Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

Some features of analysis of solutions of fullerenes C60 and C70 by their absorption spectra

The absorption spectra of a series of solutions of C₆₀ and C₇₀ and of their mixture in o-xylene are studied. The deviation from the Bouguer-Lambert-Beer law was observed in the UV absorption band of the solutions. The possible reasons for this effect were analyzed. It is shown that to calculate reliably the composition and concentrations of fullerene mixtures, it is necessary to determine preliminarily the linearity region for the 335-nm absorption band.     

Adsorption of helium on isolated C60 and C70 anions

Adsorption of helium on free, negatively charged fullerenes is studied in this work. Helium nanodroplets have been doped with fullerenes and ionised by electron attachment. For suitable experimental conditions, C^?₆₀ and C^?₇₀ anions are found to be complexed with a large number of helium atoms. Prominent anomalies in the ion abundances indicate the high stability of the commensurate 1×1 phase in which all hollow adsorption sites are occupied by one atom each. The adsorption energy for an additional helium atom is about 40% less than for atoms in the commensurate layer, similar to our previous findings for fullerene cations and in agreement with theoretical dissociation energies. Similarly, an anomaly in the adsorption energy occurs when 60 helium atoms are attached to C^?₆₀ or 65 to C^?₇₀. For C₆₀, the anomaly coincides with the one observed for cationic complexes but for C₇₀ it does not. Implications of these features are discussed in light of several theoretical studies of neutral and positively charged helium–fullerene complexes.     

Raman spectra of samarium-fullerene intercalation compounds

Samarium-fullerene intercalation compounds of nominal composition Sm_xC₆₀ (x=1,2,…,6) have been synthesized by a solid-state reaction method. We obtain a Sm_2.75C₆₀ superconducting phase with orthogonal structure and a Sm₆C₆₀ phase with body-centered cubic structure. The broadening and weakening of Raman peaks of the Sm_xC₆₀ compounds are due to the distortion of C₆₀ and electron-phonon interaction. The Raman measurements reveal that the distortion of C₆₀ decrease in Sm_xC₆₀ (x=3,4,5) exposed to air, although the fulleride solids have transformed into an amorphous state. The Raman results also show that the distortion of C₆₀ is still very large in the Sm₆C₆₀ exposed to air, or the C₆₀ molecules have been destroyed and become some amorphous carbide.     

Neutron scattering study of intermolecular excitations in solid C60 and C70

The spectrum of intermolecular excitations in solid C₆₀ and C₇₀ is investigated by neutron time-of-flight spectroscopy. The quasielastic scattering above the ordering transition is studied in detail and compared to a rotational diffusion model. Below the transition the scattering spectra have essentially a three peak structure with a prominent libronic excitation near ±2 meV for both fullerenes. A detailed analysis gives evidence for a broader distribution of librational modes with an additional pronounced maximum near 5 meV.     

Deformation-induced changes in the structure of fullerites C60/70 during their mechanical activation

Structural changes occurring during the mechanical activation of fullerites C_60/70 have been investigated using X-ray diffraction, IR and UV spectroscopy, and scanning electron microscopy. The complete destruction modes of fullerite have been determined (3.5 h at the intensity of the mill of 4.3 W/g and 28 h at 2.2 W/g). The destruction of the crystal structure of fullerites is accompanied by the destruction of fullerene molecules. The residual solvent, which enters into the composition of C_60/70, is retained during the entire time of mechanical activation. In this case, the low-frequency shift of absorption bands of toluene (729 → 725 cm^?1), which is caused by the deformation of the solvent molecule in the composition of crystal solvates, has been observed. It has been shown that the deformation stability of graphite is substantially lower than in the case of fullerite.     

Radiation-stimulated modification of C60 films on Si-oxide surfaces

The electronic structure of C₆₀/SiO_x/Si(1 0 0) interface was studied by photoemission (valence-bands, C 1s, and Si 2p core levels) and near-edge X-ray absorption fine structure (C 1s threshold) spectroscopies. It was concluded that the SiO_x/Si surface is non-reactive with respect to the interaction with C₆₀. The exposure of the C₆₀/SiO_x/Si system under non-monochromatic synchrotron radiation causes modification of the electronic structure of this system. It is explained by polymerization of the C₆₀ molecules and arising a strong ionic-like interaction of the polymerized C₆₀ with the SiO_x surface. Annealing of this system up to temperatures of 550–625 °C leads to complete desorption of the C₆₀ molecules from the non-irradiated sample areas while the modified by radiation fullerenes remain attached to the substrate.     

A comparative study of ion-induced damages in C60 and C70 fullerenes

The stability of fullerenes (C₆₀ and C₇₀) under swift heavy ion irradiation is investigated. C₆₀ and C₇₀ thin films were irradiated with 120 MeV Ag ions at fluences from 1×10¹² to 3×10¹³ ions/cm². The damage cross-section and radius of damaged cylindrical zone were found to be higher for C₆₀ than C₇₀ as evaluated by Raman spectroscopy, which shows that the C₇₀ molecule is more stable under energetic ion impact. The higher damage cross-section of the C₆₀ molecule compared with that of the C₇₀ molecule is explained on the basis of thermal conductivity in the framework of the thermal spike model. The surface morphology of pristine C₆₀ and C₇₀ films is studied by atomic force microscopy. UV-visible absorption studies revealed that band gap for C₆₀ and C₇₀ fullerenes thin films decreases with increasing ion fluence. Resistivity of C₆₀ and C₇₀ thin films decreases with increasing ion fluence but the decrease is faster for C₆₀ than C₇₀, indicating higher damage in C₆₀. Irradiation at a fluence of 3×10¹³ ions/cm² results in complete damage of fullerenes (C₆₀ and C₇₀) into amorphous carbon.     

Fullerene-like Si60C60 nanocage: Hydrogen storage capacity

For investigating the hydrogen storage capacity of the Si₆₀C₆₀ nanocage, we have performed first principle density functional theory calculations with van der waals corrections. According to the force and energy minimization of the complex structures of nH₂@Si₆₀C₆₀, we have found that the systems with n = 1 to 70 are energetically favorable. We also found that the most stable nH₂@Si₆₀C₆₀ system is related to the systems with n close to 50 which make it possible to reach 4.2 wt% of hydrogen storage. Although it is found that the nH₂@Si₆₀C₆₀ system remains stable up to n = 70 and 5.8 wt%. The NPT molecular dynamic investigation at different pressures (0–30 atm) and also different temperatures (300–1800 K) were carried out on the optimized stable system with maximum capacity of encapsulating H₂ molecules (70H₂@Si₆₀C₆₀).     

Photoinduced C70 radical anions in polymer:fullerene blends

Photoinduced polarons in solid films of polymer:fullerene blends were studied by photoluminescence (PL), photoinduced absorption (PIA) and electron spin resonance (ESR). The donor materials used were P3HT and MEH‐PPV. As acceptors we employed PC₆₀BM as reference and various soluble C₇₀‐derivates: PC₇₀BM, two different diphenylmethano‐[70]fullerene oligoether (C₇₀‐DPM‐OE) and two dimers, C₇₀–C₇₀ and C₆₀–C₇₀. Blend films containing C₇₀ revealed characteristic spectroscopic signatures not seen with C₆₀. Light‐induced ESR showed signals at g ≥ 2.005, assigned to an electron localized on the C₇₀ cage. The formation of C₇₀ radical anions also leads to a subgap PIA band at 0.92 eV, hidden in the spectra of C₇₀‐based P3HT and MEH‐PPV blends, which allows for more exact studies of charge separated states in conjugated polymer:C₇₀ blends. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-assembled growth of C60 nanowhiskers on anodic porous alumina membranes

Unique structured claw-like C₆₀ nanowhiskers were fabricated on an anodic porous alumina membrane. Tips of the C₆₀ nanowhiskers are shaped like cusps, which directly extend into the nanopores of the alumina membrane. It is revealed that the surface stress of the alumina membrane with a highly ordered nanopore arrangement due to thermal treatment is responsible for self-assembled growth of the observed C₆₀ nanowhiskers. The ordering of the nanopore arrangement in the anodic porous alumina membrane helps to form the C₆₀ nanowhiskers along the nanopore channels. The shape and structure of the C₆₀ nanowhiskers are promising characteristics for applications in nanodevices such as field-emission tips and nanoprobes. PACS 61.46.+w; 61.48.+c; 33.20.Fb     

Dielectric properties of C60 molecular complexes

The permittivity ? of the molecular complexes (BTX)C₆₀CS₂, (DAN)C₆₀(C₆H₆)₃, and (S₄N₄)_1.2C₆₀(C₆H₆)_0.8 has been studied at 100 MHz. A maximum has been observed to appear in the temperature dependence ?(T) at T≈90 K, which can be assigned to freezing of the orientational disorder. A region of anomalous growth of ? with decreasing temperature has been found in the low-temperature range T≤25 K, this feature being apparently sensitive to the actual packing pattern of C₆₀ spheres in the molecular complex.     

Thermal conductivity of C60 and C70 crystals

We have performed thermal conductivity measurements on C₆₀ and C₇₀ crystals grown by sublimation. For single crystal C₆₀, the thermal conductivity k is 0.4 W/m K at room temperature and is nearly temperature independent down to 260K. We observed a sharp orientational phase transition at 260K, indicated by a 25% jump in k. Below 90K, k is time dependent, which manifests itself as a shoulder-like structure at 85K. The temperature and time dependence of k below 260K can be described by a simple model which accounts for the thermally activated hopping of C₆₀ molecules between two nearly degenerate orientations, separated by an energy barrier of 240 meV. It is found that solvents have a strong influence on the physical properties of C₇₀ crystals. For solvent-free C₇₀ crystal, k is about constant above 300K. There is a broad first-order phase transition in k at 300K with a 25% jump. We associate this transition with the aligning of the fivefold axes of the C₇₀ molecules along the c-axis of the hexagonal lattice. Upon further cooling, k increases and is time independent.     

Optical and electrical properties of C60Tex films

The structure, composition, and electrical and optical properties of thin tellurium-intercalated fullerene films C₆₀Te_x are investigated. The samples of compositions from C₆₀Te_0.1 to C₆₀Te₆ are prepared by thermal evaporation. The sample composition and the impurity distribution are controlled by the Rutherford backscattering technique. The Raman vibrational spectra indicate changes in the symmetry of a C₆₀ molecule: the strain of the molecule increases with a decrease in the tellurium concentration and decreases as the tellurium impurity concentration increases. The evolution of the optical absorption spectra and the electrical conductivity suggests that intercalation of a tellurium impurity leads to modification of the electronic structure of the material. This process is accompanied by a shift and change in shape of the optical absorption edge and a change in the electrical conductivity of films by several orders of magnitude depending on the composition. The electrical conductivity is minimum at a low tellurium impurity content.     

Doping effect on the third-order optical nonlinearities of C70

The substitute doping effect on the third-order optical nonlinearities, characterized by the second-order hyperpolarizabilities γ, of C₇₀ is studied by substituting one carbon atom of C₇₀ with a boron or nitrogen atom. It is found that the substitute doping increases the γ magnitude of C₇₀, which is important for photonic application.     

Dielectric relaxation,ac and dc conductivities in the fullerenes C60 and C70

The complex conductivity in polycrystalline C₆₀ and C₇₀ has been investigated for frequencies 20 Hz≤ν≤10⁶ Hz and temperatures 10 K≤T≤750 K. The high-frequency dielectric constants ε_α= 2.6±0.1(C₆₀) and ε_∞= 4.6±0.1 (C₇₀) were deduced from these experiments. The observed low temperature relaxation process in C₆₀ fits well into the relaxation dynamics of the C₆₀ molecules as determined by many other experimental techniques operating on very different time scales. In addition to the study of the dipolar relaxation process, the dc and ac conductivities were determined. From the temperature dependence of the dc conductivities energy barriers of E_G=1.75±0.1 eV (C₆₀) and E_G=1.7±0.1 eV (C₇₀) were estimated. In C₇₀ we found indications for small polaron tunneling.     

A comparison of DMPC membranes mixed with melittin or C12E5

Summary The effect of adding the defect-forming molecules melittin and C₁₂E₅ to DMPC membranes has been studied and the corresponding phase diagrams established. Light, X-ray and neutron small-angle scattering have been used to characterize the mixed membranes. Both systems show a melting of the lamellar L_α into an isotropic phase upon addition of the second membrane constituent. The molar ratio, where the melting occurs is the same in concentrated and dilute samples. For the DMPC/C₁₂E₅ system not only membrane composition, but also temperature can be used to induce a transition from an isotropic to a lamellar phase. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

13C MAS NMR investigations of alkali doped C60

Results of¹³C MAS NMR measurements of the Rb _x C₆₀ system (x=2.75, 3, 4, 6) and the A₆C₆₀ compounds (A=K, Rb, Cs) are presented. Special attention was paid to sample preparation in order to suppress effects of impurities and lattice defects due to imperfect C₆₀ starting material. The¹³C MAS NMR measurements of the Rb _x C₆₀ system demonstrate the usefulness of this method to reveal valuable information about its phase diagram. The existence of underdoped Rb₃C₆₀ is proved. Well resolved lines in all investigated A₆C₆₀ compounds confirm the orientational order of the C₆₀ ions. An assignment of the signals to the three magnetically inequivalent carbon atom positions in the crystal structure is proposed.     

Dielectric relaxation,ac and dc conductivities in the fullerenes C60 and C70

The complex conductivity in polycrystalline C₆₀ and C₇₀ has been investigated for frequencies 20 Hz≤ν≤10⁶ Hz and temperatures 10 K≤T≤750 K. The high-frequency dielectric constants ε_α= 2.6±0.1(C₆₀) and ε_∞= 4.6±0.1 (C₇₀) were deduced from these experiments. The observed low temperature relaxation process in C₆₀ fits well into the relaxation dynamics of the C₆₀ molecules as determined by many other experimental techniques operating on very different time scales. In addition to the study of the dipolar relaxation process, the dc and ac conductivities were determined. From the temperature dependence of the dc conductivities energy barriers of E_G=1.75±0.1 eV (C₆₀) and E_G=1.7±0.1 eV (C₇₀) were estimated. In C₇₀ we found indications for small polaron tunneling.