Polarizability exaltation of endofullerenes X@C<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 20, 24, 28, 36, 50, and 60; X is a noble gas atom)

The polarizability exaltation in molecules of endohedral complexes of C₂₀, C₂₄, C₂₈, C₃₆, C₅₀, and C₆₀ fullerenes with He, Ne, Ar, and Kr noble gas atoms has been revealed and studied by the density functional theory method. It has been found that the sign of the Δα polarizability exaltation depends on the number of atoms in a fullerene molecule.     

Possible scheme of synthesis-assembling of fullerenes

A new scheme of fullerene formation is proposed on the basis of the similarity between the experimentally detected carbon structures. According to experimental data, the microclusters of C₂ and C₁₀ are synthesized first and then either an intermediate nucleus cluster or an obtainable lower fullerene is assembled from them. A high-symmetry fullerene can be assembled with a high probability from a nucleus cluster with a “good” symmetry. The atomic and electronic structures of molecules such as C₃₆, C₆₀, C₇₀, and C₇₆ are analyzed. For C₃₆, the NMR spectra are calculated and compared with the experimental data.     

Electronic interactions in two-dimensional polymers of the C60 fullerene

The electronic structure of tetragonal and rhombohedral polymers of the C₆₀ fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C₆₀ molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C₆₀ fullerene polymers leads to a broadening of the maxima in the CK _α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C₆₀ fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C₆₀ molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level.     

C<Subscript>60</Subscript> fullerene as an η<Superscript>6</Superscript> ligand in π complexes of transition metals

The possible existence of complexes formed by the C₆₀ fullerene or its derivatives with transition metals interacting with the carbon cage via η⁶−π type bonding is discussed. The derivatives C₆₀ R ₆ of the C₆₀ fullerene (R = −, H, F, Cl, Br, CN) are analyzed using the density functional method within the Perdew-Burke-Ernzerhof approximation. In these molecules, the R groups are attached to carbon atoms located in the α positions with respect to the common hexagon of the C₆₀ fullerene. The structure and electron configuration of complexes formed by these molecules with Cr(C₆H₆), Cr(CO)₃, MoC₆H₆, and Mo(CO)₃ particles are modeled. The “dimer” systems C₆₀R₆-M-R ₆C₆₀ (M = Cr, Mo, R =-, H, F) are investigated in which two fullerene molecules interact via a transition-metal atom. It is found that the introduction of six R groups in the α sites with respect to the common hexagon of C₆₀ favors the formation of complexes of these derivatives of the C₆₀ fullerene with the Cr(C₆H₆), Cr(CO), Mo(C₆H₆), and Mo(CO)₃ particles in which η⁶-π type bonds arise between the metal and the atoms of the hexagon fringed with the R groups. It is also demonstrated that analogous complexes with a “bare” C₆₀ fullerene are possible, but they are significantly less stable. The (C₆H₆) M-R ₆C₆₀ R ₆-M (C₆H₆) complexes of particles M(C₆H₆) (M= Cr, Mo) and derivatives R ₆C₆₀ R ₆ (R =-, H, F, Cl, Br) are studied. In the R ₆C₆₀ R ₆ molecule, six R groups are located in the α sites with respect to the common hexagon of the C₆₀ fullerene and six other groups fringe the opposite hexagon. The obtained results can be applied to planning synthesis of new complexes that C₆₀ fullerene derivatives can form with transition metals. Original Russian Text ￠ E.G. Gal’pern, A.R. Sabirov, I.V. Stankevich, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 12, pp. 2220–2223.     

Electron-induced radiation from C60 fullerene in the gas phase

The formation of radiating particles in the excitation of C₆₀ fullerene molecules by electrons with energies E _e<100 eV is investigated by the method of crossed molecular and electron beams. A quasicontinuous (with a spectral resolution of 3 nm) emission spectrum, close to the Planck emission spectrum of a heated body, is recorded in the wavelength range 300–800 nm. The temperature of the radiation corresponds to an internal energy of the C₆₀ molecule of approximately 40 eV. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 915–919 (25 June 1996)     

Long-lived excited states and photoluminescence excitation spectra in single crystals of fullerene C60

Photoluminescence, optical absorption spectra, and photoluminescence excitation spectra were measured on large (2–3 mm), very pure crystals of fullerene C₆₀ at 5 K. It is shown that the main contribution to the photoluminescence of these crystals is from singlet and triplet excitons captured on crystal defects. The concentration of these defects does not exceed 10¹⁸ cm⁻²³, and the lifetime of triplet excitons on these defects is about 3 ms. It is shown that the symmetry distortion of the C₆₀ molecules at the defects is rather large and causes the oscillator strength of the zero-phonon optical transitions to be comparable to the most intense optical transitions with the participation of intramolecular vibrations. Zh. éksp. Teor. Fiz. 113, 734–746 (February 1998)     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

C60 fullerite with a “stretched” fcc lattice

It is shown that deuteration of C₆₀ fullerite followed by thermal decomposition of the resulting deuteride C₆₀D₂₄ leads to the formation of an fcc lattice with a ₀=14.52 Å in the final product, which according to the IR spectra consists mainly of C₆₀ fullerene molecules. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 239–242 (10 August 1998)     

Electronic structure of linear chains of fullerenes

The band structure of linear chains of fullerene molecules is calculated as a function of the intermolecular π-electron overlap integral T, which increases under increasing external pressure. Chains consisting of neutral (C₆₀) and charged (C ₆₀ ⁻ ) molecules are studied. It is shown that there is a sharp transition from a metal (or narrow-gap semiconductor) to an insulator (with band gap ∼1 eV)with increasing T. The proposed model makes it possible to describe the formation of solid-carbon struc-tures, containing chains of covalently bound fullerene molecules, with different pressure-dependent semiconductor properties. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 647–650 (25 April 1997)     

Electronic and optical properties of fullerene nanostructures

Two new types of molecular/electronic fullerene nanostructures are considered: 1) highly stable hydrated clusters (I _h symmetry group) and microcrystals (T _h symmetry group) of fullerene C₆₀ in water solution and 2) the single-walled carbon nanotube from C₆₀ fullerenes. The vibrational spectra of these fullerene nanostructures are calculated using molecular dynamics. The electronic properties of a single-walled fullerene nanotube are investigated using the tight-binding method. The theoretical results obtained were compared with available experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 885–887 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Tubular fullerenes inside carbon nanotubes: optimal molecular orientation versus tube radius

We present an investigation of the orientations and positions of tubular fullerene molecules (C₉₀, ..., C₂₀₀) encapsulated in single-walled carbon nanotubes (SWCNT), a series of so-called fullerene nanopeapods. We find that increasing the tube radius leads to the following succession of energetically stable regimes: (1) lying molecules positioned on the tube's long axis; (2) tilted molecules on the tube's long axis; and (3) lying molecules shifted away from the tube's long axis. As opposed to C₇₀ and C₈₀ molecules encapsulated in a SWCNT, standing orientations do not develop. Our results are relevant for the possible application of molecular-orientation-dependent electronic properties of fullerene nanopeapods, and also for the interpretation of future experiments on double-walled carbon nanotube formation by annealing fullerene peapod systems.     

Some comparative thermodynamic characteristics of fullerite and various covalent elements

Measurements of the specific heat and elastic wave velocities for a C₆₀ fullerene sample treated at high pressure and temperature are used to estimate the Debye temperature and the function ΔC=C _p−C _v, and also to calculate the thermal expansion work in the ideal approximation. Similar calculations were made for graphite, diamond, silicon, germanium, and various refractory metals. The results were used to draw qualitative conclusions on the structural stability of a new material obtained from fullerene C₆₀ which possesses extremely high hardness. Fiz. Tverd. Tela (St. Petersburg) 40, 1387–1389 (July 1998)     

Effect of low-temperature compression on the parameters of the phase transition at 250–260 K in C60 single crystals

A thermodynamic transition in C₆₀ crystals, subjected beforehand to uniaxial compression at various temperatures and to thermal action, was investigated by differential scanning calorimetry. It was established that compression of the crystal at temperatures below the phase transition has a much smaller effect on the transition itself than a similar or even much weaker action at a temperature above the phase transition. A “quenching” effect was also found. This effect is probably due to the orientational order of individual fullerene molecules. A correlation was established between the magnitude of the effect and the preliminary deformation of C₆₀ crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 550–553 (March 1999)     

Permeability of C60 films deposited on polycarbonatesyloxane to N2, O2, CH4, and He gases

In this study, we report on the gas permeability of non-polymerized and polymerized fullerene films (thickness about 0.5 μm) grown on an organic polymer substrate, polycarbonatesyloxane (PCS), using a high vacuum deposition method. The photopolymerized C₆₀ films were prepared by a simultaneous thin film deposition and UV-vis irradiation method which was reported previously [V.A. Karachevtsev, P.V. Mateichenko, N.Y. Nedbailo, A.V. Peschanskii, A.M. Plokhotnichenko, O.M. Vovk, E.N. Zubarev, A.M. Rao, Carbon 42 (2004) 2091]. Raman spectroscopy revealed that ∼90% of the C₆₀ molecules are covalently linked to neighboring C₆₀ molecules in the photopolymerized film after 20 h of film deposition/irradiation. Permeability of the resulting membranes consisting of polymer PCS base and fullerene films to the N₂, O₂, CH₄, and He gases has been investigated. Our experiments revealed that the gas permeability properties are dependent on the age of the membrane. In particular, the aged membrane exhibited an enhanced permeability for O₂ and He gases in comparison to N₂ and CH₄, respectively.     

Interaction of C60 molecules with a (100) Mo surface

The adsorption, initial stages of film growth, and transformation of an adlayer of C₆₀ molecules on a (100) Mo surface upon heating are studied under ultrahigh-vacuum conditions. It is shown that the C₆₀ molecules remain intact in the adsorbed state up to 760 K. Layer-by-layer growth of a fullerite film is observed at room temperature, while tower-shaped crystallites grow up from a loosely packed monolayer with an approximate concentration of C₆₀ molecules equal to (1.3±0.2)×10¹⁴ molecules · cm⁻² at 500–600 K. In the latter case the percentage of the surface occupied by them depends on the temperature and the impinging flux density of fullerene molecules, but after a certain stationary value has been achieved, it scarcely depends on the exposure time. Zh. Tekh. Fiz. 69, 117–122 (November 1999)     

Temperature dependence of the spin-lattice relaxation time in local triplet-excited states of a doped C60 fullerene crystal

The temperature dependence of the spin-lattice relaxation time of triplet-excited impurity centers of a C₆₀ fullerene crystal is investigated within the framework of the spin-phonon coupling mechanism which arises as a result of the admixture of rotational to translational motions of the molecules due to acoustic lattice vibrations. General expressions are obtained for the transition probabilities for the direct single-phonon and Raman two-phonon processes. The method of atom-atom potentials is used to carry out a concrete calculation of the spin-lattice relaxation time for an isotopic impurity in a C₆₀ crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1699–1702 (September 1997)     

Molecular simulation of C60 adsorption onto a TiO2 rutile (1 1 0) surface

A Monte Carlo molecular simulation study is presented on the adsorption and growth of C₆₀ films on the surface of the (1 1 0) face of rutile. Simulations are performed for a temperature of 600 K using atomistic models both for the fullerene molecules and the TiO₂ surface. It is found in this work that C₆₀ is adsorbed preferably in an ordered arrangement along the surface depressions over the exposed undercoordinated Ti cations. At low densities adsorption occurs preferably at alternate rows, with locations in consecutive rows being occupied appreciably only at higher C₆₀ densities. At low densities, the fullerene molecules tend to aggregate into islands in the surface plane. Additional layers of C₆₀ form only as the density increases, and do so before a monolayer is completed in all consecutive rows. Full monolayer capacity obtained at the highest densities is about 0.9 C₆₀ molecules per nm², but this is only achieved by completing the packing of molecules in interstices at a slightly upper level. The fraction of the molecules that lie closest to the surface only amounts to 0.6 molecules per nm².     

Specific features of the stone-wales transformation in the C20 and C36 fullerenes

The initial stage of the defect formation in the C₂₀ and C₃₆ fullerenes has been investigated by the ab initio method and in terms of tight-binding models. A comparison with the Stone-Wales transformation in the C₆₀ fullerene has revealed the presence of two independent stages in this process, the first of which is an “incomplete” Stone-Wales transformation. At this stage, the C₂₀ and C₃₆ fullerenes transform into metastable defect configurations with two adjacent “windows” on their surface, whereas a similar configuration of the C₆₀ fullerene is unstable and corresponds to a saddle stationary point of the potential energy of the cluster. A new mechanism of plastic deformation due to the Stone-Wales transformation has been predicted for the (C₃₆) _n fullerites.     

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.     

Laser mass spectrometry study of the properties of fullerene structures

Fullerene films grown by various methods are studied using mass spectrometry. The mass spectra of the films formed onto an aluminum foil using thermal deposition (TD) or supersonic molecular beam (SMB) exhibit a small change in the mass peak distribution in the C₁₂₀ dimer range as compared to the initial fullerene powder during desorption by laser radiation irrespective of the radiation wavelength (λ = 259, 518 nm). Under the action of laser radiation with wavelength λ = 259 nm, fullerene films grown on a silicon substrate with an SMB also exhibit a small change in the mass peak distribution in the C₁₂₀ dimer range. At λ = 518 nm, the mass peak distribution in the dimer range shifts significantly toward small masses, so that the intensity maximum corresponds to M ≅ C₁₀₂. This fact is assumed to be related to the polymerization of an SMB fullerene film caused by heating due to the absorption of laser radiation with a wavelength λ = 518 nm.