Materials science applications of HREELS in near edge structure analysis and low-energy loss spectroscopy

Enhancement of resolution in core-loss and low-loss spectroscopy in a monochromated microscope

The significant enhancement of the energy resolution in the new generation of commercially available monochromated transmission electron microscopes presents new challenges in term of selecting the correct experimental conditions and understanding the various effects that can potentially influence the quality of the EELS data. In this respect we investigated the effect of point spread function of the detector and spectrum-diffraction mixing on the energy resolution and the intensity of the zero loss peak tails. Alternative approaches to improve the energy resolution by mathematical methods have been tested. By using a simple and commonly available test case (Si L(2,3) edges) we assessed the efficiency of the deconvolution algorithms to improve the resolution. The results show that the deconvolution is not always successful in improving the resolution of the core loss EELS data and the results may not always be reliable. Contrary to this, the application of the Richardson-Lucy deconvolution algorithm on some bandgap measurements data appears to be very effective. The procedure proved successful in removing the contribution of the zero-loss peak tails and allows an easier access to spectroscopic information starting at energy losses as low as of 0.5 eV with monochromated spectra and 1 eV with the non-monochromated spectra.

Angular and Local Spectroscopic Analysis to Probe the Vertical Alignment of N-doped Well-separated Carbon Nanotubes

Vertically aligned well-separated N-doped multiwalled carbon nanotubes (CNTs) were grown on a silicon substrate by plasma enhanced chemical vapor deposition (PECVD). Angular near-edge X-ray absorption fine structure (NEXAFS) was used to investigate the vertical alignment of as-grown CNTs. In addition, both individual tubes and tube bundles were characterized by high-resolution electron energy loss spectroscopy (HREELS). Simultaneous analysis of both spectroscopic techniques provides information on chemical environment, orbital orientation between carbon and heteroatoms, and local curvature effects. We demonstrate the utility of NEXAFS as an in situ probe of CNTs.

Determination of the oxidation state for iron oxide minerals by energy-filtering TEM

The oxidation state of iron oxide nanoparticles was determined using the two principally different technical realisations of energy filtering TEM, in one case using the JEOL 3010 equipped with a LaB6 cathode and a post-column GIF and in the second, the newly designed LIBRA 200FE equipped with an corrected in-column 90 degrees energy filter and a field emission gun (Schottky emitter). The samples studied were oxide-coated iron nanoparticles, and iron oxide inclusions in feldspars in granites. Five possible candidates exist for the iron-oxide phases: FeO, alpha-Fe2O3 (hematite), gamma-Fe2O3 (maghemite), Fe3O4 (magnetite) or alpha-FeO(OH) (goethite). Fingerprinting the O K-edge ELNES allows to distinguish between oxide phases with the same stochiometry and enables to make a first selection of possible candidates. The additional determination of the chemical composition allows unique identification of the phase present. For the oxide coated iron nanoparticles the most probable iron oxide phase of the shell is maghemite, which was additionally confirmed by HRTEM studies. The second studied system were iron oxide needles in alkali feldspar, where we obtained hematite as the most probable phase. There we additionally demonstrated the drastic changes of the ELNES of the O K-edge for the alkali feldspar and iron oxide needle by spatially resolved EELS.

Improving the energy resolution of X-ray and electron energy-loss spectra

We discuss some practical problems of improving the resolution of X-ray and electron spectra. Iterative Bayesian methods promise greater resolution enhancement than Fourier techniques but they also give rise to spectral artifacts. Satellite peaks are generated adjacent to strong peaks in the original spectrum and oscillatory artifacts become prominent after a large number of iterations, particularly when the original data contain high noise content. In the case of valence-electron energy-loss spectra, satellite peaks are reduced by removing the zero-loss peak prior to spectral sharpening. Even so, care should be exercised in interpreting low intensity at low energy loss (after sharpening) as evidence for a bandgap in the electronic density of states.

Practical aspects of electron energy-loss spectroscopy (EELS) calculations using FEFF8

We discuss the application of the ab initio program FEFF8 to calculations of electron energy-loss spectroscopy (EELS), focusing in particular on core-loss spectra. FEFF8 is based on a self-consistent, real space multiple scattering formalism. We focus on issues relevant to practical simulations, including the construction of well-converged potentials, the treatment of inelastic losses and exchange-correlation potentials and the core-hole. We also discuss how to account for experimental conditions, for example, sample orientation and finite temperature effects such as Debye-Waller factors. Finally we discuss the interpretation of the spectra in terms of electronic structure and local projected density of states (LDOS). As an explicit example, we illustrate various features of the code by application to the ionization edges of GaN.

First-principles study on incidence direction, individual site character, and atomic projection dependences of ELNES for perovskite compounds

The incidence direction dependence, the individual site character dependence, and the atomic projection dependence of O-K near edge fine structure of the EEL spectrum (ELNES) from YBa2Cu3O7 (YBCO) and SrTiO3 were theoretically simulated using the first-principles band structure calculation. In order to calculate ELNES, a core-hole was introduced at the oxygen 1s orbital, and sufficiently large supercells composed of more than 100 atoms were employed. We found that the intensity of the first peak of O-K ELNES from YBCO strongly depends on the atomic projection direction, and disappears when the spectrum is measured with the other projection directions. The large projection dependence was also predicted in the O-K ELNES of SrTiO3. It was found that those spectral changes according to the position of the projection are caused by the unidirectional Ti-O-Ti bond in SrTiO3.

Valence electron energy-loss spectroscopy in monochromated scanning transmission electron microscopy

With the development of monochromators for (scanning) transmission electron microscopes, valence electron energy-loss spectroscopy (VEELS) is developing into a unique technique to study the band structure and optical properties of nanoscale materials. This article discusses practical aspects of spatially resolved VEELS performed in scanning transmission mode and the alignments necessary to achieve the current optimum performance of approximately 0.15 eV energy resolution with an electron probe size of approximately 1 nm. In particular, a collection of basic concepts concerning the acquisition process, the optimization of the energy resolution, the spatial resolution and the data processing are provided. A brief study of planar defects in a Y(1)Ba(2)Cu(3)O(7-)(delta) high-temperature superconductor illustrates these concepts and shows what kind of information can be accessed by VEELS.

Theoretical electron energy-loss spectroscopy and its application in materials research

Electron energy-loss near-edge fine structure (ELNES) of group-III nitrides is calculated using a pseudopotential plan wave method within the framework of density functional theory. Core-hole effect and supercell size influence are investigated. Based on our present and earlier work, a comprehensive understanding of theoretical ELNES application in materials research is demonstrated: interpreting experimental spectra, predicating theoretical reference spectra when reliable experimental spectra are not available, identifying ELNES-structure correlation and estimating the reliability of experimental spectra.

0.23eV energy resolution obtained using a cold field-emission gun and a streak imaging technique

We demonstrate that a high energy resolution of 0.23 eV is possible by using a cold field-emission electron gun (CFEG) without a monochromator. We have used a 300 kV transmission electron microscope (Hitachi, HF-3000) equipped with a CFEG and an energy filter (Gatan, GIF2002). Since energy instability is critical for high energy resolution in electron energy-loss spectroscopy, we have applied a high-speed 'streak imaging' acquisition technique, in which a series of time-resolved spectra are acquired as a two-dimensional spectrum. With this technique, we can easily record 1000-20,000 spectra with an exposure time of 0.353 ms per spectrum. Instability of less than 1.4 kHz has been corrected in the time-resolved spectra, allowing the inherent performance of the CFEG to be realized.

Low-temperature synthesis of anatase–brookite composite nanocrystals: the junction effect on photocatalytic activity

Anatase-brookite composite nanocrystals have successfully been synthesized at 50 degrees C using a simple liquid-phase process. The photocatalytic activity of the sample for the gas-phase oxidation of CH3CHO is 5.4 times greater than that of a single-phase anatase sample with comparable crystallite size and surface area. Electron energy loss spectra suggest that this high activity results from junction between anatase and brookite crystals.

Investigation of hexagonal and cubic GaN by high-resolution electron energy-loss spectroscopy and density functional theory

High-resolution electron energy-loss spectroscopy in a transmission electron microscope is a very powerful method for the study of electronic structure of materials. The fine structure of Ga L(2,3) and N ionization edges in c-GaN and h-GaN was studied using a TEM equipped with a monochromator and high-resolution energy spectrometer. The experimental results were compared with the results of calculation based on the density functional theory using the Wien2k code and show that the best fit is achieved when the core hole effect is taken into account. The effect of the core hole value and the supercell size on the energy-loss near-edge structure have been investigated. A different behaviour was found for c-GaN and h-GaN: better agreement is obtained for a 0.5 core hole for h-GaN and for a full core hole for c-GaN. The anisotropic behaviour of the experimental spectra and calculated spectra for h-GaN have been studied and the "magic" angle was determined.

Electron energy-loss near-edge structures of 3d transition metal oxides recorded at high-energy resolution

Near-edge fine structures of the metal L(2,3) and O K-edges in transition metal-oxides have been studied with a transmission electron microscope equipped with a monochromator and a high-resolution imaging filter. This system enables the recording of EELS spectra with an energy resolution of 0.1eV thus providing new near-edge fine structure details which could not be observed previously by EELS in conventional TEM instruments. EELS-spectra from well-defined oxides like titanium oxide (TiO(2)), vanadium oxide (V(2)O(5)), chromium oxide (Cr(2)O(3)), iron oxide (Fe(2)O(3)), cobalt oxide (CoO) and nickel oxide (NiO) have been measured with the new system. These spectra are compared with EELS data obtained from a conventional microscope and the main spectral features are interpreted. Additionally, the use of monochromised TEMs is discussed in view of the natural line widths of K and L(2,3) edges.