Mechanical properties and electronic structures of compressed C<Subscript>60</Subscript>, C<Subscript>180</Subscript> and C<Subscript>60</Subscript>@C<Subscript>180</Subscript> fullerene molecules

Molecular dynamics simulations were performed for compressed C₆₀, C₁₈₀ and C₆₀@C₁₈₀ fullerene molecules, and the semi-empirical PM3 calculations were carried out to obtain the electronic structure of the compressed fullerenes. According to the obtained results, the differences of mechanical properties between these compressed fullerenes, as well as the changes of their FMO (Frontier molecular orbital) energy-levels during compression, were discussed. It is shown that (1) the C₆₀ molecule has much higher load-support and energy-absorbing capability than the C₁₈₀ and C₆₀@C₁₈₀ molecules, and the C₆₀@C₁₈₀ is only slightly superior to the C₁₈₀, (2) of the three molecules, the C₆₀ molecule has the best chemical-stability, and the C₆₀@C₁₈₀ molecule has the worst one, (3) with the increase of compressive strain, both the C₆₀ and C₆₀@C₁₈₀ molecules become more chemically active, and (4) when the compressed C₁₈₀ molecule caves in at the loading position(s), its chemical-stability decreases abruptly.     

The photoluminescence of amorphous carbon in a-C:C<Subscript>60</Subscript> films obtained by fullerene C<Subscript>60</Subscript> deposition

The results of experimental investigations of the carbon deposits obtained in the course of fullerene C₆₀ sublimation are presented. It is shown that the films formed at a high deposition rate possess a composite structure representing a mixture of fullerenes and an amorphous carbon phase with a graphitelike short-range order. The films containing amorphous nonhydrogenated carbon possess photoluminescent properties analogous to those of hydrogenated amorphous carbon films with a high hydrogen content. The possible mechanism of photoluminescence of the composite carbon films is discussed.     

Core electron level structure in C<Subscript>60</Subscript>F<Subscript>18</Subscript> and C<Subscript>60</Subscript>F<Subscript>36</Subscript> fluorinated fullerenes

The structure of C1s and F1s core electron levels in C₆₀F₁₈ and C₆₀F₃₆ fluorinated fullerenes has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. It is established that C1s levels of carbon atoms not bound to fluorine in these compounds are shifted down by 1.0 and 1.6 eV relative to the C1s level in the usual C₆₀ fullerene, so that the binding energies of the core electron levels in C₆₀F₁₈ and C₆₀F₃₆ amount to E _b (C1s, C-C) = 285.7 and 286.3 eV, respectively. These values are characteristic and can be used for the identification of both homogeneous fluorinated fullerenes and combined materials comprising a mixture of various fluorinated fullerenes with each other and with different carbon-containing based materials.     

Concentration-dependent variations in the density of C<Subscript>60</Subscript> fullerene solutions in aromatic solvents

The densities of C₆₀ fullerene solutions in benzene, toluene, and p-xylene have been determined by pyknometry and studied as a function of the fullerene concentration in solution. All dependences exhibit a non-monotonic character, whereby the density initially decreases, passes through a minimum, and then grows with increasing fullerene concentration. The position of the minimum shifts toward greater C₆₀ concentrations on the passage from benzene to toluene and p-xylene.     

Diamond synthesis from C<Subscript>60</Subscript> fullerite using boron and titanium diboride

Boron-doped diamond has been synthesized from C₆₀ fullerite at 7 GPa and 2000–2050 K using unconventional solvents for carbon: boron and titanium diboride.     

Effect of C<Subscript>60</Subscript> fullerene additives on the mechanical properties of low-density polyethylene films

We have studied the effect of C₆₀ fullerene additives on the mechanical properties (strength, Young’s modulus, elongation at break) of thin low-density polyethylene films tested for uniaxial extension.     

C<Subscript>60</Subscript> end-functionalized four-armed polymers: synthesis and optical limiting properties

A series of bromo-terminated four-armed homopolymers [polystyrene (PSt) and poly(methyl methacrylate) (PMMA)], prepared by atom transfer radical polymerization (ATRP), were subsequently functionalized with fullerene C₆₀ by atom transfer radical addition (ATRA) reaction using CuBr/2,2′-bipyridine (bipy) as the catalyst system. The C₆₀ end-capped four-armed polymer derivatives were then characterized by gel permeation chromatography (GPC), UV–vis, FT-IR, thermal gravimetric analyses (TGA) and differential scanning calorimeter (DSC). Another technique of synthesizing C₆₀-functionalized four-armed polymers from azide functional polymers was also applied for comparison. The optical limiting behavior of C₆₀ end-capped four-armed polymers prepared via ATRA reaction were measured in THF solution at 532 nm. Both groups of fullerene-functionalized polymers (C₆₀-PSts and C₆₀-PMMAs) are provided with optical limiting properties.     

Phase composition and microstructure of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> solid solutions prepared by self-propagating high-temperature synthesis

The combustion synthesis, microstructure, chemical composition, and phase composition of cast Al₂O₃-Cr₂O₃ materials are described. The synthesized solid solutions are based on corundum or chromia, depending on the Al₂O₃ and Cr₂O₃ contents. Using self-propagating high-temperature synthesis, a cast aluminum oxynitride is obtained for the first time.     

Effect of C<Subscript>60</Subscript> fullerene additives on the structure and mechanical properties of thin organic glass films

We have studied the effect of small additives of C₆₀ fullerene on the mechanical properties (strength, Young’s modulus, elongation at break) of thin organic glass films tested for uniaxial extension.     

NMR Evidence for C<Subscript>60</Subscript> Configurational Fluctuations Around Na Sites in Na<Subscript>2</Subscript>CsC<Subscript>60</Subscript>

The ²³Na nuclear magnetic resonance (NMR) spectrum, spin-lattice and spin–spin relaxation, as well as spin echo double resonance (SEDOR) are investigated in the ternary alkali fulleride compound Na₂CsC₆₀ in the temperature range of 10–300 K. The NMR line associated with the tetrahedral sodium site is split below 170 K (T and T′ lines) similarly to Rb₃C₆₀ although the crystal structures of these two materials are different. SEDOR measurements prove that the T and T′ sites are microscopically close. The merger of the two lines at about 170 K is attributed to motional narrowing resulting from a site exchange due to angular reorientations of the C₆₀ molecules. The exchange dynamics inferred from the spectra, spin–spin relaxation, and spin-lattice relaxation are all consistent and agree with inelastic neutron scattering, supporting our proposal that the observed T-T′ splitting originates from different local fullerene configurations around the tetrahedral alkaline sites.     

Synthesis of nanocrystalline TiB<Subscript>2</Subscript> powder from TiO<Subscript>2</Subscript>, B<Subscript>2</Subscript>O<Subscript>3</Subscript> and Mg reactants through microwave-assisted self-propagating high-temperature synthesis method

In this research work, microwave-assisted self-propagating high-temperature synthesis (SHS) process was employed for the fabrication of titanium diboride (TiB₂) compound from TiO₂–B₂O₃–Mg mixtures. Thermodynamic evaluations of this system and its relevant subsystems revealed that TiB₂–MgO composite powder can be easily produced by a SHS reaction. However, experimental results of a TiO₂ : B₂O₃ : 5Mg mixture heated in a domestic oven showed the formation of some intermediate compounds such as Mg₃B₂O₆, presumably due to some degree of Mg loss. The optimum amount of Mg in TiO₂ : B₂O₃ : xMg mixtures, yielding the highest amount of TiB₂ phase, was found to be around 7 mol, i.e., 40 mol% more than the stoichiometric amount. Experimental results revealed that a pure TiB₂ compound could be obtained by leaching the unwanted by-products in an HCl acid solution. Scanning electron microscopic observations and Scherrer calculations showed that the produced TiB₂ contains sub-micron (150–200 nm) particles, where each particle consists of a number of nanosized (32 nm) crystallites.     

Doped C<Subscript>60</Subscript> Study from First Principles Simulation

We have performed first principles density functional theory method simulations on electronic structures of B, N, Co, P, and Bi doped C₆₀ solids. Our electronic structure simulations show that boron, phosphorous, and cobalt doped face-centered cubic (FCC) C₆₀ solids have the electronic structures of n-type semiconductors. Nitrogen doped FCC C₆₀ solid has an electronic structure similar to those of a p-type semiconductor. P doped C₆₀ is a potential good candidate in thermoelectric application. For Bi doped C₆₀, a transformation from n-type to p-type semiconductor and gradually to metal, which corresponding to the Bi:C₆₀ dopant ratio at 1:60, 2:60, and 3:60, respectively, can be seen from our electronic density of states (DOS) analysis. There are volume contraction and charge transfer increasing in the 2:60 of Bi doped C₆₀ results compared with those of 1:60 Bi doped C₆₀ case. The charge transfer at a tetrahedral site is as three times larger as that of octahedral site. For the concentration of Bi doped C₆₀ higher than 3:60, the system is expected to be a superconductor.     

Refractive index of Al<Subscript>3</Subscript>C<Subscript>2</Subscript>B<Subscript>48</Subscript> aluminum borocarbide crystals

Aluminum borocarbide single crystals have been grown from an Al-based solution melt. The crystal lattice parameters have been determined, the dispersion of the refractive index in a 0.55–1.3 μm wavelength interval has been studied, and the temperature coefficient of the refractive index in a 300–600 K range has been measured. The crystals are characterized by a high refractive index in the visible spectral range in combination with at a high hardness, which makes them of interest for jewelry, as well as for both traditional and X-ray optics.     

Permittivity of low-concentration C<Subscript>60</Subscript> fullerene solutions in <Emphasis Type="Italic">p</Emphasis>-xylene

The frequency dependence of the permittivity of C₆₀ fullerene solutions in p-xylene has been determined at various small concentrations of fullerene molecules. The results are discussed using notions of the solution structure.     

Effect of C<Subscript>60</Subscript> fullerene additives on the thermal conductivity of poly(methyl methacrylate) films

The thermal conductivity of a composite comprising C₆₀ fullerene particles dispersed in a poly(methyl methacrylate) (PMMA) matrix exhibits a significant decrease, even at small concentrations of the fullerene. It is concluded that the main factor that determines this behavior is a decrease in the mean free-path length of phonons, which is caused by increasing inhomogeneity of the material and growing number of the scattering centers.     

Fullerene-containing C<Subscript>60</Subscript>-CdTe(CdSe) composite nanostructures

We have developed a technology for obtaining homogeneous films based on fullerene-containing C₆₀-CdTe and C₆₀-CdSe composites. The surface morphology of the initial films is characterized by an average lateral roughness size of about 150 nm. Annealing in a vacuum of 10^?5 Torr for 3 h showed that C₆₀-CdSe films are stable at temperatures up to T=180°C, while the same treatment of films of the (C₆₀)_1?x(CdTe)_x system with x<0.5 leads to the appearance of surface clusters of the semiconductor component with an average size of about 500 nm. The cluster density and size increase with the content of CdTe in the initial composite powder. The photoluminescence spectrum of a (C₆₀)_1?x(CdTe)_x film with x=0.5 upon annealing displays a dominating peak at 730 nm, which is indicative of a significant modification of the film structure as a result of this post-growth treatment. It is demonstrated that fullerene-containing composite network nanostructures with a lateral resolution up to 250 nm can be created by direct electron-beam lithography.     

Application of the multifractal concept to characterization of the structure of composite films of fullerene C<Subscript>60</Subscript> doped with CdTe

Peculiarities in the surface morphology of fullerene C₆₀ based composite films containing various amounts of CdTe (0 to 100 mass %) were studied using methods of information theory. The characteristic size of the surface structure elements for these films amounts to 150–200 nm. It is shown that the concept of multifractals and the methods of multifractal analysis can be used for a quantitative comparison of the surface topography of such composite films. The multifractal analysis reveals the interval of fullerene based film compositions containing 15–20% CdTe, which corresponds to the most equilibrium conditions of film formation and provides for the most ordered material configuration. It was established that, using films with such compositions, fullerene-containing network nanostructures with a lateral resolution of 250 nm can be obtained by direct electron-beam lithography.     

The anisotropic effect of a weak magnetic field on the photoconductivity of C<Subscript>60</Subscript> single crystals

The effect of crystal orientation on the photogeneration of free charge carriers was studied for C₆₀ single crystals in a weak magnetic field. The photoconductivity sharply depends on the orientation of magnetic field with respect to the crystallographic directions, showing a 5–8% increase for seven axes of the C₆₀ crystal.     

Crossover Behavior of Variable Range Hopping in Bi-Doped C<Subscript>60</Subscript>

Abstract: Mott variable-range hopping (VRH) is characterized by an exp [1/T ^1/4] behavior in the temperature dependence of resistivity and is generally observed near the metal–insulator transition as electron screening increases and the Coulomb gap disappears near the metallic phase, while Efros–Shklovskii VRH, characterized by exp [1/T ^1/2], is found deeper in the insulating region. We have investigated the transport properties of Bi-doped fullerene C₆₀. Samples were prepared via solid state reaction in a sealed quartz tube near 600°C. The resistivity data can be fit with a single function exp [1/T ^υ ] (υ=1/4 or 1/2, depending on the Bi concentration) over the entire temperature range below 300 K and over 5–6 orders of magnitude in resistivity. υ changes from 1/4 to 1/2 as the Bi concentration increases, suggesting a crossover from Mott VRH to intergranular tunneling at higher Bi concentration. The thermoelectric Seebeck coefficient was also measured and is about 20 μV/K at room temperature. It decreases with decreasing temperature. The thermal conductivity of the doped samples is extremely low.     

Photoluminescence and paramagnetic defect formation in PMMA-C<Subscript>60</Subscript> fullerene nanocomposite

The dynamics of thermoinduced defect formation and the photoluminescent properties of a poly(methyl methacrylate)-fullerene (PMMA-C₆₀) composite have been studied using a combination of spectroscopic techniques. Quantum-chemical calculations with implicit allowance for the presence of oxygen have been performed for a MMA-C₆₀ model complex. The generation of defects and the quenching of photoluminescence, which take place in the same temperature interval where the macromolecules exhibit low-temperature depolymerization in the presence of C₆₀, can be precursors of the depolymerization process.