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.     

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.     

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.     

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.     

Collision dynamics of He@C<Subscript>60</Subscript>+He@C<Subscript>60</Subscript> at low energies

A semi-empirical molecular dynamics model is developed. The central collisions of C₆₀+C₆₀ and He@C₆₀+He@C₆₀ at different incident energies are investigated based on this model. It is found that the dimer structures have been produced at proper incident energies and these fullerene dimers could be formed by a self-assembly of C₆₀ fullerene and He@C₆₀. The He atom has a significant effect at higher incident energy and this embedded He atom can enhance the stability of the dimer structure.     

Structure of C<Subscript>60</Subscript> polymer coatings deposited via electron-beam dispersion of fullerite

The structure and surface of thin coatings deposited via electron-beam dispersion of the C₆₀ fullerite have been investigated using IR and Raman spectroscopy, mass spectroscopy, and atomic-force microscopy. It has been demonstrated that layers with different contents of the polymerized phase, crystals of the tetragonal polymer phase, and three-dimensional polymeric forms of the C₆₀ fullerene are formed under the conditions providing for irradiation by secondary electrons in vacuum at a substrate temperature of 300 K.     

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.     

ATI of complex systems: Ar and C<Subscript>60</Subscript>

Above threshold ionization of two structurally different systems is presented namely a rare gas such as argon and the more complex C₆₀ fullerene. We show that the ionization dynamics is different and is dominated by the presence of high-lying Rydberg states in Ar and low-lying bound states in C₆₀. The study is based on a theoretical (solving the time dependent Schrödinger equation) and/or experimental (using measurements from a photoelectron imaging spectrometer) aspect.Received: 20 December 2002, Published online: 24 April 2003PACS: 33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Fb Photoionization of atoms and ions - 36.40.-c Atomic and molecular clusters - 33.60.-q Photoelectron spectra - 61.48.+c Fullerenes and fullerene-related materials     

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.     

Scanning tunneling spectroscopy and manipulation of C<Subscript>60</Subscript> on Cu(111)

The properties of C₆₀ adsorbed on Cu(111) have been studied using low temperature scanning tunnelling microscopy and spectroscopy. In the electronic spectrum of the molecule, we observe features that can be assigned to molecular orbitals. The LUMO level is split into two states, as a consequence of the charge transfer from the substrate to the carbon cage. The data from the inelastic electron tunnelling spectroscopy reveal two peaks that can be assigned to the intramolecular vibrational modes of the C₆₀ cage. We demonstrate also controlled manipulation of single molecules. The plot of the tip height, recorded during the manipulation process, indicates that the C₆₀ is pushed along the surface. PACS 68.37.Ef; 73.61.Wp; 68.43.Pq; 82.37.GK; 68.43.-h     

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.     

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.     

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.     

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.     

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.     

Anomalous character of the interaction of C<Subscript>60</Subscript> molecules with the sulfur-saturated Ta(100) surface

It is shown that, contrary to all previously studied systems, heating to ～800 K in the C₆₀? TaS² monolayer-Ta(100) adsorption system leads to the complete removal of the deposited fullerene molecules. A model is proposed that explains the observed phenomenon by a very weak nonchemisorption interaction between the C₆₀ molecules and the valence-saturated surface of tantalum disulfide that forms layered crystals with van der Waals interaction between layers.     

Effect of bismuth nanolayers on the oriented growth of fullerene C<Subscript>60</Subscript> on an amorphous substrate

The effect of a bismuth sublayer with an effective thickness of 0.5 to 4 nm on the structure of C₆₀ fullerene films grown on amorphous substrates (silicon covered with a natural oxide layer; glass) using the quasi-closed-volume method is studied. An x-ray diffraction study of fullerene films showed that the intensity ratio between the (220) and (111) peaks depends nonmonotonically on the sublayer thickness. In the bismuth sublayer thickness range 0.5–2.0 nm, fullerene films are found to exhibit a growth texture with the 〈110〉 axis; the average crystallite size was ～20 µm. The quality of the texture can be improved by varying the fullerene growth temperature.     

DFT investigation on the structures and stabilities of exohedral derivatives for D<Subscript>3</Subscript> C<Subscript>32</Subscript> fullerene: C<Subscript>32</Subscript>X<Subscript>n</Subscript> (X = H and Cl)

A systematic investigation of D₃ C₃₂ fullerene and its derivatives C₃₂X_n (X = H and Cl) has been performed using B3LYP/6-31G(d) method based on the density functional theory. The geometry structures, reaction energies, relative stabilities, and electronic properties have been studied. By investigating the possible C₃₂X_n (X = H and Cl) molecules, C₃₂H₂ and C₃₂Cl₂ behave more thermodynamically accessible with respect to other derivatives. The frontier molecular orbitals and electronic density of states calculations of C₃₂X₂ system indicate that H and Cl passivation have less contribution to the electronic structures, but significantly improve the stability of D₃ C₃₂ fullerene. Finally, the ¹³C NMR chemical shifts of C₃₂H₂ and C₃₂Cl₂ have been simulated to provide helpful information for further experiment identification.