Reflection Electron Energy Loss Spectroscopy Research Articles

Electronic and optical properties of plutonium metal and oxides from Reflection Electron Energy Loss Spectroscopy

Reflection Electron Energy Loss Spectra (REELS) have been acquired on plutonium δ-Pu-Ga alloys, α-Pu2O3 and PuO2. The position of the peaks attributable to the Pu 6p excitation can be used as ‘fingerprint’ for these species. X-ray Photoelectron Spectroscopy has been used to aid interpretation of the REELS spectra. The band gap of PuO2 was determined from the onset of the energy loss in the REELS spectrum. The REELS data have been evaluated within the dielectric response theory by means of the QUEELS-∊(k, w)-REELS software [Surf. Interface Anal. 36, (2004) 824]. The Energy Loss Function, dielectric function, refractive index and extinction coefficient for these materials have been determined in the ultraviolet and extreme ultraviolet range and are compared to previously reported results. Crown copyright (C) 2021 Published by Elsevier B. V. All rights reserved.

Individual separation of surface, bulk and Begrenzungs effect components in the surface electron energy spectra

We present the first theoretical recipe for the clear and individual separation of surface, bulk and Begrenzungs effect components in surface electron energy spectra. The procedure ends up with the spectral contributions originated from surface and bulk-Begrenzungs excitations by using a simple method for dealing with the mixed scatterings. As an example, the model is applied to the reflection electron energy loss spectroscopy spectrum of Si. The electron spectroscopy techniques can directly use the present calculation schema to identify the origin of the electron signals from a sample. Our model provides the possibility for the detailed and accurate quantitative analysis of REELS spectra.

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.

Influence of Substrate Temperature on Electrical and Optical Properties of Hydrogenated Boron Carbide Thin Films Deposited by RF Sputtering

Amorphous hydrogenated boron carbide films were deposited on silicon and glass substrates using radio frequency sputtering. The substrate temperature was varied from room temperature to 300 °C. The substrate temperature during deposition was found to have significant effects on the electrical and optical properties of the deposited films. X-ray photoelectron spectroscopy (XPS) revealed an increase in sp2-bonded carbon in the films with increasing substrate temperature. Reflection electron energy loss spectroscopy (REELS) was performed in order to detect the presence of hydrogen in the films. Metal-insulator-metal (MIM) structure was developed using Al and hydrogenated boron carbide to measure dielectric value and resistivity. Deposited films exhibited lower dielectric values than pure boron carbide films. With higher substrate deposition temperature, a decreasing trend in dielectric value and resistivity of the films was observed. For different substrate temperatures, the dielectric value of films ranged from 6.5–3.5, and optical bandgap values were between 2.25–2.6 eV.

Optical properties of amorphous carbon determined by reflection electron energy loss spectroscopy spectra.

We present the combined experimental and theoretical investigations of the optical properties of amorphous carbon. The reflection electron energy loss spectra (REELS) spectra of carbon were measured using a cylindrical mirror analyzer under ultrahigh vacuum conditions at primary electron energies of 750, 1000 and 1300 eV. The energy loss function and thereby the refractive index n and the extinction coefficient k were determined from these REELS spectra in a wide loss energy range of 2-200 eV by applying our reverse Monte Carlo method. The high accuracy of the obtained optical constants is justified with the ps- and f-sum rules. We found that our present optical constants of amorphous carbon fulfill the sum rules with the highest accuracy compared with the previously published data. Therefore, we highly recommend to replace the previous data with the present ones for practical applications. Moreover, we present the atomic scattering factors of amorphous carbon obtained from the dielectric function to predict its optical constants at a given density.

Thin Ta/Ta oxide core-shell nanoparticle film size-dependent energy structure

The paper presents the research of thin films, consisting of spherical monodisperse Ta particles. For different samples particle sizes varied from 1.3 nm to 5.8 nm. The films were also exposed to the atmosphere for oxidation, which resulted in formation of a core-shell particle structure, where core was metal and shell was Ta oxide (Ta2O5). Dependency of bandgap Egap and valence band offset (VBO) on particle size has been investigated for the structure by using the X-ray photoelectron spectroscopy (XPS) and reflected electron energy loss spectroscopy (REELS) methods. We have found, that Egap decreases from 4.72 eV to 4.47 eV whereas VBO decreases from 3.50 eV to 2.95 eV with increasing the particle size. A new method of a fine regulation of the electronic structure is suggested, particularly for the application in ultraviolet (UV) photodetector design for low-lying nuclear and electron transitions detection.

Unoccupied Atomic-Like States of GaAs

The excitation spectrum of GaAs has been studied by reflection electron energy loss spectroscopy. In addition to dominant collective excitations, a series of single-electron transitions of a core Ga 3d electron to previously unknown unoccupied states located above the Fermi level by 1.25, 3.7, and 6.8 eV have been detected in the spectrum. It has been shown that the detected states appear near the ion core of Ga because of an increase in its effective charge at excitation. Since the detected electronic levels are equidistant, they can be described by a subnanometer spherical quantum dot model. It has been shown that one of the decay channels of the detected states involves the emission of ultraviolet radiation.

Band gap measurement by nano-beam STEM with small off-axis angle transmission electron energy loss spectroscopy (TEELS)

An energy band gap measurement method based on nano-beam STEM with small off-axis angle valence band transmission electron energy loss spectroscopy (TEELS) is reported. The effect of multiple scattering event is removed by self-convolution method to obtain a single scattering loss function and a dielectric function is calculated from the single scattering valence band energy loss function through Kramers-Kronig (K-K) analysis. Optical band gaps are extracted from energy loss spectra and the imaginary part of the dielectric functions for crystalline and amorphous SiOx, SiNx, and SiON through linear fitting of on-set regions yielding results that are independent of sample thickness. The TEELS band gap data are consistent with those obtained from reflection electron energy loss spectroscopy (REELS) measurements.

Formation of graphitic films on Cu(111) via electron beam induced deposition

Electron beam induced deposition using ethylene as precursor gas is used to generate carbonaceous films on Cu(111). The study was performed in an ultrahigh vacuum chamber for two different precursor gas pressures, and two substrate temperatures. Auger electron spectroscopy and reflection electron energy loss spectroscopy were used to characterize the film growth process in-situ. These techniques allowed us to determine how the deposited film covers the Cu(111) substrate as a function of the ethylene exposure, obtaining a final coverage of up to 0.8 monolayer, depending on the growing conditions. Based on the CKLL Auger line shape and on the reflection electron energy loss (REELS) spectra we can conclude on the graphitic characteristics of the grown film and on the optimum growing conditions: lower substrate temperature and higher ethylene pressure. Raman spectroscopy and scanning tunneling spectroscopy were used as ex-situ techniques to assess the properties of the grown film. Results from both techniques indicate that the film consists of small (10–20 nm in size) nanocrystals of few (1–2) layers graphene. Once optimized, this method may allow growing graphene with a predesigned pattern, without the need to heat the substrate at high temperatures.

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.

Bandgap engineering of low-temperature CdS nanocrystalline prepared on Si(1 1 1) without post-thermal annealing

This article reports a new attempt for the bandgap formation of cadmium sulfide nanocrystals (CdS NCs) synthesized on Si(1 1 1) substrates at low temperatures: 50 and 150 °C. The influence of energy gap alteration on the optoelectronic properties was extensively considered with the aid of various tools. A wurtzite-–hexagonal structure in both CdS/Si NCs samples was confirmed by X-ray diffraction measurements. For a crystallite size around 7 nm, wider energy gaps (2.94–2.98 eV) were revealed by photoluminescence spectra. These values are in good conformity with the precise energy gaps, 2.95 eV and 3.04 eV which were estimated using a reflection-electron energy loss spectroscopy. Consequently, such expansion in the energy bandgap indicates the occurrence of a blue shift compared to the bulk CdS. Moreover, the Raman spectra show three peaks referring to the first-, second-, and third-order scattering of the longitudinal optical phonon modes. The findings demonstrate the improved crystalline quality of the sample prepared at 150 °C with no need for further thermal annealing. It is thus inferred that quantum confinement is possible in the CdS NCs prepared at low temperatures using a pulsed-laser deposition technique; thereby, they are useful for optoelectronic applications.

Spectroscopic Study of Reinforced Cross-Linked Polymeric Membranes for Fuel Cell Application.

Acid-doped reinforced polymer electrolyte membranes for high-temperature polymer electrolyte membrane fuel cell applications (HT PEMFCs) are presented and spectroscopically studied. Fully aromatic polyethers are employed bearing main chain pyridine units as the proton accepting sites, which have two different substitution patterns of the pyridine units, namely, 2,5- or 2,6-pyridine. This fact enables control of the solubility and of the acid doping ability of the polymeric membranes. Reinforcement is accomplished via incorporation of a PTFE woven fabric during the casting procedure for fabrication of the membranes. High acid uptake of the reinforced membranes was maintained for the 2,6-pyridine-based copolymers with high pyridine unit content. Studies of the swelling behavior of these reinforced membranes revealed that they expand mainly along the z-axis, which helps to avoid extensive damage in case of humidity or temperature changes during the fuel cell operation. Additionally, spectroscopic techniques are employed, namely, X-ray photoelectron spectroscopy, X-ray photoelectron spectroscopy with depth profile, near-edge X-ray absorption fine structure, and reflection electron energy loss spectroscopy, for the in-depth study of the two copolymer membranes doped with phosphoric acid. Through these spectroscopic evaluations, modifications in the membranes’ chemical structure, orientation, composition, and electronic structure after the reinforcement and doping processes were elaborated and unveiled.

Optical properties of molybdenum in the ultraviolet and extreme ultraviolet by reflection electron energy loss spectroscopy.

Optical properties of polycrystalline molybdenum are determined from ultraviolet up to extreme ultraviolet by reflection electron energy loss spectroscopy (REELS). Calculations are performed within the dielectric response theory by means of the quantitative analysis of electron energy losses at surfaces QUEELS-ϵ(k,ω)-REELS software [Surf. Interface Anal.36, 824 (2004)SIANDQ0142-242110.1002/sia.1774] that allows the simulation of inelastic scattering cross sections, using a parametric energy loss function describing the optical response of the material. From this energy loss function, the real and imaginary parts of the dielectric function, the refractive index, and the extinction coefficient are deduced and compared with previously published results.

Effect of Substrate Temperature on the Electronic Properties of MgO Thin Films on Si (100) Grown by Electron Beam Evaporation

The surface characterization of MgO thin films was investigated by using surface analysis instruments such as X-ray photoelectron spectroscopy (XPS), reflection electron energy loss spectroscopy (REELS) and ultra-violet photoelectron spectroscopy (UPS). The MgO thin films was prepared on Si substrates by using electron beam evaporation deposited at room temperature (RT) and 300 °C in air. The XPS was used to investigate the effect substrate temperature on the chemical state in the thin films surface. The O1s spectra was showed that the hydrate MgO, Mg(OH)2, was detected in the surface of film at RT and it was decreasing at substrate temperature of 300 °C in air. The band gap obtained for MgO thin films using primary energy of 1500 eV were 6.57 and 7.41 eV for film deposited at RT and 300 °C in air, respectively. The work function of MgO thin films were 4.16 and 4.60 eV for films deposited at RT and 300 °C in air, respectively. Our results suggested that the electronic properties can be improved by the heating treatment during deposition.

Origin of Intergranular Li Metal Propagation in Garnet-Based Solid Electrolyte By Direct Electronic Structure Analysis and Performance Improvement By Bandgap Engineering

Garnet-structured oxide electrolyte (Li7La3Zr2O12, LLZO) has the significant advantage of being chemically and electrochemically stable against Li metal, which allows the implementation of Li metal batteries. However, a short-circuit failure by Li penetration through the LLZO electrolyte has remained a crucial issue for safety and is a major hurdle for Li-based batteries to overcome. Here, we investigate the mechanism of Li dendrite formation for the crystalline Ta-doped LLZO (LLZTO) by examining their energy band structures and defect states using reflection electron energy loss spectroscopy (REELS), scanning photoelectron microscopy (SPEM), and nanoscale charge-based deep level transient spectroscopy (Nano Q-DLTS) techniques. The experimental results reveal that the Schottky barrier height (SBH) was lowered by 0.5 eV due to the defect states localized in grain boundaries and the metallic Li propagation along the grain boundaries is caused by the SBH reduction. Based on the analytical results, the laser annealing of LLZTO was performed as a bandgap engineering method to suppress the Li dendrite formation by forming a mixed surface layer of amorphous LLZTO and Li2O, which has a wide bandgap to block the electron injection into the grain boundaries. The electrochemical measurements for the laser treated LLZTO demonstrate that the stability and cycling performance are significantly improved. This study sheds light on the importance of electronic structure, in particular, defect states to develop high performance oxide solid electrolytes for Li metal batteries and the practicality of surface modification by laser treatment.

Chemical, morphological and optical properties of undoped and Cu-doped ZnO thin films submitted to UHV treatment

The undoped and Cu-doped zinc oxide (UZO and CZO) thin films deposited on Si substrate were treated in the UHV (Ultra-High Vacuum) system using Ar+ sputtering followed by a controlled annealing process at 650 °C. The sensitive experimental techniques XPS (X-rays Photoelectron Spectroscopy), REELS (Reflection Electron Energy-Loss Spectroscopy), PL (Photoluminescence) and AFM (Atomic Force Microscopy) have been adopted to characterize the effect of this UHV treatment on the chemical, morphological and optical properties of both samples. The XPS results show the complete removal of contamination species from the surface and bulk for both samples and display the well-defined formation of CuO and/or CuOuZn compounds in the CZO. The AFM images exhibit a significant improvement in the surface morphologies, especially CZO. Through the REELS results, the appearance of CuO oscillator (4.5 eV) at high primary energy proves more incorporation of copper into the bulk ZnO matrix. Besides, the PL measurements reveal a complete disappearance of green emission of UZO and improvement for CZO one. The slight redshift and the broadening of CZO UV-emission confirm the incorporation and substitution of copper into the ZnO matrix. Thus, this treatment is of great interest to improve the surface efficiency for reliable devices.

The effect of ion implantation on reflection electron energy loss spectroscopy: The case of Au implanted Al films

Gold-implanted aluminum films are used to investigate how reflection electron energy loss spectra (REELS) change due to the presence of a small concentration of heavy atoms at a specific depth. Au ions were implanted with 30, 100 and 300 keV energy. REELS spectra were taken at energies between 10 and 40 keV. Large changes in the REELS spectra are observed after Au implantation, but the nature of the change indicates that they are not due to modification of the dielectric function of the implanted layer, but should be interpreted as changes in the partial intensities that make up the spectrum. Two models are used to describe the results quantitatively. One method assumes v-shaped trajectories (i.e. only a single elastic deflection) and the REELS spectrum can then be calculated in a closed form. The other method is a Monte-Carlo based simulation which allows for multiple elastic deflections. Both methods describe the experimental spectra quite well, but at larger energy losses significant deviations occur between the measured and calculated intensity for both the implanted and not-implanted films. The difference in the REELS spectrum before and after implantation is less affected by these discrepancies, and can be used to obtain an estimate of both the depth and concentration of the implanted Au atoms. Due to the presence of sharp plasmon features in the energy loss spectrum of aluminum the experiment can tell us directly which partial intensities are affected by the Au impurities, as the recoil energies due to elastic scattering make it possible to identify the contribution of Au to the first few plasmons. As the Au implantation fluence is known the measurement can be used to determine the ratio of the Au and Al elastic scattering cross sections, which deviates strongly from that calculated from the Rutherford formula.

Revision of optical property of silicon by a reverse Monte Carlo analysis of reflection electron energy loss spectroscopy spectra

SynopsisThe energy loss function (ELF) of silicon in a wide photon energy region (0-200 eV) was derived from reflection electron energy loss spectroscopy spectra with a theoretical analysis of the measured data. The accuracy of our result was justified by using the f- and ps-sum rules. Based on the new ELF, individual contributions of surface excitation and the bulk excitation to the REELS spectrum have been separated, and multi-scattering effect in the reflection electron energy loss spectroscopy spectrum has been studied in detail.

Optical properties of silicon and germanium determined by high-precision analysis of reflection electron energy loss spectroscopy spectra

We present a detailed analysis and comparison of four models describing the extension of the electron-energy loss function from the optical limit of q\ensuremath{\rightarrow}0 into the (q,\ensuremath{\omega}) plane to obtain the bulk and surface terms of differential inverse inelastic mean free paths. We found that the best model that describes accurately and times efficiently the calculation of the energy loss function of free-electron-like materials is the combination of the Penn algorithm [Phys. Rev. B 35, 482 (1987)] with the Ritchie-Howie method [Philos. Mag. 36, 463 (1977)]. Applying this model in our reverse Monte Carlo method, we determined, with high-precision, electron-energy loss functions of silicon and germanium based on the theoretical analysis of the high-energy resolution reflected electron energy loss spectroscopy (REELS) spectra, measured at 3, 4, and 5 keV incident electron energies. The refractive index $n$, the extinction coefficient $k$, and the complex dielectric function ($\ensuremath{\varepsilon}={\ensuremath{\varepsilon}}_{1}+i{\ensuremath{\varepsilon}}_{2}$) were calculated from the obtained energy loss function in a wide energy loss range of 0--200 eV. The accuracy of the obtained results is justified with various sum rules. We found that the calculated optical data of Si and Ge fulfill the sum rules with an average accuracy of 0.11% or even better. Therefore, the use of these optical data in materials science and surface analysis is highly recommended for further applications.