Core-loss Spectra Research Articles

Degradation of thermoplastic polymers for fused filament fabrication under (S)TEM electron beam irradiation

The structural characterization of polymers and, in particular, of those used in Additive Manufacturing (AM) technologies, is essential to improve the understanding of their structure-property relationship for promising high-performance applications. For this, (scanning) transmission electron microscopy-electron energy loss spectroscopy, (S)TEM-EELS is an outstanding tool for exploring materials chemical and structural characteristics at high spatial resolution. However, the high beam-sensitivity of soft materials, such as polymers, hinders the possibility of probing in-depth analysis provided by (S)TEM-EELS. In this work, we analyse the electron beam irradiation damage of four polymers commonly used in Fused Filament Fabrication (FFF), namely polylactic acid (PLA), polycaprolactone (PCL), acrylonitrile butadiene styrene (ABS) and acrylonitrile styrene acrylate (ASA). For this, sequential low-loss and core-loss EEL spectra have been recorded, and the related signals have been monitored as a function of the accumulated dose. Our results show that the critical electron doses using the specimen thickness variations are larger for polymers containing aromatic groups (ABS and ASA) than for aliphatic polymers (PLA and PCL). Regarding the different elements, a larger sensitivity to the electron beam of oxygen regarding carbon and nitrogen is also evidenced. Our results have shown that polymer degradation occurs to a larger extent in the initial steps of electron irradiation, for very low accumulated electron doses, meaning that care should be taken in the selection of the microscopy settings to avoid artefacts produced by the electron beam. Degradation pathways for the four polymers studied are discussed.

Toward the Atomic-Level Analysis of Ground-State Electronic Structures of Solid Materials via Prediction from Core-Loss Spectra: A Computational Study in Si

Toward the Atomic-Level Analysis of Ground-State Electronic Structures of Solid Materials via Prediction from Core-Loss Spectra: A Computational Study in Si

The influence of residual plural scattering after deconvolution in electron magnetic chiral dichroism

This work investigated the existence and influence of residual plural scattering in electron magnetic chiral dichroism (EMCD) spectra. A series of low-loss, conventional core-loss, and q-resolved core-loss spectra at Fe-L2,3 edges were detected from areas of different thicknesses in a plane-view sample of Fe/MgO (001) thin film. It reveals by comparison that there remains noticeable plural scattering in q-resolved spectra acquired at two particular chiral positions after deconvolution, and the residual scattering is more significant in thicker areas than thinner ones. Accordingly, the orbital-to-spin moment ratio extracted from EMCD spectra, which is the difference between the two q-resolved spectra after deconvolution, would be in principle increased with increasing sample thickness. The randomly fluctuated moment ratios displayed in our experiments are greatly attributed to a slight and irregular variation of local diffraction conditions due to the bending effect and imperfect epitaxy in detected areas. We suggest EMCD spectra should be acquired from sufficiently thin samples to minimize the plural scattering effect in originally detected spectra before any deconvolution. In addition, great care should be taken for slight misorientation and imperfect epitaxy when performing EMCD investigation on epitaxial thin films using a nano beam.

Prediction of the Ground-State Electronic Structure from Core-Loss Spectra of Organic Molecules by Machine Learning.

The core-loss spectrum reflects the partial density of states (PDOS) of the unoccupied states at the excited state and is a powerful analytical technique to investigate local atomic and electronic structures of materials. However, various molecular properties governed by the ground-state electronic structure of the occupied orbital cannot be directly obtained from the core-loss spectra. Here, we constructed a machine learning model to predict the ground-state carbon s- and p-orbital PDOS in both occupied and unoccupied states from the C K-edge spectra. We also attempted an extrapolation prediction of the PDOS of larger molecules using a model trained by smaller molecules and found that the extrapolation prediction performance can be improved by excluding tiny molecules. Besides, we found that using smoothing preprocess and training by specific noise data can improve the PDOS prediction for noise-contained spectra, which pave a way for the application of the prediction model to the experimental data.

Stain-free mapping of polymer-blend morphologies via application of high-voltage STEM-EELS hyperspectral imaging to low-loss spectra

Polymer blends composed of multiple types of polymers are used for various industrial applications; therefore, their morphologies must be understood to predict and improve their physical properties. Herein, we propose a spectral imaging method based on scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy to map polymer morphologies with nanometric resolution as an alternative to the conventional electron staining technique. In particular, the low-loss spectra of the 5–30 eV energy-loss region were measured to minimize electron irradiation damage rather than the core-loss spectra, such as carbon K-shell absorption spectra, which require significantly longer recording times. Medium-voltage (200 kV) and high-voltage (1000 kV) STEM was used at various temperatures to compare the degrees of electron-beam damage resulting from various electron energies and sample temperatures. A multivariate curve resolution technique was used to isolate the constituent spectra and visualize their distributions by distinguishing the characteristic peaks derived from various chemical species. High-voltage STEM was more useful than medium-voltage STEM for analyzing thicker samples while suppressing ionization damage.

Simulated carbon K edge spectral database of organic molecules

Here we provide a database of simulated carbon K (C-K) edge core loss spectra of 117,340 symmetrically unique sites in 22,155 molecules with no more than eight non-hydrogen atoms (C, O, N, and F). Our database contains C-K edge spectra of each carbon site and those of molecules along with their excitation energies. Our database is useful for analyzing experimental spectrum and conducting spectrum informatics on organic materials.

A nanoscale investigation on the influence of anodization parameters during plasma electrolytic oxidation of titanium by high-resolution electron energy loss spectroscopy

High-resolution electron energy loss spectroscopy (EELS) in a transmission electron microscope is performed on titanium oxide coatings obtained by plasma electrolytic oxidation (PEO) with application of different electrical parameters. Core loss spectra were used to evaluate the structural evolution occurring at the two main regions characterizing a PEO coating, i.e. barrier and porous layers. Local crystalline information, extracted from EELS, was correlated to macroscopic technological parameters such as duty cycle and frequency, based on advanced data analysis. Using the spectral differences, structural maps are, for the first time, provided for titanium oxide grown anodically. Cathodic current was found to favour the growth of a mainly crystalline barrier layer, responsible for abundant O2 evolution during the treatment. A detailed mechanism regarding the stimulation of type-B discharges, when using cathodic polarization at high frequency, is given comparing outcomes from optical emission spectroscopy and structural information. As the result of the intense plasma interaction with the growing layer, the structure evolved towards the formation of 18 nm of titanium oxide characterized by a strong Ti+3 fingerprint, followed by 85 nm of Ti3O5 formed according to high temperatures and the de-oxidative condition encountered.

Combined study of the ground and excited states of carbon onions by electron energy-loss spectroscopy: Comparison with highly ordered pyrolytic graphite

Herein, an extensive electron energy loss spectroscopy (EELS) study presents the electronic structure properties of carbon nano-onions (CNOs) and graphite under same acquisition conditions, respectively. The low-loss and core-loss EELS spectra were recorded at the so-called “magic angle” on the optic axis in a transmission electron microscope. Through the “three-Gaussian” method we found the second peak in the core-loss spectrum of CNOs is markedly different from that of graphite. Valence Compton profiles were obtained by recording the EELS at a large scattering angle. The CNOs and graphite Compton profile difference indicates stronger electron localization in CNOs. From this comparison, we are able to conclude that the electronic structure properties of the ground and excited states of CNOs are quite different from those of graphite.

Characterization and understanding of the tilt-dependence of core-loss spectra for hexagonal boron nitride

Despite great interest in the thermo-mechanical, optical, and electronic features of layered materials, the complete nature of anisotropy and its effect on their spectroscopic properties remains poorly understood due to the difficulty in characterizing the chemical bonding in different crystal orientations. Here, we use electron energy loss spectroscopy (EELS) to evaluate the tilt-dependent bonding characterization of hexagonal boron nitride (hBN), a promising layered van der Waals material, by tilting the specimen and studying the changing core-loss spectra. Our experimentally calculated values match well with the theoretically obtained curve of tilt versus relative peak intensities of the core-loss EELS spectra, which illustrates the scope for precise quantitative analysis of layered materials using EELS.

Quantitative estimation of properties from core-loss spectrum via neural network

Localized structures in nano- and sub-nano-scales strongly affect material properties. Thus, some spectroscopic techniques have been used to characterize local atomic and electronic structures. If material properties can be directly ‘measured’ via spectral observations, the atomic-scale understanding of the material properties would be dramatically facilitated. In this paper, we have attempted to unveil the hidden information about the material properties directly and quantitatively based on core-loss spectra. We predicted six properties, including three geometrical and three chemical bonding properties, by a simple feedforward neural network, and achieved considerably sufficient accuracy. Moreover, we applied the constructed model to the noisy experimental spectrum and could predict the six properties precisely. This successful prediction implies that this method can pave the way for local measurement of the material properties.

An EELS signal-from-background separation algorithm for spectral line-scan/image quantification

Background removal is an important step in the quantitative analysis of electron energy-loss structure. Existing methods usually require an energy-loss region outside the fine structure in order to estimate the background. This paper describes a method for signal-from-background separation that is based on subspace division. The linear space is divided into two subspaces. The signal is recovered from a linear subspace containing no background information, and the other subspace containing the background is discarded. This method does not rely on any signal outside the energy-loss range of interest and should be very helpful for multiple linear least-squares (MLLS) regression analysis on experimental signals with little or no available smooth pre-edge region or with overlapping pre-edge features. Use of the algorithm is demonstrated with several practical applications, including closely overlapping core-loss spectra and zero-loss peak removal. Tests based on experimental data indicate that the algorithm has similar or better performance relative to conventional pre-edge power-law fitting methods in applications such as MLLS regression for electron energy-loss near-edge structure.

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.

Angle-resolved electron energy loss spectroscopy in hexagonal boron nitride

Electron energy loss spectra have been measured on hexagonal boron nitride single crystals employing a novel electron energy loss spectroscopic set-up composed by an electron microscope equipped with a monochromator and an in-column filter. This set-up provides high-quality energy-loss spectra and allows also for the imaging of energy-filtered diffraction patterns. These two acquisition modes provide complementary pieces of information, offering a global view of excitations in reciprocal space. As an example of the capabilities of the method we show how easily the core loss spectra at the $K$ edges of boron and nitrogen can be measured and imaged. Low losses associated to interband and/or plasmon excitations are also measured. This energy range allows us to illustrate that our method provides results of quality comparable to those obtained from non resonant X-ray inelastic scattering, but with advantageous specificities such as an enhanced sensitivity at low q and a much higher simplicity and versatility that makes it well adapted to the study of two-dimensional materials and related heterostructures. Finally, by comparing theoretical calculations against our measures, we are able to relate the range of applicability of ab initio calculations to the anisotropy of the sample and assess the level of approximation required for a proper simulation of our acquisition method.

(Invited) In Situ Electrochemical STEM As a Platform for Interpreting Electrochemical Phenomena

In situ electrochemical STEM (ec-STEM) is a multi-modal characterization technique that allows for the direct interpretation of nanoscale mechanisms and kinetics during electrochemical operation. The major advantage of this method is the ability to perform quantitative electrochemical measurements while simultaneously characterizing nanoscale structural and chemical changes that occur at site-specific electrode/electrolyte interfaces using high spatial resolution imaging, spectroscopy, and/or diffraction; all within a scanning transmission electron microscope (STEM). ec-STEM is being used for battery research by studying the formation mechanisms of the solid electrolyte interphase (SEI) and growth mechanisms of lithium metal dendrites within an organic electrolyte (1.2M LiPF6 EC:DMC). The initial stage of lithium metal dendrite nucleation and growth is observed on the surface and edge of the glassy carbon working electrode during cyclic voltammetry experiments below -3.0V vs. Ptpseudo reference electrode. The Li dendrites grow with a globular morphology during a sequence of bursts, whereby newly formed lithium metal deposits are passivated by the SEI followed by additional growth events associated with localized regions of SEI breakdown. Electron energy loss spectroscopy (EELS) confirms that the deposits are lithium metal with surfaces composed of inorganic compounds commonly found in the SEI. The EEL spectra reveal a Li metal plasmon peak at 7.5eV and a characteristic LiF spectral feature on the Li K-edge at 55eV. Furthermore, a framework for obtaining and analyzing quantitative EELS measurements is demonstrated to directly determine the oxidation state of battery electrodes (LiMn2O4 and Li4Ti5O12) using the “white-lines” of the L2,3 core loss ionization edges of Mn and Ti. By simultaneously acquiring low-loss and core-loss EEL spectra, plural scattering effects caused by scattering through the electrolyte can be removed to quantify the oxidation state of Mn and Ti using the white-line intensity ratio method. RF magnetron sputtering was used to deposit a thin layer of LiMn2O4, a commonly used battery cathode material, directly onto the silicon nitride membrane of the ec-STEM cell. Simultaneous low-loss and core-loss EEL spectra are acquired with and without the presence of dimethyl carbonate (DMC) in the liquid cell (Figure 3c). The low-loss portion of the EEL spectra confirms the presence of the DMC within the liquid cell and aides in the quantification of fluid layer thickness. If the fluid layer thickness is small, it is possible to extract and analyze the Mn L2,3-edge for quantification of the Mn oxidation state using the “white-line” intensity ratio method. Mixed Mn3+/4+ oxidation is measured in the as-deposited LiMn2O4, which shifts to a lower oxidation state when DMC is present in the cell, suggesting that LiMn2O4 may be prone to electron beam induced chemical reduction when immersed in a battery electrolyte during in situ ec-STEM experiments. The information obtained from these studies can help provide a deeper understanding of how batteries function at the nanoscale. Research supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Department of Energy’s Office of Basic Energy Sciences Division and by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Mapping Chemical Bonds in Semiconductor Devices by Monitoring the Shifts of EELS Edges.

Scanning transmission electron microscopy (STEM) in combination with electron energy-loss spectroscopy (EELS) can deliver information about variations of bonding at the nm scale. This is typically performed by analyzing the electron-loss near edge structure (ELNES) of given EELS edges. The present paper demonstrates an alternative way of a bonding examination through monitoring the EELS onset positions. Two conditions are essential for their accurate measurement. One (hardware) is using the dual EELS instrumentation that provides near simultaneous acquisition of low-loss and core-loss spectra. Another (software) is the least-square fitting of observed spectra to a reference spectrum. The combination of these hardware and software techniques reveals the positions of EELS onsets with the precision sufficient for mapping tiny variations of bonding. The paper shows that the method is capable of helping to solve practical tasks of nanoscale engineering like the analysis of modern CMOS devices.

Oxygen Functionalities Evolution in Thermally Treated Graphene Oxide Featured by EELS and DFT Calculations

The local atomic configuration of graphene oxide (GO) was investigated by identifying the different oxygen functionalities and following their evolution induced by thermal treatments in various environments (vacuum, nitrogen or argon flow). X-ray photoelectron spectroscopy and scanning transmission electron microscopy analyses were performed, and electron energy-loss (EEL) spectra were acquired in different regions of GO and thermally reduced GO flakes. Experimental results show a series of characteristic peaks related to C and O K-edge shells and different features of GO thermally annealed at the same temperature but in different environments. In order to understand the experimental results, density functional theory calculations of core-loss EEL spectra of GO (C and O K-edges) in the presence of oxygen functional groups have been performed for different combinations and/or concentrations. Such calculations have allowed for the association of the observed experimental peaks to the presence of specific ox...

Good reproductive preparation method of Li-intercalated hexagonal boron nitride and transmission electron microscopy – Electron energy loss spectroscopy analysis

A Li-intercalated hexagonal boron nitride (Li-h-BNIC) phase was synthesized using a highly reproducible method that involves annealing an Li3N and h-BN mixture at 1220 K. Powder X-ray diffraction, electrical conductivity measurements, transmission electron microscopy (TEM) and electron energy loss spectroscopy were performed. The stacking of BN atomic layers in the Li-h-BNIC phase is not the same as the two-layer stacking periodicity of h-BN. TEM observation suggests the existence of incommensurate periodicity along the intralayer direction. From the low-loss and core-loss spectra, the Li-h-BNIC phase is not metal as predicted by the first-principle calculations. Satellite peaks of 1 s to π* transition in the B K-edge core-loss spectrum indicate the presence of N atom vacancies modified by O atoms in the h-BN atomic layer.

Ultrafast core-loss spectroscopy in four-dimensional electron microscopy.

We demonstrate ultrafast core-electron energy-loss spectroscopy in four-dimensional electron microscopy as an element-specific probe of nanoscale dynamics. We apply it to the study of photoexcited graphite with femtosecond and nanosecond resolutions. The transient core-loss spectra, in combination with ab initio molecular dynamics simulations, reveal the elongation of the carbon-carbon bonds, even though the overall behavior is a contraction of the crystal lattice. A prompt energy-gap shrinkage is observed on the picosecond time scale, which is caused by local bond length elongation and the direct renormalization of band energies due to temperature-dependent electron–phonon interactions.

An estimation of molecular dynamic behaviour in a liquid using core-loss spectroscopy

We report an effective approach for estimating the dynamic behaviour of molecules in liquid from their core-loss spectra by combining molecular dynamics simulations and first-principles band-structure calculations. The carbon K-edge of the technologically important methanol was calculated, and the experimental spectra were well reproduced using the presented calculation method, which effectively included multiple-molecule interactions. Several peaks arose from the methanol molecules with different C-O bonding modes, and the splitting of those peaks was sensitively altered by the magnitude of the dynamic behaviour of molecules. These findings allow for estimation of the dynamic behaviour of molecules in liquids using core-loss spectroscopy, and the method offers the potential to identify the dynamic behaviour of the molecules in liquids with high spatial resolution, temporal resolution, and sensitivity.

Electron energy loss spectroscopy of polytetrafluoroethylene: experiment and first principles calculations

We have performed electron energy-loss spectroscopy (EELS) on a 200 kV transmission electron microscope (TEM) equipped with a monochromator to investigate molecular conformation of polytetrafluoroethylene (PTFE). The experimental spectra show several unique features in the low-loss region and the onset of carbon K-edge for PTFE. Density function theory (DFT) methods are employed to calculate the low-loss and core-loss spectra of PTFE with consideration of the effects of phase transitions, chain orientation and polarization. The shape and width of the characteristic peaks of the experimental spectra are well reproduced in DFT calculations. By comparing the spectra from experiments and theory, the detailed information about the conformational dependence of EEL spectra for PTFE can be obtained. In the present work, we have demonstrated an application of combining high-resolution EELS and DFT calculations in both low-loss and core-loss regions to discriminate changes of chain conformation and orientation for the polymer with complex phase transition behavior.