Energy-loss Spectroscopy In Transmission Research Articles

Complex momentum behavior of electronic excitations in β-CuPc

The electronic excitation spectrum of β-CuPc has been investigated using electron energy-loss spectroscopy in transmission. Our results demonstrate a rather strong momentum dependence of the lowest exciton features. Both main components show a negative dispersion, and the momentum dependence indicates that this negative dispersion is parallel to the molecular stacks in β-CuPc. In addition, the spectral shape also varies upon increasing momentum transfer indicating a particular momentum dependence of the inter-molecular interactions.

Impact of potassium doping on the electronic structure of tetracene and pentacene: An electron energy-loss study.

We report the doping induced changes of the electronic structure of tetracene and pentacene probed by electron energy-loss spectroscopy in transmission. A comparison between the dynamic response of undoped and potassium-intercalated tetracene and pentacene emphasizes the appearance of a new excitation feature in the former gap upon potassium addition. Interestingly, the momentum dependency of this new excitation shows a negative dispersion. Moreover, the analysis of the C 1s and K 2p core-level excitation results in a significantly lower doping level compared to potassium doped picene, a recently discovered superconductor. Therefore, the present electronic structure investigations open a new pathway to better understand the exceptional differences between acenes and phenacene and their divergent behavior upon alkali doping.

Challenging the nature of low-energy plasmon excitations in CaC6 using electron energy-loss spectroscopy

The nature of low-energy plasmon excitations plays an important role in understanding the low-energy electronic properties and coupling mechanism of different superconducting compounds such as CaC. Recent ab initio studies predict a charge carrier intraband plasmon in keeping with a low-energy acoustic plasmon. Here, we have studied the low-energy electronic excitations of CaC using high-resolution electron energy-loss spectroscopy in transmission at low temperatures. The analysis of the core level excitations leads to the conclusion that hybridization between graphite and calcium states plays an essential role in this graphite-intercalated compound. Regarding the low-energy plasmon excitation, we observe the formation of an intraband (charge carrier) plasmon with a negative dispersion at about 3.5 eV in sound agreement with the theory. Finally, a weak excitation around 1.2 eV with an almost linear dispersion relation can be observed as predicted for an acoustic plasmon that may mediate the superconducting coupling in CaC. However its optical limit at 1 eV challenges the theoretical predictions and safely rules out an electronic superconducting coupling mechanism in CaC

Electronic excitations of potassium intercalated manganese phthalocyanine investigated by electron energy-loss spectroscopy

The electronic excitations of manganese phthalocyanine (MnPc) films were studied as a function of potassium doping using electron energy-loss spectroscopy in transmission. Our data reveal doping induced changes in the excitation spectrum, and they provide evidence for the existence of three doped phases: K(1)MnPc, K(2)MnPc, and K(4)MnPc. Furthermore, the addition of electrons first leads to a filling of orbitals with strong Mn 3d character, a situation which also affects the magnetic moment of the molecule.

Characterization of the electronic excitations in Alq3 using electron energy-loss spectroscopy

We have studied the electronic excitations of tris-(8-hydroxyquinoline)-aluminium(III) (Alq3) thin films using electron energy-loss spectroscopy in transmission. This allowed us to derive an effective dielectric function of such films in a large energy range. Moreover, an analysis of the momentum dependence of the lowest lying excitation allowed us to gain information on its localization. We show that this excitation does not disperse, i.e., is localized in the condensed phase. In contrast to many other molecular organic semiconductors, its spectral weight does not follow the behavior of a simple dipole-allowed electronic transition.

The electronic excitation spectrum of CuPcF16 films

We have studied the electronic excitations of fluorinated copper phthalocyanine, CuPcF16, thin films using electron energy-loss spectroscopy in transmission. This allowed for the derivation of the dielectric function in a wide energy range. Furthermore, we have analyzed the lowest excitation feature using a Lorentz model, which enabled the determination of the dielectric background in the energy range of the gap excitation, and an analysis of the momentum dependence of the spectral intensities at low energies.

Silver filled single-wall carbon nanotubes—synthesis, structural and electronicproperties

In our study we use methodology based on a wet chemistry process to fill single-wall carbonnanotubes (SWCNTs) with silver nanowires. The novelty of the study comes from longcontinuous silver nanowires incorporated in a SWCNT with a mean diameter of 1.22 nm.Using high resolution transmission electron microscopy and electron diffraction we evidencethe formation of 100–250 nm long silver wires within SWCNTs. The electronic structure ofAg-SWCNTs is presented in comparison to the corresponding reference purified SWCNTsusing electron energy-loss spectroscopy in transmission. The structural changes and thevariation of the electronic properties are analysed with electron diffraction and thecore-level excitations.

Electronic properties of FeCl3-intercalated single-wall carbon nanotubes

The electronic properties of ${\mathrm{FeCl}}_{3}$ intercalated single-wall carbon nanotubes were investigated using high-resolution electron energy-loss spectroscopy in transmission and optical spectroscopy. The lowering of the nanotube Fermi level is directly measured via electronic excitations from C $1s$ to empty carbon $2p$ states derived from the doping. The charge induced disappearance of the optically allowed transitions is demonstrated by the evolution of the optical absorption spectrum. In the loss function the appearance of a new feature can be assigned to plasmon excitations. The optical properties of ${\mathrm{FeCl}}_{3}$ intercalated nanotubes are comparable with those of fully potassium doped nanotubes. Upon annealing, the intercalant can be removed reversibly.

Two types of charge transfer excitations in low dimensional cuprates: an electron energy-loss study

Electron energy-loss spectroscopy in transmission has been used to study the electronic excitations near the excitation onset of a number of low dimensional cuprate structures. It is shown that the spectral weight of the lowest lying charge transfer excitation significantly depends upon the dimensionality of the cuprate network, i.e., on the degree of delocalization of the involved electronic states. Furthermore, a detailed analysis of the lowest lying charge transfer excitation as a function of momentum and polarization reveals convincing evidence for the existence of two different types of charge transfer excitation located near the spectral onset. This can only be modelled with the inclusion of Cu and O orbitals beyond the standard pdσ extended Hubbard model which is widely used for the description of electronic properties of CuO networks. To the best of our knowledge this is the first experimental manifestation for the relevance of orbitals beyond the standard extended Hubbard model for the low-energy excitations in a cuprate.

Electronic properties of potassium-intercalatedC60peapods

We present a study of the electronic structure of potassium-intercalated ${\mathrm{C}}_{60}$-filled single-wall carbon nanotubes (SWCNT's), so-called peapods, in comparison to the corresponding reference SWCNT's. The structural changes and the variation of the electronic properties were characterized by electron energy-loss spectroscopy in transmission. The analysis of the $\mathrm{C}1s$ core-level excitations shows that the doping level is nearly the same for peapods and the reference SWCNT's and that a competitive charge transfer of the $\mathrm{K}4s$ electrons to both the ${\mathrm{C}}_{60}$ peas and the SWCNT pods occurs. The intercalation process causes an expansion of the intertube distance in the bundle lattices and a decrease of the intermolecular distance between the ${\mathrm{C}}_{60}$ peas in the doped peapods. Regarding the optical properties, the charge transfer to the peapods (SWCNT's) yields the formation of a free-charge-carrier plasmon at about 1.3 eV (1.45 eV). An analysis by an effective Drude-Lorentz model shows that the lower plasmon energy in the doped peapods can be explained by a higher effective screening in these hybrid compounds.

Determination of the filling factor of C60 peapods by electron energy-loss spectroscopy in transmission

We have studied electron diffracion and core-level excitations of C 60 molecules encapsulated in single-wall carbon hanotubes, so-called, peapods, by electron energy-loss spectroscopy in transmission. The overall shape of the response of the single wall nanotube and the peapods is very similar but with distinct differences in the fine structure. The filling factor of C 60 peapods can be estimated with our volume-sensitive probe.

Detailed analysis of the mean diameter and diameter distribution of single-wall carbon nanotubes from their optical response

We report a detailed analysis of the optical properties of single wall carbon nanotubes with different mean diameters as produced by laser ablation. From a combined study of optical absorption, high resolution electron energy-loss spectroscopy in transmission and tight binding calculations we were able to accurately determine the mean diameter and diameter distribution in bulk SWCNT samples. In general, the absorption response can be well described assuming a Gaussian distribution of nanotube diameters and the predicted inverse proportionality between the nanotube diameter and the energy of the absorption features. A detailed simulation enabled not only a determination of the mean diameter of the nanotubes, but also gives insight into the chirality distribution of the nanotubes. The best agreement between the simulation and experiment is observed when only nanotubes within 15$^\circ$ of the armchair axis are considered. The mean diameters and diameter distributions from the optical simulations are in very good agreement with the values derived from other bulk sensitive methods such as electron diffraction, x-ray diffraction and Raman scattering.

Electronic structure ofCuGeO3:Charge excitations between zero and one dimension

We present a joint experimental and theoretical investigation of the electronic structure of CuGeO{sub 3}. The momentum dependent loss function was measured using electron energy-loss spectroscopy in transmission. For momentum transfers parallel to the crystallographic c direction (along the CuO{sub 2} chains) the loss function agrees well with calculations based upon a Cu{sub 5}O{sub 12} cluster model. The detailed analysis of the origin of the features below 7 eV in the calculated spectra reveals two distinct energy ranges in which transitions occur either into localized or delocalized states. A shift of spectral weight towards the low energy region can be observed when the coupling between adjacent CuO{sub 4} plaquettes is increased in our model. Above 2 eV the optical conductivity derived from the experimental loss function agrees well with results from optical measurements as well as with those from calculations based on our cluster model. The small spectral weight observed at 1.8 eV in the experiment has no counterpart in the theoretical loss function and thus cannot be assigned to charge transfer transitions involving Cu and O states. The loss functions with momentum transfers parallel to the crystallographic b direction show spectral weight at 6.5 eV that shifts to highermore » energies with increasing momentum transfer. This can probably be ascribed to transitions involving unoccupied Ge 4s and 4p states.« less

One-dimensional dynamics of thedelectrons inα′−NaV2O5

We have studied the electronic properties of the ladder compound $\alpha'$-NaV$_{2}$O$_{5}$, adopting a joint experimental and theoretical approach. The momentum-dependent loss function was measured using electron energy-loss spectroscopy in transmission. The optical conductivity derived from the loss function by a Kramers-Kronig analysis agrees well with our results from LSDA+U band-structure calculations upon application of an antiferromagnetic alignment of the V~3$d_{xy}$ spins along the legs and an on-site Coulomb interaction U of between 2 and 3 eV. The decomposition of the calculated optical conductivity into contributions from transitions between selected energy regions of the DOS reveals the origin of the observed anisotropy of the optical conductivity. In addition, we have investigated the plasmon excitations related to transitions between the vanadium states within an effective 16 site vanadium cluster model. Good agreement between the theoretical and experimental loss function was obtained using the hopping parameters derived from the tight binding fit to the band-structure and moderate Coulomb interactions between the electrons within the ab plane.

Electronic structure studies of undoped and nitrogen-doped tetrahedral amorphous carbon using high-resolution electron energy-loss spectroscopy

The electronic structure of undoped and nitrogen-doped tetrahedral amorphous carbon (ta-C) has been studied using high-resolution electron energy-loss spectroscopy in transmission to determine the local bonding of the carbon and nitrogen atoms. We found that the carbon and nitrogen atoms are mainly sp3-bonded for nitrogen contents up to 10 at. %. However, the data analysis is also consistent with some N atoms occupying trivalent σ or pyrrole configurations. The real (ε1) and imaginary parts (ε2) of the dielectric function were derived over a wide energy range carrying out a Kramers–Kronig analysis of the loss spectra. The ε2 spectra show two features due to the excitation of the π and σ valence states. The π feature is very small in ta-C deposited at 100 eV, consistent with its large sp3 content (∼85%). We have determined the ratios of sp2/sp3-hybridized atoms in the samples by both the C 1s edge and the optical sum rules.

Dynamics of a hole in aCuO4plaquette: Electron energy-loss spectroscopy ofLi2CuO2

We have measured the energy and momentum dependent loss function of Li_2CuO_2 single crystals by means of electron energy-loss spectroscopy in transmission. Using the same values for the model parameters, the low-energy features of the spectrum as well as published Cu 2p_(3/2) x-ray photoemission data of Li_2CuO_2 are well described by a cluster model that consists of a single CuO_4 plaquette only. This demonstrates that charge excitations in Li_2CuO_2 are strongly localized.

Valence-band excitations inV2O5

We present a joint theoretical and experimental investigation of the electronic and optical properties of vanadium pentoxide. Electron energy-loss spectroscopy in transmission was employed to measure the momentum-dependent loss function. This in turn was used to derive the optical conductivity, which is compared to the results of band-structure calculations. A good qualitative and quantitative agreement between the theoretical and the experimental optical conductivity was observed. The experimentally observed anisotropy of the optical properties of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ could be understood in the light of an analysis of the theoretical data involving the decomposition of the calculated optical conductivity into contributions from transitions into selected energy regions of the conduction band. In addition, based upon a tight binding fit to the band structure, values are given for the effective $\mathrm{V}{3d}_{\mathrm{xy}}\ensuremath{-}\mathrm{O}2p$ hopping terms and are compared to the corresponding values for ${\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{NaV}}_{2}{\mathrm{O}}_{5}.$

Local-field effects and anisotropic plasmon dispersion in diamond

We have measured the plasmon dispersion of diamond along the high-symmetry directions using electron energy-loss spectroscopy in transmission. We found the plasmon dispersion to be considerably anisotropic. A comparison of the experimental results to ab initio calculations that take local-field effects into account demonstrates the importance of local-field effects for the dielectric response of systems with strongly inhomogeneous electron distributions.

High-resolution electron energy-loss spectroscopy of undoped and nitrogen-doped tetrahedral amorphous carbon films

Abstract Undoped and nitrogen-doped amorphous carbon (ta-C) films deposited using a filtered cathodic vacuum arc were investigated by high-resolution electron energy-loss spectroscopy in transmission. The carbon and nitrogen K-edge spectra show fine structure at the C 1s to π ∗ excitations which gives information about the hybridization and coordination of the carbon and nitrogen atoms. The loss data were collected up to 50 eV and a Kramers–Kronig analysis was used to derive their complex dielectric functions. The e 2 spectra show two features due to excitations of π and σ valence states. The π feature is very small in ta-C deposited at 100 eV, consistent with its high sp 3 content. The development of the π and σ peaks with varying sp 2 content and their oscillator strengths allow us to resolve the degree of π–σ mixing which arises from disorder in more sp 3 -bonded carbons.

Frenkel and charge-transfer excitons inC60

We have studied the low-energy electronic excitations of ${\mathrm{C}}_{60}$ using momentum dependent electron energy-loss spectroscopy in transmission. The momentum dependent intensity of the gap excitation allows for a direct experimental determination of the energy of the ${}^{1}{H}_{g}$ excitation and thus also of the total width of the multiplet resulting from the gap transition. In addition, we could elucidate the nature of the following excitations---as either Frenkel or charge-transfer excitons.