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.     

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.     

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.     

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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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.     

Cluster <Emphasis Type="Italic">ab initio</Emphasis> calculations for the C<Subscript>60</Subscript>F<Subscript>24</Subscript>, C<Subscript>60</Subscript>Cl<Subscript>24</Subscript>, and C<Subscript>60</Subscript>Br<Subscript>24</Subscript> halofullerenes

This paper reports on ab initio cluster calculations of the equilibrium geometry, electronic structure, and vibrational properties of the C₆₀, C₆₀F₂₄, C₆₀Cl₂₄, C₆₀Br₂₄ molecules.     

Cleavage of C<Subscript>60</Subscript> fullerite crystals

It is found that, under certain conditions, C₆₀ fullerite crystals can be cleaved along cleavage planes that are close-packed planes of the {111} type. Rigid gas-phase grown crystals exhibit good cleavage properties. In experiments with active compressive deformation, these crystals showed a high yield point τ_y = 2.65 MPa, a “parabolic” stress-strain curve, and brittle fracture after attaining a shear strain of about 8%. The fracture surface was clearly seen to have fragments parallel to the (111) plane. Typical microstructures observed in the cleavage plane are discussed: crystallographic cleavage steps, an indentation pattern, and a dislocation prick rosette. The fact that the activation volume V ? 60b³ is small (b is the Burgers vector of a dislocation) and strain-independent indicates the Peierls character of fullerite deformation or dislocation drag in a dense network of local defects.     

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.     

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.     

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.     

Polyaniline composites with fullerene C<Subscript>60</Subscript>

Polyaniline-fullerene composites were prepared by the introduction of fullerene during polymerization of aniline. An investigation of the composites using FTIR and ¹³C NMR spectroscopy indicated interaction between fullerene and the imine groups of polyaniline. The formation of a polyaniline-fullerene complex with a structure corresponding to a doped polyaniline was proved by wide-angle x-ray scattering analysis. The conductivity of composites is more than four orders of magnitude higher than that of undoped polyaniline and that of fullerene. Improvement in the thermal stability of composites was evaluated using TGA.     

Dislocation mobility in C<Subscript>60</Subscript> fullerite crystals

The dependences of the path of leading dislocations in indentation rosette rays on the load, the loading time, and the indentation temperature in the range 260 < T ≤ 373 K were studied for C₆₀ fullerite crystals. The dislocation mobility parameters are estimated: the exponent m characterizing the stress dependence of the dislocation velocity depends on the structural perfection of the crystal and ranges from 2.3 to 24.5, the activation energy for dislocation motion ΔH ₀ ? (0.4–0.5) eV, and the velocity of leading dislocations in indentation rosette rays v _l ? 10^?5?10^?4 cm/s. The data from micro-and macromechanical experiments are shown to agree with each other. The dislocation mobility is assumed to be controlled by the dislocation interaction with local barriers.     

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     

Stone-Wales transformation paths in fullerene C<Subscript>60</Subscript>

The mechanisms of formation of a metastable defect isomer of fullerene C₆₀ due to the Stone-Wales transformation are theoretically studied. It is demonstrated that the paths of the “dynamic” Stone-Wales transformation at a high (sufficient for overcoming potential barriers) temperature can differ from the two “adiabatic” transformation paths discussed in the literature. This behavior is due to the presence of a great near-flat segment of the potential-energy surface in the neighborhood of metastable states. Moreover, the sequence of rupture and formation of interatomic bonds is other than that in the case of the adiabatic transformation.     

Second harmonic generation in thin Ge<Subscript>35</Subscript>Sb<Subscript>5</Subscript>S<Subscript>60</Subscript> films

Generation of the second optical harmonic is investigated in 250 to 1500-nm thick films of Ge₃₅Sb₅S₆₀ chalcogenide glass. It is shown that a decrease in quadratic optical susceptibility in thicker films can result from a decrease in the bulk contribution from the electric dipole to the susceptibility and can be used in nonlinear optical diagnostics of the films.     

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.     

Growth of carbon nanotubes from C<Subscript>60</Subscript>

Carbon nanotubes can be obtained from a multitude of molecular precursors in chemical vapor deposition (CVD) processes. Here we demonstrate that the use of C₆₀ as the carbon feedstock gas in an iron-catalyzed thermal CVD experiment leads to the formation of films of multi-walled carbon nanotubes. The critical role of the diameter of the catalyst particles in determining the efficiency of nanotube growth is clearly demonstrated. Electron microscopy and Raman spectroscopy were employed for the characterisation of the nanotube material. The structural properties of the individual nanotubes show distinctive differences to acetylene-grown multi-walled nanotubes. PACS 81.07.De; 81.10.Bk     

Excitation dynamics of Rydberg states in C<Subscript>60</Subscript>

The electron and nuclear dynamics of C₆₀ fullerenes irradiated with femtosecond laser pulses are investigated with photoelectron and photoion spectroscopy. The focus of this work is the detailed exploration of the population mechanism of Rydberg levels within the excitation process of neutral C₆₀. The effect of excitation wavelength, intensity, chirp, and polarization on the kinetic energy distribution of photoelectrons in single-pulse experiments gives first insight into the underlying processes. In combination with time-resolved two-color pump-probe spectroscopy depending on either pump, or probe pulse intensity, a more complete picture of the interaction can be drawn. The results point towards a very interesting but nevertheless complex behavior including four steps: (i) non-adiabatic multielectron excitation of the HOMO (h_u) → LUMO+1 (t_1g) transition; (ii) thermalization within the hot electron cloud on a time scale below 100 fs, followed by a coupling of energy to vibrational modes of the molecule via doorway state(s); (iii) population of electronically excited Rydberg states by multiphoton absorption, and (iv) single photon ionization from the excited Rydberg states. This excitation process results in a characteristic sequence of photoelectron lines in the photoemission spectra. The comparison of the experimental results with recent theoretical work gives convincing evidence that non-adiabatic multielectron dynamics (NMED) plays a key role for the understanding of the response of C₆₀ to short-pulse laser radiation.