Stability of C<Subscript>20</Subscript> fullerene chains

The stability of (C₂₀)_N metastable chains, where C₂₀ fullerenes are joined by tight covalent bonds, is analyzed by numerical simulation using a tight-binding potential. Various channels of losing the chain-cluster structure of the (C₂₀)_N complexes have been determined including the decay of the C₂₀ clusters, their coalescence, and the separation of one C₂₀ fullerene from a chain. The lifetimes of the (C₂₀)_N chains with N = 3–7 for T = 2000–3500 K are directly calculated by the molecular dynamics method. It has been shown that, although the stability of the chains decreases with an increase in N, it remains sufficiently high even for N ? 1. An interesting lateral result is the observation of new (C₂₀)_N isomers with the combination of various intercluster bonds with the maximum binding energy of fullerenes in the chain.     

Anomalous thermal stability of metastable C<Subscript>20</Subscript> fullerene

The results of computer simulation of the dynamics of fullerene C₂₀ at different temperatures are presented. It is shown that, although it is metastable, this isomer is very stable with respect to the transition to a lower energy configuration and retains its chemical structure under heating to very high temperatures, T ≈ 3000 K. Its decay activation energy is found to be E _a ≈ 7 eV. Possible decay channels are studied, and the height of the minimum potential barrier to decay is determined to be U = 5.0 eV. The results obtained make it possible to understand the reasons for the anomalous stability of fullerene C₂₀ under normal conditions.     

Effect of heating of the electronic subsystem on the thermal stability of the C<Subscript>60</Subscript> and C<Subscript>20</Subscript> fullerenes and (C<Subscript>20</Subscript>)<Subscript>2</Subscript> cluster molecule

The effect of heating of the electronic subsystem on the thermal stability of C₆₀ and C₂₀ fullerenes and a (C₂₀)₂ cluster molecule is investigated theoretically. It is demonstrated that the excitation of electrons to upper energy levels in accordance with the Fermi-Dirac distribution function does not lead to a substantial change in the activation energy E _a for decay of the C₂₀ fullerene. The stability of the C₆₀ fullerene and the (C₂₀)₂ cluster molecule likewise does not change radically. However, the inclusion of corrections associated with the finite sizes of the heat bath leads to the activation energy E _a which is in better agreement with the calculated height of the potential barrier preventing the cluster decay.     

Vibrational properties of C<Subscript>20</Subscript>-based solids

The phonon dispersion relations and IR spectrum of a C₂₀-based solid recently identified experimentally [Iqbal et al. , Eur. Phys. J. B 31, 509 (2003)] have been computed by density functional perturbation theory. Other competitive structures made by assembling C₂₀ clusters have been considered as well. In particular, we have computed the structure and the Raman spectra of two-dimensional polymeric phases of hydrogenated C₂₀ clusters which might be formed under different synthesis conditions. Fingerprints of the different phases have been identified in the vibrational spectra which could be used in the experimental search of C₂₀-based solids.Received: 29 September 2003, Published online: 15 March 2004PACS: 61.48. + c Fullerenes and fullerene-related materials - 63.20.-e Phonons in crystal lattices - 78.30.-j Infrared and Raman spectra     

Polyaniline composites with fullerene C<Subscript>60</Subscript>

Polyaniline-fullerene composites were prepared by the introduction of fullerene during polymerization of aniline. An investigation of the composites using FTIR and ¹³C NMR spectroscopy indicated interaction between fullerene and the imine groups of polyaniline. The formation of a polyaniline-fullerene complex with a structure corresponding to a doped polyaniline was proved by wide-angle x-ray scattering analysis. The conductivity of composites is more than four orders of magnitude higher than that of undoped polyaniline and that of fullerene. Improvement in the thermal stability of composites was evaluated using TGA.     

C<Subscript>60</Subscript> fullerene decoration of carbon nanotubes

A new fully carbon nanocomposite material is synthesized by the immersion of carbon nanotubes in a fullerene solution in carbon disulfide. The presence of a dense layer of fullerene molecules on the outer nanotube surface is demonstrated by TEM and XPS. Fullerenes are redistributed on the nanotube surface during a long-term action of an electron beam, which points to the existence of a molecular bond between a nanotube and fullerenes. Theoretical calculations show that the formation of a fullerene shell begins with the attachment of one C₆₀ molecule to a defect on the nanotube surface.     

Stone-Wales transformation paths in fullerene C<Subscript>60</Subscript>

The mechanisms of formation of a metastable defect isomer of fullerene C₆₀ due to the Stone-Wales transformation are theoretically studied. It is demonstrated that the paths of the “dynamic” Stone-Wales transformation at a high (sufficient for overcoming potential barriers) temperature can differ from the two “adiabatic” transformation paths discussed in the literature. This behavior is due to the presence of a great near-flat segment of the potential-energy surface in the neighborhood of metastable states. Moreover, the sequence of rupture and formation of interatomic bonds is other than that in the case of the adiabatic transformation.     

X-ray diffraction analysis of C<Subscript>60</Subscript> fullerene powder and fullerene soot

C₆₀ fullerene powder and fullerene soot are examined by the method of small-angle X-ray diffraction. Small-angle diffraction patterns constructed in the Guinier coordinates make it possible to find the radii of inertia of scattering elements. The small-angle diffraction scattering findings agree well with large-angle X-ray diffraction data.     

A new allotropic form of carbon [C<Subscript>28</Subscript>]<Subscript>n</Subscript> based on fullerene C<Subscript>20</Subscript> and cubic cluster C<Subscript>8</Subscript> and Si and Ge analogs of this form: Computer simulation

The structure of a new allotropic form of carbon [C₂₈]_n having a simple cubic lattice and space group \(Pm \bar 3\) is proposed. The geometrical parameters of the building block of such a hypothetic crystal are preliminarily determined from DFT-PBE calculations of the cluster C₈@(C₂₀)₈ and the polyhedral hydrocarbon molecule C₈@(C₂₀H₁₃)₈, in which the centers of the cubic clusters C₈ coincide with the centers of the cluster C₈@(C₂₀)₈ and of the molecule C₈@(C₂₀H₁₃)₈, respectively, and dodecahedral C₂₀ carbon cages are located at the vertices of a cube. The energy of dissociation of the cluster C₈@(C₂₀)₈ into a cubic cluster C₈ and eight dodecahedral clusters C₂₀ is calculated to be 1482 kcal/mol, and the energy of each C₈-C₂₀ bond is equal to 74.2 kcal/mol. The structure of the [C₂₈]_n crystal is refined using the DFT-PBE96/FLAPW method and optimized geometry. Calculations show that the crystal is a dielectric with an energy gap of 3.3 eV. The lattice parameter a of the crystal is equal to 5.6 Å, and its density is 3.0 g/cm³. The possible existence of analogous allotropic forms of elements Si and Ge is discussed. A method is proposed for designing a hypothetic allotropic form [C₂₈]_n from C₂₀(CH₃)₈ molecules with T _h symmetry.     

Thermodynamic and dilatometric properties of the dimerized phase of a C<Subscript>60</Subscript> fullerene

This paper reports on the results of complex investigations into the structural, thermodynamic, and dilatometric properties of the C₆₀ dimerized phase prepared under compression of a C₆₀ fullerite at a pressure up to 8 GPa and a temperature of 290 K. It is demonstrated that the dimerized phase has a face-centered cubic structure with a lattice parameter a=14.02±0.05 Å. The dimeric structure of the studied sample is confirmed by x-ray diffraction analysis. According to the dilatometric data, the volume jump observed in the vicinity of the orientational transition for the dimerized phase is estimated to be approximately 30 times less than that for the C₆₀ fullerite. The temperature dependence of the heat capacity of the (C₆₀)₂ crystalline dimer is examined using precision adiabatic vacuum calorimetry under normal pressure in the temperature range from T → 0 K to 340 K. The results obtained are used in the calculations of thermodynamic functions, namely, the heat capacity C _p ⁰ (T), the enthalpy H⁰(T)-H⁰(0), the entropy S⁰(T), and the Gibbs function G⁰(T)-H⁰(0). The fractal dimension D is determined as a function of the heat capacity. The standard entropy of the formation of the (C₆₀)₂ crystalline dimer from a simple compound (graphite) at T=298.15 K and normal pressure is calculated.     

Molecular properties of C<Subscript>60</Subscript> fullerene complexes with cycle-containing polymers in solutions

The donor-acceptor complexes of the C₆₀ fullerene with cycle-containing polymers, namely, poly(2,6-dimethyl-1,4-phenylene oxide) (PPhO) and poly(N-vinylpyrrolidone) (PVP), are studied. A comparative analysis of the hydrodynamic and electrooptical properties of the initial polymers and their complexes with C₆₀ in solutions demonstrates that the C₆₀ fullerene has a restructuring effect on the polymer macromolecule, thus decreasing the degree of asymmetry of the macromolecular structure.     

Tuning electronic properties of In<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires by doping control

We present two effective routes to tune the electronic properties of single-crystalline In₂O₃ nanowires by controlling the doping. The first method involves using different O₂ concentrations during the synthesis. Lightly (heavily) doped nanowires were produced by using high (low) O₂ concentrations, respectively, as revealed by the conductances and threshold voltages of nanowire-based field-effect transistors. Our second method exploits post-synthesis baking, as baking heavily doped nanowires in ambient air led to suppressed conduction and a positive shift of the threshold voltage, whereas baking lightly doped nanowires in vacuum displayed the opposite behavior. Our approaches offer viable ways to tune the electronic properties of many nonstoichiometric metal oxide systems such as In₂O₃, SnO₂, and ZnO nanowires for various applications. PACS 85.35.-p     

Effect of C<Subscript>60</Subscript> fullerene additions on the mechanical properties of a polybutadiene-styrene raw rubber

The effect of small C₆₀ fullerene additions on the mechanical properties (upon uniaxial tension) and structure of a polybutadiene-styrene raw rubber is studied.     

Photoinduced optical activity of C<Subscript>70</Subscript> fullerene in organic solvents

We have experimentally studied for the first time the effect of photoinduced rotation of the plane of polarization for pulsed laser radiation in solutions of C₇₀ fullerene in organic solvents and their mixtures. We have shown that the effect is observed for elliptical polarization of the laser radiation and is absent for linear polarization. We present the results of a study of the nonlinear optical characteristics of the C₇₀ solutions. We discuss the physical mechanisms by which nonlinear gyrotropy is induced in solutions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 93–99, January–February, 2009.     

Thermodynamic properties of C<Subscript>60</Subscript> polyfullerites

This paper discusses the results of calorimetric studies of the 1D C₆₀ (orthorhombic) and 2D C₆₀ (tetragonal and rhombohedral) fullerites, as well as of the graphite-like polyfullerite, which are produced from a starting C₆₀ fullerite subjected to a pressure of 1–8 GPa at temperatures ranging from 300 to 1270 K. The analysis is made primarily of the C _p ⁰ heat capacity measurements performed in adiabatic calorimeters in the 5-to 350-K range.     

Stepwise computational synthesis of fullerene C<Subscript>60</Subscript> derivatives. Fluorinated fullerenes C<Subscript>60</Subscript>F<Subscript>2<Emphasis Type="Italic">k</Emphasis></Subscript>

Reactions of fullerene C₆₀ with atomic fluorine are studied by the unrestricted broken spin symmetry Hartree-Fock (UBS HF) approach implemented in semiempirical codes based on the AMI technique. The calculations are focused on a successive addition of a fluorine atom to the fullerene cage following the indication of the highest chemical susceptibility of the cage atom, which is calculated at each step. The proposed computational synthesis is based on the effectively unpaired-electron concept of the chemical susceptibility of fullerene atoms. The obtained results are analyzed from the standpoints of energy, symmetry, and the composition abundance. A good fitting of the data to experimental findings proves a creative role of the suggested synthetic methodology.     

Rotation of the inner shell in a C<Subscript>20</Subscript>@C<Subscript>80</Subscript> nanoparticle

The stability of a C₂₀@C₈₀ nanoparticle and the rotation of its inner shell are studied theoretically within the tight-binding approximation. It is found that the C₂₀ skeleton in the free state is described by space group D_3d; in the case where C₂₀ is placed into the C₈₀(I _h ) fullerene field, the space group of C₂₀ is raised to I _h due to isomerization. The total energy surface of the C₂₀@C₈₀ compound is scanned over two rotation angles. Based on an analysis of the surface relief and energy isoline map, orientational melting of the nanoparticle is predicted. A nanoparticle gyroscope—C₂₀ rotating in the field of C₈₀ at a certain relative orientation and energy supply—is also predicted to exist.     

The effect of water on the stability of C<Subscript>60</Subscript> fullerene nanowhiskers

The morphology of C₆₀ precipitates synthesized by using isopropyl alcohol (IPA) added with water was investigated in order to know the effect of water on the growth of C₆₀ nanowhiskers (C₆₀NWs) in C₆₀–toluene–IPA solution systems. The stability of C₆₀NWs decreased and granular crystals of C₆₀ were formed in the solutions when IPA added with an excess amount of water was used in the liquid–liquid interfacial precipitation method. The C₆₀NWs were found to be destabilized with time in the solutions added with water. The C₆₀NWs dried in air showed similar Raman profiles irrespective of the use of IPA with and without water addition. The Raman profiles of granular C₆₀ single crystals showed the base lines much flatter than those of C₆₀NWs, indicating that C₆₀NWs possess a disordered crystal structure. By optimizing the growth condition, short C₆₀NWs with aspect ratios ranging from 3 to 10 and an average length of about 1.8 μm were successfully fabricated. The short C₆₀NWs are expected to be applicable for electrodes of organic thick film solar cells.     

Thermal desorption states of C<Subscript>60</Subscript> fullerene molecules in polymer matrices

Polymer-C₆₀ fullerene composite coatings are studied using thermal desorption mass spectrometry. It is found that thermal desorption spectra of C₆₀ fullerene molecules can exhibit several resolved peaks (at a specified heating rate) corresponding to thermal desorption states. The relative intensity of the thermal desorption peaks depends on the procedure used for preparing the composite coatings, in particular, on the time of sedimentation of the polymer-fullerene suspension. The occurrence of different stages in thermally stimulated desorption of C₆₀ fullerene molecules is explained by the fact that the fullerene molecules can exist in several phase states characterized by different densities and degrees of ordering in the polymer matrix.