Reflection Electron Energy Loss Spectroscopy Research Articles

Optical properties of InSb derived from reflection electron energy loss spectroscopy spectrum

The energy loss function (ELF) of the narrow bandgap semiconductor, InSb, was derived from the reflection electron energy loss spectroscopy (REELS) spectrum measured at 4 keV incident electron energy in an energy loss range of 1.6–200 eV using a high energy resolution electron spectrometer. The experimental spectrum was analyzed with the reverse Monte Carlo (RMC) method, which integrates the classical trajectory Monte Carlo simulation with the simulated annealing method for optimizing the trial ELF. Subsequently, the complex dielectric function ε(ω), refractive index and extinction coefficient were determined from the obtained ELF in the energy loss (i.e. photon energy) range of 1.6–200 eV. The validity of the obtained data was verified by the f-sum rule, ps-sum rule, inertial sum rule and dc-conductivity sum rule. We found that our calculated data of InSb fulfill the sum rules with very high accuracy; therefore, the use of these calculated optical data in material science and surface analysis is highly recommended for further applications.

Advanced Fabrication of 3D Micro/Nanostructures of Gallium Oxide with a Tuned Band Gap and Optical Properties.

Gallium oxide (Ga2O3) is a promising material for high-power semiconductor applications due to its wide band gap and high breakdown voltage. However, the current methods for fabricating Ga2O3 nanostructures have several disadvantages, including their complex manufacturing processes and high costs. In this study, we report a novel approach for synthesizing β-Ga2O3 nanostructures on gallium phosphide (GaP) using ultra-short laser pulses for in situ nanostructure generation (ULPING). We varied the process parameters to optimize the nanostructure formation, finding that the ULPING method produces high-quality β-Ga2O3 nanostructures with a simpler and more cost-effective process when compared with existing methods. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the samples, which indicated the presence of phosphorous. X-ray photoelectron spectroscopy (XPS) confirmed the formation of gallium oxide, along with a minor amount of phosphorus-containing compounds. Structural analysis using X-ray diffraction (XRD) revealed the formation of a monoclinic β-polymorph of Ga2O3. We also measured the band gap of the materials using reflection electron energy loss spectroscopy (REELS), and found that the band gap increased with higher nanostructure formation, reaching 6.2 eV for the optimized sample. Furthermore, we observed a change in the heterojunction alignment, which we attribute to the change in the oxidation of the samples. Our results demonstrate the potential of ULPING as a novel, simple, and cost-effective method for fabricating Ga2O3 nanostructures with tunable optical properties. The ULPING method offers a green alternative to existing fabrication methods, making it a promising technology for future research in the field of Ga2O3 nanostructure fabrication.

Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering.

Ta-doped Li7La3Zr2O12 (LLZTO) garnet is a promising Li-ion-conducting ceramic electrolyte for solid-state batteries. However, it is still challenging to use LLZTO in Li metal batteries operating at high current densities because of the tendency for Li metal to nucleate and propagate along the grain boundaries. In this study, we carry out a detailed investigation to elucidate the effect of microstructure and grain size on the electrochemical properties and short circuit behavior in LLZTO. Pellets were prepared using reactive sintering from pyrochlore precursors (a method called pyrochlore-to-garnet, P2G) and compared with LLZTO synthesized using solid-state reaction (SSR) followed by conventional pressureless sintering. Both preparation methods were controlled to keep the phase and elemental composition, ionic and electronic conductivity, relative density, and area-specific resistance of the pellets constant. Reflection electron energy loss spectroscopy and X-ray photoelectron spectroscopy confirm that both types of LLZTO have similar band gaps and chemical states. Microstructure analysis shows that the P2G method results in LLZTO with an average grain size of around 3 μm, which is much smaller than the grain sizes (as large as 20 μm) seen in SSR LLZTO. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm2 in bidirectional cycling and >1 mA/cm2 for unidirectional) than those seen in SSR LLZTO. Post-mortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. The improved cycling behavior in P2G LLZTO despite the higher grain boundary area could be from more homogeneous current density at the interfaces and different grain boundary properties arising from the liquid-phase, reactive sintering method. These results suggest that the effect of grain size on Li dendrite propagation in LLZO may be highly dependent on the synthesis and sintering method employed.

Probing the Electronic Structure of Prussian Blue and Analog Films by Photoemission and Electron Energy Loss Spectroscopies.

X-ray photoelectron spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS) were employed to characterize the electronic properties of Prussian blue (PB) and its analogs when electrodeposited over metal-decorated carbon nanotubes (CNTs). Through an investigation of the influence of carbon nanotubes (CNTs) and preparation conditions on the electronic structure, valuable insights were obtained regarding their effects on electrochemical properties. XPS analysis enabled the probing of the chemical composition and oxidation states of the film materials, unveiling synthesis-driven variations in their electronic properties. REELS provided information on energy loss and electronic transitions, enabling further characterization of the changes in the electronic structure induced by different preparation methods. Such findings emphasize the importance of surface characterization to understand how the unique electronic properties of such materials can be harnessed to enhance their performance and functionality.

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.

Mechanistic studies of Yb2O3/HAT-CN connection electrode in tandem semiconductor devices

The optically transparent connecting electrode is much desired in fabrication of tandem optoelectronic devices. Yet, optically transparent materials, such as oxides, are electrically insulating. In this work, we show that low work function oxides Yb2O3 combing with high work function 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) molecule can be used as effective connecting electrodes to make high performance tandem organic light emitting diodes with negligible voltage loss. For instance, in a tandem device with two emission zones, yielding a brightness of 100 cd/m2, the voltage required is 5.3 V, which is approximately twice that of a single emission zone device. To gain insights into the band alignment of this electrode, we conducted the measurements, including ultraviolet photoelectron spectroscopy to analyze the electronic structures of occupied valence and gap states and reflection electron energy loss spectroscopy to study the unoccupied states. To understand the charge transport and injection behavior of this electrode, we conducted variable temperature charge transport measurements. Our findings reveal the presence of localized gap states within the Yb2O3/HAT-CN structure. These gap states effectively form a conduction pathway for facilitating the transport of charge carriers. At higher temperatures (≥200 K), charge transport is primarily limited by the Efros–Shklovskii type of hopping conduction through the localized states in the Yb2O3. Conversely, at lower temperatures (<200 K), the electrical current is limited by the properties of HAT-CN. These discoveries suggest that localized gap states at the oxides/organic heterojunctions can be effectively utilized in the fabrication of tandem semiconductor devices.

Optimizing energy storage performance of ALD YSZ thin film devices via yttrium concentration variations

In the last decade, research has focused on fabricating new materials to accelerate the development of energy storage devices. In this work, metal-electrolyte-metal (Au-YSZ-Ru) structures were fabricated on silicon substrates using atomic layer deposition, sputtering, and thermal evaporation techniques. The effect of the yttrium concentration in YSZ films on their chemical, structural, optical, and electrical properties were studied. XPS analysis showed yttrium concentrations of 14.6, 10.3, 6.8, and 5.3 at.% for films with ALD cycle ratios Zr:Y of 2:1, 4:1, 6:1, and 8:1. Deconvolution of the oxygen XPS signal demonstrate the yttrium to oxygen vacancies ratio. The band gap was calculated through UV–vis indicating an increase in values with yttrium concentration. In addition, the trend was confirmed by reflection electron energy loss spectroscopy (REELS). The structure was investigated by performing X-ray diffraction (XRD) and infrared attenuated total reflectance spectroscopy (IR-ATR) measurements. Both results indicate a better crystallization of the cubic phase for the samples with lower concentrations, as the intensity of (111) diffraction peaks and IR band increased for lower yttrium concentrations. Impedance spectroscopy revealed an activation energy of 1.69, 1.32, and 1.28 eV for the 10.3, 6.8, and 5.3 at.% film concentration, respectively. According to cyclic voltammetry and charge-discharge tests, a lower yttrium concentration, 5.3 at.%, promotes electrode REDOX reactions improving the energy-storage properties.

Capabilities of a New Compact SEM / STEM Electron Detector for Energy Resolved Scanning Imaging, Reflection Electron Energy Loss Spectroscopy (REELS) and Elastic Peak Electron Spectroscopy (EPES).

Capabilities of a New Compact SEM / STEM Electron Detector for Energy Resolved Scanning Imaging, Reflection Electron Energy Loss Spectroscopy (REELS) and Elastic Peak Electron Spectroscopy (EPES).

Determination of electron inelastic mean free path and stopping power of hafnium dioxide

We present inelastic mean free path (IMFP) and stopping power data of the hafnium dioxide applying the relativistic dielectric response theory. The energy loss function (ELF) derived from reflection electron energy loss spectroscopy spectrum with the reverse Monte Carlo method was used for the first time to obtain the IMFP and stopping power of HfO2. The probability of the energy loss is determined by the dielectric response function εq,ω as a function of the frequency ω and the wavenumber q of the electromagnetic disturbance. Two algorithms, namely the full Penn algorithm (FPA) and the super-extended Mermin algorithm (SMA), were employed to expand the optical energy loss function, Im-1/ε0,ω, into the q,ω-plane. The results indicate that the IMFP and the stopping power obtained by using both algorithms are consistent at high electron energies, but show differences at low electron energies (less than ∼ 70 eV). This discrepancy arises from the consideration of the finite plasmon lifetimes effect in the SMA model, while it is neglected in the FPA model. Additionally, we observed that the band gap has a significant influence on the IMFP and the stopping power at low electron energies. Typically, the inclusion of the band gap leads to an increase in the IMFP, because transition channels with energies larger than E-Eg-Ev are prohibited.

Energy loss function of samarium

We present a combined experimental and theoretical work to obtain the energy loss function (ELF) or the excitation spectrum of samarium in the energy loss range between 3 and 200 eV. At low loss energies, the plasmon excitation is clearly identified and the surface and bulk contributions are distinguished. For the precise analysis the frequency-dependent energy loss function and the related optical constants (n and k) of samarium were extracted from the measured reflection electron energy loss spectroscopy (REELS) spectra by the reverse Monte Carlo method. The ps- and f-sum rules with final ELF fulfils the nominal values with 0.2% and 2.5% accuracy, respectively. It was found that a bulk mode locates at 14.2 eV with the peak width ~6 eV and the corresponding broaden surface plasmon mode locates at energies of 5-11 eV.

Demonstration of Bixbyite-Structured δ-Ga2O3 Thin Films Using β-Fe2O3 Buffer Layers by Mist Chemical Vapor Deposition

Gallium oxide (Ga2O3) possesses five polymorphs: α, β, γ, κ (ε), and δ. Although the first four polymorphs have been well-studied, there are few reports on δ-Ga2O3. Here, we demonstrate the epitaxial growth of metastable δ-Ga2O3 thin films by mist chemical vapor deposition using β-Fe2O3 buffer layers. X-ray diffraction (XRD) 2θ–ω scan pattern revealed that (004) κ-Ga2O3 grew on (111) yttria-stabilized zirconia (YSZ) without a buffer layer or with a bcc-In2O3 buffer layer, whereas (222) δ-Ga2O3 grew on (222) β-Fe2O3. The β-Fe2O3 buffer layer led to the epitaxial growth of the δ-Ga2O3 thin film. The lattice mismatch between the equivalent crystal structures of β-Fe2O3 and δ-Ga2O3 triggered this growth. XRD analysis shows that δ-Ga2O3 grew epitaxially on the β-Fe2O3 buffer layer/YSZ substrate in both the out-of-plane and in-plane orientations, and the lattice constant inferred from the diffraction peaks was estimated to be 9.255 Å. Reciprocal space mapping results indicated that the δ-Ga2O3 grown on β-Fe2O3 was fully relaxed. Selected area electron diffraction images confirmed that the δ-Ga2O3 exhibited a cubic bixbyite structure. The optical band gap of δ-Ga2O3 was 4.3 or 4.9 eV, as calculated from reflection electron energy loss spectroscopy. We successfully grew a δ-Ga2O3 epitaxial thin film for the first time.

Synthesis of Optoelectronic Nanostructures on Silicon and Gold-Coated Silicon via High-Intensity Laser Pulses at Varied Pulse Durations

This work defines the generation of nanostructures on silicon and gold-coated silicon substrates by tuning the pulse duration of our proposed method: ultra-short laser pulses for in situ nanostructure generation (ULPING) under ambient conditions. The method is a single-step novel method which is efficient in synthesizing nanostructures on the substrates. We observed a higher nanofiber generation at a shorter pulse duration using Scanning Electron Microscopy (SEM) imaging. Silicon oxide formation was confirmed by Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analysis and a band gap of 8.19 eV was achieved for the Si + Au sample, which was determined by the Reflection Electron Energy Loss Spectroscopy (REELS) spectra. A high valence band offset of 4.93 eV was measured for the silicon-based samples for the Si/SiO2 interface. The addition of gold nanoparticles decreased the band gap and we observed a blue shift in optical conductivity for samples with nanofibers using optical spectroscopy.

Low Leakage Current Metal–Insulator–Metal Device Based on a Beryllium Oxide Insulator Created by a Two-Step Spin-Coating Method as a Novel Type of Modified Pechini Synthesis

Beryllium oxide (BeO) is considered to be an attractive alternative material for use in future industries in areas such as semiconductors, spacecraft, aircraft, and rocket technologies due to its high bandgap energy, useful melting point, good thermal conductivity, and dielectric constants. In this context, our approach is a novel method to produce BeO thin films based on a two-step spin-coating innovation of the conventional powder synthesis method. The surface morphology and the crystal structure of BeO thin films were observed to be dependent on the citric acid/beryllium sulfate ratio and the sintering temperature, respectively. To characterize the BeO films, X-ray photoelectron spectroscopy was conducted for an elemental analysis. Furthermore, the bandgap of the BeO thin films was determined by reflection electron energy loss spectroscopy. Finally, the leakage current of a planar metal–insulator–metal device consisting of Au/Ti/BeO thin film/Ti/Au electrodes was determined to be below the nA range over the linear voltage sweeping range of −20 V to +20 V. These results can assist researchers in the areas of morphology control strategies, phase transfer theories, and applications that utilize BeO thin film manufactured by a solution process.

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).

Optical, structural and composition properties of silicon nitride films deposited by reactive radio-frequency sputtering, low pressure and plasma-enhanced chemical vapor deposition

We present a comparative study of optical properties of silicon nitride thin films deposited by reactive radio-frequency (R-RF) sputtering, low pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD). For LPCVD process, two different proportions of mixed gases were used (LPCVD (A) and LPCVD (B) processes) and PECVD deposition were conducted in three regimes: low frequency (LF), mixed frequency and high frequency. Dielectric functions were extracted from ellipsometric measurements for the wavelength range from ultraviolet to near-infrared wavelengths, spanning from 210 nm to 1690 nm. To understand how different deposition parameters affect the optical properties of thin films, additional structures and composite analysis was done by using X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, X-ray reflectometry, atomic force microscopy, reflection electron energy loss spectroscopy, Fourier-transform infrared spectroscopy and stress measurements. The series of analysis show that the influence of deposition method on optical properties is significant especially for in the range of 200 nm–400 nm. For these UV wavelengths, LPCVD (A)-deposited films give a transparency window at the shortest wavelength up to 275 nm, while R-RF-sputtering and PECVD (LF) lead to transparency windows starting up to 320 nm wavelengths. Hence, appropriate techniques and recipes should be selected to account for various peculiarities in optical and structural properties of silicon nitride films towards their potential applications in photonic and nanostructured systems.

Surface and Bulk Plasmon Excitation on Aluminum Surface at Small to High Grazing Angles

We present a series of spectra measured by reflected electron energy loss spectroscopy on an aluminum sample using a cylindrical mirror analyzer. The measurements were performed in the energy range between 250 eV and 2000 eV and with various incident angles, including the grazing geometry of an 88° incident angle. The observed spectra were evaluated and decomposed for surface and bulk excitation. The determined surface plasmon excitations were compared to the elastic peak and to the bulk excitation. We found a slight surface plasmon energy shift with altering glancing angles. We show that this shift exists independently from the bulk plasmon interference.

Preparation and characterization of polythiophene/gold nanoparticles/carbon nanotubes nanocomposites thin films: Spectroscopy and morphology

Hybrid nanocomposite films containing polythiophene/gold nanoparticles and polythiophene/gold nanoparticles/carbon nanotubes, synthesized through a liquid-liquid interfacial polymerization route at different reaction times and amounts of carbon nanotubes, were investigated through spectroscopic and morphological techniques. X-ray photoelectron spectroscopy showed the formation of polythiophene aggregates and the presence of large gold particles, corroborating the results of transmission and scanning electron microscopy. Near-edge X-ray absorption fine structure showed films without any preferential molecular orientation of the polymer chains. The bandgap obtained by reflection electron energy loss spectroscopy is reduced with the increase in the synthesis reaction time and the decrease in the amount of carbon nanotubes, which is followed by ultrafast electron dynamics studied with the core-hole clock method. These films were heat-treated at 200 °C for 30 min and faster charge transfer times were observed. Current x Voltage curves showed a photoactive response of the materials.

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.

Energy band offsets of BeO dielectrics grown via atomic-layer deposition on β-Ga2O3 substrates

We report the energy-band alignment of atomic layer-deposited (ALD) beryllium oxide (BeO) films and β-Ga2O3 substrates. BeO is a unique oxide with a high dielectric constant and bandgap energy that can be used as a gate dielectric; however, it also has an extremely high thermal conductivity. It has great potential to improve the heat dissipation of β-Ga2O3 power devices. In this study, the conduction band offset between the BeO film and β-Ga2O3 substrate was found to be 3.4 eV, which was larger than those of conventional high-k gate dielectrics. In addition, the bandgap energies (8.6 eV and 4.7 eV for BeO and β-Ga2O3, respectively) were determined using reflection electron energy loss spectroscopy. The valence band offset (0.5 eV) was calculated using Kraut's method with the core level and valence band maximum energies of the BeO film and β-Ga2O3 substrate. The high conduction band offset provided by the ALD BeO dielectric on the β-Ga2O3 substrate lowered the gate leakage current density of a β-Ga2O3 power device.