Electron Energy Loss Measurements Research Articles

Polarization-Resolved Electron Energy Gain Nanospectroscopy With Phase-Structured Electron Beams

Free-electron-based measurements in scanning transmission electron microscopes (STEMs) reveal valuable information on the broadband spectral responses of nanoscale systems with deeply subdiffraction limited spatial resolution. Leveraging recent advances in manipulating the spatial phase profile of the transverse electron wavefront, we theoretically describe interactions between the electron probe and optically stimulated nanophotonic targets in which the probe gains energy while simultaneously transitioning between transverse states with distinct phase profiles. Exploiting the selection rules governing such transitions, we propose phase-shaped electron energy gain nanospectroscopy for probing the 3D polarization-resolved response field of an optically excited target with nanoscale spatial resolution. Considering ongoing instrumental developments, polarized generalizations of STEM electron energy loss and gain measurements hold the potential to become powerful tools for fundamental studies of quantum materials and their interaction with nearby nanostructures supporting localized surface plasmon or phonon polaritons as well as for noninvasive imaging and nanoscale 3D field tomography.

Electronic structure of nickel ferrite thin films growth on strontium titanate

In this study nickel ferrite thin films have been grown on strontium titanate-100 single crystal substrate by using RF magnetron sputtering. To investigate the electronic structure of nickel ferrite film, X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy were employed. In the X-ray photoelectron spectra of Fe 3p, the presence of the valence states Fe2+ and Fe3+ was evidenced; also, Ni2+ and Ni3+ were identified in the Ni 3p spectra. The atomic ratio of 2.01 was found, which is agreeing to the nominal atomic ratio. By reflection electron energy loss measurements found a band gap between 1.77 eV. The magnetic properties were analyzed by vibrating sample magnetometry, indicating in-plane easy magnetization.

On the variety and formation sequence of second-phase particles in nickel-based superalloys fabricated by laser powder bed fusion

We examine by transmission electron microscopy the precipitation state in the as-built microstructure of a hot-cracking sensitive nickel-based superalloy fabricated by laser powder bed fusion. Most observations are carried out on carbon extraction replica to isolate the precipitate signals from those of the matrix. Chemical maps measured by energy dispersive X-ray spectrometry are compared to phase and orientation maps obtained on the same precipitates by automated crystal orientation mapping. The chemical analyses are completed by electron energy-loss spectrometry measurements made on thin foils. The large fields of view investigated allow for achieving a representative picture of the second-phase particles in the as-built microstructure. A large variety of nano-particles is found: (Ti,Nb)(C,N) carbonitrides, (δ) Al2O3 alumina, (Cr,Mo)3B2 and (Cr,Mo)5B3 borides, and the intermetallic phase Ni7Zr2. The precipitates themselves are also complex in terms of their inner structure: carbonitrides can present a core-shell structure of composition and are frequently twinned, alumina particles serve as nucleation sites for carbonitride aggregates, and several crystallographic forms of borides may coexist next to each other. These observations provide the necessary information to retrace the solidification path of the microstructure during fabrication and to discuss the precipitation mechanisms under the extreme processing conditions associated with laser powder bed fusion.

Discrete Sources Method to Solve Nonlocal Scattering Problems in Plasmonic Applications

Current paper presents a comprehensive up-to-date review of one of the most recently developed numerical schemes based on the Discrete Sources Method. The aim of these developments is to implement and justify new efficient mathematical models allowing to accurately simulate response of small plasmonic nanoparticles with scale less than 10 nm to the different types of incident fields. Spatial dispersion effects of the material that are non-negligible at the given scales are incorporated into the numerical technique via Generalized Nonlocal Optical Response approach. Electron energy loss and plane wave scattering problems are considered, with the latter additionally featuring account for the presense of the substrate in the medium. Validity of the obtained results is ensured via a posteriori residual estimation, via comparison of computed scattering properties to the other available simulation techniques, and via comparison to the experimental electron energy loss measurements available in reference literature.

Anisotropic plasmons due to carrier electrons in Cs-doped hexagonal WO3 studied by momentum transfer resolved electron energy-loss spectroscopy

Cs-doped hexagonal WO3 (CWO) is used as a solar heat-shielding material for windows, in which plasma oscillation due to carrier electrons (carrier plasmon) plays an important role for near infrared scattering. Despite the hexagonal crystal structure of CWO, the anisotropic properties of the carrier plasmons have not been investigated. This study reports the momentum transfer resolved electron energy-loss spectroscopic measurements of CWO to investigate the anisotropic properties of carrier plasmons. The experimental results clarified that the two plasma oscillation modes at 1.2 and 1.8 eV have different excitation properties in CWO. One plasma oscillation at 1.2 eV was excited for q along the ab plane with a large damping effect, which indicated that electron excitations occur for the q//ab plane. Another mode at 1.8 eV was an oscillation excited for q along the c-axis with a small damping effect, i.e., a long plasmon relaxation time. These two modes can be interpreted by the anisotropic energy dispersion of the electronic states around the Fermi level of CWO. Such anisotropic properties of the carrier plasmons led to an accurate understanding of the heat-shielding mechanism.

Theory of momentum-resolved phonon spectroscopy in the electron microscope

We provide a theoretical framework for the prediction and interpretation of momentum dependent phonon spectra due to coherent inelastic scattering of electrons. We complete the approach with first principles lattice dynamics using periodic density functional theory and compare to recent electron energy loss measurements on cubic and hexagonal boron nitride performed within a scanning transmission electron microscope (STEM). The combination of theory and experiment provides the ability to interpret momentum dependent phonon spectra obtained at nanometer spatial resolution in the electron microscope.

Excitation and desorption of physisorbed H2 via theΣu2 electron scattering resonance.

Our high-resolution electron energy-loss measurements concern physisorbed H2 and comprise differential cross sections for the excitation of the internal H2 modes and the H2-surface bonding mode and their combinations and extend over the electron impact energy range of the classical low-energy H2 Σu2 resonance. Comparison with corresponding data for the excitation of the internal modes of gas phase H2 reveals that strong elastic electron reflectivity from the Cu(100) substrate profoundly distorts the inelastic scattering pattern for physisorbed H2. We find that this influence can be corrected for and that the resulting peak cross sections agree with the H2 gas phase data, in accordance with theoretical predictions for the excitation of the internal H2 vibration. We have used corrected cross sections for the rotational mode spectra of physisorbed H2, HD, and D2 in a model concerning electron induced desorption via rotation-translation energy conversion. These spectra include transitions from the ground state as well as excited levels of the physisorption potential well. H2 and HD can desorb from all levels while D2, for energetic reason, can only desorb from the excited levels. This model gives a satisfactory account of the observed desorption cross sections and predicts characteristic velocity distributions of the desorbing molecules. The cross section data for H2 and HD reveals that direct bound-free transitions also contribute to the electron induced desorption.

Semiconductor-to-metal transition in the bulk of WSe2 upon potassium intercalation

We present electron energy-loss spectroscopic measurements of potassium (K) intercalated tungsten diselenide (WSe2). After exposure of pristine WSe2 films to potassium, we observe a charge carrier plasmon excitation at about 0.97 eV, which indicates a semiconductor-to-metal transition. Our data reveal the formation of one particular doped K-WSe2 phase. A Kramers–Kronig analysis allows the determination of the dielectric function and the estimation of the composition of K0.6WSe2. Momentum dependent measurements reveal a substantial plasmon dispersion to higher energies.

Monte Carlo simulations of measured electron energy-loss spectra of diamond and graphite: Role of dielectric-response models

In this work we compare Monte Carlo (MC) simulations of electron-transport properties with reflection electron energy-loss measurements in diamond and graphite films. We assess the impact of different approximations of the dielectric response on the observables of interest for the characterization of carbon-based materials. We calculate the frequency-dependent dielectric response and energy-loss functions of these materials in two ways: a full ab initio approach, in which we carry out time-dependent density functional simulations in linear response for different momentum transfers, and a semi-classical model, based on the Drude–Lorentz extension to finite momenta of the optical dielectric function. Ab initio calculated dielectric functions lead to better agreement with measured energy-loss spectra compared to the widely used Drude–Lorentz model. This discrepancy is particularly evident for insulators and semiconductors beyond the optical limit (q≠0), where single-particle excitations become relevant. Furthermore, we show that the behaviour of the energy-loss function obtained at different accuracy levels has a dramatic effect on other physical observables, such as the inelastic mean free path and the stopping power in the low energy (<100 eV) regime and thus on the accuracy of MC simulations.

Desorption ofH2, HD, andD2through Temporary Negative Ion Formation

Physically adsorbed H(2), HD, and D(2) desorbs via the low-energy 2Σ(u)+ electron scattering resonance with characteristic electron energy dependent cross sections as we show by desorption and electron energy-loss measurements. Only H(2) and HD desorb, for energetic reasons, via excitation of the j = 0 → 2 rotational mode and its subsequent decay into free translational motion, while all three isotopes desorb by direct transitions to unbound continuum states, with a probability close to unity and an unexpectedly weak mass dependence ~m(-1/2).

High resolution electron energy loss spectroscopy of clean and hydrogen covered Si(001) surfaces: First principles calculations

Surface phonons, conductivities, and loss functions are calculated for reconstructed (2×1), p(2×2) and c(4×2) clean Si(001) surfaces, and (2×1) H and D covered Si(001) surfaces. Surface conductivities perpendicular to the surface are significantly smaller than conductivities parallel to the surface. The surface loss function is compared to high resolution electron energy loss measurements. There is good agreement between calculated loss functions and experiment for H and D covered surfaces. However, agreement between experimental data from different groups and between theory and experiment is poor for clean Si(001) surfaces. Formalisms for calculating electron energy loss spectra are reviewed and the mechanism of electron energy losses to surface vibrations is discussed.

Electronic structure of thermoelectric Zn–Sb

The electronic structures of the two main compounds of thebinary zinc antimonides that are stable at room temperature,Zn1Sb1 andβ-Zn4Sb3, were probed with x-ray photoelectron spectroscopy. Additionally, electron energy lossmeasurements and density functional theory calculations are presented. The compounds arefound to share a very similar electronic structure. They both feature only small chargetransfers and differ moderately in their screening potentials. These results are in line withrecent theoretical works on the Zn–Sb system and are discussed in light of the reportedthermoelectric performance of the materials.

Dissociation of PhysisorbedH2through Low-Energy Electron Scattering Resonances

Electron induced dissociation of physisorbed H2, HD, and D2 proceeds, as we observe in electron energy-loss measurements of the resulting atomic species, with a high quantum efficiency via the 2Σg(+) core excited electron scattering resonances. We find that the predominant decay of the temporary H2(-) state to the neutral excited 3Σu(+) parent state, which is intramolecularly antibonding, provides a sufficiently long-lived channel for dissociation to occur with high probability, even in the proximity of a metal surface.

Relationship between resistive switching characteristics and band diagrams ofTi/Pr1−xCaxMnO3junctions

We investigated the Ca-composition dependence of the resistive switching (RS) characteristics and band diagrams in $\text{Ti}/{\text{Pr}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ $[\text{PCMO}(x)]$ junctions and the impact of oxygen vacancies on the band diagrams. Hysteretic current-voltage characteristics, i.e., the RS effect, were observed for $\text{Ti}/\text{PCMO}(x)$ junctions with $x&lt;0.8$, whereas junctions consisting of $n$-type semiconducting $\text{PCMO}(x)$ with $x&gt;0.8$ showed almost no RS effect. The RS ratio ${R}_{\text{H}}/{R}_{\text{L}}$, where ${R}_{\text{H}}$ and ${R}_{\text{L}}$ are resistances of high- and low-resistance states, respectively, showed a clear $x$ dependence: ${R}_{\text{H}}/{R}_{\text{L}}$ increased with increasing $x$ and was maximized at $x\ensuremath{\sim}0.4$. Cross-sectional transmission electron microscope images of the switched $\text{Ti}/\text{PCMO}(x)$ junctions confirmed the formation of amorphous ${\text{TiO}}_{y}$ layers at the interfaces. Electron energy-loss measurements of the $\text{Mn-}L$ edge indicated that oxygen-deficient $\text{PCMO}(x)$ layers were formed at the interface due to the electrochemical migration of oxygen ions. Optical-absorption measurements of oxygen-deficient $\text{PCMO}(x)$ films revealed that the formation of oxygen vacancies increased the band gap of $\text{PCMO}(x)$. On the basis of the results, we propose a possible model involving the change in barrier height and/or width for the hole-carrier conduction at the $\text{PCMO}(x)$ interface induced by the electrochemical migration of oxygen vacancies as the mechanism of the RS effect.

Electron-Induced Desorption of PhysisorbedH2via Resonance Vibrational Excitation

We show by electron energy-loss measurements that desorption of physisorbed H(2) and D(2) induced by low-energy electrons takes place with large cross sections, predominantly via resonance excitation of the molecule-surface vibrational mode. The observed H(2), D(2) cross-section ratio supports a picture where rotation-translation conversion of the resonance excited j = 0 --> 2 rotational transition contributes to the desorption of H(2), while this channel is energetically closed for D(2).

Monochromated, spatially resolved electron energy-loss spectroscopic measurements of gold nanoparticles in the plasmon range

Gold nanoparticles show optical properties different from bulk material due to resonance phenomena which depend on local structure and geometry. Electron energy-loss spectrometry (EELS) in scanning transmission electron microscopy (STEM) allows the spatially resolved measurement of these properties at a resolution of few nanometers. In this work, the first monochromated measurements of gold nanoparticles (spheres, rods and triangles) are presented. Due to the improved energy resolution of about 0.2 eV, surface plasmon excitations at energies below 1 eV could be accurately measured from raw experimental data.

High resolution electron energy loss measurements of Na∕Cu(111) and H2O∕Na∕Cu(111): Dependence of water reactivity as a function of Na coverage

Collective electronic excitations occurring in Na layers grown on Cu(111) and in H2O/Na/Cu(111) have been investigated at room temperature by high resolution electron energy loss spectroscopy. Loss spectra taken for a coverage between 0.55 and 0.70 ML of Na are characterized by a feature at 3.0 eV assigned to a Mie resonance. Further increasing the Na coverage leads to the appearance of the Na surface plasmon at 3.9 eV. Water molecules dissociate on Na layers as shown by the appearance of the OH-Na vibration. Upon water adsorption, relevant effects on both electronic excitations and vibrational modes were observed as a function of Na coverage.

Inelastic mean-free paths and surface excitation parameters by absolute reflection electron-energy loss measurements

An analytical approach is proposed for simultaneously determining the inelastic mean-free path (IMFP), the surface excitation parameter (SEP), and the differential SEP (DSEP) in absolute units from an absolute reflection electron energy loss spectroscopy (REELS) spectrum under the assumption that the normalized differential inelastic mean-free path for bulk excitations and the elastic scattering cross section are known. This approach was applied to an analysis of REELS spectra for Ni, and the IMFP, SEP, and DSEP in Ni for $300--3000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ electrons were determined. The resulting IMFPs showed good agreement with those calculated using the TPP-2M predictive equations and with those calculated from optical data. The deduced DSEPs show a reasonable agreement with those theoretically predicted. The obtained SEPs were compared with those calculated using several predictive equations. The present SEP results agreed well with the Chen formula with a material parameter proposed for Ni. The present approach has high potential for the experimental determination of IMFPs, SEPs, and DSEPs in absolute units.

Disordered matter under extreme conditions: X-ray diffraction, electron spectroscopy and electroresistance measurements

In the last decade, we have developed a set of experimental techniques for in-house X-ray diffraction measurements under high-temperature (up to about 2300K) and high-pressure (up to 10GPa and 1500K, up to 100GPa and 700K) conditions, electron energy-loss and Auger measurements for surface electronic structure measurements at high-temperature (up to about 1800K), and electrical resistance measurements covering both low-temperatures (2K) and high-temperatures/high-pressures (1500K, 7GPa) conditions. In this paper we discuss some important technical features and possibilities of these new equipments and present novel data collected for phase transitions and structural modifications occurring in liquid and solid systems. In particular, we present new results about phase transitions and undercooling of bismuth under pressure, extreme undercooling and metastable states in gallium films, and surface phase transitions of Si at high-temperatures. The relevance of these experiments to the exploitation of the potential of equipments available at synchrotron radiation facilities is emphasized.

Full Recovery of Electron Damage in Glass at Ambient Temperatures

An unusually complete recovery of extensive electron-beam-induced damage in a thin film of a CaO-Al2O3-SiO2 glass was discovered. Nanoscale measurements show that the Ca ions migrate about 10 nm away during irradiation and return during recovery. Oxygen atoms are trapped largely as molecular oxygen and do not migrate. Electron energy loss measurements demonstrate that the glass returns completely to the original compositional and structural state thus indicating that the glass is in a deep thermodynamic energy minimum.