Reflection Electron Energy Loss Spectroscopy Spectra Research Articles

Determination of the energy loss function of tungsten from reflection electron energy loss spectroscopy spectra

We incorporate experimental reflection electron energy loss spectroscopy (REELS) spectrum data with theoretical analysis to precisely determine the energy loss function (ELF) of tungsten (W) in the energy loss range of 0.1–110 eV at the incident electron energies of 1 keV, 2 keV and 3 keV. Employing the reverse Monte Carlo (RMC) method, we have obtained an averaged ELF whose relative errors of the perfect-screening sum rule and oscillator-strength sum rule were ∼ 0.70 % and 0.68 %, respectively. This ELF was then used to derive the optical constants and complex dielectric function. Furthermore, we have successfully differentiated between bulk and surface contributions to the REELS spectra throughout the entire considered energy loss range.

Effects of Oxygen Partial Pressure on the Electronic Properties and Chemical State of MgO/SiO<sub>2</sub>/Si(100) Thin Films

Magnesium Oxide (MgO) thin films were deposited on SiO2/Si substrate by electron beam evaporation. The properties of MgO thin film with and without oxygen partial pressure have been investigated by X-ray photoelectron spectroscopy (XPS), Reflection Electron Energy Loss Spectroscopy (REELS), and Ultra-Violet Photoemission Spectroscopy (UPS). The XPS was used to investigate the chemical state of the film. REELS spectra revealed that MgO thin films deposited under oxygen partial pressure had band gaps of 6.07 eV. Meanwhile, the band gap for MgO thin films grown without oxygen partial pressure was 7.17 eV. The UPS results showed that the work functions of MgO thin film with and without oxygen partial pressure are 4.75 and 4.84 eV, respectively. In the MgO thin film with oxygen partial pressure, the intensity for the valence band peak at 12.16 eV decreased, but the work function remained relatively the same. Our results demonstrated that the oxygen partial pressure played a crucial role in improving the electronic properties of MgO thin films.

A Comparative Study of Un-Doped ZnO and in Doping ZnO Thin Films with Various Concentrations, Subjected to Appropriate UHV Treatment and Characterized by Sensitive Spectroscopy Techniques XPS, AES, Reels and PL

Abstract In this study, we use complementary and sensitive experimental techniques XPS (X-rays Photoelectron Spectroscopy), AES (Auger Electron Spectroscopy, REELS (Reflection Electron Energy-Loss Spectroscopy) and PL (photoluminescence) to investigate and compare the chemical, structure, electronic and optical properties of Un-doped ZnO (UZO) and Indium-doped ZnO (IZO) (4% In; 6% In) thin films. Spray method is used for the growth of these thin films on Si substrate. A treatment process UHV (Ultra-High -Vacuum: Ar+ sputtering followed by checked successive heating until 650°C) is performed. XPS and AES results allow to confirm the clean state of samples and the incorporation of indium into the ZnO matrix to form chemical species of (In-O-Zn) type. The recorded REELS spectra at different primary energies and the PL measurements justify that the UHV treatment plays an important role to improve the physical structure of IZO (6% In).

The more accurate determination of surface excitation parameters, the more accurate quantitative surface analysis

The accurate quantitative surface analysis for the study of nanostructures practically used for e.g. photocatalysis, electrocatalysis, energy storage and sensing applications is of high importance. Moreover, the accurate quantitative electron spectroscopy and analytical microscopy of semiconducting nanomaterials such as nano-photocatalysts relies on the accurate determination of surface excitation parameters (SEP). Reflection electron energy loss spectroscopy (REELS) as a surface sensitive technique could be applied for the determination of SEP. The chemical, mechanical, thermal stability, optical and dielectric properties of Ni6MnO8 as binary metal oxide makes such structure suitable for practical applications. Therefore, here Ni6MnO8 due to its great importance in its special applications was typically studied by REELS using the Yubero-Tougaard algorithm, which is based on dielectric response theory. After an extrapolation process applied to the obtained REELS spectrum, the energy gap (Eg) of this material was obtained to be 3 eV. The surface and bulk energy loss functions of the material were also determined. The values of inelastic mean free path (IMFP) of the electrons of different energies in the range of 500–4000 electron volts travelled in Ni6MnO8 were determined. In addition, from the obtained energy loss function (ELF), by using the Kramers-Kronig transformations of the real and imaginary parts of dielectric function (ε), the important optical parameters of Ni6MnO8 such as refractive index (n), attenuation coefficient (k) and absorption coefficient (μ) were determined. Finally, as a critical parameter in quantitative electron spectroscopy, the SEP and its angular distribution were determined. It was found that the Pauli-Tougaard model requires a modification to make it better descriptive to well fit the SEP data for a given material. Therefore, a more accurate model was obtained to make a better surface analysis and analytical microscopy.

Evidence of sp2-like Hybridization of Silicon Valence Orbitals in Thin and Thick Si Grown on α-Phase Si(111)√3 × √3R30°-Bi.

One-monolayer (ML) (thin) and 5-ML (thick) Si films were grown on the α-phase Si(111)√3 × √3R30°-Bi at a low substrate temperature of 200 °C. Si films have been studied in situ by reflection electron energy loss spectroscopy (REELS) and Auger electron spectroscopy, as a function of the electron beam incidence angle α and low-energy electron diffraction (LEED), as well as ex situ by grazing incidence X-ray diffraction (GIXRD). Scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS) were also reported. The REELS spectra, taken at the Si K absorption edge (~1.840 KeV), reveal the presence of two distinct loss structures attributed to transitions 1s→π* and 1s→σ* according to their intensity dependence on α, attesting to the sp2-like hybridization of the silicon valence orbitals in both thin and thick Si films. The synthesis of a silicon allotrope on the α-phase of Si(111)√3 × √3R30°-Bi substrate was demonstrated by LEED patterns and GIXRD that discloses the presence of a Si stack of 3.099 (3) Å and a √3 × √3 unit cell of 6.474 Å, typically seen for multilayer silicene. STM and STS measurements corroborated the findings. These measurements provided a platform for the new √3 × √3R30° Si allotrope on a Si(111)√3 × √3 R30°-Bi template, paving the way for realizing topological insulator heterostructures from different two-dimensional materials, Bi and Si.

Optical constants of organic insulators in the UV range extracted from reflection electron energy loss spectra

Reflection electron energy loss spectroscopy (REELS) spectra were measured for seven insulating organic compounds (DNA, Irganox 1010, Kapton, polyethylene [PE], poly(methyl methacrylate) [PMMA], polystyrene [PS] and polytetrafluoroethylene [PTFE]). Optical constants and energy band gaps were extracted from the measured REELS spectra after elimination of multiple electron scattering via a deconvolution and fitting the normalised single scattering energy loss spectra to Drude and Drude–Lindhard model dielectric functions, constrained by the Kramers–Kronig sum and f‐sum rules. Satisfactory agreement is found for those optical constants for which literature data exists. For PTFE, the observed features in the optical data correspond to its electronic structure.

تعیین خواص دیالکتریک و نوری چارچوب زئولیتی ZIF-8 به وسیله طیف سنجی افت انرژی الکترون بازتابی (REELS)

Specific properties such as pore tunability, structural variety as well as chemical, mechanical, and thermal stability, make zeolitic imidazolate frameworks (ZIFs) suitable and of high importance for practical applications. Optical and chemical sensors and supercapacitors are among these applications which necessitate the detailed study of optical and dielectric properties of ZIFs. Therefore, ZIF-8 as a zinc-based mesoporous material was studied by reflection electron energy loss spectroscopy (REELS) applying the Yubero-Tougaard algorithm which is based on dielectric response theory. The band gap energy (Eg) of ZIF-8 was determined to be 4.2 eV using an extrapolation procedure applied to an experimental REELS spectrum. Its bulk and surface energy loss functions were also determined. Inelastic mean free path (IMFP) values of electrons of different energies transported in ZIF-8 were determined and their large differences with values calculated from the Tanuma-Powell-Penn (TPP) formula were discussed. In addition, the obtained ELF was used to apply the Kramers-Kronig transformation to obtain the real part (e1) and imaginary part (e2) of the dielectric function (e), refractive index (n), extinction coefficient (k), reflection coefficient (R) and absorption coefficient (µ) of ZIF-8 as important optical properties of this widely applicable material.

Elucidating the capability of electron backscattering for 3D nano-structure determination

Reflection electron energy loss spectroscopy (REELS) is well established for the study of homogeneous materials with flat surfaces. Here we extend the use of this technique to nano-structures consisting of silicon and silica and show that the experimentally-observed peculiar dependence of the REELS spectra on the sample orientation can be reproduced by Monte Carlo simulations using the known sample morphology. A sample with a 3D structure, resembling those found in FinFET transistors, was analyzed through electron Rutherford backscattering (ERBS, revealing the mass of the atoms near the surface) and REELS (revealing the electronic structure). ERBS/REELS spectra were taken at two incoming electron energies (5 and 40 keV) and in two experimental geometries with the component of the outgoing propagation direction along the surface being either parallel or perpendicular to the fins. The measured spectra were different for the two geometries due to attenuation effects within the fins, especially at 5 keV where the inelastic mean free path is of the order of the fin dimensions. This means that the 3D structure induces shadowing effects which suppress the elastic peaks and enhanced the inelastic signal. A Monte Carlo code was used to simulate multiple elastic and inelastic interactions of the electrons with these 3D structures and was indeed able to reproduce these experimental results, including the shadowing effects. A sub-angstrom layer of Au was evaporated on the sample and the changes induced by the Au layer were dependent on the orientation of the fins and were again reproduced by the simulation.

Monte Carlo simulation study of reflection electron energy loss spectroscopy of an Fe/Si overlayer sample

Reflection electron energy loss spectroscopy (REELS) has been used to study the optical and electronic properties of semi‐infinite solid samples, aided by a theoretical model of the interaction between electrons and a solid. However, REELS has not been used to its full capacity in studying nanomaterial samples because of the difficulty in modeling the electron interaction with a layered nanostructure. In this study, we present a numerical calculation result on the spatially varying inelastic mean free path for a sample comprising an Fe layer of varying thickness on an Si substrate. Furthermore, a Monte Carlo model for electron interaction with this Fe‐Si layered structure sample is built based on this inelastic scattering cross section and used to reproduce the REELS spectra of Fe‐Si layered structures. The simulated spectra of the sample with varying Fe layer thickness on top of a Si substrate were compared with the experimental spectra. This comparison clearly identifies that the Fe layer remaining on top of the experimental Si substrate after Ar+ beam sputtering is in the form of a homogeneous mixed layer, where the Fe/Si interface excitation is absent in the experimental spectra owing to pulverization of the Fe/Si interface during the Ar+ sputtering process.

Stopping powers and inelastic mean free path from quantitative analysis of experimental REELS spectra for electrons in Ti, Fe, Ni, and Pd

Stopping power (SP) and inelastic mean free path (IMFP) of electrons in Ti, Fe, Ni, and Pd have been determined by using dielectric models. We have used energy loss function (ELF) determined from quantitative analysis of experimental reflection electron energy loss spectroscopy (REELS) spectra as the input parameter for this model. ELF in this study was determined from the previously published quantitative analysis of REELS spectra. The SP of Fe, Ni, Pd, and Ti was compared with several calculation methods for energies from 100 eV to 10 keV and shows SP in this study, which are in best agreement for medium to high energies (greater than or equal to 300 eV). The IMFP obtained in this study shows the best agreement with online database TPP2M and NIST and also calculation by Tanuma with a root mean square (rms) less than 12%. The present approach shows ELF from quantitative analysis of REELS spectra has a high potential for the experimental determination of SP and IMFP of metals.

Surface Study of Fe3O4 Nanoparticles Functionalized With Biocompatible Adsorbed Molecules.

Surfaces of iron oxide of ferrimagnetic magnetite (Fe3O4) nanoparticles (MNPs) prepared by Massart's method and their functionalized form (f-MNPs) with succinic acid, L-arginine, oxalic acid, citric acid, and glutamic acid were studied by dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR-S), UV-vis, thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and reflection electron energy loss spectroscopy (REELS). The XPS analysis of elements and their chemical states at the surface of MNPs and f-MNPs revealed differences in chemical bonding of atoms, content of carbon–oxygen groups, iron oxide forms, iron oxide magnetic properties, adsorbed molecules, surface coverage, and overlayer thickness, whereas the Auger parameters (derived from XPS and Auger spectra) and elastic and inelastic scattering probabilities of electrons on atoms and valence band electrons (derived from REELS spectra) indicated modification of surface charge redistribution, electronic, and optical properties. These modified properties of f-MNPs influenced their biological properties. The surfaces biocompatible for L929 cells showed various cytotoxicity for HeLa cells (10.8–5.3% of cell death), the highest for MNPs functionalized with oxalic acid. The samples exhibiting the largest efficiency possessed smaller surface coverage and thickness of adsorbed molecules layers, the highest content of oxygen and carbon–oxygen functionalizing groups, the highest ratio of lattice O2− and OH− to C sp2 hybridizations on MNP surface, the highest ratio of adsorbed O− and OH− to C sp2 hybridizations on adsorbed molecule layers, the closest electronic and optical properties to Fe3O4, and the lowest degree of admolecule polymerization. This high cytotoxicity was attributed to interaction of cells with a surface, where increased content of oxygen groups, adsorbed O−, and OH− may play the role of additional adsorption and catalytic sites and a large content of adsorbed molecule layers of carboxylic groups facilitating Fenton reaction kinetics leading to cell damage.

The influence of shallow core levels on the shape of REELS spectra

It is well established that the shape of spectra acquired using reflection electron energy loss spectroscopy (REELS) is determined by the dielectric function. Extracting the dielectric function from REELS spectra is a real challenge as it is governed by strong multiple scattering. It is generally assumed that the contribution of shallow core levels (with binding energies over 100 eV) to the REELS data can be neglected when one interprets valence band REELS data that usually extends to, at most, 100 eV energy loss. Here we show that, especially when incoming energies over 1 keV are used, this is not the case and that the intensity of the REELS spectrum can only be calculated correctly if the shallow core levels are taken into account. The implications for this for the extraction of the dielectric function from REELS data is discussed.

Lithium analysis in Li‐ion battery materials via a scanning electron microscopy‐based approach

We performed Li analysis by reflection electron energy loss spectroscopy (REELS) with a scanning electron microscopy‐based apparatus. It was possible to distinguish between Li compounds containing different transition metal elements spatially, via the spectrum imaging scheme of REELS spectra. We also acquired the Li spectrum for an Li‐inserted graphite negative electrode. REELS measurements with hemispherical analyzer were performed to obtain high‐quality spectra with sufficient energy resolution and compared with those from cylindrical mirror analyzer measurements; the former provided a more detailed chemical state evaluation of Li.

Absolute determination of optical constants of three transition metals using reflection electron energy loss spectroscopy

The optical constants, n and k, of three transition metals (Cr, Co, and Pd) were determined from the measured reflection electron energy-loss spectroscopy (REELS) spectra, covering the spectral energy range from visible to vacuum ultraviolet. To do this, a spectral data analysis technique [Xu et al., Phys. Rev. B 95, 195417 (2017)], which combines a sophisticated Monte Carlo simulation for modelling the experimental REELS spectrum and the simulated annealing algorithm for the determination of the true energy loss function (ELF) was adopted. The validity of the obtained ELFs was discussed by comparing with the previous data derived by optical methods and by applying the oscillator strength and the perfect screening-sum rules. Besides, the consistency of the calculated data was evaluated for three in situ measurements for each sample at three primary energies. The complex dielectric function, the refractive index n and the extinction coefficient k were then derived from the obtained ELF via the analytical Kramers-Kronig relation.

Lifshitz analysis of dispersion forces based on quantitative reflection electron energy loss spectroscopy

HypothesisThe energy loss experienced by an electron while moving through a solid is determined by the optical properties of the surrounding. Hence, quantitative Reflection Electron Energy Loss Spectroscopy (REELS) should allow the determination of optical data required for the calculation of Hamaker coefficients using Lifshitz theory. This approach might improve the accuracy of calculated Hamaker coefficients and should also enable to harness the unique capabilities of REELS to analyse nanostructured surfaces and thin-films with great spatial resolution and surface sensitivity. ExperimentsREELS spectra of a survey of insulating polymers and of metal-like Ti0.23Al0.32N0.44 (TiAlN) were measured, the complex dielectric functions determined and the corresponding Hamaker coefficients across vacuum and water calculated. The sensitivity of the quantification procedure towards typical systematic errors was investigated. For polystyrene the results were comparatively analysed using vacuum ultraviolet spectroscopy (VUV). FindingsThe accuracy especially of the non-retarded vacuum Hamaker constants of the polymers was increased when compared to VUV reflectance spectroscopy due to the higher spectral range of REELS. Furthermore, a new correction procedure for the intricate case of unresolved inelastic losses in the REELS spectrum, such as encountered in the case of TiAlN, could be developed using spectroscopic ellipsometry as a complementary mean.

Study of optical and electronic properties of nickel from reflection electron energy loss spectra

We use the classical Monte Carlo transport model of electrons moving near the surface and inside solids to reproduce the measured reflection electron energy-loss spectroscopy (REELS) spectra. With the combination of the classical transport model and the Markov chain Monte Carlo (MCMC) sampling of oscillator parameters the so-called reverse Monte Carlo (RMC) method was developed, and used to obtain optical constants of Ni in this work. A systematic study of the electronic and optical properties of Ni has been performed in an energy loss range of 0–200eV from the measured REELS spectra at primary energies of 1000eV, 2000eV and 3000eV. The reliability of our method was tested by comparing our results with the previous data. Moreover, the accuracy of our optical data has been confirmed by applying oscillator strength-sum rule and perfect-screening-sum rule.

Composition dependence of dielectric and optical properties of Hf-Zr-silicate thin films grown on Si(100) by atomic layer deposition

Composition dependence of the dielectric and optical properties of (HfZrO4)(1−x) (SiO2)(x) (0≤x≤0.2) gate dielectric thin films, grown on Si(100) by the atomic layer deposition method, was investigated by means of reflection electron energy loss spectroscopy (REELS). The quantitative analysis of REELS spectra was carried out by using the QUASES-XS-REELS and QUEELS-ε(k,ω)-REELS software, which determine the dielectric function and optical properties through an analysis of experimental REELS in terms of a simulated energy loss function (ELF). For HfZrO4, the ELF showed peaks in the vicinity of 10.5, 15.6, 18, 21.5, 26.7, 34.5, 39.5, 46.5 and 57eV. For HfZr-silicates with low SiO2 concentration (x=0.10 and 0.15), the peak positions were similar to those of HfZrO4, but for x=0.20, the number of peaks were reduced and are at 14.8, 23.5, 34.5, 40.5, and 46.5eV. These peaks originate from the superposition among d electron states of Zr, f electron states of Hf and p electron states of Si. The strength of the peak at 46.5eV decreased as we increased the amount of SiO2 in the compounds, which indicates that it is due to excitation between Hf 4f and Zr 3d electron states. Changes in the complex dielectric function, optical properties and band gap related to the SiO2 concentration in the films were discussed systematically. In addition, the inelastic mean free path (IMFP) was also calculated from the determined dielectric function. The IMFP of the HfZr-silicates increased with increasing SiO2 content and with increasing primary energy. This method is of high importance in terms of determining the dielectric and optical properties and inelastic mean free path from REELS spectra. The advantage of the method applied here is that no information on the morphology and components of the sample is required. This means that this method presented here turned out to be a convenient and efficient tool for investigation of optical properties and inelastic mean free path of ultrathin high-k alloy materials.

Li Distribution in a LiNi1/3Mn1/3Co1/3O2 Positive Electrode Analyzed Using Reflection Electron Energy-Loss Spectroscopy

Li-ion batteries are commonly used as energy-storage devices, where it is important to examine the spatial distribution and chemical state of Li with high spatial resolution to understand various phenomena pertaining to their use. To this end, analytical techniques using electron beam sources are advantageous due to their high spatial resolution. Observations of the Li distribution on the nanometer scale can be successfully obtained using electron energy-loss spectroscopy (EELS) with transmission electron microscopy (TEM) or scanning TEM (STEM) [1, 2]. However, TEM/STEM have inherent requirements in terms of sample preparation such as size or thickness, which limits its application range. Scanning electron microscopy (SEM) is much more commonly used and has a high spatial resolution on the nanometer scale, and it can observe larger particles or multiple samples in one sample holder. Ishida et al. reported the Li distribution of carbon and Li metal by using scanning auger electron spectroscopy (AES) [3]. Lithium-transition metal oxides with low electron conductivity are frequently used as electrode materials. However, the Li KLL signal in AES often overlaps with AES signals from other transition metal elements. The charge-up phenomena of a sample can cause problems during AES analysis, which complicates the discussions around chemical shifts. Despite this, the technique is applicable in analyzing light elements in materials with high spatial resolution. We performed Li analysis by reflection EELS (REELS) with SEM, and found that this technique is effective in obtaining the K-edge core-loss spectrum of Li. We identified the differences in chemical state in Li K-edge spectra for each sample, where the features of these REELS spectra were similar to those observed by TEM-EELS. As such, we proposed REELS as a method for lithium analysis in Li-metal oxides, and reported the successful detection of different chemical states of Li in various Li compounds. Using a single crystal Li4Ti5O12 substrate, elemental Li mapping was successfully carried out before and after electrochemical Li insertion and extraction [4]. In the present study, we expand the application of REELS to the analysis of positive electrode materials. The positive electrode material in Li-ion batteries is typically a Li-transition metal oxide of micron particle size. We applied REELS to determine the Li distribution in a LiNi1/3Mn1/3Co1/3O2 positive electrode after a charging/discharging cycle. Combined with scanning spreading resistance microscopy (SSRM), the changes in electrical conductivity of the Li-poor particles inside the electrode were visualized. [1] J. Kikkawa, T. Akita, M. Tabuchi, M. Shikano, K. Tatsumi, and M. Kohyama, Electrochem. Solid-State Lett. 11 183 (2008).[2] N. Taguchi, T. Akita, H. Sakaebe, K. Tatsumi, and Z. Ogumi, J. Electrochem. Soc. 160 2293 (2013).[3] N. Ishida, D. Fujita, J. Electron Spectrosc. Relat. Phenom. 186 39 (2013).[4] N. Taguchi, M. Kitta, H. Sakaebe and T. Akita, J. Electron Spectrosc. Relat. Phenom. 203 40 (2015).

Lithium analysis using reflection EELS for lithium compounds

Analyses of Li has been performed by reflection electron energy-loss spectroscopy (REELS) technique, using a scanning electron microscopy (SEM) apparatus, where Li signals can be detected with high spatial resolution without severe requirements for sample shapes. For Li4Ti5O12 and Li7Ti5O12 samples as electrode materials of Li-ion batteries, the Li distribution was successfully revealed by the spectrum-imaging scheme. The fine structure of REELS spectra was able to be applied to the chemical analysis of Li-containing compounds.

Electronic, electrical and optical properties of undoped and Na-doped NiO thin films

The electrical and optical properties as well as the electronic structure of sodium-doped (Na-doped) nickel oxide (NiO) thin films were investigated using X-ray photoelectron spectroscopy (XPS), reflection electron energy loss spectroscopy (REELS), X-ray absorption near edge structure, and extended X-ray absorption fine structure. The XPS results confirmed the presence of NiO bonds of the NiO phase for all films via the Ni 2p spectra. The NiO thin films annealed above 200°C have both nickel oxide and nickel metal phase. The REELS spectra showed that the band gaps of NiO thin films after Na doping were decreased. Na-doped NiO thin films exhibited relatively low resistivity compared to undoped NiO thin films. In addition, the Na-doped NiO thin films deposited at room temperature have p-type conductivity with a low electrical resistivity of 11.57Ωcm and high optical transmittance of 80% in the visible light region. Our results demonstrate that Na doping plays a crucial role in enhancing the electrical and optical properties of NiO thin films.