Superconductivity and spectroscopy of heterofullerides Rb<Subscript>2</Subscript>MC<Subscript>60</Subscript>, K<Subscript>2</Subscript>MC<Subscript>60</Subscript>, and KM<Subscript>2</Subscript>C<Subscript>60</Subscript> (M = Mg,Be)

A series of new heterofullerides with compositions Rb₂MC₆₀, K₂MC₆₀, and KM₂C₆₀ (M = Mg, Be) have been synthesized. Measurements of the temperature dependences of the magnetic susceptibility in the temperature interval from 4.2 to 300 K reveal a superconducting transition in heterofullerides K₂MgC₆₀, KMg₂C₆₀, K₂BeC₆₀, and Rb₂BeC₆₀ at temperatures T _c = 13–22 K. The electron states with uncompensated spin are studied by the electron paramagnetic resonance technique. The contributions of conduction electrons and localized electrons to the paramagnetic susceptibility are extracted.     

The oxidation of fullerenes (C<Subscript>60</Subscript>, C<Subscript>70</Subscript>) with various oxidants under ultrasonication

The reaction of C₆₀, under ultrasonication, with various oxidants, such as 3-chloroperoxy benzoic acid (Fluka 99%), 4-methyl morpholine N-oxide (Aldrich 97%), chromium (VI) oxide (Aldrich 99.9%), and the oxone® monopersulfate compound, causes the oxidation of fullerenes at room temperature. The FAB-MS spectra and HPLC profile confirmed that the products of fullerene oxidation were [C₆₀(O)_n] (n=1～3 or n=1). C₇₀ also reacted, under ultrasonication, with various oxidants, but the reaction rate of C₇₀ was lower than that of C₆₀.     

Equilibrium state of C<Subscript>60</Subscript>, C<Subscript>70</Subscript>, and C<Subscript>72</Subscript> nanoclusters and local defects of the molecular skeleton

The stability of C₆₀ and C₇₀ fullerenes and C₆₀ and C₇₂ nanotubes devoid of 2–12 atoms of the cluster skeleton was theoretically studied. It was established that C_n molecules with an even number of atoms remain stable, which was confirmed by experimental studies of monomolecular decay of clusters with the number of atoms n≥30. The change in the internuclear distances and in the ionization potential of nanoclusters was determined depending on the number of eliminated atoms. Such defects were shown to decrease the ionization potential of nanoclusters by 0.5–0.8 eV. The electron spectrum was calculated within the Harrison semiempirical tight-binding model in the Goodwin modification. A new parametrization of interatomic matrix elements of the Hamiltonian and atomic terms for carbon nanoclusters was suggested.     

Investigation of fluorination reactions of the C<Subscript>60</Subscript>Br<Subscript>24</Subscript> bromofullerene with xenon difluoride

The products of the reaction between the C₆₀Br₂₄ bromofullerene and xenon difluoride are studied by IR spectroscopy, laser desorption mass spectrometry, ¹⁹F NMR spectroscopy, and thermogravimetry. It is revealed that the degree of fluorination of the substrate is primarily determined by the duration of contact between the reactants. An attempt is made to perform nucleophilic substitution of fluorine for bromine in the C₆₀Br₂₄ bromofullerene in a solution with potassium fluoride.     

X-ray emission and X-ray photoelectron spectroscopic studies of fullerene fluoride C<Subscript>60</Subscript>F<Subscript>24</Subscript>

The structure of the fullerence fluoride C₆₀F₂₄ of the T _h symmetry contains two types of chemically different carbon atoms, namely, atoms of isolated double bonds and atoms of CF groups. X-ray photoelectron and x-ray emission spectroscopic studies of C₆₀F₂₄ revealed a difference in the widths of the x-ray bands corresponding to these types of atoms. Nonempirical quantum-chemical calculations performed for C₅₉NF ₂₄ ⁺ ions with a hole in the C 1s core level of the fullerence fluoride showed that the difference in the bandwidths may be due to the fact that the vibrational states of the system are different when 1s electrons are removed from chemically nonequivalent atoms.     

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.     

Photopolymerization of C<Subscript>60</Subscript> and Li@C<Subscript>60</Subscript> studied by second-harmonic generation and infrared spectroscopy

The photopolymerization of C₆₀ and Li@C₆₀ films was investigated by means of optical second-harmonic generation. The films were deposited under ultra-high-vacuum conditions and irradiated in situ with an Ar⁺ laser at 514 nm. The second harmonic generated by a Nd:YAG laser working at 1064 nm was monitored after different steps of irradiation. Photopolymerization was observed after very low irradiation doses, of the order of 10²⁰ photons/cm², and confirmed with infrared absorption spectroscopy. Similar kinetics for C₆₀ and Li@C₆₀ were observed. The measurements give evidence for photopolymerization of the endohedral fullerene Li@C₆₀. PACS 78.30.Na; 82.50.Hp; 81.05.Tp     

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.     

Photophysics of C<Subscript>60</Subscript> and C<Subscript><Emphasis Type="Bold">60</Emphasis></Subscript><Superscript><Emphasis Type="Bold">-</Emphasis></Superscript> in faujasite zeolites

While the intercalation of C₆₀ and the formation of C ₆₀ ^- in the supercages of NaX and NaY are confirmed by using ¹²⁹Xe NMR and ESR, the photophysical properties of C₆₀ and C ₆₀ ^- are characterized by monitoring transient reflectance spectra and kinetics, fluorescence kinetics, and diffuse reflectance spectra. C ₆₀ ^- is considerably more abundant in NaY than in NaX. This difference is explained in terms of polarity difference between two zeolites. Both C₆₀ and C ₆₀ ^- have remarkably elongated excited-state lifetimes due to their collision-free environment in zeolitic nanocavities although C ₆₀ ^- has much shorter lifetimes than C₆₀. C ₆₀ ^- , in particular, shows intense absorption and emission due to its reduced symmetry in zeolites. Received 13 July 2001 and Received in final form 8 October 2001     

Translocation mechanism of C<Subscript>60</Subscript> and C<Subscript>60</Subscript> derivations across a cell membrane

Carbon-based nanoparticles (NPs) such as fullerenes and nanotubes have been extensively studied for drug delivery in recent years. The permeation process of fullerene and its derivative molecules through membrane is essential to the utilization of fullerene-based drug delivery system, but the mechanism and the dynamics of permeation through cell membrane are still unclear. In this study, coarse-grained molecular dynamics simulations were performed to investigate the permeation process of functionalized fullerene molecules (ca. 0.72 nm) through the membrane. Our results show that single functionalized fullerene molecule in such nanoscale could permeate the lipid membrane in micro-second time scale. Pristine C₆₀ molecules prefer to aggregate into several small clusters while C₆₀OH₁₅ molecules could aggregate into one big cluster to permeate through the lipid membrane. After permeation of C₆₀ or its derivatives into membrane, all C₆₀ and C₆₀OH₁₅ molecules disaggregated and monodispersed in the lipid membrane.

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Electrodiffusion phenomena in C<Subscript>60</Subscript> thin films

As part of our ongoing research program to produce semiconductor devices based on C₆₀ thin films, we report here on our first attempts at the intercalative doping of C₆₀ thin films through the diffusion of metals. Two techniques were employed: (a) chemically induced counter electrodiffusion of Cu and I₂ into a C₆₀ matrix and (b) Au diffusion under the action of an external electric field.     

Ordering phenomena in C<Subscript>60</Subscript>-tetraphenylphosphonium bromide

The fulleride salt C₆₀-tetraphenylphosphonium bromide is investigated as a function of temperature by single crystal X-ray diffuse scattering and diffraction. At room temperature, the C₆₀ orientational disorder is found to be more complex than previously expected. Moreover, a structural phase transition, due to the C₆₀ orientational ordering, is evidenced around 120 K. Its relation with the stabilization of a static Jahn-Teller effect is discussed. Received 3 November 1999     

Thermally stimulated desorption of C<Subscript>60</Subscript> and C<Subscript>70</Subscript> fullerenes from rigid-chain polyimide films

Polyimide-fullerene composite thin coatings are investigated using thermal desorption mass spectrometry in the temperature range 20–800°C. It is found that, at temperatures below the temperature of decom-position of the polymer matrix, thermally stimulated desorption of fullerene molecules is limited by the diffusion of fullerene molecules in the matrix. The diffusion coefficients and activation energies of diffusion of C₆₀ and C₇₀ fullerene molecules are determined from the experimental data on thermally stimulated desorption in the framework of several approaches. It is revealed that the diffusion of C₇₀ molecules in the polyimide matrix is more hindered than the diffusion of C₆₀ molecules in the same matrix.     

High-pressure hydrogenated fullerenes: Optical spectra and stability of C<Subscript>60</Subscript>H<Subscript>36</Subscript> at high pressure

The optical Raman and photoluminescence (PL) spectra of the high-pressure hydrogenated fullerene C₆₀ are studied at normal conditions and at high pressure. The Raman spectrum of the most stable hydrofullerene C₆₀H₃₆ contains a large number of peaks related to various isomers of this molecule. Comparison of the experimental data with the results of calculations shows that the most abundant isomers have the symmetries S₆, T, and D_3d. The Raman spectrum of deuterofullerene C₆₀H₃₆ is similar to that of C₆₀H₃₆, but the frequencies of the C-H stretching and bending modes are shifted due to the isotopic effect. The PL spectrum of hydrofullerene C₆₀H₃₆ is shifted to higher energies by approximately 1 eV with respect to that of pristine C₆₀. The effect of hydrostatic pressure on the Raman and PL spectra of C₆₀H₃₆ has been investigated up to 12 GPa. The pressure dependence of the phonon frequencies exhibits peculiarities at approximately 0.6 and 6 GPa. The changes observed at approximately 0.6 GPa are probably related to a phase transition from the initial orientationally disordered body-centered cubic structure to an orientationally ordered structure. The peculiarity at approximately 6 GPa may be related to a pressure-driven enhancement of the C-H interaction between the hydrogen and carbon atoms belonging to neighboring molecular cages. The pressure-induced shift of the photoluminescence spectrum of C₆₀H₃₆ is very small up to 6 GPa, and a negative pressure shift was observed at higher pressure. All the observed pressure effects are reversible with pressure.     

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.     

Grain Boundary Diffusion in C<Subscript>60</Subscript> Thin Films

This paper focuses on the effect of grain boundaries on the diffusion processes in polycrystalline C₆₀ thin films. Electrically induced diffusion of Au was investigated by in situ measurements of the film conductivity. Electron Paramagnetic Resonance (EPR) spectroscopy was used to study diffusion of oxygen. Increase in grain sizes in polycrystalline C₆₀ thin films was found to result in the acceleration of gold and oxygen diffusion. The results are interpreted assuming that these impurities diffuse in C₆₀ films dominantly along grain boundaries.     

Photoionization of the C<Subscript>60</Subscript> and C<Subscript>240</Subscript> fullerenes by ultrashort electromagnetic pulses

The photoionization of the C₆₀ and C₂₄₀ fullerenes by ultrashort electromagnetic pulses of subfemtosecond duration is studied. The probability for the process to occur during the action of the pulse as a function of the pulse duration is calculated for different carrier frequencies. The spectrum of photoelectrons emitted during the ionization of the fullerenes by a pulse with a corrected Gaussian shape is calculated.     

Impact of a physiological medium on the aggregation state of C<Subscript>60</Subscript> and C<Subscript>70</Subscript> fullerenes

The C₆₀ and C₇₀ fullerene-cluster size distribution in aqueous solutions and a physiological medium is studied via dynamic light scattering. The initial aqueous solutions of fullerenes obtained via different methods are found to contain clusters with a characteristic size of about 100 nm. The additional aggregation of fullerenes is observed after their transfer into a physiological medium (0.9% NaCl) and is established to depend on the preparation method. The cluster-size distribution in a fullerene–pectic-acid mixture is found to vary in water and a physiological medium. The results reveal the need for additional studies of the structure and properties of C₆₀ and C₇₀ molecules, as well as their complexes with medicines, in a physiological medium for medical applications.     

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.     

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.