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.     

New property of the fullerenes: the anomalously effective quenching of electronically excited states owing to energy transfer to the C<Subscript>70</Subscript> and C<Subscript>60</Subscript> molecules

The quenching of the electronically excited states of various energy donors—Tb³⁺; 9,10-anthracene dibromide; and adamantanone—by C₇₀ fullerene has been detected and analyzed. The quenching is characterized by anomalously high biomolecular quenching rate constants, which are obtained from the Stern-Volmer dependences of the energy-donor photoluminescence intensity on the concentration of the C₇₀ molecules. It has been shown that the high efficiency of quenching by the C₇₀ fullerene as compared to the C₆₀ fullerene is due to the higher polarizability of the C₇₀ molecule and large overlap integrals of the energy-donor photoluminescence spectra with the absorption spectrum of the C₇₀ fullerene.     

Vibrational spectra of 2-cyclooctylamino-5-nitropyridine and its mixtures with C<Subscript>60</Subscript> and C<Subscript>70</Subscript> fullerenes

The vibrational spectra of 2-cyclooctylamino-5-nitropyridine (COANP) solutions and the evolution of the spectra upon changing over from the solutions to solid-phase COANP are investigated. The bands observed in the spectra are assigned to the corresponding vibrational modes. The nature of the interaction of COANP with C₆₀ and C₇₀ fullerenes is elucidated by analyzing the transmission spectra of these compounds. No interaction of the COANP compound with C₆₀ and C₇₀ fullerenes is revealed under the studied conditions. It is assumed that the physical nature of this phenomenon can be associated with the formation of liquid-crystal clusters consisting of fullerene molecules.     

Phase Transitions in a Mixture of Amorphous C<Subscript>60</Subscript> and C<Subscript>70</Subscript> Fullerene Phases at High Temperatures and Pressures

Phase transitions in two types of amorphous fullerene phases (C₆₀–C₇₀ (50/50) mixtures and an amorpous C₇₀ fullerene phase) are studied via neutron diffraction at pressures of 2–8 GPa and temperatures of 200–1100°C. Fullerenes are amorphized by grinding in a ball mill and sintered under quasi-hydrostatic pressure in a toroidal-type chamber. Diffraction studies are performed ex situ. It is shown that the amorphous phase of fullerenes retains its structure at temperatures of 200–500°C, and amorphous graphite is formed at 800–1100°C with a subsequent transition to crystalline graphite. This process is slow in a mixture of fullerenes, compared to C₇₀ fullerene. According to neutron diffraction data, the amorphous graphite formed from amorphous fullerene phases has anisotropy that is much weaker in a fullerene mixture.     

Study of the Initial Stage of Fluorinated C<Subscript>60</Subscript>F<Subscript>18</Subscript> Fullerene Adsorption on the Cu(001) Surface

The dynamics of the adsorption and evolution of fluorinated C₆₀F₁₈ fullerene molecules on the Cu(001) surface are studied by real-time ultra-high vacuum scanning tunneling microscopy. Fluorinated fullerene molecules are shown to decompose with time on the Cu(001) surface transforming to C₆₀ molecules. The decay rate depends on the initial molecular coverage. The rapid decay of fluorinated fullerene molecules is observed when the coverage is no higher than 0.2 single layers. As a result, two-dimensional islands consisting of pure C₆₀ molecules are formed on the Cu(001) surface. 2D islands consisting of fluorinated fullerene molecules are formed when the initial molecular coverage is higher than 0.5 single layers. The molecules inside these islands also tend to decompose with time. It is found experimentally that fluorine atoms are removed completely from the initial C₆₀F₁₈ molecules adsorbed on the Cu(001) surface after 250 h when the initial molecular coverage is 0.6 single layers.     

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.     

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.     

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.     

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.     

Withdrawal of Electrodynamical Forbiddance and Peculiarities of the SERS Spectra in Fullerene C<Subscript>70</Subscript>

It is demonstrated that in fullerene C₇₀, which can be considered as a deformed fullerene C₆₀ in some mean sense there is a withdrawal of an Electrodynamical forbiddance of a strong quadrupole light-molecule interaction, which is realized in the fullerene C₆₀. This situation occurs because of the reduction of symmetry of C₆₀ from the icosahedral symmetry group Y_h to the group D_5h. The withdrawal results in appearance of the lines in the SERS spectra of C₆₀, which are forbidden in usual Raman scattering and are active in the infrared absorption spectra. The experimentally measured SERS spectra of C₇₀ demonstrates existence of such lines that strongly confirms our ideas about the dipole-quadrupole SERS mechanism.     

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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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.     

Processing of C<Subscript>60</Subscript> thin films by Matrix-Assisted Pulsed Laser Evaporation (MAPLE)

Thin films of fullerenes (C₆₀) were deposited onto silicon using matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out from a frozen homogeneous dilute solution of C₆₀ in anisole (0.67 wt%), and over a broad range of laser fluences, from 0.15 J/cm² up to 3.9 J/cm². MAPLE has been applied for deposition of fullerenes for the first time and we have studied the growth of thin films of solid C₆₀. The fragmentation of C₆₀ fullerene molecules induced by ns ablation in vacuum of a frozen anisole target with C₆₀ was investigated by matrix-assisted laser desorption/ionization (MALDI). Our findings show that intact fullerene films can be produced with laser fluences ranging from 0.15 J/cm² up to 1.5 J/cm².     

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.     

Kinetics of the sorption of <Superscript>3</Superscript>He by C<Subscript>60</Subscript> fullerite. The quantum diffusion of <Superscript>3</Superscript>He and <Superscript>4</Superscript>He in fullerite

The kinetics of the sorption and subsequent desorption of gaseous ³He in a C₆₀ fullerite powder has been studied in the temperature range of 2–292 K. The temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite have been plotted using the measured characteristic times of filling of octahedral and tetrahedral interstices, as well as previous data. These temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite are qualitatively similar. A decrease in the temperature from 292 to 79 K is accompanied by a decrease in the diffusion coefficients, which corresponds to the dominance of the thermally activated diffusion of helium isotopes in fullerite. A further decrease in the temperature to 8–10 K leads to an increase in the diffusion coefficients by more than an order of magnitude. The diffusion coefficients of ³He and ⁴He are independent of the temperature below 8 K, indicating the tunnel character of the diffusion of helium in C₆₀ fullerite. The isotope effect is manifested in the difference between the absolute values of the diffusion coefficients of ³He and ⁴He atoms at the same temperatures.     

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.     

The structure of fullerite C<Subscript>60</Subscript> intercalated with molecular oxygen

A (O₂) _x C₆₀ sample with a high content of oxygen (x ≥ 0.4) and free of technological solvent impurities was obtained by precipitation from solution. For the first time, the results of the determination of the x coefficients using ¹³C NMR and elemental analysis were compared. It was shown by Raman spectroscopy, mass spectrometry, and NMR that the inclusion of oxygen into fullerite was accompanied by a decrease in the frequency of O=O stretching vibrations by no less than 12 cm⁻¹ compared with gaseous O₂. Nevertheless, oxygen exists in the molecular form in (O₂)_0.4C₆₀ and is released in the form of O₂ as the sample is heated to 373 K. The number of oxygen molecules occupying octahedral pores closets to the fullerene molecule takes on all the possible values, from 0 to 6. At room temperature, the (O₂) _x C₆₀ sample lost oxygen much more slowly than similar products prepared by diffusion saturation of pure fullerite with oxygen.     

Modeling of the structure and electronic structure of condensed phases of small fullerenes C<Subscript>28</Subscript> and Zn@C<Subscript>28</Subscript>

A comparative analysis of the stability factors and electronic structure of two possible crystalline forms of small fullerene C₂₈ and endohedral fullerene Zn@C₂₈ with diamond and lonsdaleite structures is performed using a cluster model. Atoms of elements that, when placed inside C₂₈ cages, have no significant effect on the stability of free small-fullerene molecules are shown to be able to dramatically change the electronic properties and reactivity of the C₂₈ skeleton and to be favorable for forming small-fullerene crystalline modifications, which are covalent crystals. In contrast, if the presence of foreign atoms inside C₂₈ cages stabilizes the isolated nanoparticles, then molecular crystals (such as C₆₀ fullerites) are formed due to weak van der Waals forces.     

The Energy Spectrum and Optical Properties of Fullerene C<Subscript>70</Subscript> within the Hubbard Model

The energy spectrum of fullerene C₇₀ is calculated within the Hubbard model in the mean-field approximation. Using group-theory methods, irreducible representations are determined that correspond to the energy states, as well as allowed transitions in the energy spectrum of fullerene C₇₀. On the basis of this spectrum, an interpretation of experimentally observed optical absorption bands of fullerene C₇₀ is proposed.