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.     

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.     

Mass thickness determination by electron energy loss for quantitative X-ray microanalysis in biology

As is well known, electron energy loss spectroscopy can be used to determine the relative sample thickness in the electron microscope. This paper considers how such measurements can be applied to biological samples in order to obtain the mass thickness for quantitative X-ray microanalysis. The important quantity in estimating the mass thickness from an unknown sample is the total inelastic cross section per unit mass. Models for the cross section suggest that this quantity is constant to within ±20% for most biological compounds. This is comparable with the approximation made in the continuum method for measuring mass thickness. The linearity of the energy loss technique is established by some measurements on evaporated films and quantitation is demonstrated by measurements on thin calcium standards. A significant advantage of the method is that the energy loss spectrum can be recorded at very low dose, so that mass thickness determination can be made before even the most sensitive samples suffer damage resulting in mass loss. The energy loss measurements avoid the necessity to correct the continuum measurement for stray radiation produced in the vicinity of the sample holder. Unlike the continuum method the energy loss technique requires uniform mass thickness across the probe area, but this is not usually a problem when small probes (<100 nm diameter) are used.     

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.     

Transmission electron microscopy at 20 kV for imaging and spectroscopy

The electron optical performance of a transmission electron microscope (TEM) is characterized for direct spatial imaging and spectroscopy using electrons with energies as low as 20 keV. The highly stable instrument is equipped with an electrostatic monochromator and a C_S-corrector. At 20 kV it shows high image contrast even for single-layer graphene with a lattice transfer of 213 pm (tilted illumination). For 4 nm thick Si, the 200 reflections (271.5 pm) were directly transferred (axial illumination). We show at 20 kV that radiation-sensitive fullerenes (C₆₀) within a carbon nanotube container withstand an about two orders of magnitude higher electron dose than at 80 kV. In spectroscopy mode, the monochromated low-energy electron beam enables the acquisition of EELS spectra up to very high energy losses with exceptionally low background noise. Using Si and Ge, we show that 20 kV TEM allows the determination of dielectric properties and narrow band gaps, which were not accessible by TEM so far. These very first results demonstrate that low kV TEM is an exciting new tool for determination of structural and electronic properties of different types of nano-materials.     

Deconvolution of core electron energy loss spectra

Different deconvolution methods for removing multiple scattering and instrumental broadening from core loss electron energy loss spectra are compared with special attention to the artefacts they introduce. The Gaussian modifier method, Wiener filter, maximum entropy, and model based methods are described. Their performance is compared on virtual spectra where the true single scattering distribution is known. A test on experimental spectra confirms the good performance of model based deconvolution in comparison to maximum entropy methods and shows the advantage of knowing the estimated error bars from a single spectrum acquisition.     

In situ high resolution electron microscopy/electron energy loss spectroscopy observation of wetting of a Si surface by molten Al

Electron energy loss spectroscopy was used to observe the segregation of Al on a Si surface above the melting point of Al. A mixture of Al and Si particles was heated above the melting point of Al in a vacuum of 1 × 10⁻⁵ Pa. The Si surface, which initially had been covered with an amorphous oxide layer before heating, became clean and atomically facetted when the Al melted. It was shown that the Si surface was segregated with Al.     

Determination of the inelastic mean free path of electrons in vitrified ice layers for on-line thickness measurements by zero-loss imaging

The inelastic mean free path of 120 keV electrons in vitrified ice layers has been determined in an energy-filtering TEM. From the ratio of the unfiltered and zero-loss-filtered image intensities recorded with a slow-scan CCD camera, the relative sample thickness t/ Λ can be calculated. For calibration, the geometric ice thickness was measured by imaging a tilted view of a cylindrical hole which had been burnt into the ice layer. The total inelastic mean free path was found to be 161 nm, and the partial inelastic mean free path for an acceptance angle of 4.2 mrad was 232 nm. These results were built into a standard protocol for use in cryo-electron microscopy allowing on-line measurements of local ice-layer thicknesses by zero-loss-filtered/unfiltered imaging.     

Extracting physically interpretable data from electron energy-loss spectra

Principal component analysis is routinely applied to analyze data sets in electron energy-loss spectroscopy (EELS). We show how physically meaningful spectra can be obtained from the principal components using a knowledge of the scattering of the probe electron and the geometry of the experiment. This approach is illustrated by application to EELS data for the carbon K edge in graphite obtained using a conventional transmission electron microscope. The effect of scattering of the probe electron is accounted for, yielding spectra which are equivalent to experiments using linearly polarized X-rays. The approach is general and can also be applied to EELS in the context of scanning transmission electron microscopy.     

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.     

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.     

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.     

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.     

The secondary electron emission yield for 24 solid elements excited by primary electrons in the range 250-5000 ev: a theory/experiment comparison

The secondary electron (SE) yield, delta, was measured from 24 different elements at low primary beam energy (250-5,000 eV). Surface contamination affects the intensity of delta but not its variation with primary electron energy. The experiments suggest that the mean free path of SEs varies across the d bands of transition metals in agreement with theory. Monte Carlo simulations suggest that surface plasmons may need to be included for improved agreement with experiment.     

The relative importance of multiple inelastic scattering in the quantification of EELS

A comparison is given of energy loss results obtained for the L and K edges of aluminium as a function of specimen thickness, crystallographic orientation and collection angle. It is demonstrated that as the thickness is increased post-loss elastic scattering is generally important in reducing the fraction of electrons collected. The implications for the quantification of EELS data are discussed while a comparison of the Fe/C ratio in cementite demonstrates the improved consistency which can be obtained when comparing K and L losses at lower energy separation than are the losses for aluminium.     

EELS log-ratio technique for specimen-thickness measurement in the TEM

We discuss measurement of the local thickness t of a transmission microscope specimen from the log-ratio formula t = λ In (I_t/I₀) where I_t and I₀ are the total and zero-loss areas under the electron-energy loss spectrum. We have measured the total inelastic mean free path λ in 11 materials of varying atomic number Z and have parameterized the results in the form λ = 106F (E₀/E_m)/ln (2βE₀/E_m) where F = (1 + E₀/1,022)/(1 + E₀/511)², the incident energy E₀ is in keV, the spectrum collection semiangle β is in mrad, and E_m = 7.6Z^0.36. This formulation should allow absolute thickness to be determined to an accuracy of ±20% in most inorganic specimens.     

An empirical energy loss equation of electrons

A modified Love-Cox-Scott (1978) equation of electron energy loss has been suggested. The stopping powers predicted by the modified Love-Cox-Scott equation are compared with those by the Tung et al. (1979) model, the Joy and Luo (1989) equation, and the experimental data given in database of Joy at: http://web.ukt. edu/-scrutk. In the energy range of E0< or = 5 keV, the Monte Carlo simulations of the electron scattering in Al, Ag, and Au have been performed, applying the Mott cross section for elastic scattering and the modified Love-Cox-Scott equation (1978) and the equations by Love et al. (1978) and Joy and Luo (1989), respectively, for the inelastic scattering. The calculated results on the backscattering coefficients, the energy distributions of the backscattered electrons, and the energy dissipation of the electron based on the three equations are compared.     

A computational test for the removal of plural scattering from angle-resolved energy loss spectra

A recently developed method based on matrix analysis for the removal of plural scattering from angle-resolved energy loss spectra is tested. A single loss function, Lorentzian in the energy and Gaussian in the angular variable is assumed as input for the test. Multiple scattering probabilities are simulated by summing up n-fold self-convolutions of the input function according to the Poisson distribution for incoherent n-fold scattering. The simulated profile serves as input for the retrieval algorithm, the result of which is compared with the original single-loss probability. It is concluded that the method is feasible, but not likely to be suited for routine investigations.     

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.     

Determination of mean free path for energy loss and surface oxide film thickness using convergent beam electron diffraction and thickness mapping: a case study using Si and P91 steel

Determining transmission electron microscope specimen thickness is an essential prerequisite for carrying out quantitative microscopy. The convergent beam electron diffraction method is highly accurate but provides information only on the small region being probed and is only applicable to crystalline phases. Thickness mapping with an energy filter is rapid, maps an entire field of view and can be applied to both crystalline and amorphous phases. However, the thickness map is defined in terms of the mean free path for energy loss (λ), which must be known in order to determine the thickness. Convergent beam electron diffraction and thickness mapping methods were used to determine λ for two materials, Si and P91 steel. These represent best‐ and worst‐case scenario materials, respectively, for this type of investigation, owing to their radically different microstructures. The effects of collection angle and the importance of dynamical diffraction contrast are also examined. By minimizing diffraction contrast effects in thickness maps, reasonably accurate (±15%) values of λ were obtained for P91 and accuracies of ±5% were obtained for Si. The correlation between the convergent beam electron diffraction‐derived thickness and the log intensity ratios from thickness maps also permits estimation of the thickness of amorphous layers on the upper and lower surfaces of transmission electron microscope specimens. These estimates were evaluated for both Si and P91 using cross‐sectional transmission electron microscopy and were found to be quite accurate.