distances larger than . At smaller ball separation, a special charge-density-wave ground state occurs, which exhibits polarized spheres. At around experimental bond distances, both CDW and SDW are present with rather small amplitudes. Received: 13 September 1996/Accepted: 11 November 1996     

Thermal conductivity of pressure polymerized C60

60 polymerization in the temperature interval at pressures below by measurements of the time dependence of the thermal conductivity. It has been found at that the polymerization process at is slower than the reverse transformation from “polymeric” to “monomeric” phase at . The thermal conductivity of polymerized C₆₀ was measured in the temperature range and found to increase with increasing temperature, which reflects strong phonon scattering. Both the presence of non-bonded C₆₀ molecules and a high degree of structural disorder in the crystalline lattice of the polymeric phase might be responsible for the behaviour of . The results for are qualitatively similar to those reported previously for C₆₀ polymerized at higher , but an order of magnitude smaller. Received: 20 September 1996/Accepted: 11 November 1996     

Atomic nitrogen in C60:N@C60

60 (atomic nitrogen inside C₆₀) is produced by ion implantation. Two different production methods are employed: Kaufman ion source and glow discharge. After the bombarded material is dissolved in toluene or CS₂ and is filtered, several milligrams of C₆₀ containing N@C₆₀ in a concentration of 10^-4 to 10^-5 are obtained. N@C₆₀ gives a very clear hyperfine-split electron paramagnetic resonance signal. The most prominent features of N@C₆₀ are: (i) Nitrogen in C₆₀ keeps its atomic electronic configuration and occupies the on-center position. (ii) N@C₆₀ is stable at ambient conditions, the thermal instability starts at 260 °C. (iii) The complex survives exohedral addition reactions and is a sensitive detector of cage distortions caused by addends. (iv) C₆₀ and N@C₆₀ exhibit slightly different retention times in column chromatography, thus permitting an enrichment of N@C₆₀ by this method. Received: 2 October 1997/Accepted: 16 October 1997     

Investigation of size-driven phase transition in bismuth titanate nanocrystals by Raman spectroscopy

Raman spectroscopy was used to investigate the lattice dynamics and structural transformations in bismuth titanate (Bi₄Ti₃O₁₂) nanocrystals prepared by a chemical coprecipitation technique. The crystal structure of the samples of different grain sizes was determined by X-ray-diffraction analysis. The evolution of the Raman spectrum with grain size was characterized by an intensity decrease, a broadening of the line width, a frequency shift, and the disappearance of the Raman mode. The results revealed the appearance of a size-driven phase transition from orthorhombic to tetragonal phases at a critical size of 44 nm. This result is quite consistent with the X-ray-diffraction measurement and differential thermal analysis. The origin was attributed to the grain-size effect and explained by the surface-energy mechanism. Received: 26 June 2002 / Accepted: 18 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: msz@nju.edu.cn     

Spectroscopic characterization of pressure modified C60

60 are reported. The material associated with the mixture of rhombohedral and tetragonal phases was synthesized under pressure of when treatment temperature was only a little lower than the high-temperature limit of C₆₀ stability. The substance exhibits very rich infrared and Raman spectra as well as luminescence spectra of an unusual shape. They show that vibrational transitions in the prepared carbon architecture substantially deviate from icosahedral symmetry of buckyballs and exhibit a similarity with lattice phonons in graphite. It may originate from mixing of C₆₀ modes and vibrations of a layer structure on deformed and weakly bound fullerene molecules. The luminescence spectra reveal three distinct electronic states located below . One of them responsible for the emission peak at is very characteristic for the pressure modified fullerene. The data should be useful for the accurate determination of structural changes in C₆₀ induced by pressure. Received: 6 September 1996/Accepted: 10 October 1996     

Photoluminescence in anatase titanium dioxide nanocrystals

Titanium dioxide (TiO₂) nanocrystals were prepared by a hydrolysis process of tetrabutyl titanate. X-ray diffraction and Raman scattering showed that the as-prepared TiO₂ nanocrystals have anatase structure of TiO₂, and that the monophase anatase nanocrystals can be achieved through a series of annealing treatments below 650 °C. We measured photoluminescence (PL) spectra of the TiO₂ nanocrystals. Under 2.41–2.71 eV laser irradiation, the TiO₂ nanocrystals displayed strong visible light emission with maxima of 2.15–2.29 eV even at excitation power as low as 0.06 W/cm². To identify the PL mechanism in the TiO₂ nanocrystals, the dependences of the PL intensity on excitation power and irradiation time were investigated. The experimental results indicated that the radiative recombination is mediated by localized levels related to surface defects residing in TiO₂ nanocrystallites. Received: 7 April 1999 / Revised version: 23 August 1999 / Published online: 30 November 1999     

EXAFS study on ultrafine Ni-Co-B amorphous catalysts

\chem{\chemvar{A}C_{60}} with , , . From the temperature dependent line shift of we have extracted information about the electron density distribution on the molecules. The data also support the existence of an like bonding type between the molecules. The magnetic phase transition observed at low temperature in and is monitored by an increase of the second moment as well as an increase of the spin lattice relaxation rates of the and alkali nuclei. The results are compared to NMR data of where no signs of a magnetic ordering are found. The magnetically ordered phases are discussed in terms of antiferromagnetic spin density wave. We also report on NMR measurements of orthorhombic one dimensional and rhombohedral two dimensional polymers of obtained under high pressure. By spinning the samples up to , we are able to identify two resonances at and for the orthorhombic polymer and six resonances at , , , , and for the rhombohedral one. The static distortion of the molecules induced by the transformation under pressure must be at the origin of the observed inequivalent carbons in both systems. The NMR lineshape simulations of the obtained spectra are compatible with the suggested polymeric structures where the molecules are connected by cycloadditions. Received: 10 October 1996/Accepted: 13 November 1996     

Raman study on the pressure-induced disordered state of polystyrene

The pressure-induced disordered state was first reported for iPS, the polymer showing thermal shrinkage. The low-frequency Raman study showed clearly that pressure introduces irregularity into the polymer backbone of iPS. It has also been revealed that there exist two types of vibrational modes, which show different sensitivities on the regularity of the main chain. These modes indicate two distinctive features of dispersion in the ordered form. In the disordered state, the dispersive modes smear out and the dispersionless one survives to be observed. We conclude that pressure causes the ordered-disordered conversion like thermal shrinkage. This conversion will be a common feature of the polymers showing thermal shrinkage and would be able to take place at relatively low pressure.     

Causes of different catalytic activities of metals in formation of single-wall carbon nanotubes

When single-wall carbon nanotubes (SWNTs) were formed by pulsed Nd:YAG laser ablation or arc discharge, the yield depended on the metal catalyst: NiCo> Ni∼NiFe≫Co∼Fe>Pd∼Pt. It appears that an effective catalyst for SWNT growth must satisfy three conditions: it must be a good graphitization catalyst, have low solubility in carbon, and have a stable crystallographic orientation on graphite. NiCo, Ni, and NiFe satisfy these three conditions. The poor catalytic activities of Co, Fe, Pd, and Pt for SWNT formation would be explained by the ineffectiveness of Pt and Pd as graphitization catalysts, crystallographic orientation instability of Co crystals on graphite, and high solubility of Fe in graphite. Received: 29 October 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Atomistic processes and the strain distribution in the early stages of thin film growth

Structural relaxations in small Co islands on the Cu(001) surface are investigated performing atomistic calculations. We demonstrate that the strain relief at the metal interface in the early stages of heteroepitaxy is more complicated than suggested by simple considerations based on the small mismatch between the Co and Cu bulk metals. We found that the strain distribution in the surface region near the islands varies strongly on an atomic scale. The effect of strain on the shape of the Co islands is revealed. Diffusion on the top of strained islands and edge diffusion are considered. Received: 10 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

Raman scattering characterization of vertical aligned 1D IrO2 nanocrystals grown on single crystal oxide substrates

Raman scattering (RS) has been used as a technique for characterization of IrO₂ one dimensional (1D) nanocrystals (NCs) deposited on sapphire(100) and LiNbO₃(100) substrates under various conditions. The IrO₂ NCs were grown via metalorganic chemical vapor deposition method using (MeCp)(COD)Ir as the precursor and reactive magnetron sputtering using Ir metal target. The red-shifts and asymmetric broadening of the Raman lineshape for the NCs were analyzed by a modified spatial correlation (MSC) model, which includes the factor of stress induced shift. The proposed MSC model showed that the effects of stress and nanometric size can be separated in analyzing the observed Raman features. The usefulness of the experimental RS together with the MSC model analysis as a residual stress and structural characterization technique for 1D NCs has been demonstrated.     

Molecular dynamics study of effects of radius and defect on oscillatory behaviors of C60-nanotube oscillators

Using the micro-canonical ensemble, we investigate the oscillatory behaviors of some selected C60-nanotube oscillators by the classical molecular dynamics (MD) simulations method. The second-generation empirical bond-order potential and the van der Waals potential are used to describe bonding and nonbonding atomic interactions, respectively. In the process of simulation, two factors of the radius and vacancy defect of single-walled carbon nanotubes (SWCNTs) are discussed to investigate their effects on the oscillatory behaviors of C60-nanotube oscillators. The simulation results show that the energy dissipation of the C60-nanotube oscillator is sensitive to the radius and vacancy defect, and that the effect of the vacancy defect on the oscillatory behaviors of oscillator depends obviously on the radius of the outer tube. It is found that a single vacancy defect placed on the outer tube of the C60-(17,0) nanotube oscillator can significantly reduce energy dissipation. For C60-(18,0), C60-(19,0) and C60-(11,11) nanotube oscillators, however, the results show that an oscillator containing a vacancy defect is less stable than the one without defect.     

Nanotube growth during annealing of mechanically milled Boron

Boron powder, finely ground in a tungsten carbide ball mill in an ammonia atmosphere, has been annealed at 1200 °C in flowing nitrogen to produce small quantities of cylindrical BN nanotubes, both as isolated individuals and grouped into ropes. Thick-walled conical BN tubes are abundant in specimens annealed for longer times, and their growth was catalysed once WC debris was converted into W metal particles. Some catalytic effect of small W nanoparticles could be necessary for nanotube formation, though no tip particles have been imaged here. Given the low temperature of mechanical milling and annealing, BN growth must involve surface diffusion and solid-state reconfiguration. It could be possible to engineer desirable physical and chemical properties by exploiting the variation in cylindrical versus conical BN structures as a function of annealing time. Received: 19 December 2001 / Accepted: 3 April 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-8253, E-mail: john.fitzgerald@anu.edu.au     

Kinetics of nanocrystallization in FeCoNbB(Cu) alloys

HITPERM alloys (FeCoMBCu; M=Nb, Zr, Hf...) have been recently developed and proposed as competitive soft magnetic materials for high-temperature applications. To our knowledge, this work contains the first results on nanocrystallization isothermal kinetics for these alloys. Analysis of nanocrystallization of the studied FeCoNbB(Cu) alloys in the frame of the Johnson–Mehl–Avrami theory shows a slowing-down of the kinetics and anomalously low values of the Avrami exponent, in a similar way to that reported for FeSiBNbCu (FINEMET)-type alloys. Compositional effects of Co substitution and Cu addition are considered. A more realistic kinetic model developed by Hermann et al. accounts for the experimental data. Received: 14 November 2001 / Accepted: 24 June 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +34-95/461-2097, E-mail: conde@us.es     

Deposition and diffusion of size-selected (Ag400+) clusters on a stepped graphite surface

400 clusters on a stepped graphite surface by a combination of scanning electron microscopy experiments and computer simulations (molecular dynamics and Monte Carlo methods). We find that the shape of the clusters is only partially deformed by the impact with the surface, moreover the clusters do not create surface defects upon landing, and so are able to diffuse freely over the surface. Many clusters are found to become trapped at surface steps, where their mobility is reduced by the higher binding energy. Exploring the 1-D diffusion of clusters along the steps reveals the low mobility for larger islands, as well as the importance of defects on the step (for example kinks), which trap the mobile clusters. Received: 9 April 1998/Accepted: 25 August 1998     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998