An Analytical Technique to Extract Surface Information of Negatively Stained or Heavy-Metal Shadowed Organic Materials within the TEM

Using a series of uranyl acetate stained or platinum-palladium shadowed organic samples, an empirical analytical method to extract surface information from energy-filtered transmission electron microscopy (EFTEM) images is described. The distribution of uranium or platinum-palladium atoms, which replicate the sample surface topography, have been mathematically extracted by dividing the image acquired in the valence bulk plasmon energy region (between 20 and 30 eV) by the image acquired at the carbon K ionization edge (between 284 and 300 eV). The resulting plasmon-to-carbon ratio (PCR) image may be interpreted as a precise metal replica of the sample surface. In contrast to conventional EFTEM elemental mapping, including an absolute quantification approach, this technique can be applied to 200–600 nm thick organic samples. A combination of conventional TEM and PCR imaging allows one to detect complementary transmission and topographical information with nanometer precision of the same area of carbon-based samples. The advantages and limitations of PCR imaging are highlighted.

Interface Stoichiometry and Structure in Anodic Niobium Pentoxide

High-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS) were performed on electrochemically anodized niobium and niobium oxide. Sintered anodes of Nb and NbO powders were anodized in 0.1 wt% H3PO4 at 10, 20, and 65 V to form surface Nb2O5 layers with an average anodization constant of 3.6 ± 0.2 nm/V. The anode/dielectric interfaces were continuous and the dielectric layers were amorphous except for occurrences of plate-like, orthorhombic pentoxide crystallites in both anodes formed at 65 V. Using EELS stoichiometry quantification and relative chemical shifts of the Nb M4,5 ionization edge, a suboxide transition layer at the amorphous pentoxide interface on the order of 5 nm was detected in the Nb anodes, whereas no interfacial suboxide layers were detected in the NbO anodes.

Analysis of Extended Energy-Loss Fine Structure of Nanometerscale Clusters

Small atomic clusters are of great importance for applications such as catalysts whose activity depends on the surface of the cluster. Attempts to determine the atomic short-range order and size of clusters have been made by analyzing the extended X-ray absorption fine structure (EXAFS). However, the analysis was made on an average of many small clusters. Analysis of extended energy-loss fine structure (EXELFS) in an electron energy-loss spectrum (EELS) has developed to the point where in some cases, the quality of the results is comparable to its X-ray analogue, EXAFS. No other technique provides nanometer-scale spatial resolution of the analyzed area for determining the atomic structure. Most EXELFS analysis has been performed on the K-ionization edge of lighter elements. For heavier elements, a more complex ionization edge such as the L-edge has to be used, due to the inefficiency of collecting high quality EEL spectra at higher energy-losses (Z > 18).

The Origin of the Zanstra Discrepancy: UV Excess in the Central Star Continuum?

The temperature of a planetary nebula central star (CPN) may be determined by observing the nebular flux in an H I or He II recombination line and the stellar flux in a continuum band. The former measures the integrated UV stellar continuum blueward of the ionization edge of the recombined ion. By assuming a continuum shape (usually a blackbody), the ratio of these two fluxes yields an effective temperature for the CPN. This particular method, first introduced by Zanstra (1931), has an advantage over others in that the observables are relatively straightforward to obtain. However, this method also carries a troublesome ambiguity with it: CPN temperatures determined using He II recombination lines. This Zanstra discrepancy is reviewed by Kaler (1985) and Henry and Shipman (1986). Examples of He II and H I temperatures for numerous CPNs are given in Pottasch (1984), where it is shown that the He II Zanstra temperature often exceeds the H I temperature by several times 104K.

Plasmon lineshape analysis in EELS of semiconductors

Many materials display plasmon peaks in their low-loss EELS spectra. The plasmon peak shape, energy, and linewidth are characteristic of each material and are sensitive to the outer-shell electron density and details of the electronic band and energy-level structures. As these properties are a function not only of the composition but also the structure and chemistry of a sample, plasmon spectroscopy can potentially become a materials characterization tool which goes beyond the elemental analyses provided by EDXS and ionization-edge EELS. However, analysis of plasmon spectra requires considerably more sophistication than either of the aforementioned techniques due to the possibility of overlapping spectrum features, the prevalence of plural scattering, and the difficulty in detecting and characterizing the often subtle differences between the plasmon spectra of similar materials. As yet, no systematic approach to plasmon analysis analogous to that available commercially for EDXS or EELS core-edge analysis has been developed. We present here an approach which, for the simple case of semiconductors, makes some progress in this direction.

Background modeling alternatives in EELS and their implications on microanalysis

With the advent of parallel detectors, electron energy-loss spectroscopy (EELS) is expected to be employed increasingly in the routine microanalysis of light elements. The quantitation formulae that are used are relatively simple and straightforward and require only a measurement of the integrated core-loss intensity over a particular energy window beyond the ionization edge (Δi) and most often, a calculated ionization cross-section. Even though the hydrogenic cross-sections that are used routinely for microanalysis can be a source of error, it is observed that the procedure that largely determines the accuracy of quantification is the removal of the contribution of the background below the ionization edge. Based on empirical observations, an inverse power law function of the form I = AE-r, where the exponent ‘r’ takes values from 2 to 6, is now commonly used for the background. A background fitting region preceding the ionization edge (Δb) is chosen, the constants A and r determined by least squares refinement and the background extrapolated beyond the ionization edge for the required energy window (Δi).

Determination of ionization cross section by electron spectroscopic imaging

Electron energy loss spectroscopy (EELS) has become a significant technique for high resolution elemental microanalysis and mapping. Theoretically, quantitative analysis requires only one simple equation:Eqn.(1)where N is the number of atoms per unit area analysed; I(net)(a,δ) is the core loss intensity integrated over an energy range δ beyond the ionization edge and with a collection angle a; I(total) is the total intensity integrated beneath the whole spectrum; σ(a,δ) is the corresponding ionization cross section.To obtain I(net) according to current convention and some theoretical justification, the simplest way of removing the background beneath an ionization edge is simply fitting at least two pre-edge measurements to the equation of I=AE-r, where I is the intensity of electrons that have energy loss E; A and r are constants to be determined from the fitted pre-edge region. Several other methods have also been derived for better accuracy in specific applications.