Electron Energy Loss Spectroscopy Experiments Research Articles

Textural, Microstructural and Chemical Characterization of Ferritic Stainless Steel Affected by the Gold Dust Defect

The "gold dust defect" (GDD) appears at the surface of ferritic stainless steels (FSS) and degrades their appearance. Previous research showed that this defect might be related to intergranular corrosion and that the addition of aluminium improves surface quality. However, the nature and origin of this defect are not properly understood yet. In this study, we performed detailed electron backscatter diffraction analyses and advanced monochromated electron energy-loss spectroscopy experiments combined with machine-learning analyses in order to extract a wealth of information on the GDD. Our results show that the GDD leads to strong textural, chemical, and microstructural heterogeneities. In particular, the surface of affected samples presents an α-fibre texture which is characteristic of poorly recrystallised FSS. It is associated with a specific microstructure in which elongated grains are separated from the matrix by cracks. The edges of the cracks are rich in chromium oxides and MnCr2O4 spinel. In addition, the surface of the affected samples presents a heterogeneous passive layer, in contrast with the surface of unaffected samples, which shows a thicker and continuous passive layer. The quality of the passive layer is improved with the addition of aluminium, explaining the better resistance to the GDD.

Nanoscale Characterization of Individual Three-Dimensional Split Ring Resonator Systems

In this work, we investigate the hybridization of three-dimensional plasmonic split ring resonator (SRR) pairs using focused electron beam nanospectroscopy and model their combined electric and magnetic responses using electromagnetic theory and numerical calculations. Specifically, we fabricate three-dimensional (3D) SRR dimers with varying in-plane rotations and out-of-plane tilts and perform electron energy loss spectroscopy (EELS) measurements to elucidate the impact of their 3D electric and magnetic interactions on the EEL spectrum of each individual SRR dimer. On the basis of the competition between the SRR’s electric and magnetic interactions within our model, we find that varying the 3D tilt angle diminishes the magnetic coupling but increases the level of overall mode mixing. Through further modeling of the experimental data, we determine the system’s electric and magnetic coupling constants, as well as the overall effective coupling constant, a useful metric for characterizing the strength of light–matter interaction. We additionally explore the potential for geometric frustration in the magnetic mode ordering of a coupled SRR trimer using both EELS experiment and companion theoretical modeling. Taken together, the insight gained into the behavior of coupled 3D SRRs serves as a stepping stone to the rational design of 3D photonic metamaterials endowed with even richer optical functionality.

Structural Evolution in an Annealed (Eu, Tb)-Doped ZnO/Si Nanoscale Junction: Implication for Red LED Development

Codoped (Tb,Eu) ZnO films grown by magnetron sputtering on a silicon substrate and annealed up to 1200 °C showed intense photoluminescence (PL) emission from Eu3+ ions. The high-temperature annealing led to diffusion and segregation of rare earth (RE) elements toward the bottom of the film, which induced the formation of nanometric Zn-free inclusions responsible for remarkable PL emission intensity. Combined electron diffraction, chemical contrast imaging, and optical studies of these nanometric phases have been carried out. Large inclusions of zinc silicates and RE silicates a few hundreds of nanometers in size were observed, embedded in a silica phase. The structural determination of these RE-rich inclusions was carried out by combining atomic Z contrast imaging (high-angle annular dark-field imaging) and precession electron diffraction data. Upon annealing at 1200 °C, it appeared that the structure was related to an F-type disilicate structure. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy experiments were carried out to determine the ratios between elements and the oxidation states of the RE elements in the abovementioned inclusions. A (Tb,Eu)2Si2O7 formulation is proposed from dynamical precession electron diffraction tomography refinements, leading to a +III valence state for the RE species in agreement with spectroscopic results. PL modeling is also in good agreement with the experimental data. These results complete those obtained at 1100 °C for which the inclusions were identified as some RE10–x(SiO4)6O2–x oxyapatite structures and pointed out a combined structural and PL properties’ evolution between 1100 and 1200 °C annealing temperatures.

Theory of spin-polarized high-resolution electron energy loss spectroscopy from nonmagnetic surfaces with a large spin-orbit coupling

The scattering theory of low-energy (slow) electrons has been developed by Evans and Mills [Phys. Rev. B 5, 4126 (1972)]. The formalism is merely based on the electrostatic Coulomb interaction of the scattering electrons with the charge-density fluctuations above the surface and can describe most of the interesting features observed in the high-resolution electron energy-loss spectroscopy experiments. Here we extend this theory by including the spin-orbit coupling in the scattering process. We discuss the impact of this interaction on the scattering cross section. In particular, we discuss cases in which a spin-polarized electron beam is scattered from nonmagnetic surfaces with a strong spin-orbit coupling. We show that under some assumptions one can derive an expression for the scattering cross section, which can be used for numerical calculations of the spin-polarized spectra recorded by spin-polarized high-resolution electron energy-loss spectroscopy experiments.

Comparison of detection limits of direct-counting CMOS and CCD cameras in EELS experiments

The noise performance and the detection limits of a direct-counting complementary metal-oxide semiconductor (CMOS) K2 camera and a charge-coupled device (CCD) camera in electron energy loss spectroscopy (EELS) experiments were evaluated. In the case of a single spectrum acquired at the shortest dwell times (2.5 ms for K2 and 1 μs for CCD), the detection limit, defined as three times the standard deviation of the spectral noise (3σ), was very low (1 e−/channel) in the counting-mode spectrum acquired with the K2 camera compared with that acquired with the CCD camera (5 e−/channel). By contrast, the spectral noise of the K2 camera changed depending on the dwell time because of the multiple read-outs related to its fixed frame rate (400 fps). The spectral noise of the K2 camera was greater than that of the CCD camera when the dwell time was longer than ∼30 ms. Thus, the CCD camera was found to still be useful when detecting a very small number of electrons using a long acquisition time. In the case of an accumulated spectrum obtained by acquiring 10,000 spectra after subtracting the ultra-high-quality dark reference signal, the detection limits per read-out were ∼0.016 and ∼0.025 e−/channel/read-out for the K2 and CCD cameras, respectively. Because both cameras have advantages and disadvantages with respect to their detection limit, speed, and dynamic range, their proper use is important.

The effects of bending on plasmonic modes in nanowires and planar structures.

In this work, we investigate the effects of bends on the surface plasmon resonances in nanowires (NWs) and isolated edges of planar structures using electron energy loss spectroscopy experiments and theoretical calculations. Previous work showed that the sharp bends in NWs do not affect their resonant modes. Here, we study previously overlooked effects and analyze systematically the evolution of resonant modes for several bending angles from 30° to 180°, showing that bending can have a significant effect on the plasmonic response of a nanostructure. In NWs, the modes can experience significant energy shifts that depend on the aspect ratio of the NW and can cause mode intersection and antinode bunching. We establish the relation between NW modes and edge modes and show that bending can even induce antinode splitting in edge modes. This work demonstrates that bends in plasmonic planar nanostructures can have a profound effect on their optical response and this must be accounted for in the design of optical devices.

Collective electronic excitations in charge density wave systems: The case of CuTe

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative.

Bridging nano-optics and condensed matter formalisms in a unified description of inelastic scattering of relativistic electron beams

In the last decades, the blossoming of experimental breakthroughs in the domain of electron energy loss spectroscopy (EELS) has triggered a variety of theoretical developments. Those have to deal with completely different situations, from atomically resolved phonon mapping to electron circular dichroism passing by surface plasmon mapping. All of them rely on very different physical approximations and have not yet been reconciled, despite early attempts to do so. As an effort in that direction, we report on the development of a scalar relativistic quantum electrodynamic (QED) approach of the inelastic scattering of fast electrons. This theory can be adapted to describe all modern EELS experiments, and under the relevant approximations, can be reduced to most of the last EELS theories. In that aim, we present in this paper the state of the art and the basics of scalar relativistic QED relevant to the electron inelastic scattering. We then give a clear relation between the two once antagonist descriptions of the EELS, the retarded dyadic Green function, usually applied to describe photonic excitations and the quasi-static mixed dynamic form factor (MDFF), more adapted to describe core electronic excitations of material. Using the photon propagator in a material, expressed in the relevant gauges, as a tool to understand the interaction between a fast electron and a material, we extend this relation to a newly defined quantity, the relativistic MDFF. The relation between the dyadic Green function and the relativistic MDFF does depend only on the photon propagator and not on the specifics of the particle (here, a fast electron) probing the target. Therefore, it can be adapted to any spectroscopy where a relation between the electromagnetic and electronic properties of a material is needed. We then use this theory to establish two important EELS-related equations. The first one relates the spatially resolved EELS to the imaginary part of the photon propagator and the incoming and outgoing electron beam wavefunction, synthesizing the most common theories developed for analyzing spatially resolved EELS experiments. The second one shows that the evolution of the electron beam density matrix is proportional to the mutual coherence tensor, proving that quite universally, the electromagnetic correlations in the target are imprinted in the coherence properties of the probing electron beam.

Electronic Structure Sensitivity to Surface Disorder and Nanometer-Scale Impurity of 2D Titanium Carbide MXene Sheets as Revealed by Electron Energy-Loss Spectroscopy

Two-dimensional (2D) carbides and/or nitrides (so-called MXenes) are among the latest and largest family of 2D materials. Due to their 2D nature and their unique properties of hydrophilicity, good metallic conductivity, and structural diversity, these materials are intensively studied for sensing applications or as supports for nanomaterials toward, e.g., plasmonics, catalytic, or energy storage applications. For these potential usages, the extent to which the electronic properties of MXene sheets are modified upon functionalization or intercalation is critical, and an optimized nondestructive probing of the interaction between MXene layers and functionalization is important to be determined. Here, these issues are addressed using a combination of first-principles simulations and electron energy loss spectroscopy (EELS) experiments performed at the nanoscale on Ti3C2Tx and Ti2CTx MXene multilayers, where T denotes the surface functionalization groups. Based on a detailed analysis of the carbon and surface group K edge fine structure, we show that the C-K edge is an ideal marker for surface-induced electronic structure modifications in the Tin+1Cn conducting core. These results highlight how a nanometer-scale impurity can very locally interact with a Ti3C2Tx multilayer and modify its electronic structure. This approach allows discrimination between the surface and core alteration of the Ti3C2Tx layers. Finally, the higher sensitivity to surface states of the Tin+1Cn conducting core in Ti2CTx as compared to Ti3C2Tx is discussed. We expect these results to offer an approach for understanding MXenes’ behavior and especially characterize their interactions with other nanomaterials when used in composites.

Interface-Induced Near-Infrared Response of Gold-Silica Hybrid Nanoparticles Antennas.

We proposed an IR absorber hybrid nanoantenna comprise of two overlapping gold nanoparticles residing over larger a silica nanoparticle. A wet chemical route was employed to prepare the hybrid structure of nanoantenna. High-resolution transmission electron microscope was used to measure the size and morphology of the nanoantenna. The Hybrid nanoantenna was excited by electron beam to investigate the optical response over a large wavelength range using Electron Energy Loss Spectroscopy. The beam of the electron was focused and we measured the electron energy loss spectra at different point of interest, which confirmed the of Low Energy Surface Plasmon Politron resonances in the IR region. The optical response of the nanoantenna was simulated numerically by employing Electric Hertzian dipole using finite element method with frequency domain solver in CST Microwave Studio. We used the Electric Hertzian dipole approach for the first time to model the Electron Energy Loss Spectroscopy experiment. The Electron Energy Loss Spectroscopy experimental results with their numerically simulated values confirmed the plasmonic resonance at the interface of the two overlapped gold nanoparticles.

Chemical and bonding analysis of liquids using liquid cell electron microscopy

Abstract

Dynamical charge susceptibility in the Hubbard model

We compute the dynamical charge susceptibility in the two-dimensional Hubbard model within the dynamical cluster approximation. In order to understand the connection between charge susceptibility and pseudogap, we investigate the momentum, doping, and temperature dependence. We find that as a function of frequency, the dynamical charge susceptibility is well represented by a single peak at a characteristic frequency. It shows little momentum or temperature dependence, while the doping dependence is more evident, and no clear signature of the pseudogap is observed. Data for the doping evolution of the static susceptibility and for fluctuation diagnostics are presented. Our susceptibilities should be directly measurable in future Momentum-resolved electron energy-loss spectroscopy experiments.

Water-induced hydrogenation of graphene/metal interfaces at room temperature: Insights on water intercalation and identification of sites for water splitting

Though it is well recognized that the space between graphene cover and the metal substrate can act as a two-dimensional (2D) nanoreactor, several issues are still unresolved, including the role of the metal substrate, the mechanisms ruling water intercalation and the identification of sites at which water is decomposed. Here, we solve these issues by means of density functional theory and high-resolution electron energy loss spectroscopy experiments carried out on graphene grown on (111)-oriented Cu foils. Specifically, we observe decomposition of H2O at room temperature with only H atoms forming bonds with graphene and with buried OH groups underneath the graphene cover. Our theoretical model discloses physicochemical mechanisms ruling the migration and decomposition of water on graphene/Cu. We discover that the edge of graphene can be easily saturated by H through decomposition of H2O, which allows H2O to migrate in the subsurface region from the decoupled edge, where H2O decomposes at room temperature. Hydrogen atoms produced by the decomposition of H2O initially form a chemical bond with graphene for the lower energy barrier compared with other routes. These findings are essential to exploit graphene/Cu interfaces in catalysis and in energy-related applications.

Directing Gold Nanoparticles into Free-Standing Honeycomb-Like Ordered Mesoporous Superstructures.

2D mesoporous materials fabricated via the assembly of nanoparticles (NPs) not only possess the unique properties of nanoscale building blocks but also manifest additional collective properties due to the interactions between NPs. In this work, reported is a facile and designable way to prepare free-standing 2D mesoporous gold (Au) superstructures with a honeycomb-like configuration. During the fabrication process, Au NPs with an average diameter of 5.0 nm are assembled into a superlattice film on a diethylene glycol substrate. Then, a subsequent thermal treatment at 180 °C induces NP attachment, forming the honeycomb-like ordered mesoporous Au superstructures. Each individual NP connects with three neighboring NPs in the adjacent layer to form a tetrahedron-based framework. Mesopores confined in the superstructure have a uniform size of 3.5 nm and are arranged in an ordered hexagonal array. The metallic bonding between Au NPs increases the structural stability of architected superstructures, allowing them to be easily transferred to various substrates. In addition, electron energy-loss spectroscopy experiments and 3D finite-difference time-domain simulations reveal that electric field enhancement occurs at the confined mesopores when the superstructures are excited by light, showing their potential in nano-plasmonic applications.

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.

Electron energy loss and X-ray absorption behaviour of high density nonmagnetic cobalt

We have studied the absorption behaviour at the L-edge of the recently discovered high density nonmagnetic (HDNM) cobalt in a polycrystalline cobalt thin film. Soft X-ray absorption spectroscopy (SXAS) experiment has shown a higher edge jump energy for HDNM cobalt compared to normal ferromagnetic cobalt. Electron energy loss spectroscopy (EELS) measurement has shown a lower absorption cross-section for HDNM cobalt compared to normal cobalt. As the thin film contained layers of HDNM Co and normal Co at different depths (Sci. Rep. 7, 41856 (2017)) a depth selective SXAS experiment was performed. In SXAS experiment the thin film sample was sputtered in order to obtain absorption spectra from different depths. We clearly observe a shift of the L-III edge energy to lower energy as soon as the high density layer is removed and normal Co layer is exposed to the X-ray beam. EELS experiments were carried out on high density grains, identified by transmission electron microscopy and Co fluorescence imaging. The results were compared with those from lower density or normal density grains. The energy loss peaks are less intense for high density cobalt.

Electron energy loss spectroscopy simulation by a frequency domain surface integral equation solver

Plasmonic nanoparticles have been mostly studied using conventional light sources. Recently, electron energy loss spectroscopy (EELS) has started to be used to analyze plasmonic nanoparticles where incident electromagnetic fields are created by swift electrons. To accurately simulate EELS experiments, several numerical methods have been adapted. In this paper, a frequency domain surface integral equation (FDSIE) solver is modified to simulate EELS for plasmonic nanoparticles of gold and silver. Accuracy and versatility of the proposed FDSIE solver are shown by several numerical examples and compared to existing numerical, analytical, and experimental results.

Optoelectronic properties of calcium cobalt oxide misfit nanotubes

We report on the optoelectronic properties of a recently discovered nanotubular phase of misfit-layered calcium cobalt oxide, CaCoO2–CoO2. Individual nanotubes are investigated by spatially resolved electron energy-loss spectroscopy experiments performed in a transmission electron microscope, and complementary first-principles, time-dependent hybrid density-functional theory calculations are performed to elucidate the electronic structure and optical spectra. We find that the band gap is independent of the geometry of the nanotubes, and experimental and calculated results independently confirm an optical gap of 1.9–2.1 eV for the CaCoO2–CoO2 nanotubes. The time-dependent hybrid density-functional theory calculations also suggest the existence of strongly bound intralayer excitons (up to 0.5 eV binding energy), which could allow for optoelectronic applications of these nanotubes at near-infrared to visible (∼1.5–2 eV) wavelengths.

Magnesium Nanoparticle Plasmonics.

Nanoparticles of some metals (Cu/Ag/Au) sustain oscillations of their electron cloud called localized surface plasmon resonances (LSPRs). These resonances can occur at optical frequencies and be driven by light, generating enhanced electric fields and spectacular photon scattering. However, current plasmonic metals are rare, expensive, and have a limited resonant frequency range. Recently, much attention has been focused on earth-abundant Al, but Al nanoparticles cannot resonate in the IR. The earth-abundant Mg nanoparticles reported here surmount this limitation. A colloidal synthesis forms hexagonal nanoplates, reflecting Mg's simple hexagonal lattice. The NPs form a thin self-limiting oxide layer that renders them stable suspended in 2-propanol solution for months and dry in air for at least two week. They sustain LSPRs observable in the far-field by optical scattering spectroscopy. Electron energy loss spectroscopy experiments and simulations reveal multiple size-dependent resonances with energies across the UV, visible, and IR. The symmetry of the modes and their interaction with the underlying substrate are studied using numerical methods. Colloidally synthesized Mg thus offers a route to inexpensive, stable nanoparticles with novel shapes and resonances spanning the entire UV-vis-NIR spectrum, making them a flexible addition to the nanoplasmonics toolbox.

A carbonate-free, sulfone-based electrolyte for high-voltage Li-ion batteries

Abstract Practical implementation of next-generation Li-ion battery chemistries is to a large extent obstructed by the absence of an electrolyte that is capable of simultaneously supporting reversible electrochemical reactions at two extreme electrochemical potentials—above 4.5 V at the positive electrode and near 0 V vs. Li at the negative electrode. Electrolytes based on carbonate esters have been reliable in satisfying state-of-the-art Li-ion battery (LIB) chemistries below 4.4 V), releasing CO2. In this work, we explore a carbonate-free electrolyte system based on a single sulfone solvent, in which a newly discovered synergy between solvent and salt simultaneously addresses the interfacial requirements of both graphitic anode and high-voltage cathode (LiNi0.5Mn1.5O4 (LNMO)). Experimental measurements, quantum chemistry (QC) calculations, and molecular dynamics simulations reveal the system’s fast ion conduction, stability over a wide temperature range, and non-flammability. At the anode, a LiF-rich interphase generated by early-onset reduction of the salt anion effectively suppresses solvent co-intercalation and subsequent graphite exfoliation, enabling unprecedented and highly reversible graphite cycling in a pure sulfone system. Under oxidative conditions, QC calculations predict that high salt concentration promotes complex/aggregate formation which slow the decomposition of sulfolane and leads to polymerizable rather than gaseous products—a fundamental improvement over carbonate solvents. These predictions are corroborated by X-ray photoelectron spectroscopy (XPS), cryogenic-transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS) experiments, which revealed the presence of a thin, conformal, sulfur-based cathode electrolyte interphase (CEI). Together, the functional interphases (SEI/CEI) generated by this electrolyte system supported long term operation of a high-voltage (4.85 V) LNMO/graphite full cell, which retained ∼70% of its original first-cycle discharge capacity after the 1000th cycle. Based on these results, this new carbonate-free electrolyte system, supported by the mechanistic understanding of its behavior, presents a promising new direction toward unlocking the potential of next generation Li-ion battery electrodes.