Energy-loss Spectroscopy In Transmission Research Articles

Electronic structure studies of pressure-polymerized C 60

We have investigated the electronic structure of one and two-dimensional C 60 polymers with regard to both their low-lying excitations and C 1s excitation spectra by means of electron energy-loss spectroscopy in transmission. We compare the results with those for pristine C 60. In general, the spectra for the polymers resemble those for pristine C 60 but show a broadening due to both the lowering of the symmetry and the increased intermolecular overlap. This is also reflected by a reduction of the optical gap in comparison with pristine C 60.

Plasmon dispersion in quasi-one-dimensional (TaSe 4) 2I and K 0.3MoO 3

We present results of experimental and theoretical studies of the plasmon dispersion in the quasi-one-dimensional conductors (TaSe 4) 2I and K 0.3MoO 3. For both compounds, the dispersion relation, as measured by electron energy-loss spectroscopy in transmission, is quasilinear over a wide momentum range. Different theoretical approaches are discussed to explain the data, all based on a one-band model within the framework of the random-phase approximation. From this analysis, we find in contrast to photoemission measurements no hint for singular one-dimensional properties.

Electron energy-loss spectroscopy in transmission of undoped and doped diamond films

We have prepared thin diamond films by microwave assisted plasma chemical vapour deposition (MWCVD) and hot filament chemical vapour deposition (HFCVD). Diamond powder pre-treatment of the silicon substrates or bias potential were used for nucleation enhancement. Doping of the films was carried out in two ways: in-situ during the CVD process or after the deposition by ion implantation. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been applied to characterize the morphology and texture. Electron energy-loss spectroscopy in transmission was then used to investigate the electronic structure of the diamond films as a function of the preparation parameters and the doping level.

Plasmon excitations in quasi-one-dimensionalK0.3MoO3

We present an investigation of the plasmon excitations in a quasi-one-dimensional metal, the blue bronze ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_{3}.$ The dispersion relation along the one-dimensional direction, as measured by electron energy-loss spectroscopy in transmission, is quasilinear over a wide momentum range before it exhibits a negative curvature. We show that the quasilinear part can be explained within an essentially three-dimensional model based on the random-phase approximation. Band-structure effects are thereby considered through the Ehrenreich-Cohen formula, tailor-made to the specific, strongly anisotropic material. From this analysis, we find no hint for exceptional properties caused by one dimensionality as is currently discussed in the context of photoemission measurements. The genuine dependence of the plasmon modes on the propagation angle relative to the one-dimensional axis is masked by interband transitions lying at about the same energy as the plasmon excitations itself.

Unusual plasmon dispersion in the quasi-one-dimensional conductor(TaSe4)2I: Experiment and theory

We present a combined experimental and theoretical study of the plasmon dispersion in a quasi-one-dimensional conductor, the transition metal tetrachalco-halogenide $({\mathrm{TaSe}}_{4}{)}_{2}\mathrm{I}$. The dispersion relation, as measured by electron energy-loss spectroscopy in transmission, is quasilinear, which is not expected from the literature. However, we show that the data can be explained within a one-band model, which additionally allows the derivation of the conduction-band dispersion. Thus, contrary to evidence from photoemission measurements, we find no hint for singular properties caused by one dimensionality.

The metallofullerene Tm@C 82 : isomer-selective electronic structure

using photoemission and electron energy-loss spectroscopy in transmission. In particular, the impact of the isomer symmetry of the C82 cage (Cs or C3v) on the electronic structure and on the optical properties is analysed in detail. For both isomers, combined XPS and XAS measurements of the Tm 4f and 4d states offer conclusive proof of a divalent encaged Tm ion. No significant change of the valence of the rare earth ion upon changes in the cage symmetry is observed. From the C 1s excitation spectra we find a more pronounced LUMO for the C3v(8) isomer, as is expected for the higher symmetry of the cage. Additional information about the optical properties was obtained from the low-energy loss function. With use of a Kramers–Kronig analysis, the dielectric function, e, and the optical conductivity, σ, have been derived. e(0) and the onset of the spectral weight are found to be 7.7 [12.8] and 0.6 eV [0.8 eV] for the Cs(6) {C3v(8)} isomer, respectively.

Momentum-dependent excitations in highly ordered films ofpara-hexaphenyl

Highly textured para-hexaphenyl films have been investigated using electron energy-loss spectroscopy in transmission. The alignment of the crystallites in the free standing para-hexaphenyl films is determined by electron diffraction. Energy-loss functions for momentum transfers parallel and perpendicular to the molecular axis have been recorded. It is shown directly that a bandlike model---the so-called quasi-band-structure---is an appropriate tool to describe the momentum-dependent electronic excitations in such finite size systems, even if the molecules under consideration consist of only six repeat units as in hexaphenyl.

On-Ball Doping of Fullerenes: The Electronic Structure ofC59N Dimers from Experiment and Theory

We present the first studies of the electronic structure of the heterofullerene $({\mathrm{C}}_{59}\mathrm{N}{)}_{2}$ using electron energy-loss spectroscopy in transmission, photoemission spectroscopy, and density functional theory calculations. Both the C 1s excitation spectra and valence band photoemission show negligible occupation of the C-derived lowest unoccupied electronic states and indicate localization of the excess electrons at the N atoms. The experimental results, together with the detailed analysis of our theoretical data, provide compelling evidence for the chemical picture of a triply coordinated N atom with a lone pair in each heterofullerene unit, and confirm the theoretically predicted ``6,6 closed'' structure for the dimer.

The electronic structure of polymerized fullerenes and dimerized heterofullerenes

We present studies of the electronic structure of polymerized orthorhombic and dimerised using electron energy-loss spectroscopy in transmission. From the C1s excitation spectra a reduced density of π ? states is ob- served for polymerized . This is in contrast to and can be explained by the different type of 'doping' and by the different bonding between the fullerene molecules in the two systems. Additional information about the optical prop- erties was obtained from the low energy loss function. Using a Kramers-Kronig analysis, the dielectric function, (e), and the optical conductivity, (σ), have been derived. e and the onset of the spectral weight have been compared between the polymer, the dimer and . This onset of spectral weight is found to be at : and : for o- and for , respectively.

EELS and XPS investigations of Ba2Cu3O4Cl2

Electron energy-loss spectroscopy in transmission and x-ray photoelectron spectroscopy have been applied to investigate single crystalline Ba2Cu3O4Cl2 to obtain the loss-function along the high symmetry directions of the cuprate plane, the Cu2p and O1s excitation edges, and to study the valence band and the core level electronic structure. The lossfunction shows a gap of 1.65eV, practically dispersionless structures between 2 and 8eV, and a volume plasmon located at 25eV. The dielectric function is derived. The excitation edges are a measure for the number of holes located in the Cu like or O like states parallel or perpendicular to the Cu3O4 planes. The Cu2p3/2 XPS shows three features in the main line and a satellite structure due to two independent copper sites and indicating a strong 3d electron correlation. By interpreting forward scattering features the valence band structure can be assigned to its atomic origin.

Electronic structure of the two C 78 isomers with C 2v symmetry

The electronic structure of the two C 78 isomers with C 2v symmetry has been studied in the solid state using electron energy-loss spectroscopy in transmission and photoemission spectroscopy. We show that the two isomers differ considerably in both their occupied and unoccupied structure as well as regarding their electronic excitation spectrum although they poses the same number of C atoms and have the same molecular symmetry.

Core electron spectroscopic studies of YNi 2B 2C

High-energy electron energy-loss spectroscopy in transmission has been applied to obtain the absorption spectra at the B 1s, C 1s and Ni 2 p 3 2 excitation edges of bulk polycrystalline YNi 2B 2C. We find a close similarity in the near-edge structures and confirm the strongly covalent character of the bonding between Ni, B and C. We compare our results to band structure calculations of the local unoccupied density of states generated within the local density approximation. From x-ray photoemission spectroscopy measurements we determine the core level binding energies and from a comparison of photoemission and Auger spectroscopic measurements we derive a value of the Ni d-d Coulomb energy U A of about 4eV.

Core electron spectroscopic studies of YNi 2B 2C

High-energy electron energy-loss spectroscopy in transmission has been applied to obtain the absorption spectra at the B 1s, C 1s and Ni 2p 3 2 excitation edges of bulk polycrystalline YNi 2B 2C. We find a close similarity in the near-edge structures and confirm the strongly covalent character of the bonding between Ni, B and C. We compare our results to band structure calculations of the local unoccupied density of states generated within the local density approximation. From x-ray photoemission spectroscopy measurements we determine the core level binding energies and from a comparison of photoemission and Auger spectroscopic measurements we derive a value of the Ni d-d Coulomb energy U A of about 4 eV.

Electron energy-loss studies of NaxC60 compounds

The crystal and electronic structure of NaxC60 (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 t1G level of C60 leads to additional electronic transitions near the Fermi energy.

Preparation and electronic structure of phase pure K3C60

Phase pure K3C60 films have been grown using vacuum distillation. The structure of such films could be shown to be face centered cubic consistent with X-ray diffraction studies. The electronic structure of the films has been studied using electron energy-loss spectroscopy in transmission. From C1s core excitation measurements the unoccupied density of states has been determined. Performing the dielectric function has been derived in a wide energy range (0–45 eV). It is shown that the low energy part of the optical conductivity cannot be understood within a simple free electron model but that interband transitions between the three conduction bands have to be taken into account. The spectral weight of interband transitions between valence and conduction bands shows strong momentum dependence due to optical selection rules demonstrating the molecular-like nature of the electronic states.

Crystal and electronic structure of solid C76

The crystal and electronic structure of solidC76 has been studied using transmission electron microscopy and electron energy-loss spectroscopy in transmission. C76 forms a close packed structure with an average facecentred cubic symmetry. From valence band and core electron excitations information on the dielectric function and the unoccupied density of states has been obtained.

Electron energy-loss and photoemission studies of solid C84.

The electronic structure of pure solid ${\mathrm{C}}_{84}$ has been studied using high-energy electron energy-loss spectroscopy in transmission and photoemission spectroscopy. Information is obtained on the occupied and unoccupied \ensuremath{\sigma} and \ensuremath{\pi} bands. The reduced degeneracy of the electronic states and smaller band gap of ${\mathrm{C}}_{84}$ in comparison with ${\mathrm{C}}_{60}$ are evident, as well as the excitonic nature of the transition across the band gap of solid ${\mathrm{C}}_{84}$.

Electron energy-loss spectroscopy studies on C60 and C70 fullerite

The electronic structure of the crystalline fullerites C60 and C70 has been investigated by high-energy electron energy-loss spectroscopy in transmission. From valence band excitations and from core-level excitations of the C 1s level information on occupied and unoccupied π and ϖ bands has been obtained.

Dielectric function of YBa2Cu3O7-? between 50 meV and 50 eV

Electronic excitations with polarization parallel to the CuO2-planes in single-crystalline YBa2Cu3O7 and YBa2Cu3O6 have been investigated by optical reflectance and by highenergy electron energy-loss spectroscopy in transmission in the energy ranges from 50 meV to 6 eV and 0.2 eV to 150 eV, respectively. From a combination of these experimental data the dielectric function, the reflectivity and the optical conductivity have been obtained in a wide energy range. The observed spectra are interpreted in terms of optical phonons, free-carrier absorption, transitions across the charge-transfer gap, further interband transitions, low-lying core-level excitations, excitonic transitions and valence conservingd-d transitions within the Cu 3d shell combined with O 2p intraband transitions. The charge carrier plasmon and the 4 eV-excitation in YBa2Cu3O6 show quadratic dispersions in momentum transfer. From the dispersion constant of the plasmon a mean value of the Fermi velocity of the charge carriers parallel to the CuO2 planes has been derived.

Electron energy-loss studies on high-temperature superconductors

The electronic structure of the high-temperature superconductors La2−xSrxCuO4, Bi2Sr2Ca1−x YxCu2O8, and Nd2−xCexCuO4−δ have been studied by high-energy electron energy-loss spectroscopy in transmission. Core-level spectroscopy, in particular on the O 1s and Cu 2p levels, yields information on the partial unoccupied density of states, i.e., on the character of hole states close to the Fermi level.