EELS Spectra Research Articles

Exceptionally tunable electronic, optical and transport properties of two dimensional GaS doped with group II and group IVa elements

In this present study, we systematically investigated the structural, electronic, optical and transport properties of pristine and group II, group IVa doped GaS monolayers using density functional theory (DFT). The strong formation energy suggests realization of group II & group IVa doped GaS. A semiconductor to metallic transition occurs in GaS monolayer with doping. Moreover, doped GaS monolayers have shown tunable optical properties. Doped GaS monolayers show optical activity in both ultraviolet (UV) as well as visible region. EEL spectra of GaS shift towards high energy region (red shift) with group II elements doping while no significant shift is observed for group IVa elements doping in both the polarizations. We found quantum conductance of 4G0 for doped GaS monolayer except for Be-doped GaS. The metallic character of doped GaS is clearly captured in tunneling current characteristics showing ohmic-like characteristics for doped GaS monolayer. This is due to finite density of states associated with S atoms of doped GaS. The doping extends the functionalities of pristine GaS with tunable properties suggesting doped GaS as a potential candidate for use in photo-diodes, photo-catalysts, photo-detectors and bio-sensing.

Catalysts for control of long chain branching

We present a multi-region extension of power law background subtraction for core-level EEL spectra to improve the robustness of background removal. This method takes advantage of the post-edge shape of core-loss EEL edges to enable simultaneous fitting of pre- and post-edge background regions. This method also produces simultaneous and consistent background removal from multiple edges in a single EEL spectrum. The stability of this method with respect to the fitting energy window and the EELS signal to noise ratio is also discussed.

Use of turbidimeter for measurement of solid catalyst system component in a reactor feed

We present a multi-region extension of power law background subtraction for core-level EEL spectra to improve the robustness of background removal. This method takes advantage of the post-edge shape of core-loss EEL edges to enable simultaneous fitting of pre- and post-edge background regions. This method also produces simultaneous and consistent background removal from multiple edges in a single EEL spectrum. The stability of this method with respect to the fitting energy window and the EELS signal to noise ratio is also discussed.

Investigation of the structure and chemical nature of Pd fission product agglomerations in irradiated TRISO particle SiC

Tristructural-isotropic (TRISO)-coated fuel particles are used in high-temperature gas-cooled nuclear reactors. Although the polycrystalline 3C–SiC layer acts as the main barrier to fission product release, post-irradiation examinations have shown that certain metallic fission products are found on the outside of the TRISO coated particle, with no observable evidence of micro-cracks or mechanical failure. In this study, an atomic resolution transmission electron microscopy investigation of a SiC layer from a neutron irradiated (19.38% fissions per initial metal atom average burnup; 1072 °C time-averaged temperature) TRISO-coated particle from the first fuel irradiation experiment as part of the US DOE Advanced Gas Reactor Fuel Qualification and Development (AGR) Program, was conducted. The fission product Pd was found to be present at dislocation cores associated with twins, stacking faults, and their intersections as atomic scale silicide inclusions. EELS elemental quantification of the silicide region showed a reduction of C and increase of Si in the Pd containing inclusion. Pd was also observed as distributed along grain boundaries down to the atomic scale. The local corrosion and dissociation of C from Pd containing grain boundary agglomerations was observed at the atomic level. Elemental quantification of atomically dispersed Pd grain boundary regions using EELS spectrum imaging similarly showed a measurable decrease in C concentration at Pd regions and an increase in Si concentration.

Electron energy loss spectroscopy of thin slabs with supercell calculations

Electron energy loss spectroscopy in the low-loss regime is widely used to access to the screening of the Coulomb potential as a function of the momentum transfer. This screening is strongly reduced for low-dimensional materials and this spectroscopy is a technique of choice to study the resulting quantum confinement. Time-dependent density-functional theory within an ab initio formalism is particularly suited to simulate angular-resolved electron energy loss spectra, taking benefit from the reciprocal space description. For an isolated object, the standard procedure based on the supercell approach dramatically fails for the out-of-plane optical response of the surface and we have proposed a scheme called Selected-$G$ [N. Tancogne-Dejean, C. Giorgetti, and V. V\'eniard, Phys. Rev. B 92, 245308 (2015)], leading to a slab potential. In this paper, we show that the standard procedure also affects the in-plane components of the EEL spectra. Applying the Selected-$G$ procedure, we show that the full expression of the slab potential is crucial to describe slabs of finite thickness. We compare our formalism to other cutoff procedures, and show that if they provide spectra with the correct spectral weight, allowing the good description of plasmon dispersion, the amplitude of the peaks depends on the choice of the supercell. Our results, which provide spectra independent of vacuum, will have a strong impact on the calculation of properties such as quasiparticle corrections.

Absolute vibrational and electronic cross sections for low-energy electron scattering from condensed Thymidine

SynopsisAbsolute vibrational and electronic excitation cross sections (CSs) for low-energy electrons (1-18 eV) scattering from condensed thymidine (dT) are measured with High-Resolution Electron Energy Loss Spectros-copy (HREELS). EEL spectra are recorded and the vibrational and electronic modes of dT identified. Absolute CSs for these modes are extracted from each spectrum by a fitting procedure. Resonances are observed in the incident energy dependence of the CSs, below 3 eV and near 4, 8 and 10 eV. The CSs are compared to those of its constituents: thymine and tetrahydrofuran.

Simultaneous multi-region background subtraction for core-level EEL spectra

We present a multi-region extension of power law background subtraction for core-level EEL spectra to improve the robustness of background removal. This method takes advantage of the post-edge shape of core-loss EEL edges to enable simultaneous fitting of pre- and post-edge background regions. This method also produces simultaneous and consistent background removal from multiple edges in a single EEL spectrum. The stability of this method with respect to the fitting energy window and the EELS signal to noise ratio is also discussed.

Analysis of XPS and REELS spectra of beryllium

The differential inverse inelastic mean free paths (DIIMFP) of beryllium were derived from energy spectra acquired using X-ray photoelectron spectroscopy and electron energy loss spectroscopy techniques by means of the fitting of calculated spectra to experimental data. The calculation of the energy spectra is performed employing the partial intensity approach (PIA). The EELS and XPS spectra were acquired in different laboratories using different Be samples. The comparison of the obtained DIIMFPs with literature data is presented.

Extremely low count detection for EELS spectrum imaging by reducing CCD read-out noise

Extremely low count detection for EELS spectrum imaging is required to overcome problems with electron irradiation and widen the range of available applications. We have made a systematic statistical study of the reduction of CCD noise for EELS. We propose a calculation method to estimate the properties of noise and a procedure to reduce it. Since the dominant noise is a practically random component, it can be reduced by subtracting the population mean of the dark reference and a summation over an appropriate number of spectra, depending on the standard deviation of the noise. A gain-averaging method can further improve the signal-to-noise (SN) ratio. It is thereby demonstrated that a high-SN spectrum can be obtained even for a single-count core-loss signal. The present method would be useful for measuring low signal spectrum such as monochromated spectra and for radiation sensitive materials.

Ultra-thin h-BN substrates for nanoscale plasmon spectroscopy

Probing plasmonic properties of surface deposited nanoparticles with high spatial resolution requires the use of a low absorption support. In this work, ultra-thin hexagonal boron nitride (h-BN) flakes are employed as substrates for scanning transmission electron microscopy. The thicknesses of only a few atomic layers, the flat surface, and the large bandgap provide a unique set of properties, which makes h-BN ideally suitable for high resolution plasmon spectroscopy by means of electron energy loss spectroscopy (EELS), especially for small nanoparticles. A facile fabrication process allows the production of h-BN substrates with a thickness of only a few atomic layers. The advantages of h-BN, especially for the low-loss energy region of EEL spectra, are shown in a direct comparison with a silicon nitride substrate. Furthermore, results of the investigation of localized surface plasmon resonances (LSPRs) of Ag and Ag–Au core–shell nanoparticles in the sub-20 nm size regime are presented, confirming the advantages of the fabricated substrate for LSPR mapping. The plasmonic nanoparticles were assembled utilizing the helium nanodroplet synthesis approach, which allows for a very soft deposition and the preservation of the integrity of the ultra-thin substrate. Moreover, it provides a completely solvent and surfactant free environment for the assembly of tailored nanoparticles.

Vibrational electron energy loss spectroscopy in truncated dielectric slabs

Specially designed instrumentation for electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope makes it possible to probe very low-loss excitations in matter with a focused electron beam. Here we study the nanoscale interaction of fast electrons with optical phonon modes in silica. In particular, we analyze the spatial dependence of EEL spectra in two geometrical arrangements: a free-standing truncated slab of silica and a slab with a junction between silica and silicon. In both cases, we identify different loss channels, involving polaritonic and nonpolaritonic contributions to the total electron energy loss, and we obtain the corresponding energy-filtered maps. Furthermore, we present a comparison of the theoretical simulations for a silica-silicon junction with experimental results, and we discuss the spatial resolution attainable from the energy-filtered map considering optical phonon excitations in a conventional experimental arrangement.

Copper Oxidation Effect in the EMC/Cu Interfacial Adhesion Improvement for a Novel Copper Interconnection Substrate Application

Abstract Copper oxidation structure, cupric oxide (CuO) and cuprous oxide (Cu2O), under Ar/H2 plasma reaction mechanism for the EMC/Cu interface adhesion improvement was studied in this work. This work is utilized TGA to figure out Cu oxidized state and sample preparation, and using plasma treatment Cu oxidation layer to evaluate the EMC/Cu interface adhesion strength by shear testing method. Results show a plasma reduction on Cu oxidation layer provide a better interface adhesion, and the layer structure has a significant composition change, Cu/Cu2O/CuO/EMC → Cu/Cu2O/CuO/Cu2O/EMC. These layer structures were identified by high-resolution TEM mapping with EELS spectrum fitting, it was also verified for CuO reduction to form Cu2O, following the Cu2O hydration would provide much hydrogen-bonding in the EMC/Cu interface. This kind of chain reaction mechanism including CuO reduction and Cu2O hydration was described by 2CuO + H2 → Cu2O +H2O → 2CuOH (hydration molecular). The reaction mechanism of the EMC/Cu bonding has been investigated and verified in this experimental study and our conclusion is that hydrogen bonding on the Cu oxidation layer surface can strengthen the EMC/Cu interface adhesion.

Spectroscopic properties of a freestandingMnPS3single layer

Quasi-two-dimensional manganese thiophosphate, $\mathrm{MnP}{\mathrm{S}}_{3}$, is interesting due to its two-dimensional antiferromagnetic structure observed at low temperatures as well as possible technological applications. While the spectroscopic properties of bulk $\mathrm{MnP}{\mathrm{S}}_{3}$ structures have been extensively studied, the spectroscopic characteristics of freestanding $\mathrm{MnP}{\mathrm{S}}_{3}$ layers are yet to be explored. We present an experimental study on the spectroscopic properties of a freestanding $\mathrm{MnP}{\mathrm{S}}_{3}$ single layer obtained through exfoliation from bulk $\mathrm{MnP}{\mathrm{S}}_{3}$. We find that the position of the main peak in the electron-energy-loss spectrum (EELS) is shifted from approximately 19 eV in bulk $\mathrm{MnP}{\mathrm{S}}_{3}$ to 15 eV in single $\mathrm{MnP}{\mathrm{S}}_{3}$ layer. Theoretical calculations show that this peak corresponds to a volume plasmon in bulk $\mathrm{MnP}{\mathrm{S}}_{3}$ and a damped plasmon peak in single-layer $\mathrm{MnP}{\mathrm{S}}_{3}$. In addition, the dispersion of this peak was investigated using momentum-resolved electron-energy-loss spectroscopy. The peak dispersion for the single-layer $\mathrm{MnP}{\mathrm{S}}_{3}$ displays a square-root behavior characteristic of a two-dimensional plasmon. We show that EELS spectra provide both a means to identify single-layer $\mathrm{MnP}{\mathrm{S}}_{3}$ from bulk structures and also show the effects of low dimensionality on the electronic excitations.

Novel synthesis and properties of hydrogen-free detonation nanodiamond

Practically hydrogen-free nanodiamond (HFND), a prospective neutron-optical material, was prepared (in up to 3% yield) by detonating pure or graphite-doped RDX in an ice shell and comprehensively characterized by elemental analysis, X-ray diffraction, TEM, SAED, EELS, Raman and IR spectra, ESR, DSC/TGA, and specific-surface (BET) determination. The primary diamond grains are larger (15–16 nm) than in ordinary nanodiamond from RDX/TNT blends (ca. 5 nm) but smaller than in HFND from benzotrifuroxan (27 nm), with electron spins distributed in volume. HFND is much less hygroscopic and more prone to oxidation than ordinary nanodiamond. When heated in air up to 400°С, HFND shown a drastic increase of oxygen/carbon ratio without an apparent loss of mass. The mechanism of hydrogen non-contamination (through segregation of detonation gases) and particle growth is discussed.

The local structure in heavily boron-doped diamond and the effect this has on its electrochemical properties

Transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS), and first principles calculations of EEL spectra were utilized to elucidate the relationship between the microscopic structure and the electrochemical properties of heavily boron-doped diamond (h-BDD). The electrochemical properties of h-BDD containing 1 at.% and 3 at.% boron are very different. TEM observations showed that 1 at.% h-BDD consists of small densely packed diamond crystallites, while 3 at.% h-BDD contains small voids and a graphite phase partly along the grain boundaries. The EEL spectrum of the grain interior in 1 at.% h-BDD and comparison of this with a theoretical spectrum shows that the boron atoms are mostly dispersed as single isolated substitutional atoms on diamond lattice sites in the grain interior and that only a small amount of sp2-bonded carbon is present. In contrast, in the grain interior of 3 at.% h-BDD, the boron atoms are mostly associated with nearest neighbor boron pairs, and consequently sp2-bonded carbon is formed. Thus, the local structure has a significant effect on the amount of sp2-bonded carbon. The quite different electrochemical properties of the samples are ascribed to the amount of sp2-bonding arising from the different local structures.

Analysis of Ti valence states in resistive switching regions of a rutile TiO2−x four-terminal memristive device

We have performed Ti valence state analysis of our four-terminal rutile TiO2−x single-crystal memristors using scanning transmission electron microscopy–electron energy loss spectroscopy (STEM–EELS). Analysis of Ti-L2,3 edge EELS spectra revealed that the electrocolored region formed by the application of voltage includes a valence state reflecting highly reduced TiO2−x due to the accumulation of oxygen vacancies. Such a valence state mainly exists within ∼50 nm from the crystal surface and extends along specific crystal directions. These electrically reduced surface layers are considered to directly contribute to the resistive switching (RS) in the four-terminal device. The present results add new insights into the microscopic mechanisms of the RS phenomena and should contribute to further development and improvements of TiO2−x based memristive devices.

DFT calculations of graphene monolayer in presence of Fe dopant and vacancy

In the present work, the effects of Fe doping and vacancies on the electronic, magnetic and optical properties of graphene are studied by density functional theory based calculations. The conductive behavior is revealed for the various defected graphene by means of electronic density of states. However, defected structures show different magnetic and optical properties compared to those of pure one. The ferromagnetic phase is the most probable phase by substituting Fe atoms and vacancies at AA sublattice of graphene. The optical properties of impure graphene differ from pure graphene under illumination with parallel polarization of electric field, whereas for perpendicular polarization it remains unchanged. In presence of defect and under parallel polarization of light, the static dielectric constant rises strongly and the maximum peak of Im ε(ω) shows red shift relative to pure graphene. Moreover, the maximum absorption peak gets broaden in the visible to infrared region at the same condition and the magnitude and related energy of peaks shift to higher value in the EELS spectra. Furthermore, the results show that the maximum values of refractive index and reflectivity spectra increase rapidly and represent the red and blue shifts; respectively. Generally; substituting the C atom with Fe has more effect on magnetic and optical properties relative to the C vacancies.

Surface-Enhanced Molecular Electron Energy Loss Spectroscopy.

Electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) is becoming an important technique in spatially resolved spectral characterization of optical and vibrational properties of matter at the nanoscale. EELS has played a significant role in understanding localized polaritonic excitations in nanoantennas and also allows for studying molecular excitations in nanoconfined samples. Here we theoretically describe the interaction of a localized electron beam with molecule-covered polaritonic nanoantennas, and propose the concept of surface-enhanced molecular EELS exploiting the electromagnetic coupling between the nanoantenna and the molecular sample. Particularly, we study plasmonic and infrared phononic antennas covered by molecular layers, exhibiting either an excitonic or vibrational response. We demonstrate that EEL spectra of these molecule-antenna coupled systems exhibit Fano-like or strong coupling features, similar to the ones observed in far-field optical and infrared spectroscopy. EELS offers the advantage to acquire spectral information with nanoscale spatial resolution, and importantly, to control the antenna-molecule coupling on demand. Considering ongoing instrumental developments, EELS in STEM shows the potential to become a powerful tool for fundamental studies of molecules that are naturally or intentionally located on nanostructures supporting localized plasmon or phonon polaritons. Surface-enhanced EELS might also enable STEM-EELS applications such as remote- and thus damage-free-sensing of the excitonic and vibrational response of molecules, quantum dots, or 2D materials.

Imaging and electron energy-loss spectroscopy using single nanosecond electron pulses

We implement a parametric study with single electron pulses having a 7 ns duration to find the optimal conditions for imaging, diffraction, and electron energy-loss spectroscopy (EELS) in the single-shot approach. Photoelectron pulses are generated by illuminating a flat tantalum cathode with 213 nm nanosecond laser pulses in a 200 kV transmission electron microscope (TEM) with thermionic gun and Wehnelt electrode. For the first time, an EEL spectrometer is used to measure the energy distribution of single nanosecond electron pulses which is crucial for understanding the ideal imaging conditions of the single-shot approach. By varying the laser power, the Wehnelt bias, and the condenser lens settings, the optimum TEM operation conditions for the single-shot approach are revealed. Due to space charge and the Boersch effect, the energy width of the pulses under maximized emission conditions is far too high for imaging or spectroscopy. However, by using the Wehnelt electrode as an energy filter, the energy width of the pulses can be reduced to 2 eV, though at the expense of intensity. The first EEL spectra taken with nanosecond electron pulses are shown in this study. With 7 ns pulses, an image resolution of 25 nm is attained. It is shown how the spherical and chromatic aberrations of the objective lens as well as shot noise limit the resolution. We summarize by giving perspectives for improving the single-shot time-resolved approach by using aberration correction.

Probing plasmon resonances of individual aluminum nanoparticles

The plasmon resonances of individual aluminum nanoparticles are investigated by electron energy-loss spectroscopy (EELS) in scanning transmission electron microscope (STEM). Surface plasmon mode and bulk plasmon mode of Al nanoparticles are clearly characterized in the EEL spectra. Discrete dipole approximation (DDA) calculations show that as the particle diameter increases from 20 nm to 100 nm, the plasmon resonance shifts to lower energy and higher mode of surface plasmon arises when the diameter reaches 60 nm and larger.