Low-loss Electron Energy Loss Spectroscopy Research Articles

Design and application of a relativistic Kramers–Kronig analysis algorithm

Low-loss electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope probes the valence electron density and relevant optoelectronic properties such as band gap energies and other band structure transitions. The measured spectra can be formulated in a dielectric theory framework, comparable to optical spectroscopies and ab-initio simulations. Moreover, Kramers–Kronig analysis (KKA), an inverse algorithm based on the same name relations, can be employed for the retrieval of the complex dielectric function. However, spurious contributions traditionally not considered in this framework typically impact low-loss EELS modifying the spectral shapes and precluding the correct measurement and retrieval of the dielectric information. A relativistic KKA algorithm is able to account for the bulk and surface radiative-loss contributions to low-loss EELS, revealing the correct dielectric properties. Using a synthetic low-loss EELS model, we propose some modifications on the naive implementation of this algorithm that broadens its range of application. The robustness of the algorithm is improved by regularization, applying previous knowledge about the shape and smoothness of the correction term. Additionally, our efficient numerical integration methodology allows processing hyperspectral datasets in a reasonable amount of time. Harnessing these abilities, we show how simultaneous relativistic KKA processing of several spectra can share information to produce an improved result.

Layer Rotation-Angle-Dependent Excitonic Absorption in van der Waals Heterostructures Revealed by Electron Energy Loss Spectroscopy.

Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide (TMDC) monolayers are a fascinating class of two-dimensional (2D) materials. The presence of interlayer excitons, where the electron and the hole remain spatially separated in the two layers due to ultrafast charge transfer, is an intriguing feature of these heterostructures. The optoelectronic functionality of 2D heterostructure devices is critically dependent on the relative rotation angle of the layers. However, the role of the relative rotation angle of the constituent layers on intralayer absorption is not clear yet. Here, we investigate MoS2/WSe2 vdW heterostructures using monochromated low-loss electron energy loss (EEL) spectroscopy combined with aberration-corrected scanning transmission electron microscopy and report that momentum conservation is a critical factor in the intralayer absorption of TMDC vdW heterostructures. The evolution of the intralayer excitonic low-loss EEL spectroscopy peak broadenings as a function of the rotation angle reveals that the interlayer charge transfer rate can be about an order of magnitude faster in the aligned (or anti-aligned) case than in the misaligned cases. These results provide a deeper insight into the role of momentum conservation, one of the fundamental principles governing charge transfer dynamics in 2D vdW heterostructures.

Orbital Angular Momentum and Energy Loss Characterization of Plasmonic Excitations in Metallic Nanostructures in TEM

Recently, a new device to measure the orbital angular momentum (OAM) electronic spectrum after elastic/inelastic scattering in a transmission electron microscope has been introduced. We modified the theoretical framework needed to describe conventional low-loss electron energy loss spectroscopy (EELS) experiments in transmission electron microscopes (TEM) to study surface plasmons in metallic nanostructures, to allow for an OAM post selection and devise new experiments for the analysis of these excitations in nanostructures. We found that unprecedented information on the symmetries and on the chirality of the plasmonic modes can be retrieved even with limited OAM and energy resolutions.

A solution to FIB induced artefact hydrides in Zr alloys

The preparation of TEM samples using focused-ion beam (FIB) techniques allows for characterization of electron-transparent site-specific areas, such as oxide-metal interfaces and crack tips, leading to improved understanding of material degradation mechanisms. Using conventional FIB techniques to prepare zirconium alloy TEM samples results in the production of artefact δ hydrides during the final thinning stage, which can confound analyses and mechanistic interpretation, especially when understanding hydride morphology and crystallography is critical. A comparison of various preparation methods was performed to determine their influence on artefact hydrides in TEM samples. As-received Zircaloy-4 material with <10 μg/g hydrogen was quenched, fabricated into electropolished (EP) foils, TEM lamella were extracted using FIB, then the lamella were final thinned using different ion beam preparation techniques. Low-loss electron energy loss spectroscopy (EELS) was performed on the EP and FIB-prepared samples before and after final thinning. The expected post-quench distribution of γ hydrides was present in the EP and FIB samples before final thinning, however, after FIB or broad ion beam final thinning at room temperature the hydride phase was predominately δ with a larger volume fraction. After TEM lamella were cooled with liquid nitrogen during FIB or broad ion beam final thinning only γ hydrides were present, which was confirmed with diffraction analysis by observing γ hydride superlattice reflections. The large surface area to volume ratio, reactive surface, and beam heating during conventional FIB final thinning is believed to cause hydrogen-containing impurities in the vacuum chamber to dissociate and allow hydrogen to enter the specimen. Specimen cooling and the presence of a nearby cold finger reduce the hydrogen partial pressure near the sample surface and significantly reduce the hydrogen adsorption, absorption, and diffusion kinetics. Future investigations using FIB-prepared TEM samples of zirconium should apply cryo-techniques to prevent artefact hydrides.

Effect of Electron Beam Irradiation on Polymers

Electron energy loss spectroscopy (EELS) in combination with transmission electron microscopy (TEM) is widely used for chemical state analysis of variety of chemical compounds. High beam sensitivity of substances like polymers hinders the possibility of exploring in-depth analysis provided through the high spatially resolved EELS spectroscopy. In this study, the electron beam irradiation damage on polymers were analyzed with varying dose of electron beams. The stability of the polymers under electron beam exposure depends on the chemical structure on the polymers. In this study the polymers with and without phenyl groups namely Polycarbonate, Polyethylene terephthalate, Polystyrene, Styrene Maleic Anhydride and Polymethylmethacrylate are selected for the comparative degradation study. Effect of varying the electron dose on the stability of polymers were monitored by recording the low-loss EELS spectrum in π to π* transition and (π+σ) to (π+σ)* transition region.

Assessing Oxygen Vacancies in Bismuth Oxide through EELS Measurements and DFT Simulations

Pioneering electron energy loss spectroscopy (EELS) measurements of α-Bi2O3 are performed on three samples obtained through different synthesis methods. Experimental low-loss and core-loss EELS spectra are acquired. By combining them with detailed structural characterization and Density Functional Theory (DFT) simulations, we are able to detect and evaluate the presence of oxygen vacancies in the samples. This type of information has not been accessed previously from EELS data in bismuth oxide, because high-resolution EELS spectra or how vacancies reflect in Bi2O3 spectra were unreported. This novel measurement is further validated through comparison with photoluminescence data. Therefore, the technique has the ability to probe oxygen vacancies in Bi2O3 at an unprecedented resolution, which might allow solving material science and technological issues related to this material.

Correlations between structure, composition and electrical properties of tungsten/tungsten oxide periodic multilayers sputter deposited by gas pulsing

W/WOx multilayered thin films have been deposited by DC reactive sputtering using the reactive gas pulsing process. It is implemented to produce regular alternations of metal-oxide compounds at the nanometric scale. Structure and growth have been investigated by high resolution transmission electron microscopy, scanning transmission electron microscopy, X-ray energy dispersive spectroscopy and electron energy loss spectroscopy. Regularity of tungsten-based alternations, quality of interfaces as well as oxygen presence through the multilayered structure have been determined and linked to the growth conditions. Chemical information was obtained from the energy dispersive X-ray spectroscopy and low-loss electron energy loss spectroscopy. As they can be related to the chemical composition of the periodic layers, the position and the broadening of the bulk plasmon peak were studied. For the smallest periods (<10 nm), the presence of oxygen has been pointed out in the metal-rich layer whereas for the thickest ones (100 nm), pure metal is only present. Finally, relationships have been established between in situ growth conditions, structural and chemical parameters and electrical properties in periodic multilayers.

Electron Energy Loss Spectroscopy of Surface Plasmon Resonances on Aberrant Gold Nanostructures

Effects of structural elongation, annulation, and irregularity on emergent plasmonic modes were examined computationally and experimentally using low-loss electron energy loss spectroscopy (EELS). Resonant eigenstates were compared for multiple gold nanodisc shapes: circular, elongated, annulated, and acircular annulated. Losses of resonant eigenstates were mapped to distinguish discrete bright and dark resonance modes that emerged from morphological changes. Single bright and dark resonances exhibited by circular nanodiscs were supplemented by additional bright modes arising from axial asymmetry induced by radial aberrations in elongated discs. An antibonding bright mode appeared in annulated discs due to different dipole patterns arising from charge interactions on adjacent surfaces. Eccentricity on annulated discs hybridized resonant mode structures from discs, rings, and ellipses. Direct comparison between experimental measures and theory for a fabricated elongated nanoring with morphological distorti...

Transmission electron microscopy of carbon-coated and iron-doped titania nanoparticles

We present a study on the properties of iron (Fe)-doped and carbon (C)-coated titania (TiO2) nanoparticles (NPs) which has been compiled by using x-ray diffraction (XRD), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). These TiO2 NPs were prepared by using the flame synthesis method. This method allows the simultaneous C coating and Fe doping of TiO2 NPs. XRD investigations revealed that the phase of the prepared NPs was anatase TiO2. Conventional TEM analysis showed that the average size of the TiO2 NPs was about 65 nm and that the NPs were uniformly coated with the element C. Furthermore, from the x-ray energy dispersive spectrometry analysis, it was found that about 8 at.% Fe was present in the synthesized samples. High-resolution TEM (HRTEM) revealed the graphitized carbon structure of the layer surrounding the prepared TiO2 NPs. HRTEM analysis further revealed that the NPs possessed the crystalline structure of anatase titania. Energy-filtered TEM (EFTEM) analysis showed the C coating and Fe doping of the NPs. The ratio of L3 and L2 peaks for the Ti-L23 and Fe-L23 edges present in the core loss electron energy loss spectroscopy (EELS) revealed a +4 oxidation state for the Ti and a +3 oxidation state for the Fe. These EELS results were further confirmed with XPS analysis. The electronic properties of the samples were investigated by applying Kramers–Kronig analysis to the low-loss EELS spectra acquired from the prepared NPs. The presented results showed that the band gap energy of the TiO2 NPs decreased from an original value of 3.2 eV to about 2.2 eV, which is quite close to the ideal band gap energy of 1.65 eV for photocatalysis semiconductors. The observed decrease in band gap energy of the TiO2 NPs was attributed to the presence of Fe atoms at the lattice sites of the anatase TiO2 lattice. In short, C-coated and Fe-doped TiO2 NPs were synthesized with a rather cost-effective and comparatively easily scalable method. The presented analysis enables us to predict the excellent efficiency of these NPs for solar-cell and photo-catalysis applications.

Investigation of phase separation in InGaN alloys by plasmon loss spectroscopy in a TEM

ABSTRACTPhase separation of InxGa1-xN alloys into Ga-rich and In-rich regions was observed by a number of research groups for samples grown with high indium content, x. Due to the radiation sensitivity of InGaN to beam damage by fast electrons, high-resolution imaging in transmission electron microscopy (TEM) or core-loss electron energy-loss spectroscopy (EELS) may lead to erroneous results. Low-loss EELS can yield spectra of the plasmon loss regions at much lower electron fluxes. Unfortunately, due to their delayed edge onset, the low energetic core losses of Ga and In partially overlap with the plasmon peaks, all of which shift with indium content.Here we demonstrate a method to quantify phase separation in InGaN thin films from the low-loss region in EELS by simultaneously fitting both plasmon and core losses over the energy range of 13-30eV. Phase separation is shown to lead to a broadening of the plasmon peak and the overlapping core losses, resulting in an unreliable determination of the indium concentration from analyzing the plasmon peak position alone if phase separation is present. For x=0.3 and x=0.59, the relative contributions of the binary compounds are negligibly small and indicate random alloys. For xnom.=0.62 we observed strong broadening, indicating phase separation.

Isolated energy level in the band gap ofYb2Si2O7identified by electron energy-loss spectroscopy

We report the detection of an isolated energy level in the band gap of crystalline ${\mathrm{Yb}}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$ in the low-energy-loss region of its electron energy-loss (EEL) spectrum, obtained using a monochromated scanning transmission electron microscope. The experimental results are corroborated by first-principles calculations of the theoretical EEL spectrum. The calculations reveal that unoccupied Yb $4f$ orbitals constitute an isolated energy level about 1 eV below the conduction band minimum (CBM), resulting in a terrace about 1 eV wide at the band edge of the EEL spectrum. In the case of ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$, no band edge terrace is present because the unoccupied $f$ level lies just below the CBM. We also examined optical absorption properties of ${\mathrm{Yb}}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$ using UV-vis diffuse reflectance spectroscopy, which shows that the isolated energy level could not be detected in the band edge of the obtained absorbance spectrum. These findings demonstrate the utility of low-loss EEL spectroscopy with high energy resolution for probing semilocalized electronic features.

Density Functional Theory Modeling of Low-Loss Electron Energy-Loss Spectroscopy in Wurtzite III-Nitride Ternary Alloys.

In the present work, the dielectric response of III-nitride semiconductors is studied using density functional theory (DFT) band structure calculations. The aim of this study is to improve our understanding of the features in the low-loss electron energy-loss spectra of ternary alloys, but the results are also relevant to optical and UV spectroscopy results. In addition, the dependence of the most remarkable features with composition is tested, i.e. applying Vegard's law to band gap and plasmon energy. For this purpose, three wurtzite ternary alloys, from the combination of binaries AlN, GaN, and InN, were simulated through a wide compositional range (i.e., Al x Ga1-x N, In x Al1-x N, and In x Ga1-x N, with x=[0,1]). For this DFT calculations, the standard tools found in Wien2k software were used. In order to improve the band structure description of these semiconductor compounds, the modified Becke-Johnson exchange-correlation potential was also used. Results from these calculations are presented, including band structure, density of states, and complex dielectric function for the whole compositional range. Larger, closer to experimental values, band gap energies are predicted using the novel potential, when compared with standard generalized gradient approximation. Moreover, a detailed analysis of the collective excitation features in the dielectric response reveals their compositional dependence, which sometimes departs from a linear behavior (bowing). Finally, an advantageous method for measuring the plasmon energy dependence from these calculations is explained.

Quantitative parameters for the examination of InGaN QW multilayers by low-loss EELS.

We present a detailed examination of a multiple InxGa1-xN quantum well (QW) structure for optoelectronic applications. The characterization is carried out using scanning transmission electron microscopy (STEM), combining high-angle annular dark field (HAADF) imaging and electron energy loss spectroscopy (EELS). Fluctuations in the QW thickness and composition are observed in atomic resolution images. The impact of these small changes on the electronic properties of the semiconductor material is measured through spatially localized low-loss EELS, obtaining band gap and plasmon energy values. Because of the small size of the InGaN QW layers additional effects hinder the analysis. Hence, additional parameters were explored, which can be assessed using the same EELS data and give further information. For instance, plasmon width was studied using a model-based fit approach to the plasmon peak; observing a broadening of this peak can be related to the chemical and structural inhomogeneity in the InGaN QW layers. Additionally, Kramers-Kronig analysis (KKA) was used to calculate the complex dielectric function (CDF) from the EELS spectrum images (SIs). After this analysis, the electron effective mass and the sample absolute thickness were obtained, and an alternative method for the assessment of plasmon energy was demonstrated. Also after KKA, the normalization of the energy-loss spectrum allows us to analyze the Ga 3d transition, which provides additional chemical information at great spatial resolution. Each one of these methods is presented in this work together with a critical discussion of their advantages and drawbacks.

Low-loss electron energy loss spectroscopy: An atomic-resolution complement to optical spectroscopies and application to graphene

This paper reports on successful application of electron energy loss spectroscopy (EELS) for probing low-energy electronic excitations at all values of momentum transfer with atomic-scale spatial resolution. In this regard EELS appears to be complementary to optical spectroscopy which probes only direct excitations with limited spatial resolution. The authors suggest that EELS can be used to study atomic-scale structural, electronic, magnetic and vibrational properties of systems having defects and impurities as well.

Electron energy loss spectroscopy on semiconductor heterostructures for optoelectronics and photonics applications

In this work, we present characterization methods for the analysis of nanometer-sized devices, based on silicon and III-V nitride semiconductor materials. These methods are devised in order to take advantage of the aberration corrected scanning transmission electron microscope, equipped with a monochromator. This set-up ensures the necessary high spatial and energy resolution for the characterization of the smallest structures. As with these experiments, we aim to obtain chemical and structural information, we use electron energy loss spectroscopy (EELS). The low-loss region of EELS is exploited, which features fundamental electronic properties of semiconductor materials and facilitates a high data throughput. We show how the detailed analysis of these spectra, using theoretical models and computational tools, can enhance the analytical power of EELS. In this sense, initially, results from the model-based fit of the plasmon peak are presented. Moreover, the application of multivariate analysis algorithms to low-loss EELS is explored. Finally, some physical limitations of the technique, such as spatial delocalization, are mentioned.

On the impact of indium distribution on the electronic properties in InGaN nanodisks

We analyze an epitaxially grown heterostructure composed of InGaN nanodisks inserted in GaN nanowires in order to relate indium concentration to the electronic properties. This study was achieved with spatially resolved low-loss electron energy-loss spectroscopy using monochromated electrons to probe optical excitations—plasmons—at nanometer scale. Our findings show that each nanowire has its own indium fluctuation and therefore its own average composition. Due to this indium distribution, a scatter is obtained in plasmon energies, and therefore in the optical dielectric function, of the nanowire ensemble. We suppose that these inhomogeneous electronic properties significantly alter band-to-band transitions and consequently induce emission broadening. In addition, the observation of tailing indium composition into the GaN barrier suggests a graded well-barrier interface leading to further inhomogeneous broadening of the electro-optical properties. An improvement in the indium incorporation during growth is therefore needed to narrow the emission linewidth of the presently studied heterostructures.

Spatially Resolved Characterization of Phases in LiFePO4 Battery Cathodes Using Low Loss Electron Energy-loss Spectroscopy

Lithium iron phosphate (LiFePO4) has considerable potential for automotive applications due to its high rate capability, reasonable energy density and environmentally benign nature [1]. However, performance degradation seen after thousands of cycles at high charging-rates (C-rates) has been a point of major concern [2]. Studies of the aging mechanism suggest that phases (LiFePO4/FePO4) formed in the cathode during discharge influence the aging profile [3]. These phases have been investigated recently using x-ray and neutron diffraction [4, 5]. While these methods provide insights into crystal structure and unit cell volume changes associated with phase changes between LiFePO4 and FePO4, their ability to provide spatially resolved measurements of quantities such as Li and Fe concentration at the nanoscale is severely limited. Such measurements are key to confirming the presence or absence of the phases mentioned above and solid solution phases (LixFePO4). With recent advances in aberration corrected electron microscopy, electron energy-loss spectroscopy (EELS) performed in the scanning transmission electron microscope (STEM) has emerged as a leading technique to obtain spatially resolved measurements of chemistry, structure and bonding. Previously, researchers have used shifts observed in core-loss peaks of iron and oxygen to study the delithiation of FePO4 host lattice [6-8]. However, these ionization edges have relatively small inelastic scattering cross sections, and consequently, a relatively high electron dose is required to achieve acceptable signal-to-noise ratios in the data resulting in significant electron beam damage to the specimen. The low energy-loss region of the spectrum (0-40 eV) can be very useful since (i) the intensity is much larger compared to the coreloss spectrum and (ii) lesser collection time is required which significantly lowers the risk of beam damage to the material. In this study, we explore the use of low loss EELS to characterize the nominally pure LiFePO4 powder with trace amounts of FePO4 which is typical of phase mixtures that is present during phase transformation within the LiFePO4 battery cathode at the nanoscale during aging.

Compositional analysis of InAs-GaAs-GaSb heterostructures by Low-Loss Electron Energy Loss Spectroscopy

As an alternative to Core-Loss Electron Energy Loss Spectroscopy, Low-Loss EELS is suitable for compositional analysis of complex heterostructures, such as the InAs-GaAs-GaSb system, since in this energy range the edges corresponding to these elements are better defined than in Core-Loss. Furthermore, the analysis of the bulk plasmon peak, which is present in this energy range, also provides information about the composition. In this work, compositional information in an InAs-GaAs-GaSb heterostructure has been obtained from Low-Loss EEL spectra.

Electron Energy Loss Spectroscopy Characterization of TANOS (TaN/Al2O3/Si3N4/SiO2/Si) Stacks

The interfacial layer between the Al₂O₃ layer and the Si₃N₄ layer formed after postdeposition annealing (PDA) of TaN/Al₂O₃/Si₃N₄/SiO₂/Si (TANOS) stacks was investigated using transmission electron microscopy (TEM), scanning transmission electron microscopy, and electron energy loss spectroscopy (EELS). From the result of the TEM analysis, it was found that the 2-nm-thick interface layer between Al₂O₃ and Si₃N₄ layers was amorphous. The high-loss EELS analysis showed that the phases of the interfacial layer weakly bound together instead of the substoichiometric silicon oxide and amorphous Al₂O₃ near the bottom interface of the crystalline Al₂O₃. The low-loss EELS analysis showed that aluminum existed in metallic state at the interface. Therefore, we speculated that SiO(x)N(y) could be formed by oxidation of Si₃N₄ during PDA and that metallic aluminum could be formed by the decomposition of weakly bound amorphous Al₂O₃ during electron irradiation. These complicated reactions near the interface could induce oxygen deficiency in the Al₂O₃ layer and finally degrade the retention properties of TANOS stacks.

Numerical simulation of Electron Energy Loss Spectroscopy using a Generalized Multipole Technique

We numerically simulate low-loss Electron Energy Loss Spectroscopy (EELS) of isolated spheroidal nanoparticles, using an electromagnetic model based on a Generalized Multipole Technique (GMT). The GMT is fast and accurate, and, in principle, flexible regarding nanoparticle shape and the incident electron beam. The implemented method is validated against reference analytical and numerical methods for plane-wave scattering by spherical and spheroidal nanoparticles. Also, simulated electron energy loss (EEL) spectra of spherical and spheroidal nanoparticles are compared to available analytical and numerical solutions. An EEL spectrum is predicted numerically for a prolate spheroidal aluminum nanoparticle. The presented method is the basis for a powerful tool for the computation, analysis and interpretation of EEL spectra of general geometric configurations.