Orientation-dependent ionization potential of CuPc and energy level alignment at C60/CuPc interface

The electronic structure evolution of interfaces of fullerene (C₆₀) with copper phthalocyanine (CuPc) on highly oriented pyrolytic graphite (HOPG) and on native silicon oxide has been investigated with ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The LUMO edge of C₆₀ was found to be pinned at the interface with CuPc on SiO₂. A substantial difference in the electron affinity of CuPc on the two substrates was observed as the orientation of CuPc is lying flat on HOPG and standing up on SiO₂. The ionization potential and electron affinity of C₆₀ were not affected by the orientation of CuPc due to the spherical symmetry of C₆₀ molecules. We observed band bending in C₆₀ on the standing-up orientation of CuPc molecules, while the energy levels of C₆₀ on the flat-lying orientation of CuPc molecules were observed to be flat. The observation points to a dependence of photoexcited charge transfer on the relative molecular orientation at the interface.     

Electron energy-loss studies of NaxC60 compounds

The crystal and electronic structure of Na_xC₆₀ (4< x < 10) has been investigated using electron energy-loss spectroscopy in transmission. Electron scattering experiments confirm that the crystal symmetry remains face centered cubic over the entire intercalation range. The electronic structure shows a close similarity for the Na₆C₆₀ and Na₄C₆₀ compounds compared to the corresponding K and Rb systems. Upon intercalation to higher stochiometries up to Na₁₀C₆₀ the charge transfer from sodium to C₆₀ turns out to be incomplete, something that has never been observed before in alkali intercalated systems. Current theoretical calculations indicate that this can be understood by taking into account additional states related to the Na atoms in the octahedral site. In Na₁₀C₆₀ the partial filling of the t_1G level of C₆₀ leads to additional electronic transitions near the Fermi energy.     

Electronic interactions in two-dimensional polymers of the C60 fullerene

The electronic structure of tetragonal and rhombohedral polymers of the C₆₀ fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C₆₀ molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C₆₀ fullerene polymers leads to a broadening of the maxima in the CK _α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C₆₀ fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C₆₀ molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level.     

Spectroscopic study of phase transformations between orthorhombic and tetragonal C60 polymers

We present a detailed study of transformations between the orthorhombic and tetragonal polymeric states of C₆₀. The transformations are characterised by Raman spectroscopy and X-ray diffraction. We show that the transformation from the orthorhombic (O) phase to the tetragonal (T) phase is very fast and our results indicate that the transformation goes via an intermediate dimer (D) state in a two-stage process, O↦D and, D↦T transformations, where the second process is slower than the first. On the other hand, the transformation from the tetragonal to the orthorhombic phase is significantly slower, indicating a high-energy threshold to break the polymer bonds at the temperatures used. The results also support earlier suggestions that the tetragonal phase contains disordered dimers that can be viewed as lattice defects in the formation of higher polymers.     

On interface dipole layers between C60 and Ag or Au

C₆₀ layers on polycrystalline Ag and Au are studied by photoelectron spectroscopy. At these metal/C₆₀ interfaces an electron transfer occurs from the metal to the lowest unoccupied orbital of C₆₀. We found in the case of the polycrystalline Ag/C₆₀ interface a dipolar layer with its associated electric field in the direction corresponding to the charge transfer, so pointing from the substrate to the adsorbent. Yet, at the Au/C₆₀ interface we observed an overall electric field pointing from C₆₀ towards the metal. We discuss our observations in terms of charge transfer, screening and hybridization effects and propose the occurrence of a hybridization mechanism similar to back-bonding at the Au/C₆₀ interface. We show that the alignment of energy levels at the metal/C₆₀ interface cannot simply be deduced using the metal workfunction and the frontier orbitals of C₆₀, including screening effects, since hybridization effects may strongly alter the interfacial energy level structure. Our experimental findings on the polycrystalline metal/C₆₀ interfaces indicate an at-most weak dependence of the Fermi level of the C₆₀ overlayer on the workfunction of the polycrystalline metal substrate. These interfaces are found in donor–acceptor-based organic photovoltaic devices and our results may help to understand the electrical characteristics of these devices. Received: 26 September 2001 / Accepted: 15 January 2002 / Published online: 3 June 2002     

Mechanisms of inelastic scattering of low-energy protons by C6H6, C60, C6F12, and C60F48 molecules

The mechanisms of inelastic scattering of low-energy protons with a kinetic energy of 2–7 eV by C₆H₆, C₆F₁₂, C₆₀, and C₆₀F₄₈ molecules are studied using the methods of quantum chemistry and nonempirical molecular dynamics. It is shown that, for the C₆H₆ + proton and C₆₀ + proton systems, starting from a distance of 6 Å from the carbon skeleton, the electronic charge transfer from the aromatic molecule to H⁺ occurs with a probability close to unity and transforms the H⁺ ion into a hydrogen atom and the neutral C₆H₆ and C₆₀ molecules into cation radicals. The mechanism of interaction of low-energy protons with C₆F₁₂ and C₆₀F₄₈ molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the noncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized, on the one hand, on the positively charged hydrogen ion and, on the other hand, on the C₆F₁₂ and C₆₀F₄₈ molecules. As a result, the neutral molecule + proton state becomes the ground state. In turn, this inversion makes the electronic charge transfer energetically unfavorable. Quantum-chemical and molecular-dynamics calculations on different levels of theory showed that, for fluorine derivatives of some aromatic structures (C₆F₁₂, C₆₀F₄₈), the barriers to proton penetration through carbon hexagons are two to four times lower than for the corresponding parent systems (C₆H₆, C₆₀). This effect is explained by the absence of active π-electrons in the case of fluorinated molecules.     

BOND STRUCTURE AND ELECTRON STATES OF CHARGED C60

In the alkali-metal doped C₆₀, the charge transfer induces the distortion of the bond structure and forms the self-trapping electronic bound states. Our theory manifests: 1, the charge transfer reduces the symmetry of C₆₀ from I_h to D_5d. 2, Both the bond distortion and the self-trapping states possess layer structures and are localized in the equatorial area. 3, The carbon atoms in charged C₆₀ are divided into eight layers with an inversion center, then there exist four nonequivalent groups of carbon atoms. It makes the NMR line split into a fine structure with strength ratio 1:1:2:2. 4, The charged C₆₀ has two self-trapping bound states, one is 0.06eV above HOMO with odd parity and the other is 0.05 eV below LUMO with even parity.     

A Raman study of polymerised C60

60 are reported. Although prepared according to different routes the Raman spectra of the two polymeric phases of C₆₀ show a quantitative agreement with respect to mode positions and intensity. We conclude from this that both materials have the same structure at least in the short range order, i.e. the same type of bonding and co-ordination between neighbouring C₆₀ molecules. An investigation of the time dependence of the thermal decomposition of high pressure polymerised C₆₀ is also presented. The rate of decomposition of the polymeric phase is found to be multi-exponential at all temperatures investigated. From an Arrhenius-type analysis of the short time data and the long time data, respectively, the activation energy for thermal dissociation of polymeric bonds was found to increase with time. Received: 20 September 1996/Accepted: 11 November 1996     

Crystalline structure of a C60/C70 membrane

The crystalline structure of a C₆₀/C₇₀ membrane prepared by an original technique has been studied by x-ray diffraction and Raman spectroscopy. The effects of purification of the starting C₆₀/C₇₀ mixture to C₇₀ composition and of spatial separation of the C₆₀ and C₇₀ phases in the membrane have been revealed. The samples studied were established to have a composition gradient from C₆₀ to C₇₀. The main structure of the membrane is shown to be an fcc lattice with a=14.308 Å.     

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.     

Fullerene-C60 and crown ether doped on C60 sensors for high sensitive detection of alkali and alkaline earth cations

Fullerenes are effective acceptor components with high electron affinity for charge transfer. The significant influences of chemical adsorption of the cations on the electrical sensitivity of pristine C₆₀ and 15-(C₂H₄O)₅/C₆₀ nanocages could be the basis of new generation of electronic sensor design. The density functional theory calculation for alkali and alkaline earth cations detection by pristine C₆₀ and 15-(C₂H₄O)₅/C₆₀ nanocages are considered at B3LYP level of theory with 6–31 G(d) basis set. The quantum theory of atoms in molecules analysis have been performed to understand the nature of intermolecular interactions between the cations and nanocages. Also, the natural bond orbital analysis have been performed to assess the intermolecular interactions in detail. Furthermore, the frontier molecular orbital, energy gap, work function, electronegativity, number of transferred electron (∆N), dipole moment as well as the related chemical hardness and softness are investigated and calculated in this study. The results show that the adsorption of cations (M=Na⁺, K⁺, Mg²⁺ and Ca²⁺) are exothermic and the binding energy in pristine C₆₀ nanocage and 15-(C₂H₄O)₅/C₆₀ increases with respect to the cations charge. The results also denote a decrease in the energy gap and an increase in the electrical conductivity upon the adsorption process. In order to validate the obtained results, the density of state calculations are employed and presented in the end as well.     

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.     

Enhanced high-rate performance of sub-micro Li4Ti4.95Zn0.05O12 as anode material for lithium-ion batteries

Zn-doped Li₄Ti₅O₁₂ was prepared by a ball milling-assisted solid-state method, and the characters were determined by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, cyclic voltammetry, and galvanostatic charge–discharge testing. The results show that Li₄Ti_5?x Zn _x O₁₂ (x?=?0, 0.05) exhibits the pure phase structure, and Zn doping does not change the electrochemical reaction process and basic spinel structure of Li₄Ti₅O₁₂. The particle size of both samples is about 300–500 nm. The prepared Li₄Ti_4.95Zn_0.05O₁₂ presents an excellent rate capability and capacity retention. At the charge–discharge rate of 1C, the initial discharge capacity of Li₄Ti_4.95Zn_0.05O₁₂ is 268 mAh g^?1. After 90 cycles at 5C, the discharge capacity of Li₄Ti_4.95Zn_0.05O₁₂ is obviously higher than that of Li₄Ti₅O₁₂. The excellent electrochemical performance of the Li₄Ti_4.95Zn_0.05O₁₂ electrode could be attributed to the improvement of reversibility by doping zinc and the sub-micro particle size.     

Magnetism of C60-based molecular complexes: High-field-magnetization and magnetooptical study

Magnetization study of the C₆₀ · TMTSF · 2CS₂ molecular complex in magnetic fields up to 47 T for the temperature range 1.8–300 K and ESR spectroscopy of the molecular complex (ET)₂C₆₀ at T=1.8 K for the frequency range 60–90 GHz in magnetic fields up to 32 T provide experimental evidence that paramagnetic centers with reduced g-factor values g<1 control the magnetic properties of these solids. A model is suggested in which the renormalization of the g factor is due to the dynamic Jahn-Teller effect involving negative C₆₀ ions that appear as defects in the crystalline structure with a weak charge transfer.     

Templating a face-centered cubic (110) termination of C60

Scanning tunneling microscopy has been used to study C₆₀ overlayers on TiO₂(100)-(1 × 3). Initial adsorption preferentially occurs on O vacancies (Ti³⁺), evidencing a site-specific interaction dominated by substrate-adsorbate charge transfer. At saturation coverage the molecules are incommensurate with Ti³⁺ sites along [001], suggesting a delicate balance between intermolecular and substrate interactions. The unit cell of the C₆₀ overlayer is 13.8Å× 10.0 ± 0.5Å, consistent with the first layer of fcc C₆₀(110); molecules also adopt sites in the troughs of the (1 × 3) structure to form the second layer.     

2D polymerization and doping of fullerenes under pressure

Abstract

Tetragonal polymeric C₆₀ has been studied by Raman spectroscopy and other methods. Attempts have been made to transform samples from the tetragonal to the orthorhom-bic phase and vice versa. The results suggest that the transformation is direct with no intermediate stage with free molecules. Tetragonal C₆₀ has also been intercalated by potassium metal.     

Spectroscopic characterization of pressure modified C60

60 are reported. The material associated with the mixture of rhombohedral and tetragonal phases was synthesized under pressure of when treatment temperature was only a little lower than the high-temperature limit of C₆₀ stability. The substance exhibits very rich infrared and Raman spectra as well as luminescence spectra of an unusual shape. They show that vibrational transitions in the prepared carbon architecture substantially deviate from icosahedral symmetry of buckyballs and exhibit a similarity with lattice phonons in graphite. It may originate from mixing of C₆₀ modes and vibrations of a layer structure on deformed and weakly bound fullerene molecules. The luminescence spectra reveal three distinct electronic states located below . One of them responsible for the emission peak at is very characteristic for the pressure modified fullerene. The data should be useful for the accurate determination of structural changes in C₆₀ induced by pressure. Received: 6 September 1996/Accepted: 10 October 1996     

Synthesis and electrochemical performance of hole-rich Li4Ti5O12 anode material for lithium-ion secondary batteries

Hole-rich Li₄Ti₅O₁₂ composites are synthesized by spray drying using carbon nanotubes as additives in precursor solution, subsequently followed calcinated at high temperature in air. The structure, morphology, and texture of the as-prepared composites are characterized with XRD, Raman, BET and SEM techniques. The electrochemical properties of the as-prepared composites are investigated systematically by charge/discharge testing, cyclic voltammograms and AC impedance spectroscopy, respectively. In comparison with the pristine Li₄Ti₅O₁₂, the hole-rich Li₄Ti₅O₁₂ induced by carbon nanotubes exhibits superior electrochemical performance, especially at high rates. The obtained excellent electrochemical performances of should be attributed to the hole-rich structure of the materials, which offers more connection-area with the electrolyte, shorter diffusion-path length as well faster migration rate for both Li ions and electrons during the charge/discharge process.     

Electron-energy-loss spectroscopy of C60 monolayer films on active and inactive surfaces

The characteristics of interaction between C₆₀ molecules and Si(1 1 1)-7×7, Ag/Si(1 1 1)-√3×√3 R30° and layered material MoS₂ surfaces have been investigated using electron-energy-loss spectroscopy (EELS). The EEL spectrum of C₆₀/Si(1 1 1)-7×7 shows a new peak at loss energy of 2.7 eV. This indicates the existence of charge transfer from the substrate to C₆₀ molecules. The EEL spectrum of a C₆₀ monolayer film grown on a cleaved surface of MoS₂ is almost the same as that of bulk C₆₀. The EEL spectrum of a C₆₀ monolayer film on an Ag/Si(1 1 1) surface is quite different from that on a clean Si(1 1 1)-7×7 surface, although the films on those substrates have the same epitaxial arrangement. Furthermore, intensities of energy-loss peaks of C₆₀/Ag/Si(1 1 1) are slightly smaller than those of C₆₀/MoS₂ in spite of having the same loss-energy. This suggests that the interaction between C₆₀ molecules and the Ag/Si(1 1 1) surface is stronger than that between C₆₀ molecules and the MoS₂ surface.     

A DFT study on Diels–Alder cycloadditions of trans‐1,3‐butadiene to C60 and C70

Investigation of the relative reactivity of bonds in fullerenes will provide fundamental theory for the design of fullerene‐based materials. We have theoretically investigated the reactivity of the Diels–Alder (DA) cycloaddition of cis‐1,3‐butadiene to all types of bonds in C₆₀ and C₇₀ using the M06‐2X hybrid density functional theory (DFT) calculations (J. Phys. Org. Chem. 2012, 25 850–855) and have pointed out that the DA cycloadditions of cis and trans forms of 1,3‐butadiene to ethylene have a specially intimate relationship (J. Phys. Org. Chem. 2014, 27 652–660). For the aim of telling a whole story of the DA cycloaddition concerning C₆₀ and C₇₀, the DA cycloadditions of trans‐1,3‐butadiene to all types of bonds in C₆₀ and C₇₀ were explored at the same theoretical level as those of the cis‐1,3‐butadiene. The calculated results related with the trans‐ and cis‐1,3‐butadienes were compared. The potential energy curves of DA cycloadditions of trans‐ and cis‐1,3‐butadiene to C₆₀ and C₇₀ were discussed. The distortion–interaction energy model was employed to elucidate the origin of different reactivity of all kinds of C?C bonds. The solvent effects were examined using the continuum solvent model. These current results, along with our previous research, will help to obtain an overall view of the DA cycloadditions of 1,3‐butadiene to C₆₀ and C₇₀. Copyright © 2014 John Wiley & Sons, Ltd.