EELS and optical response of a noble metal nanoparticle in the frame of a discrete dipole approximation

Surface plasmons of noble metal nanoparticles have recently been studied by Electron Energy-Loss Spectroscopy (EELS) performed in an electron microscope. We present here the basic formalism for EELS simulations in a Discrete Dipole Approximation (DDA) framework for such surface excitations. We compare EELS data and optical properties of a silver triangular nanoprism and show that the spatial variation of surface plasmon excitation probabilities allows to discriminate modes of similar energies. We also emphasize the importance of the substrate polarization effect to reliably describe experimental data.     

Breakdown of the dipole approximation in core losses

The validity of the dipole approximations commonly used in the inelastic scattering theory for transmission electron microscopy is reviewed. Both experimental and numerical arguments are presented, emphasizing that the dipole approximations cause significant errors of the order of up to 25% even at small momentum transfer. This behavior is attributed mainly to non-linear contributions to the dynamic form factor due to the overlap of wave functions.     

On the measurement of thickness in nanoporous materials by EELS

This work discusses thickness measurements in nanoporous MgO using the log-ratio method in electron energy-loss spectroscopy (EELS). In heterogeneous nanoporous systems, the method can induce large errors if the strength of excitations at interfaces between pores and the matrix is large. In homogeneous nanoporous systems, on the other hand, the log-ratio method is still valid, but the inelastic scattering mean-free-path is no longer equal to that in the same bulk system.     

Position stabilization of EELS spectra

The position of the zero-loss peak in electron energy-loss spectra was sensed with a double photo diode. A signal, proportional to the disturbances from a central position, was amplified and fed back into a deflection coil in order to compensate for the origin of the disturbances. Thus, slow variations of the position of characteristic edges in the EEL spectrum could be reduced by a factor 100, and 60 Hz oscillations could be reduced by a factor 5.     

Optimal conditions for the use of correspondence analysis for element determination in EELS images

Correspondence analysis is used for the determination of element localization and element concentration in electron energy–loss imaging. The introduction of a new method for the quantification of element concentration embedded in the protocols of correspondence analysis may be regarded as a useful replacement of the power–law estimation technique, especially in regions of the spectrum where the latter method cannot be used. Conditions for the optimum use of the method are established.     

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.     

Near-atomic-resolution EELS in silicon-germanium alloys

Recent advances in instrumental capabilities now allow us to combine spatially resolved EELS with annular darkfield (ADF) imaging to relate the electronic or bonding behaviour of a small region to its atomic structure. Although still limited to projections of structures at the 0·2 nm level, the ADF imaging gives excellent information about the structure and composition of crystalline interfaces and defects. Spatially resolved EELS can then obtain the local bonding and electronic structure, with a similar spatial resolution. Using the silicon 2p core excitation as a probe of the conduction band density of states, it is possible to follow changes of these bands as a function of composition and defect structure in the Ge-Si alloy system. Effects due to local strain are also observable.     

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.     

Weighted least squares estimation of background in EELS imaging

In quantitative Electron Energy Loss Spectrometry, a weighted least squares estimation should theoretically be used to estimate the background law below core edge energy, since the variances of the data vary. However, it is found that proper weighting makes the above edge signal-to-noise ratio decrease rather than increase. This result is discussed, and the influence of the bias introduced by the logarithmic transformation of the data is quantified.     

Interpretation of O K-edge EELS in zircon using a structural variation approach

This work describes an approach to interpret the near-edge fine structure of electron energy-loss spectroscopy (EELS) of O K-edge in zircon using a structural variation method. The positions and intensities of several peaks in the O K-edge EELS spectrum are assigned to specific structural parameters. It suggests that the near-edge structures in EELS can be used to measure atomic structure changes.     

Surface microanalysis by reflection electron energy-loss spectroscopy

Several basic physical concepts of applying eq. I_k = IσN^xt to surface microanalysis by reflection electron energy-loss spectroscopy (REELS) are clarified. Here I^k and I are the integrated intensities of the core ionization edge and the low loss part, σ is the scattering cross section of element x with atomic concentration N^x, and t is the specimen thickness. The reflected inelastic electrons are found to be distributed almost symmetrically around the Bragg sports and can be reasonably described by a Lorentzian function. EELS microanalysis can be performed by using the diffracted sports. The ω correction, arising from the angular contributions of the neighbouring spots into the spectrometer collecting aperture, is required to be considered.     

Advances in EELS spectroscopy by using new detector and new specimen preparation technologies

First results obtained with a Gatan UHV Enfina system, which was attached to a VG HB 501 UX dedicated STEM, are reported. The Enfina system is based on a CCD detector and offers, compared to the previously used photodiode array, a narrower point‐spread function, higher sensitivity, and faster read‐out capabilities. These improvements are demonstrated with electron energy‐loss measurements on various oxides, such as Al₂O₃, TiO₂ and SrTiO₃. It is shown that a better energy resolution is achieved and that acquisition of high‐energy absorption edges with a reasonable signal‐to‐noise ratio becomes possible. Furthermore, we report on the influence of the TEM specimen quality on the energy‐loss spectra. Thin amorphous layers at the specimen surfaces, which are induced by ion‐milling processes, can modify specific electron energy‐loss near‐edge structure features. We found that for the investigated ceramics the use of low‐energy ion‐milling systems is highly recommended, since the loss of energy‐loss near‐edge structure details by the presence of the amorphous layers is considerably reduced. This is especially true for very thin specimens.     

Comparison of EFTEM and STEM EELS plasmon imaging of gold nanoparticles in a monochromated TEM

We present and compare two different imaging techniques for plasmonic excitations in metallic nanoparticles based on high energy-resolution electron energy-loss spectroscopy in a monochromated transmission electron microscope. We demonstrate that a recently developed monochromated energy-filtering (EFTEM) approach can be used in addition to the well established scanning technique to directly obtain plasmon images in the energy range below 1 eV. The EFTEM technique is described in detail, and a double experiment performed on the same, triangular gold nanoparticle compares equivalent data acquired by both techniques, respectively.     

Electron spectroscopic study (ESI,EELS) of Nanoplast-embedded mammalian lung

The potential of Nanoplast melamine resin embedding for the study of mammalian lung parenchyma was examined by means of electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS). Samples were either fixed with glutaralde-hyde-paraformaldehyde or glutaraldehyde-tannic acid, or were directly transferred to the embedding medium without prior fixation. Organic dehydrants, as well as fixatives containing heavy metals and stains, were omitted. A very high level of ultrastructural detail of chromatin, ribosomes, mitochondria and plasma membranes was achieved by ESI from the Nanoplast-embedded samples. The most prominent gain in ultrastructural detail was achieved when moving from an energy loss just below the L_2,3 edge of phosphorus at 132 eV to an energy loss just beyond this edge. This reflects the prominent P L_2,3 edge observed by EELS of Nanoplast-embedded samples in comparison with conventionally processed samples. Thus, taking into account possible sectioning artefacts, excellent heterochromatin images which rely on the phosphorus distribution can be obtained from Nanoplast-embedded samples by computer-assisted analysis of electron spectroscopic images. In this respect glutaraldehyde-paraformaldehyde fixation is preferable to glutaraldehyde-tannic acid fixation because the presence of silicon, revealed by EELS, in tannic-acid-fixed samples may introduce artefacts in phosphorus distribution images obtained by the three-window method because of the close proximity of the L_2,3 edges of silicon and phosphorus.     

Characterization of the fibre–matrix interfacial structure in carbon fibre-reinforced polycarbosilane-derived SiC matrix composites using STEM/EELS

This paper presents a characterization study of the microstructural evolution of various carbon fibre-reinforced polycarbosilane (PCS)-derived SiC matrix composites during high temperature heat treatment. Both surface-treated and untreated carbon fibre reinforcements were investigated. The STEM/EELS technique was found to be a particularly useful characterization tool. The results of quantitative EELS linescans have been interpreted in terms of the migration of gaseous SiO and CO, produced by the reaction between the small amount of SiO₂ and excess carbon within the PCS-derived SiC matrix, from the central matrix region towards the fibre–matrix interfaces. Generally, the migration of gaseous SiO and CO results in an enrichment of SiO₂ at the region adjacent to the fibre–matrix interface. However, differing final composite microstructures are formed depending on the strength of the fibre–matrix bonding. In the case of strong fibre-matrix interfacial bonding where few escape channels are present, a distinct Si–C–O layer was identified within the matrix adjacent to the fibre–matrix interface; both crystalline β-SiC and the segregated Si–O–C phase coexist in this microstructure up to at least 1450 °C. In the case of weak fibre–matrix bonding this oxygen segregated interfacial layer is eventually removed at high enough temperatures. The final interfacial microstructure has important consequences for the mechanical properties of the composite material.     

Optimized acquisition parameters and statistical detection limit in quantitative EELS

Optimizing the acquisition parameters for EELS recording has to be accomplished simultaneously from the physical and the statistical points of view; the statistical aspect of the question is covered here. Approximate probability density functions of the variables of interest are derived, which provide a global measure of signal-to-noise ratio taking into account every step of the EELS edge area estimation process. Qualitative and quantitative advice is given regarding the critical choice of the estimation and integration energy regions. The notion of visual contrast is presented; it permits the introduction of the concept of statistical detection limit. It is found that for typical experimental conditions, when other factors are equal, the required analysis time for the sample varies approximately as the inverse square of the concentration.     

Physical and chemical changes in polystyrene during electron irradiation using EELS in the TEM: contribution of the dielectric function

We have performed electron energy-loss spectroscopy analysis of polystyrene in the analytical transmission electron microscope in order to evaluate the possibility of obtaining both chemical and dielectric information on the polymer at the submicrometre scale. Irradiation has also been studied, particularly the influence of the specimen temperature on the kinetics of degradation.
The main results show that polystyrene is resistant to electron beam with a critical dose of about 10⁴ C m⁻² at 127 K. Spectra could be acquired with doses less than this critical dose. We were thus able to propose an identification of the different chemical bonds of carbon (including the C–H bond), in agreement with previous X-ray absorption near-edge structure experiments. The chemical changes in polystyrene due to severe irradiation damage are also visible in the carbon K-edge near-edge structure.
At the same time, we calculated the dielectric function from the low-loss part of the spectra. Interestingly, this part of the spectrum is the most sensitive to irradiation, since great changes can be seen during exposure.
Lastly, we propose a degradation process, in agreement with all these results.     

A High-Resolution TEM/EELS Study of the Effect of Doping Elements on the Sliding Mechanisms of Sputtered WS2 Coatings

It has been shown many times that cosputtering low-friction coatings of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) with other elements can improve the structural, mechanical, and tribological properties. To achieve the lowest friction, MoS₂ or WS₂ should be doped with element(s) improving the hardness and density of the coatings. On the other hand, such elements, or their compounds, should not be present in the outermost molecular layers at the sliding interface. This article suggests that there are important differences between how MoS₂ and WS₂ coatings respond to or react with doping elements, despite the almost identical structure and behavior of the undoped materials. Two systems have been investigated by high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM) electron energy loss spectroscopy (EELS), W-S-C-Cr and W-S-C-Ti, and showed significant amounts of oxides, which typically formed a layer just underneath the crystalline WS₂ top layer. Further, carbon was almost completely absent in the tribofilms, despite the fact that the as-deposited coatings contained as much as 40–50 at% C. An interesting observation here is that WS₂ basal planes surround or embed Fe wear particles, suggesting a relatively strong adhesion or a Fe-S chemical bonding between iron/steel and WS₂. The result of this is that the wear particles become pacified and remain in the contact as low-friction material.