Image-EELS: Simultaneous recording of multiple electron energy-loss spectra from series of electron spectroscopic images

A new approach for element microanalysis with energy-filtering transmission electron microscopy (EFTEM) is presented which was accomplished with the CEM 902 electron microscope (Zeiss, Germany). This method is called Image-EELS, because it is a synthesis of electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Series of energy-filtered images at increasing energy losses are recorded from one area with a TV camera. In a second step the intensity of selected regions in the image stack is measured with an image analysis system and plotted as a function of the energy loss. Thus many spectra from different objects can be calculated from one image series and compared with each other. The spatial resolution of EELS is considerably enhanced, the noise is decreased because many pixels from irregular objects are integrated, and the information from ESI can be analysed as a function of the energy loss.     

Point defect characterization in HAADF-STEM images using multivariate statistical analysis

Quantitative analysis of point defects is demonstrated through the use of multivariate statistical analysis. This analysis consists of principal component analysis for dimensional estimation and reduction, followed by independent component analysis to obtain physically meaningful, statistically independent factor images. Results from these analyses are presented in the form of factor images and scores. Factor images show characteristic intensity variations corresponding to physical structure changes, while scores relate how much those variations are present in the original data. The application of this technique is demonstrated on a set of experimental images of dislocation cores along a low-angle tilt grain boundary in strontium titanate. A relationship between chemical composition and lattice strain is highlighted in the analysis results, with picometer-scale shifts in several columns measurable from compositional changes in a separate column.     

The electron energy-loss spectroscopic analysis of inhaled smoke particles

This paper details the use of electron spectroscopic imaging in the elemental analysis of smoke particles inhaled by smoke-death victims. The results show that these particles have a varied structure and composition. Because of this, these particles may play a far more significant role in smoke inhalation injuries than has previous been recognized.     

Development,quantitative performance and applications of a parallel electron energy-loss spectrum imaging system

The development of a parallel electron energy-loss spectrum imaging system is presented. The analytical performance of the imaging technique was investigated and the system applied to materials science problems. The system, which allows acquisition and storage of a parallel electron energy-loss spectrum at each pixel of an image, was developed by interfacing a multichannel analyser and a microscope to a computer workstation. In the experimental conditions used for imaging, detection limits and quantification errors were large and varied as a function of spatial resolution and the range of chemical elements of interest in the image. Applications of this imaging technique in materials science showed that quantitative chemical information is provided by the system and that the use of relative thickness maps and detailed statistical analysis of the spectrum-image allowed an unbiased interpretation of the images. As energy-loss spectra are available after processing, spectroscopic information about the analysed material can be used to provide supplementary information.     

Stoichiometry and valence measurements of niobium oxides using electron energy-loss spectroscopy

Qualitative and quantitative electron energy‐loss spectroscopy analyses have been performed on niobium and stable niobium oxides (NbO, NbO₂ and Nb₂O₅). At integration windows (Δ) greater than 75 eV, k‐factor analysis can be used to distinguish between the stoichiometry of the three oxides within 5.7% error. As seen in other metal oxides, with increasing oxidation state the metal ionization edges shift to higher energies relative to the O‐K edge. Normalized M_2,3 white‐line intensities show a strong correlation with 4d occupancy for each compound. The data are in correspondence with that observed in the literature for 4d transition metals using normalized L_2,3 white lines. Lastly, a distinctive energy‐loss near‐edge, structure of the O‐K edge was observed for each oxide, which could be used as a fingerprint for analysis of unknowns.     

Quantitative analysis of valence electron energy-loss spectra of aluminium nitride

The optical properties and electronic structure of aluminium nitride are determined using valence electron energy-loss spectroscopy in a dedicated scanning transmission electron microscope. Quantitative analysis of the experimental valence electron energy-loss spectra to determine the electronic structure encompasses single scattering deconvolution of the valence electron energy-loss spectra to calculate the energy-loss function, Kramers–Kronig analysis of the energy-loss function to reveal the complex dielectric function, transformation of the dielectric function into the optical interband transition strength via optical property relations and finally critical-point analysis of the interband transition strength. The influence of both experimental and analytical parameters on the final result was studied systematically to define and improve the understanding of the methods. To check the reliability of this technique the interband transition strength determined was compared with results of vacuum ultraviolet spectroscopy. Good agreement was found if sample preparation was taken into account. The preparation of the specimen for the transmission electron microscopy has an effect on the electronic structure. Quantitative analysis of valence electron energy-loss spectroscopy, using the methods presented, is an important and capable method to determine the electronic structure of materials and it has the benefit of high spatial resolution.     

Quantitative analysis of electron energy-loss spectra from ultrathin-sectioned biological material

A computer program for quantitative spectral analysis is proposed for the elemental analysis of biological material by electron energy-loss spectroscopy in a conventional transmission electron microscope, the Zeiss EM902. Bio-standards are used to test the performance of this program. The application of a simplex optimization method for curve-fitting is proposed to separate the ionization edge from the background. Making use of Ce-, Ca- and Fe-bio-standards, this method is compared with Egerton's well-known two-area method.     

Multivariate statistical analysis of electron energy-loss spectroscopy in anisotropic materials

Recently, an expression has been developed to take into account the complex dependence of the fine structure in core-level electron energy-loss spectroscopy (EELS) in anisotropic materials on specimen orientation and spectral collection conditions [Y. Sun, J. Yuan, Phys. Rev. B 71 (2005) 125109]. One application of this expression is the development of a phenomenological theory of magic-angle electron energy-loss spectroscopy (MAEELS), which can be used to extract the isotropically averaged spectral information for materials with arbitrary anisotropy. Here we use this expression to extract not only the isotropically averaged spectral information, but also the anisotropic spectral components, without the restriction of MAEELS. The application is based on a multivariate statistical analysis of core-level EELS for anisotropic materials. To demonstrate the applicability of this approach, we have conducted a study on a set of carbon K-edge spectra of multi-wall carbon nanotube (MWCNT) acquired with energy-loss spectroscopic profiling (ELSP) technique and successfully extracted both the averaged and dichroic spectral components of the wrapped graphite-like sheets. Our result shows that this can be a practical alternative to MAEELS for the study of electronic structure of anisotropic materials, in particular for those nanostructures made of layered materials.     

Changes in the iron white-line ratio in the electron energy-loss spectrum of iron-copper multilayers

We report changes in the iron L₃/L₂ white-line ratio of the electron energy-loss spectrum of a series of iron-copper multilayers which we discuss in terms of the lattice plane spacings present in the multilayer system. Momentum-resolved data from one multilayer is also considered.     

Compression of image information in electron images

Image compression for electron imaging is considered by the method of transform coding using the discrete cosine transform and an adaptive method. Using examples drawn from conventional transmission microscopy, data compression by at least 10 to 12 times (from the original 8 bits per pixel image) and sometimes 20 to 30 times is shown to give acceptable image quality. A modification for electron images is necessary to published procedures of sub-image classification within the adaptive procedure. This is because of the nature of the spatial frequency spectra of electron images. On average, the action of data compression is shown to be analogous to the action of a point spread function for the images chosen for study.     

In situ analysis of gas composition by electron energy-loss spectroscopy for environmental transmission electron microscopy

We have developed methods for using in situ electron energy-loss spectroscopy (EELS) to perform quantitative analysis of gas in an environmental transmission electron microscope. Inner-shell EELS was able to successfully determine the composition of gas mixtures with an accuracy of about 15% or better provided that some precautions are taken during the acquisition to account for the extended gas path lengths associated with the reaction cell. The unique valence-loss spectrum associated with many gases allowed simple methodologies to be developed to determine gas composition from the low-loss region of the spectrum from a gas mixture. The advantage of the valence loss approach is that it allows hydrogen to be detected and quantified. EELS allows real-time analysis of the volume of gas inside the reaction cell and can be performed rapidly with typical acquisition times of a few seconds or less. This in situ gas analysis can also be useful for revealing mass transport issues associated with the differential gas diffusion through the system.     

Precession technique and electron diffractometry as new tools for crystal structure analysis and chemical bonding determination

We have developed a new fast electron diffractometer working with high dynamic range and linearity for crystal structure determinations. Electron diffraction (ED) patterns can be scanned serially in front of a Faraday cage detector; the total measurement time for several hundred ED reflections can be tens of seconds having high statistical accuracy for all measured intensities (1-2%). This new tool can be installed to any type of TEM without any column modification and is linked to a specially developed electron beam precession "Spinning Star" system. Precession of the electron beam (Vincent-Midgley technique) reduces dynamical effects allowing also use of accurate intensities for crystal structure analysis. We describe the technical characteristics of this new tool together with the first experimental results. Accurate measurement of electron diffraction intensities by electron diffractometer opens new possibilities not only for revealing unknown structures, but also for electrostatic potential determination and chemical bonding investigation. As an example, we present detailed atomic bonding information of CaF(2) as revealed for the first time by precise electron diffractometry.     

3D spectrum imaging of multi-wall carbon nanotube coupled pi-surface modes utilising electron energy-loss spectra acquired using a STEM/Enfina system

Numerous studies have utilised electron energy-loss (EEL) spectra acquired in the plasmon (2-10 eV) regime in order to probe delocalised pi-electronic states of multi-wall carbon nanotubes (MWCNTs). Interpretation of electron energy loss (EEL) spectra of MWCNTs in the 2-10 eV regime. Carbon (accepted for publication); Blank et al. J. Appl. Phys. 91 (2002) 1657). In the present contribution, EEL spectra were acquired from a 2D raster defined on a bottle-shaped MWCNT, using a Gatan UHV Enfina system attached to a dedicated scanning transmission electron microscope (STEM). The technique utilised to isolate and sequentially filter each of the volume and surface resonances is described in detail. Utilising a scale for the intensity of a filtered mode enables one to 'see' the distribution of each resonance in the raster. This enables striking 3D resonance-filtered spectrum images (SIs) of pi-collective modes to be observed. Red-shift of the lower energy split pi-surface resonance provides explicit evidence of pi-surface mode coupling predicted for thin graphitic films (Lucas et al. Phys. Rev. B 49 (1994) 2888). Resonance-filtered SIs are also compared to non-filtered SIs with suppressed surface contributions, acquired utilising a displaced collector aperture. The present filtering technique is seen to isolate surface contributions more effectively, and without the significant loss of statistics, associated with the displaced collector aperture mode. Isolation of collective modes utilising 3D resonance-filtered spectrum imaging, demonstrates a valuable method for 'pinpointing' the location of discrete modes in irregularly shaped nanostructures.     

Assessment of electron irradiation damage to biomolecules by electron diffraction and electron energy-loss spectroscopy

Electron radiation damage is one of the most severe problems in high resolution electron microscopy of biomolecules. The techniques of electron diffraction and electron energy-loss spectroscopy were applied to gain a better understanding of radiation damage in amino acids and nucleic acid bases. The results when compared with G-values for the release of ammonia and hydrogen sulphide from amino acids seem to indicate that bond scission is an important cause of radiation damage at moderate doses of irradiation. High resolution structural disorder in nucleic acid bases was found to involve loss of atoms peripheral to the main ring structure.     

Measurement of local thickness by electron energy-loss spectroscopy

We review various methods which can be used to derive the thickness of an electron-microscope specimen from its transmission energy-loss spectrum. We have applied a sum-rule technique to various kinds of specimen, using a variety of electron-optical conditions, and estimate its accuracy to be typically ±10% (or±2nm if larger) over the thickness range 10–150 nm. This method requires no knowledge of the physical or chemical properties of the specimen other than its refractive index (∞ for metal). It involves only a low radiation dose, allowing good lateral resolution (<10 nm) to be achieved even by selected-area techniques.     

Automated magnification calibration in transmission electron microscopy using Fourier analysis of replica images

The magnification factor in transmission electron microscopy is not very precise, hampering for instance quantitative analysis of specimens. Calibration of the magnification is usually performed interactively using replica specimens, containing line or grating patterns with known spacing. In the present study, a procedure is described for automated magnification calibration using digital images of a line replica. This procedure is based on analysis of the power spectrum of Fourier transformed replica images, and is compared to interactive measurement in the same images. Images were used with magnification ranging from 1,000 x to 200,000 x. The automated procedure deviated on average 0.10% from interactive measurements. Especially for catalase replicas, the coefficient of variation of automated measurement was considerably smaller (average 0.28%) compared to that of interactive measurement (average 3.5%). In conclusion, calibration of the magnification in digital images from transmission electron microscopy may be performed automatically, using the procedure presented here, with high precision and accuracy.     

Measurement of radiation damage by electron energy-loss spectroscopy

Inner shell ionization edges in the energy-loss spectrum were used to measure the loss of gaseous elements from thin samples of nitrocellulose, polyvinyl formal, copper phthalocyanine, graphite bisulphate and sodium chloride during exposure to 80 keV electrons. The irradiation kinetics were measured for electron doses up to 10⁴ C m^?2 and for dose rates up to 10 mA m^?2. The energy-loss technique is discussed in relation to other methods for assessing radiation damage.     

Mapping and fuzzy classification of macromolecular images using self-organizing neural networks

In this work the effectiveness of the fuzzy kohonen clustering network (FKCN) in the unsupervised classification of electron microscopic images of biological macromolecules is studied. The algorithm combines Kohonen's self-organizing feature maps (SOFM) and Fuzzy c-means (FCM) in order to obtain a powerful clustering technique with the best properties inherited from both. Exploratory data analysis using SOFM is also presented as a step previous to final clustering. Two different data sets obtained from the G40P helicase from B. Subtilis bacteriophage SPP1 have been used for testing the proposed method, one composed of 2458 rotational power spectra of individual images and the other composed by 338 images from the same macromolecule. Results of FKCN are compared with self-organizing feature maps (SOFM) and manual classification. Experimental results prove that this new technique is suitable for working with large, high-dimensional and noisy data sets and, thus, it is proposed to be used as a classification tool in electron microscopy.     

Extended electron energy-loss fine structure and selected-area electron diffraction studies of small palladium clusters

Extended electron energy-loss fine structure (EXELFS) and selected-area electron diffraction (SAED) techniques have both been applied to the study of the crystalline structure of Pd clusters of average diameters ranging from bulk to 24 Å. The combined use of these techniques gives complementary information about the crystalline structure of Pd clusters. Both techniques show the same lattice parameter expansion, about 4% for the smallest Pd cluster, with respect to the bulk. The EXELFS analysis performed on the Pd-M_4,5 edge shows a sizeable increase of structural disorder in the smallest cluster. SAED gives additional information about the Pd bulk sample, showing the occurrence of crystalline regions about 50 Å in diameter.