Electron-energy-loss spectroscopy of KxC60 and K-halides: comparison in the K3p excitation region

We have investigated the electronic structure of K_xC₆₀ (x=0–6) using low-energy electron energy loss spectroscopy (LEELS), especially focusing on the K_3p core-electron excitation spectra. It is found that the structure of the K_3p-excitation spectrum of K_xC₆₀ quite differs from that of KCl. Furthermore, the K_3p-excitation LEELS of K₃C₆₀ has been revealed to be different from that of K₆C₆₀. K_3p electrons are excited into K_4s- and K_3d-derived empty states in both K_xC₆₀ and KCl, but in the case of K_xC₆₀ the K_3d-derived empty states have a rather complicated structure where several levels are not well separated. Consequently, K_3p-excitation LEEL spectra of K_xC₆₀ show wide-spread plateau-like structures. The difference in the K_3p-excitation spectra of K₃C₆₀ and K₆C₆₀ is considered to originate from the different crystal field around K⁺ cations in the C₆₀ molecular crystal.     

Electrochemically intercalated MxC60 thin films in a solid state cell (M = Li,K): Optical and photoelectrochemical characterization

Solid-state electrochemical cells have been prepared with C₆₀ vacuum-evaporated thin films, a Li- or K-source counter electrode and a polymer PEO-LiClO₄ (PEO-KClO₄) electrolyte. The electrochemical intercalation in C₆₀ of Li⁺ (or K⁺) ions has been performed under constant current conditions up to a formal stoichiometry of the fulleride film equal to Li₁₂C₆₀ (K₅C₆₀). A complete charge-transfer pocess from the intercalated alkali to the alkali-metal compound has been assumed. Several quasi-equilibrium potential plateaux were observed during intercalation, that we associate with the coexistence of phases with different intercalant concentration. The electrochemical intercalation process is irreversible to a large extent. Optical and photoelectrochemical spectroscopy of the fulleride films was done in-situ at different moments of the intercalation reaction by illuminating the film electrodes through the transparent and conducting glass substrates. The photo electrochemical spectral response agrees well with the optical absorption spectra, both indicating a bandgap of 2.2 eV. The photoelectrochemical response shows a minimum forx = 3 (in the K_xC₆₀ compound) and a maximum atx = 4 (in both K_xC₆₀ andLi_xC₆₀ compounds), in agreement with previous conductivity results. NIR diffuse reflectance spectra of the solid-state cell show absorption bands in the fulleride films at a wavelength of 1100 nm. A band-energy diagram has been proposed for the M_xC₆₀/PEO-MClO₄/M electrochemical cell.On leave from Instituto de Fisica, Facultad de Ingenleria, C.C. 30, 11000 Montevideo, Uruguay     

Resonant photoemission study ofK-derived valence-band states in K X C60

The electronic structure of K-doped C₆₀ was investigated by photoemission (PE) and X-ray absorption near-edge structure (XANES) studies at the C-1s and K-2p thresholds. In addition, information on the local K-derived partial density of states in superconducting K₃C₆₀ was obtained by resonant PE at the K-2p _1/2 threshold. The experimental observations support a complete charge transfer from K to C₆₀ and we clearly observe a finite density of states atE _F . From resonant PE, occupied states with K-p, d character could be identified in the binding-energy region from 1.5 to 8 eV below, but not directly at the Fermi level. This partial-density-of-states structure agrees well with the results of our band-structure calculations based on the local-density approximation.     

Compression properties of finite nuclei in a semi-classical relativistic approach

Skyrme type potentials are known to lead — in the framework of the scaling model — to a finite-nucleus incompressibility valueK _A where the volume coefficientK _v equals roughly the negative surface coefficient K_s. This is found for Skyrme interactions with K_v between 200 and 360 MeV. In a semi-classical relativistic approach on the basis of the — model (linear as well as non-linear) using in addition local density approximations, we findK _s to depend in particular on the surface energy coefficienta _s , and not so much on the value ofK _v . For a realistic value of a_s, both the linear and the non-linear — model (with NL1 parameter set) yield a ratio ¦K _s K _v ¦ of approximately 1. We discuss implications of this finding with a particular view on recent empirical results onK _v andK _s .Dedicated to Prof. Dr. H. J. Mang on the occasion of his 60th birthday     

ON THE SUPERCONDUCTING MECHANISM OF K3C60 AND Ba1-xKxBiO3

A superconducting mechanism for K₃C₆₀ and Ba_1-xK_xBiO₃ has been suggested. This is the combining picture of the Cooper pair and the Ogg pair. Numerical calculations within the framework of this picture have been done for K₃C₆₀ and Ba_1-xK_xBiO₃, and the overall consistency with the experimental data of superconducting properties is good.     

X-ray photoelectron spectroscopy and near-edge X-ray-absorption fine structure of C60 polymer films

We report core-level and valence-band X-ray photoelectron spectroscopy (XPS) and carbon [ _]K near-edge X-ray-absorptionfine structure spectroscopy (NEXAFS) results of plasma-polymerized C₆₀. In comparison with evaporated C₆₀ the C 1s peak is broader and asymmetric for the C₆₀ polymer and its shake-up satellites diminished. Furthermore, the features of the valence-band as well as the features of the π^* antibonding orbitals of the C₆₀ polymer are broader and reduced in intensity. Changes in the electronic structure are attributed to the polymerization of C₆₀, the post-plasma functionalization of the surface by oxygen after exposure to atmosphere, and the occurrence of amorphous carbon. Received: 28 May 1999 / Accepted: 31 August 1999 / Published online: 8 March 2000     

Electron attenuation lengths in fullerene and fullerides

Using X-ray photoemission measurements, we have determined the attenuation length of C 1s photoelectrons in C₆₀ film to be 21.5 Å with the incident photon energy of Mg Kα radiation. The inelastic mean free path calculated with the TPP-2M algorithm coincides fairly well with the experimentally determined attenuation length, indicating the validity of the algorithm to fullerene and fullerides. The inelastic mean free paths for some fullerides, i.e. K₃C₆₀, K₆C₆₀, Ba₄C₆₀, Sm_2.75C₆₀ and Sm₆C₆₀ are calculated to help the quantitative analyses of the photoemission spectra for these compounds.     

Ab-initio molecular dynamical study of a single transition metal atom on fullerene C<Subscript>60</Subscript>: the case of Ta

We report the first-principles Car-Parrinello molecular dynamics study of the behaviour of a single transition metal Ta atom on fullerene C₆₀, at different temperatures, and for both neutral and charged clusters. We seek to characterise the motion of the lone Ta metal atom on the C₆₀ surface, contrasting its behaviour both with that of three Ta atoms, as well as with a single alkali metal atom on the cage surface. Our earlier simulations on C₆₀Ta₃ had revealed that the Ta atoms on the surface of the fullerene are affected by a rather high mobility, and that the motion of these atoms is highly correlated due to Ta-atom-Ta-atom attraction. Earlier, experimental studies of a single metal atom (K, Rb) on the surface of a C₆₀ molecule had led to the inference that at room temperature the metal atom skates freely over the surface, the first direct evidence for which was presented by us in earlier first principles molecular dynamical simulations.     

A new approach to analyse H⊗(2h⊕g) Jahn-Teller system for C60

It is now well-known that electron (hole)-vibron coupling and hence Jahn-Teller (JT) effect is important understanding the properties of C₆₀ and related molecules. In this paper, we study H(2) coupling case to find the potential energy surfaces for the positively charged C₆₀ molecule due to distortion. The H(2) Jahn-Teller system is of particular importance as this will be the JT effect displayed by C₆₀ molecules removed with an electron. C₆₀ ⁺ is obtained by removing one electron from fivefold degenerate H_u highest occupied molecular orbital (HOMO) and a hole in HOMO interacts with the vibrational modes of C₆₀ and symmetry is broken. We apply the method of symmetry breaking mechanism to obtain expressions for the potential energy surface. Received 27 December 1999 and Received in final form 15 May 2000     

Rheological behaviour and shear thickening exhibited by aqueous CTAB micellar solutions

In this experimental work we carefully investigate the rheological behaviour and in particular the shear thickening exhibited by aqueous micellar solutions of CTAB with NaSal as added counterion. We are particularly interested in the evolution of the critical shear rate (at which shear thickening occurs) versus C _D , the surfactant concentration. We show that , at fixed salt concentration C _S , increases with C _D following a power law evolution with a positive exponent of + 5.8. On the other hand we show that if the ratio C _D / C _S is fixed, decreases with C _D with a negative exponent of -2.0. Nevertheless investigations of the zero shear viscosity indicate that in all situations (implying variation of the surfactant concentration C _D , or the salt concentration C _S or the temperature) is a decreasing function of the length of the micelles. All these evolutions are compatible with a gelation mechanism which could possibly be associated with entanglement effects of large interacting flowing structures. Received: 3 March 1998 / Revised: 16 June 1998 / Accepted: 3 July 1998     

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.     

Characterization of fullerenes using positrons

Positron distributions and lifetimes in the K_xC₆₀ crystals for x=0, 3, 4, 6 have been calculated using the superposed-atom model and the numerical relaxation technique. It is revealed that positrons are distributed predominantly at the octahedral interstices in the fcc lattice of pure C₆₀. The distributions and lifetimes of positrons are significantly changed by potassium doping. In the fcc K₃C₆₀, the positron density shows its maxima in the interstitial region between the octahedral and tetrahedral interstices and has some amplitude in the inner space of the C₆₀ molecules. In contrast to this, positrons are distributed one-dimensionally along the [001] direction in the bct K₄C₆₀ while positrons are nearly localized into the C₆₀ molecules in the bcc K₆C₆₀.     

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.     

Atomic force microscopy of C60/C70 single-crystal fullerenes under ethanol

C₆₀/C₇₀ crystal surfaces were imaged by atomic force microscopy under ethanol with resolution of single molecules. Spherical and elongated elliptical fullerenes can be distinguished corresponding most likely with C₆₀, respectively C₇₀. Determination of the maximum diameter for a large number of molecules confirms the presence of two species of fullerenes, one with 9.4 Å, the other with 11.2 Å. The measured ratio C₆₀:C₇₀ is 81:19 which resembles the spectroscopical data. The molecules are arranged either in hexagonal (hcp) or cubic (fcc) packing, in some areas the two arrangements alternate within a few nm. Elongated fullerenes apparently prefer the hexagonal packing.     

First-principles studies on Ti3Si0.5Ge0.5C2 under pressure

The structural, electronic and elastic properties of Ti₃Si_0.5Ge_0.5C₂ have been investigated by using the pseudopotential plane-wave method within the density-functional theory. Our calculated equation of state (EOS) is consistent with the experimental results. The density of states (DOS) indicates that Ti₃Si_xGe_1−xC₂ (x=0, 0.5, 1.0) are metallic, and these compounds have nearly the same electrical conductivity. The elastic constants for Ti₃Si_0.5Ge_0.5C₂ are obtained at zero pressure, which is compared to Ti₃SiC₂ and Ti₃GeC₂. We can conclude that Ti₃Si_0.5Ge_0.5C₂ is brittle in nature by analyzing the ratio between bulk and shear moduli. There appears to be little effect on the electronic and elastic properties with the Ge substitution to Si atoms in Ti₃SiC₂.     

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.     

Electroconductivity and pressure–temperature states of step shocked C60 fullerite

A study of electrophysical and thermodynamic properties of C₆₀ single crystals under step shock loading has been carried out. The increase and the following reduction in specific electroconductivity of C₆₀ fullerite single crystals at step shock compression up to pressure 30 GPa have been measured. The equations of state for face centred cubic (fcc) C₆₀ fullerite as well as for two-dimensional polymer C₆₀ and for three-dimensional polymer C₆₀ (3D-C₆₀) were constructed. The pressure–temperature states of C₆₀ fullerite were calculated at step shock compression up to pressure 30 GPa and temperature 550 K. The X-ray diffraction studies of shock-recovered samples reveal a mixture of fcc C₆₀ and a X-ray amorphous component of fullerite C₆₀. The start of the formation of the X-ray amorphous component occurs at a pressure P _m≈ 19.8 GPa and a temperature T _m≈ 520 K. At pressures exceeding P _m and temperatures exceeding T _m, the shock compressed fullerite consist of a two-phase mixture of fcc C₆₀ fullerite and an X-ray amorphous component presumably consisting of the nucleators of polymer 3D-C₆₀ fullerite. The decrease in electroconductivity of fullerite can be explained by the percolation effect caused by the change of pressure, size and number of polymeric phase nuclei.