UPS and EELS study of zirconium oxidation

The electronic structures of metallic zirconium, zirconium oxide, and zirconium surfaces with intermediate degrees of oxidation have been studied by photoemission spectroscopy using synchrotron radiation and by electron energy loss spectroscopy. Both methods are used to analyze the same samples in one experimental cycle. Some specific features of the electronic structures that had not been detected earlier are revealed. The experimental data obtained are explained using the first-principles calculations of the electronic states of hcp metallic zirconium and cubic or monoclinic zirconia. The dielectric function and the electron-energy-loss function are calculated for comparison with the experimental data. Despite certain quantitative differences, the experimental and calculated data on the electronic structures of zirconium and its oxide are in good qualitative agreement.     

Electron energy-loss spectroscopy of V2O5 nanofibers synthesized by electro-spinning

The dielectric properties of V₂O₅ nanofibers, synthesized by the electrospinning method, are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) in a transmission electron microscope. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the Generalized Gradient Approximation (GGA) is presented, having found an excellent agreement between them. Although the experimental EELS has been acquired for the nanoparticles composing the fibers, and numerical calculations were carried out for bulk material, agreement between experimental and calculated results shows that no difference exists between the electronic structure in calculated bulk material and the nanoparticles. Furthermore, our results from EELS confirm that we accomplished the expected crystalline phase. The origins of interband transitions are identified in the electronic band structure by calculating the partial imaginary part of the dielectric function and the partial density of states.     

Electronic structure of MgS and MgYb2S4: Electron Energy-Loss Spectroscopy and self-consistent multiple scattering calculations

The electronic structure of MgS and MgYb₂S₄ have been studied using the fine structure of the Mg-K, S-K, Mg-L_2,3, S-L_2,3 and Yb-N₅ edges measured by electron energy-loss spectroscopy (EELS). Our experimental results are compared with real-space full multiple scattering calculations as incorporated in the FEFF9.6 code. All edges are very well reproduced. Total and partial densities of states have been calculated. The calculated densities of states of Mg and S are similar in both compounds. The energy distribution of these states suggests a covalent nature for both materials. For MgYb₂S₄ a band gap smaller than for MgS is predicted. In this compound the top of the valence band and the bottom of the conduction band are dominated by Yb states.     

Electronic structure and X-ray photoelectron spectra of TiC and ZrC: Cluster and band structure approaches

The electronic structure for titanium and zirconium monocarbides have been calculated with the SCF MS X_α-cluster method. The results are compared with the experimental X-ray photoelectron spectra and augmented plane wave (APW) calculations.     

Atomic resolution STEM analysis of defects and interfaces in ceramic materials

Atomic resolution scanning transmission electron microscopy (STEM) analysis, in particular the combination of Z-contrast imaging and electron energy-loss spectroscopy (EELS) has been successfully used to measure the atomic and electronic structure of materials with sub-nanometer spatial resolution. Furthermore, the combination of this incoherent imaging technique with EELS allows us to correlate certain structural features, such as defects or interfaces directly with the measured changes in the local electronic fine-structure. In this review, we will discuss the experimental procedures for achieving high-resolution Z-contrast imaging and EELS. We will describe the alignment and experimental setup for high-resolution STEM analysis and also describe some of our recent results where the combined use of atomic-resolution Z-contrast imaging and column-by-column EELS has helped solve important materials science problems.     

Nanoscale EELS analysis of dielectric function and bandgap properties in gaN and related materials

The low loss region of an EEL spectrum (<50 eV) contains information about excitations of outer shell electrons and thus the electronic structure of a specimen which determines its optical properties. In this work, dedicated electron energy loss spectroscopy (EELS) methods for the experimental acquisition and analysis of spectra are described, which give improved information about the electronic structure near the bandgap region at a spatial resolution in the range of nanometers. For this purpose, we made use of a cold field emission scanning transmission electron microscope (STEM) equipped with a dedicated EELS system. This device provides a subnanometer electron probe and offers an energy resolution of 0.35 eV. Application of suitable deconvolution routines for removal of the zero loss peak extracts information on the bandgap region while the Kramers-Kronig transformation deduces the dielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap materials for the case of III-nitride compounds, which are currently the most promising material for applications on optoelectronic devices working in the blue and ultraviolet spectral range. The obtained results are in excellent agreement with experimental measurements by synchrotron ellipsometry and theoretical studies. The potential of the superior spatial resolution of EELS in a STEM is demonstrated by the analysis of dielectric properties of individual layers of heterostructures and individual defects within wurtzite GaN.     

Investigation on optical properties of ZnO nanowires by electron energy-loss spectroscopy

Electron energy-loss spectroscopy (EELS) and ab initio band structure calculations have been performed to determine the optical properties of wurtzite structured zinc oxide (ZnO) nanowires. Compared with other techniques, EELS significantly extends the energy range and is a useful technique for analysis of the dielectric properties on a microstructure level. The first-principles calculations allow deep insight into the experimental results. Furthermore, the polarization dependencies of optical properties have been discussed. Our results give some reference to the thorough understanding of optical properties of ZnO.     

Electronic structure of zirconium dihydride

Using linearized muffin-tin orbitals, with the atomic spheres corrected for overlap, we have performed a self-consistent calculation of the electronic structure of the cubic phase of zirconium dihydride. We have calculated the electronic structure of nonstoichiometric ZrH_1.75. We have studied the effects of a hydrogen vacancy on the electronic characteristics. Assuming a low positron density, we calculated the positron spectrum, the positron lifetime, and the contributions to electron-positron annihilation of various electronic states. We obtained a reasonably good agreement between our theoretical results and the experimental data. Institute of Strength Physics and Meterials Research, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 96–103, august, 1996.     

Electronic structure of graphite: Single particle and collective excitations studied by EELS,SEE and K edge loss techniques

We present some EELS, SEE and K edge loss measurements on in situ cleaved graphite. Single particle excitations in the EELS measurements have been identified accordingly with some previous results on pure polycrystalline iron and are compared with available band structure calculations of graphite. The core edge loss spectroscopy performed in the reflection mode at low primary electron energy proved to be a powerful and rapid technique to study the partial density of empty states.     

Experimental and theoretical determination of the low-loss electron energy loss spectroscopy of LiMn2O4

The dielectric properties of cubic spinel-type LiMn₂O₄, used as cathode material in lithium ion secondary batteries, are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) spectrum in a transmission electron microscope. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA) is presented. The origins of interband transitions are identified in the electronic band structure, by calculating the partial imaginary part of the dielectric function and the partial density of states of Li, Mn and O. Good agreement with experimental spectra is observed which allowed interpreting main features of the EELS spectrum.     

Development of electron energy-loss spectroscopy for nanoscience

Electron energy-loss spectroscopy (EELS) has been well established in providing the composition and chemical bonding information of materials, particularly for light elements. Its potential for structural determination has long been known but has yet to be fully explored. With the convergence of rapid development in computing power and improvement in the efficiency of the material specific electronic structure simulation, plus the recent breakthrough in the development of C(s)-corrected electron microscopy, the reconstruction of the local three dimensional structure of nanomaterial using EELS in conjunction with advanced structural imaging and diffraction techniques is becoming increasingly feasible. In this paper, we will review from our own examples the progress in EELS instrumentation, methods and simulation to illustrate the progress that has been made. They include the density-function-theory-based ab initio spectroscopic simulation for standard-less fingerprint applications for metastable polymorph identification, magic angle electron energy-loss spectroscopy as well as recent results from the dual-detectors EELS system which allows the energy instability of the spectrometer to be analyzed in real-time and eventually compensated on-line.     

Electron-energy-loss and X-ray photoelectron spectra. Part 1. Electronic structure of tetraphenylporphyrin and its nickel derivative

Specific information on the energy level diagram and on the energy forbidden gap of tetraphenylporphyrin and its Ni derivative has been obtained by means of electron energy-loss spectroscopy (EELS) and X-ray photoelectron spectroscopy techniques. Comparison of the free ligand and its metal derivative emphasizes the importance of the metal 3d levels in the conduction processes. The interband and plasmon transition present in the EELS spectra of these species are compared with the theoretically calculated energy level diagram and experimental data.     

HRTEM and EELS study of Y2O3/MgO thin films

Y2O3 thin films deposited on (001)-MgO substrate have been investigated by high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy. Digital processing of the HRTEM images reveals the presence of grains with a crystallographic structure different from that of the rest of the film (Ia3). The spectrum imaging technique has been applied in vicinity of the Y2O3/MgO interface to get a better knowledge of the phases nucleated on the substrate surface. Fine structures of the O K-edge have been studied in detail; actually two kinds of spectra have been detected in the yttrium oxide thin film. These spectra have been compared to self-consistent full multiple scattering calculations (SC-FMS). One family of spectra has then been associated to the well-known Ia3 structure. The other family of spectra has been compared to calculations performed for the other known structures (such as hexagonal or monoclinic) of Y2O3 with a little success. We have finally compared these spectra to calculations performed with a particular atomic arrangement (octahedral) of Y and O atoms, which leads to a good match between experimental and calculated spectra. Our results emphasize the benefit of coupling several techniques such as HRTEM, EELS and SC-FMS for the determination of structures at the nanometric scale.     

Unraveling the symmetry of the hole states near the Fermi level in the MgB2 superconductor

We use x-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to study the fine structure at the K edge of boron in MgB(2). We observe in XAS a peak of width 0.7 eV at the edge threshold, signaling a narrow energy region with empty boron p states near the Fermi level. The changes in the near edge structure observed in EELS with direction of the momentum transfer imply that these states have p(x)p(y) symmetry. Our observations are consistent with electronic structure calculations indicating a narrow energy window of empty p(x)p(y) states that falls to zero at 0.8 eV above the Fermi level. The disappearance of the p(x)p(y) feature in EELS at grain boundaries suggests that this signature may become powerful in probing superconductivity at nanoscale.     

Initial stage of oxidation of titanium

The electronic structure of titanium with different oxidation levels of the surface was studied by photoelectron spectroscopy and electron energy loss spectroscopy (EELS). The spectra of the valence band and the core level of Ti3p and EEL spectra were recorded for a titanium surface with oxidation exposures from 0.12 to 900 L at room temperature. In general, the titanium oxidation process is similar to that for zirconium, which was investigated earlier; however, certain features of the electronic structure that were not observed earlier were revealed for titanium. Even at high oxidation exposures of ∼1000 L, both O2p valence band and valence band corresponding to titanium metal states were observed. During consecutive oxidation, one can see four different positions of the Ti3p peak corresponding to titanium atoms with different valences. It is probable that, at the initial oxidation stage, islands of complex composition oxide were formed on the surface and grew into the bulk during further oxidation.     

Experimental and theoretical determination of the low-loss electron energy loss spectroscopy of nanostructured ZnO

The dielectric properties of nanostructured wurtzite-type ZnO are studied by analyzing the low-loss region of the electron energy loss spectroscopy (EELS) in a transmission electron microscope. Characteristic peaks at about 12 and 32 eV in the imaginary part of the dielectric function shift to lower energies as particle size decreases. A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA), GGA+U and modified Becke-Johnson (mBJ) is presented. The origins of interband transitions are identified in the electronic band structure by calculating the partial imaginary part of the dielectric function and the partial density of states of Zn and O.     

On the role of the image force in the electron-energy-loss spectrum of a dielectric target

In reflection electron-energy-loss spectroscopy (EELS), the attraction of a probing particle by its image below the surface of the target affects the electron trajectory close to the surface. The reduction of the interaction time that results from the acceleration of the electrons has important effects on the EELS intensity measured at grazing incidences. The classical formulation of EELS developed in this paper takes the image force into account when the target is a dielectric medium. Calculations of the loss intensities performed for MgO are found to be in good agreement with experimental data.     

The near edge structure of cubic boron nitride

We compare the near edge structure (NES) of cubic boron nitride (cBN) measured using both electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) with that calculated using three commonly used theoretical approaches. The boron and nitrogen K-edges collected using EELS and XAS from cBN powder were found to be nearly identical. These experimental edges were compared to calculations obtained using an all-electron density functional theory code (WIEN2k), a pseudopotential density functional theory code (CASTEP) and a multiple scattering code (FEFF). All three codes were found to reproduce the major features in the NES for both ionisation edges when a core-hole was included in the calculations. A partial core hole (1/2 of a 1s electron) was found to be essential for correctly reproducing features near the edge threshold in the nitrogen K-edge and to correctly obtain the positions of all main peaks. CASTEP and WIEN2k were found to give almost identical results. These codes were also found to produce NES which most closely matched experiment based on χ² calculations used to qualitatively compare theory and experiment. This work demonstrated that a combined experimental and theoretical approach to the study of NES is a powerful way of investigating bonding and electronic structure in boron nitride and related materials.     

X-ray emission spectra and chemical bonding in TiC,TiN and TiO

The experimental X-ray emission spectra of titanium carbide, nitride and oxide have been obtained. Quantum-chemical calculations of the electronic structure of clusters in TiC, TiN and TiO have been carried out by the semiempirical Mulliken-Wolfsberg-Helmholtz method with self-consistency on charges and configurations. The results of these calculations are in good agreement with the X-ray spectroscopy data and offer a reasonable explanation of the experimental spectra. Chemical bonding and electronic structure of the compounds are discussed. Ionicity is shown to increase from TiC to TiO according to the electronegativity principle, the calculated charges on the metal ions being close to experimental estimates. The role of metal-metal and metal-nonmetal interactions in the chemical bonding is analysed. Vacancy models for TiO and their effect on the X-ray emission spectra are investigated. By the CNDO method with configurational interactions the optical spectrum of titanium carbide has been calculated. It is shown that this spectrum may be interpreted from the results for the [TiC₆] cluster, without introducing the Lye-Logothetis band scheme with negative charge on the metal ion.     

Electronic structure of the Zr-He system

Ab initio calculations of the electronic structure of hexagonal close-packed and face-centered cubic zirconium with the impurity of helium atoms of about 6 at % have been performed. It has been established that the presence of helium significantly changes the electronic structure of zirconium and leads to a considerable redistribution of its electron density. Calculated values of chemical shifts of the core states of Zr atoms due to the presence of helium atoms in its lattice have been discussed.