Current applications of electron energy loss spectroscopy

It is undoubtedly true that the advent of efficient energy loss spectrometers for transmission microscopes over the last few years has been of considerable assistance, at least qualitatively, for the analysis of light elements and, to a more limited extent, in structure interpretation. Rather frustratingly, given the potentially better spatial resolution of EELS over EDX, realistic quantitative analysis remains difficult, and similarly - while fascinating effects are seen in, for example, the crystallographic orientation dependence of the signal - these are currently only broadly interpretable in relation to those observed in EDX. The reasons for this are discussed, as are the relative advantages of large and small collection angles for different types of experiment.     

Contribution of electron energy loss spectroscopy to the development of analytical electron microscopy.

The combined use of an electron energy loss spectrometer and an electron microscope provides some chemical information at the nanometer scale. The physics of the interaction processes between the incident electron beam and the thin sample foil is reviewed in terms of energy and momentum transfer. This analysis of the content of an electron energy loss spectrum allows us to establish rules for a satisfactory use of the information and to discuss the detection limits of this newly developed microanalytical technique.     

Quantitative electron energy loss spectroscopy in biology

The potential for applying electron energy loss spectroscopy (EELS) in biology is assessed. Some recent developments in instrumentation, spectrometer design, parallel detection and elemental mapping are discussed. Quantitation is demonstrated by means of the spectrum from DNA which gives an elemental ratio for N:P close to the expected value. A range of biologically important elements that can be usefully analyzed by EELS is tabulated and some possible applications for each are indicated. Detection limits and the effects of radiation damage are illustrated by spectra from the protein, insulin, and from the fluorinated amino-acid, histidine. Calcium detectability under optimum conditions may be as low as 1 mmol/kg dry weight. The application of EELS to analysis of cryosectioned adrenomedullary (chromaffin) cells is described in order to help determine the composition of the secretory granule. Water content can be determined from the amount of inelastic scattering as measured by the low-loss spectrum. The nitrogen/phosphorus ratio can be measured to provide information about the relative concentrations of ATP, chromogranin, and catecholamines. Quantitative EELS elemental maps are obtained in the STEM mode from chromaffin cells in order to measure the distribution of light elements.     

New developments in electron energy loss spectroscopy

In the electron microscope, spectroscopic signals such as the characteristic X-rays or the energy loss of the incident beam can provide an analysis of the local composition or electronic structure. Recent improvements in the energy resolution and sensitivity of electron spectrometers have improved the quality of spectra that can be obtained. Concurrently, the calculations used to simulate and interpret spectra have made major advances. These developments will be briefly reviewed. In recent years, the focus of analytical electron microscopy has moved away from single spectrum acquisition to mapping and imaging. In particular, the use of spectrum imaging (SI), where a full spectrum is acquired and stored at each pixel in the image is becoming widespread. A challenge for the application of spectrum imaging is the processing of such large datasets in order to extract the significant information. When we go beyond the mapping of composition and look to map bonding and electronic structure this becomes both more important and more difficult. Approaches to processing spectrum imaging data sets acquired using electron energy loss spectroscopy (EELS) will be explored in this paper.     

Thickness measurements with electron energy loss spectroscopy

Measurements of thickness using electron energy loss spectroscopy (EELS) are revised. Absolute thickness values can be quickly and accurately determined with the Kramers-Kronig sum method. The EELS data analysis is even much easier with the log-ratio method, however, absolute calibration of this method requires knowledge of the mean free path of inelastic electron scattering lambda. The latter has been measured here in a wide range of solids and a scaling law lambda approximately rho(-0.3) versus mass density rho has been revealed. EELS measurements critically depend on the excitation and collection angles. This dependence has been studied experimentally and theoretically and an efficient model has been formulated.     

Some effects of electron channeling on electron energy loss spectroscopy

As an electron beam (of order 100 keV) travels through a crystalline solid it can be channeled down a zone axis of the crystal to form a channeling peak centered on the atomic columns. The channeling peak can be similar in size to the outer atomic orbitals. Electron energy loss spectroscopy (EELS) measures the losses that the electron experiences as it passes through the solid yielding information about the unoccupied density of states in the solid. The interaction matrix element for this process typically produces dipole selection rules for small angle scattering. In this paper, a theoretical calculation of the EELS cross section in the presence of strong channeling is performed for the silicon L23 edge. The presence of channeling is found to alter both the intensity and selection rules for this EELS signal as a function of depth in the solid. At some depths in the specimen small but significant non-dipole transition components can be produced, which may influence measurements of the density of states in solids.     

The thickness determination of organic crystals under low dose conditions using electron energy loss spectroscopy.

In the 3-dimensional (3-D) reconstruction of protein crystals with variable thicknesses the electron images and diffraction patterns can only be merged if the crystal thickness is known. Measurement of the thickness using the ratio of the number of inelastically scattered electrons to the number of electrons in the zero loss peak can be accomplished with parallel electron energy loss spectrometry (PEELS). A theoretical analysis of the accuracy of the technique on paraffin crystals of different thicknesses is presented. Our experimental studies with paraffin crystals show the feasibility of measuring a single layer of 47A with good accuracy under low dose and low temperature conditions. A simple experimental apparatus is proposed to obtain thicknesses from small regions of unstained protein crystals prior to collecting the 3-D data sets from the unexposed area of the same crystal.     

Optimization of quantitative electron energy loss spectroscopy in the low loss region: phosphorus L-edge.

The purpose of this study was to optimize quantitative electron energy loss spectroscopy (EELS) of elements that have characteristic edges in the low energy loss region and are components of organic matrices. The optimum parameters for phosphorus L2,3-edge (at 135 eV) detection were determined by numerical analysis of computer-generated, Poisson-noisy spectra and by experimental measurements (at 80 keV) of films of the phosphoprotein, phosvitin. When the first, second and third valence electron/plasmon scatterings are included in the multiple least-squares (MLS) fit, the background subtraction of (first-difference) spectra is significantly more accurate than that obtained with the "inverse power law" method, even for a specimen thickness of only 0.25 lambda. Taking into account the effects of plural scattering, the optimal thickness for P quantitation is approximately 0.3 lambda. Signal-to-noise (S/N) ratio decreases rapidly with thickness, and at 1.0 lambda, it is only about 60% of the optimum S/N. The combined effects of the statistical uncertainty of measurements and of the systematic error due to gain variations of the parallel detector were evaluated, and the relative sensitivities of the no-difference (raw spectrum), first-difference and second-difference methods were compared. For channel-to-channel gain variations greater than 0.1% and up to 0.8%, the first-difference method results in the lowest uncertainty of P measurements. In the absence of gain variations, direct fitting provides the greatest sensitivity (least uncertainty), whereas at larger gain variations it may be necessary to use the second-difference method. The optimum energy shift for collecting a first-difference spectrum, approximately 15 eV, did not show any great variation between 5 and 25 eV, and is, in general, specimen dependent. Quantitation with EELS showed excellent correlation with simultaneous electron probe X-ray microanalysis, but, for the detection of P in a 0.25 lambda thick specimen, EELS was approximately five to six times more sensitive than X-ray. The minimal detectable P concentration, with 0.5 nA beam current for 100 s in a 0.25 lambda thick specimen, was 8.4 mmol/kg (0.01 at%) at the 99% confidence level, equivalent to 34 phosphorus atoms for a 15 nm probe. This value is close to the theoretical prediction of 7.5 mmol/kg, and can be improved only by further reducing the gain variation and directly fitting the non-difference spectrum. Appropriate reduction of the gain variations to less than 0.1% would result in a further, approximately two-fold, improvement in the parallel EELS detection system.(ABSTRACT TRUNCATED AT 400 WORDS)     

Inner shell edge profiles in electron energy loss spectroscopy

Inner shell edge profiles for K, L and M edges that are most likely to be used in microanalysis have been calculated using Hartree-Slater wave functions and are compared to experimental data. The aim is to identify those features that are not predicted by a one-electron atomic theory and to get some estimate of the accuracy of quantitative analysis using these calculations. In general, the fit between theory and experiment is quite good for those edges which do not have maxima delayed by more than 40 eV. In addition, solid state effects are averaged out if large (100 eV) integration windows are used. Accuracy can be improved in the transition metals and the rare earths by excluding the “white line” portion of the spectrum in any comparison.     

Performance and applications of electron energy loss spectroscopy in stem

When coupled in the image mode to a VG-HB501 microscope, the spectrometer designed by O. Krivanek and manufactured by Gatan Inc. is well suited for resolving analytical problems with a high spatial resolution. It actually records energy loss spectra from areas as small as 0.5 nm with a typical energy resolution of 1 eV over the energy loss range and with a good efficiency in collecting inelastic electrons. During the last few months, this high performance combination of microscope and spectrometer has been used to investigate (a) detection limits in EELS which are presently estimated of the order of ten atoms in a test situation such as metallic clusters deposited on a very thin carbon layer; (b) quantitative chemical analysis of representative nanovolumes of complex oxide specimens, emphasizing several aspects of elemental segregation in the neighborhood of grain boundaries and within vitreous areas; (c) changes of fine structures close to the K-oxygen threshold, due to different bonding states; and (d) efficient Z-contrast imaging modes on sections of embedded biological material without metallic staining.     

Accurate composition determination of Be-Ti alloys by electron energy loss spectroscopy

Accurate quantification of the Be content in Be-Ti alloys on a submicrometre scale can be accomplished with electron energy loss spectroscopy in an analytical electron microscope. The three major steps required to ensure the accuracy of the numerical results are analysed. The first step is the choice of the specimen thickness which should be such that the influence of the specimen surface effects can be ignored yet thin enough so that deconvolution of the spectra is unnecessary. The second step is the background extrapolation under the ionization edge of interest. In this study, a direct least-squares fit with a progressive weighting is used to avoid the drawbacks of the conventional linear least-squares fit. The third step is the calibration of the partial ionization cross-section ratio with the use of a standard specimen. Without this calibration step, the error in the final microanalysis result could be excessive, as demonstrated. With all these precautions taken into consideration, we are able to show that the intermetallic phase TiBe₁₂ exhibits a great deviation from its nominal stoichiometry.     

New examples for near-edge fine structures in electron energy loss spectroscopy

The electron energy loss near-edge structures of the K and L edges of elements occurring in sulfides and oxides of copper and zinc have been investigated. Furthermore, the K edge of carbon in some carbonates and the L edges of sulfur and phosphorus in sulfates and phosphates have been measured and the near-edge fine structures of these edges have been found to be characteristic for the complex ions CO^2-₃, SO^2-₄. Thus, near-edge fine structures of EELS edges can be very useful as a fingerprint for rapid identification of chemical compounds such as carbonates, sulfates, phosphates and some oxides in the TEM.     

The potential of energy loss spectroscopy for electronic characterization of structures on the nanometer scale

The last few years have seen a dramatic increase in the use of electron spectroscopy coupled with microscopy. Though dominant consideration has been aimed at elemental analysis and the use of ELS as a complementary tool with EDX for elemental determination, much less consideration has been given to one of the more powerful aspects of this spectroscopy, namely, electronic structure determination on the nanometer scale. It is the purpose here to consider certain aspects of such determination on the nanometer scale (with reference to elucidation and evaluation of man-made nanostructures) and to discuss some aspects of spectrometer design relevant to such determinations.     

Deadtime corrections for a pulse counting system in electron energy loss spectroscopy

We have measured the relationship between the input and output pulse rates for an EELS pulse counting system. Two simple formulae for predicting the behaviour of such a system are compared with the data. One is for a system which has extendible deadtime. The other is for the counting system which has non-extendible deadtime. The latter provides agreement with our experimental results over the range 0–20 MHz.     

Demonstration of lanthanum in liver cells by energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy and high-resolution transmission electron microscopy

The appearance of lanthanum in liver cells as a result of the injection of lanthanum chloride into rats is investigated by advanced transmission electron microscopy techniques, including electron energy loss spectroscopy and high‐resolution transmission electron microscopy. It is demonstrated that the lysosomes contain large amounts of lanthanum appearing in a granular form with particle dimensions between 5 and 25 nm, whereas no lanthanum could be detected in other surrounding cellular components.     

Extended fine structure in electron energy loss spectra of MgO crystallites

EXAFS-like features in energy loss spectra (known as EXELFS) were observed by using an imaging energy filter built into an electron microscope. Single crystal micro-platelets of MgO were used as test samples. Extended fine structures were obtained reproducibly in the energy range up to 150 eV beyond the OK, MgK and MgL edges. Comparison with single scattering calculations for the K edges showed a fair agreement, indicating the feasibility of this type of analysis.     

Experimental and theoretical evidence for the magic angle in transmission electron energy loss spectroscopy

We present experimental measurements of the C K-ELNES of high temperature pyrolysed graphite and related crystalline materials as a function of collection angle and sample tilt. These results together with a corresponding theoretical analysis indicate that the so-called "magic angle" for EELS measurements of an anisotropic crystal such as graphite, where spectra are independent of sample orientation, is approximately two times the characteristic scattering angle. We briefly discuss the implications of this result for the experimental measurement of anisotropic structures, including interfaces, as well as for the detailed modelling of ELNES structures using advanced electronic structure calculations.