Electron Energy Loss Spectrometer Research Articles

Automatic and Quantitative Measurement of Spectrometer Aberrations.

The performance of electron energy loss spectrometers can often be limited by their electron optical aberrations. Due to recent developments in high energy resolution and momentum-resolved electron energy loss spectroscopy (EELS), there is renewed interest in optimizing the performance of such spectrometers. For example, the "ω - q" mode of momentum-resolved EELS, which uses a small convergence angle and requires aligning diffraction spots with the slot aperture, presents a challenge in the realignments of the spectrometer required by the adjustment of the projection lenses. Automated and robust alignment can greatly benefit such a process. The first step toward this goal is automatic and quantitative measurement of spectrometer aberrations. We demonstrate the measurement of geometric aberrations and distortions in EELS within a monochromated scanning transmission electron microscope (STEM). To better understand the results, we present a wave mechanical simulation of the experiment. Using the measured aberration and distortion coefficients as inputs to the simulation, we find a good match between the simulation and experiment, verifying formulae used in the simulation. From verified simulations with known aberration coefficients, we can assess the accuracy of measurements. Understanding the errors and inaccuracies in the procedure can guide further progress in aberration measurement and correction for new spectrometer developments.

Plasma-relevant fast electron impact study of difluoromethane

Difluoromethane () is a commonly used etching gas in manufacturing very-large-scale integrated circuits as the fluorine source. Electron impact excitation cross sections for are important input data for modeling the reactive etching plasmas. In this work, we present the experimental generalized oscillator strengths for the , , and transitions on the absolute scale. The measurement was realized by employing the crossed-beam based relative flow technique at an angle-resolved electron energy loss spectrometer, operating at an incident energy of 1500 eV and a resolution of 80 meV. The dipole-forbidden transition at around 9.7 eV is identified for the first time. The measured data are fitted with the well-known Lassettre formula to yield analytical expressions and optical oscillator strengths at for dipole-allowed transitions. The analytical expressions enable integrations over momentum transfer to obtain the Born integral cross sections for electron impact excitations from the threshold to 5000 eV. The integral cross sections for dipole-allowed transitions are scaled to a more reliable level by using the BE-scaling method. The experimental data can benchmark theoretical calculations and supplement the molecular database for plasma modelers.

Oscillator strengths and cross sections of the valence-shell excitations of CH[formula omitted]I studied by fast electron scattering

Oscillator strengths and cross sections of the valence-shell excitations in CH3I are important for studying molecular photodissociation and monitoring the destruction of ozone in the Earth’s atmosphere. Based on the relative flow technique, absolute generalized oscillator strengths of the valence-shell excitations in CH3I have been determined by using an angle-resolved electron energy loss spectrometer. The measurement was carried out at an incident electron energy of 1.5 keV and an energy resolution of 80 meV. Optical oscillator strengths for the dipole-allowed transitions have been obtained at the zero limit of the squared momentum transfer, providing an independent cross check with the previous experimental results. The integral cross sections from the excitation threshold to 5000 eV have also been obtained by means of the BE-scaling method. The present experimental oscillator strengths and cross sections can supplement the fundamental spectroscopic data of CH3I and will play an important role in the studies of molecular photodissociation dynamics.

Oscillator strengths and cross sections of the valence shell excitations in nitrous oxide studied by high-energy electron scattering

Generalized oscillator strengths (GOSs) of the valence shell excitations to the A1Σ−+B1Δ, C1Π, D1Σ+ and 21Π states in nitrous oxide have been determined by employing an angle-resolved electron energy loss spectrometer operated at an incident energy of 1500 eV and an energy resolution of about 80 meV. Detailed comparisons with the previous data indicate that the higher-order Born terms play a significant role in the GOSs of C1Π and D1Σ+ states at either low impact energies or large momentum transfers, and the GOSs of A1Σ−+B1Δ and C1Π states measured with the gas cell may suffer from the pressure effect. The measured GOS data are fitted with the Lassettre formula to determine the corresponding optical oscillator strengths (OOSs) and calculate the integral cross sections (ICSs) based on the BE-scaling method. The oscillator strengths and cross sections in this work provide a cross check on the previous data and can serve as a benchmark for testing the developed theoretical models and computational codes.

Oscillator strengths and cross sections of high Rydberg states in ammonia studied by fast electron scattering

We report the experimental generalized oscillator strengths (GOSs) for transitions from the ground state to the Rydberg states , , , and an ‘undefined’ state in ammonia. The experiment was performed using an angle-resolved electron energy loss spectrometer operated at an incident energy of 1500 eV and an energy resolution of about 80 meV. The experimental observation confirms the recent theoretically predicted appreciable vibrational effects on the GOS for the transition (2021 J. Phys. B: At. Mol. Opt. Phys. 54 135202). The measured GOS data are fitted with the Lassettre formula to determine the corresponding optical oscillator strengths for dipole-allowed transitions. Furthermore, the fitted curves enable the integrations of GOSs over the momentum transfer squares to obtain the Born excitation cross sections. The integral cross sections are scaled to an accurate scale by using the BE-scaling method. The data in this work can supplement the molecular database and deepen our understanding of the scattering mechanism.

Valence-shell electronic excitations of CF4 studied by fast electron scattering

Oscillator strengths and cross sections of the valence-shell excitations in CF4 are of crucial importance in modeling the etching plasma and monitoring the greenhouse effect. In this work, generalized oscillator strengths of the valence-shell excitations in CF4 were measured by using an angle-resolved electron energy loss spectrometer, operating at an incident electron energy of 1.5 keV and a high energy resolution of 80 meV. The experimental GOSs are analytically fitted with the well-known Lassettre formula, and, thus, the optical oscillator strengths are determined at the limit of squared momentum transfer K2=0. The analytical GOS also enables an integration over K2, determining the Born cross sections with respect to the incident electron energy from the excitation threshold to 5000 eV. The BE-scaled integral cross sections for dipole-allowed transitions are obtained by adopting the BE-scaling method. Oscillator strengths and cross sections of the valence-shell excitations in CF4 provide an independent cross-check to the previous experimental and theoretical works. Moreover, the cross-checked data supplement and benchmark the molecular database for use in plasma models.

Plasma-relevant fast electron impact study of trifluoromethane

Abstract Trifluoromethane is an important etching gas in semiconductor industries as the fluorine source in reactive etching plasma. Electron impact excitations into the dissociation states are essential for plasma modeling. By employing an angle-resolved electron energy loss spectrometer, the generalized oscillator strengths (GOSs) for the valence-shell excited states have been determined at an absolute scale utilizing the crossed-beam based relative flow technique. The measurement was performed with an incident electron energy of 1500 eV at an energy resolution of about 70 meV. The corresponding optical oscillator strengths are also obtained by extrapolating the measured GOSs to the limit of zero momentum transfer. The extrapolated data and fitted linewidths are used to reproduce the photoabsorption spectra and compare with the available experimental and theoretical results. In addition, the integral cross sections from the excitation threshold to 5000 eV of the associated excited states have been derived by using the BE-scaling method. The resulted data can not only be used in plasma models but also serve as a benchmark for further theoretical calculations.

Momentum-transfer-dependence behavior of the 3s autoionization resonances of argon studied by high-resolution fast electron scattering

The asymmetric Fano profile, which is caused by the two-channel interference between discrete states and relevant continuum states, presents a great challenge to atomic and molecular physics. This study investigates the inner-valence excitations of argon $3s$ electrons in the energy range 24.5--29.5 eV using an electron energy-loss spectrometer. The absolute generalized oscillator strength densities of the $3s$ excitations and the Fano profile parameters (i.e., $q$, ${f}_{a}$, $\ensuremath{\rho}{}^{2}$, $f$, $S$) in $0<{K}^{2}<2.2$ a.u. for the $3s3{p}^{6}ns$ ($n=4$ and 5), $3s3{p}^{6}np$ ($n=4$ and 5), and $3s3{p}^{6}3d$ autoionization states are determined. For the same Rydberg series, similar momentum-transfer-dependence behaviors are observed for the Fano profile parameters $q$, ${f}_{a}$, $\ensuremath{\rho}{}^{2}$, $f$, and $S$. The magnitudes of $q$ and ${f}_{a}$ are found to be nearly equal for the same Rydberg series, while the corresponding $f$ and $S$ parameters decrease as the principal quantum number increases, confirming the theoretical predictions of Fano and Cooper [U. Fano and J. W. Cooper, Phys. Rev. 137, A1364 (1965)]. Moreover, the typical dipole-allowed or dipole-forbidden ${K}^{2}$-dependence behaviors of the Fano profile parameters are used to elucidate the properties of electric transition multipolarities. The parameters of the Fano profile derived in a large ${K}^{2}$ region offer an opportunity to understand the wave functions far away from the nucleus.

Correction of EELS dispersion non-uniformities for improved chemical shift analysis

We outline a simple routine to correct for non-uniformities in the energy dispersion of a post-column electron energy-loss spectrometer for use in scanning transmission electron microscopy. We directly measure the dispersion and its variations by sweeping a spectral feature across the full camera to produce a calibration that can be used to linearize datasets post-acquisition, without the need for reference materials. The improvements are illustrated using core excitation electron energy-loss spectroscopy (EELS) spectra collected from NiO and diamond samples. The calibration is rapid and will be of use in all EELS analysis, particularly in assessments of the chemical states of materials via the chemical shift of core-loss excitations.

The performance evaluation of direct detection electron energy-loss spectroscopy at 200 kV and 80 kV accelerating voltages

Direct electron detectors (DeDs) have been widely used for imaging studies because of their higher beam sensitivity, lower noise, improved pixel resolution, etc. However, there have been limited studies related to the performance in spectroscopic applications for the direct electron detection. Hereby, taking the advantage of the DeD installed on a high-performance electron energy-loss spectrometer, we systematically studied the performance of a DeD (Gatan's K2 IS) fitted on an aberration-corrected transmission electron microscope (TEM) equipped with an electron monochromator. Using SrTiO3 as the model system, the point spread function in the zero-loss region of the spectrum and the performance for core loss spectroscopy have been investigated under both 200 kV and 80 kV operating conditions. We demonstrate that the K2 detector can achieve an overall better performance at 200 kV than a charge coupled device (CCD) detector. At 80 kV, the K2 DeD is still better than a CCD, except for the relative broad tails of the zero-loss peak. The signal-to-noise ratio is very close for DeD and CCD under 80 kV. Based on our data obtained at different operating voltages, it is clear that DeD will benefit the microscopy community and boost the development of cutting-edge materials science studies by pushing the frontiers in electron energy-loss spectroscopy.

Atomic-scale observation of phase transition of MgMn2O4 cubic spinel upon the charging in Mg-ion battery

Direct observation of demagnesiated structure of MgMn2O4 spinel oxides at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. Upon the Mg ions extraction from MgMn2O4 spinel oxides, structural transition from cubic to tetragonal phases was observed. The phase transition is a result of Mn octahedral distortion due to cooperative Jahn–Teller distortion of six-coordinate MnIII (t2g3–eg1). On the basis of HAADF/ABF micrographs and electron energy-loss spectrometer (EELS), it is suggested that Mn3+/Mn4+ and/or Mn3+/Mn5+ redox couples play a crucial role in the first electrochemical Mg deintercalation. This is the first report on observation of cubic-tetragonal phase transition in charged spinel MgMn2O4 which shed new insight into the magnesium storage mechanism in this important cathode material for Mg-ion batteries.

On the thermal stability of mesoporous metal oxide systems decorated with metallic nanoparticles for gas sensing applications

Mesoporous metal oxide systems decorated with metallic nanoparticles are among the most studied functional materials for their applications as chemo-resistive gas sensors. The temperature at which such materials are supposed to operate as gas sensors in real working conditions for an optimum device sensitivity ranges around 150-300oC. In our work, SnO2 mesoporous systems have been prepared by solvothermal methods. As a method for tailoring the chemo-resistive properties, the obtained mesoporous matrix has been further decorated with metallic Fe nanoparticles by wet spraying methods. The as-prepared systems have been extensively investigated by analytical transmission electron microscopy using a JEOL ARM200F instrument equipped with X-ray Energy Dispersive and Electron Energy Loss spectrometers (EDS/EELS). Our work has been focused on the morphological and structural aspects of the above mentioned system under the influence of in-situ heating up to 550oC (a meaningful temperature from the point of view of sample preparation). The morpho-structural evolution with temperature has been emphasized by High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED), Scanning Transmission Electron Microscopy (STEM) and Electron Energy Loss Spectroscopy Spectrum Imaging (EELS-SI).

The Major Developments in Camera and Electron Energy Loss Spectrometer Technology Since the Turn of the Century

The Major Developments in Camera and Electron Energy Loss Spectrometer Technology Since the Turn of the Century

Electron energy-loss spectroscopy (EELS) with a cold-field emission scanning electron microscope at low accelerating voltage in transmission mode

A commercial electron energy-loss spectrometer (EELS) attached to a high-resolution cold-field emission scanning electron microscope in transmission mode (STEM) is evaluated and its potential for characterizing materials science thin specimens at low accelerating voltage is reviewed. Despite the increased beam radiation damage at SEM voltages on sensitive compounds, we describe some potential applications which benefit from lowering the primary electrons voltage on less-sensitive specimens. We report bandgap measurements on several dielectrics which were facilitated by the lack of Cherenkov radiation losses at 30 kV. The possibility of volume plasmon imaging to probe local composition changes in complex materials was demonstrated using energy-filtered STEM, either via spectrum imaging or elemental mapping using the “three-windows” method. As plasmonic materials are increasing used for energy, electronics or biomedical applications, the ability of reliably evaluate their properties at low accelerating voltage in a SEM is very appealing and is demonstrated. The energy resolution of the spectrometer, taken as the full width at half maximum of the zero-loss peak, was routinely measured at around 0.55 eV and it is demonstrated that t/λ ratios up to 1.5 allowed practical EEL spectroscopy at 30 kV.

Lithium Storage Behavior of Three-Dimensional Graphene-like Ordered Microporous Carbon Synthesized in a Zeolite Template

Nanostructured carbons with high porosity are of great interest as high-performance anode materials for lithium-ion battery (LIB) applications. In the present work, we investigated the Li storage behavior of three-dimensional graphene-like ordered microporous carbon, which was synthesized via a zeolite-templating route. The microporous carbon exhibited an extremely high initial Li storage capacity as an anode for LIBs. Approximately 60% of the initial capacity was reversible during the subsequent cycles, while 40% was irreversible. The reversible capacity was comparable to the highest value reported so far with carbon alone. In order to investigate the chemical state of the Li species (i.e., ionic or metallic), the charged and discharged zeolite-templated carbon sample was characterized with 7Li nuclear magnetic resonance spectroscopy. The location of lithium was investigated using a transmission electron microscope equipped with an electron energy loss spectrometer and an energy-dispersive X-ray spectrometer. The result indicated that most of the reversible and irreversible lithium existed in nonmetallic +1 oxidation state inside the carbon micropores. The huge amount of Li storage is attributed to the three-dimensionally ordered microporous structure with a large surface area built with the single-layer graphene-like framework. In this study, we provides insights on the Li storage behavior in narrow carbon pore environment.

Absolute vibrational excitation cross sections for 1-18 eV electron scattering from condensed dimethyl phosphate (DMP).

Absolute cross sections (CSs) for vibrational excitation by 1-18 eV electrons incident on condensed dimethyl phosphate (DMP) were measured with a high-resolution electron energy loss (EEL) spectrometer. Absolute CSs were extracted from EEL spectra of DMP condensed on multilayer film of Ar held at about 20 K under ultra-high vacuum (∼1 × 10-11 Torr). Structures observed in the energy dependence of the CSs around 2, 4, 7, and 12 eV were compared with previous results of gas- and solid-phase experiments and with theoretical studies on dimethyl phosphate and related molecules. These structures were attributed to the formation of shape resonances.

Structural proprieties of the Al–Al 4 C 3 nanocomposite produced via mechanical alloying and annealing

The Al–Al4C3 nanocomposite was produced via mechanical alloying of Al 6 wt% C mixture for a predetermined time (up to 20 h), followed by annealing. The structural evolution was characterised via X-ray diffraction and transmission electron microscope equipped with electron energy loss spectrometer. In addition, focused ion beam–scanning electron microscopy was used for locating and analysing the reinforcing particles. During milling, the size of aluminium particles reached the nanometre scale with a 54 nm size. After annealing, carbide was homogeneously distributed in the nanostructured aluminium particles with an average size of 50 nm, result in an average hardness of 320 HV. This was observed for the powder that was mechanically milled for 20 h and that underwent annealing from room temperature to 540°C and was maintained at this temperature for 4 h.

Application of EELS and EFTEM to the life sciences enabled by the contributions of Ondrej Krivanek

The pioneering contributions of Ondrej Krivanek to the development of electron energy loss spectrometers, energy filters, and detectors for transmission and scanning transmission electron microscopes have provided researchers with indispensible tools across a wide range of disciplines in the physical sciences, ranging from condensed matter physics, to chemistry, mineralogy, materials science, and nanotechnology. In addition, the same instrumentation has extended its reach into the life sciences, and it is this aspect of Ondrej Krivanek's influential contributions that will be surveyed here, together with some personal recollections. Traditionally, electron microscopy has given a purely morphological view of the biological structures that compose cells and tissues. However, the availability of high-performance electron energy loss spectrometers and energy filters offers complementary information about the elemental and chemical composition at the subcellular scale. Such information has proven to be valuable for applications in cell and structural biology, microbiology, histology, pathology, and more generally in the biomedical sciences.

透過電子顕微鏡法の最近の動向とグリーンナノテクノロジーへの応用

Transmission electron microscopy has been playing an important role in research and development of nanotechnology which is common and fundamental in various fields of science and technology such as energy, environmental sciences, information and telecommunications, and life sciences and etc. A transmission electron microscope (TEM) provides us with crystallographic information and positional information from a local area of specimen. Furthermore information on local chemical composition and atomic bonding state is obtained provided the TEM is equipped with an X-ray energy dispersive spectrometer (XEDS) and an electron energy loss spectrometer (EELS). The image resolution and special resolution of elemental analysis have been improved to an atomic level together with the development of an analytical TEM that has a field emission gun, an aberration corrector and a function of scanning TEM (STEM). It has been possible to extract high quality and various information on substance and materials, owing to the development of electron detectors and imaging techniques of STEM. Objects of TEM observation have been widely spread after the development of related techniques such as preparation of TEM samples and controlling of specimen environment (heating, cooling, gas pressure, stress application etc.). We introduce recent trend of transmission electron microscopy and demonstrate some applications to green nanotechnology which is closely related to development of renewable energy, energy saving, energy storing, saving of rear resource, removal of harmful substance from air and water and so on.

An electron energy loss spectrometer based streak camera for time resolved TEM measurements

We propose an experimental setup based on a streak camera approach inside an energy filter to measure time resolved properties of materials in the transmission electron microscope (TEM). In order to put in place the streak camera, a beam sweeper was built inside an energy filter. After exciting the TEM sample, the beam is swept across the CCD camera of the filter. We describe different parts of the setup at the example of a magnetic measurement. This setup is capable to acquire time resolved diffraction patterns, electron energy loss spectra (EELS) and images with total streaking times in the range between 100ns and 10μs.