Thin Samples Research Articles

Laser-induced selective local patterning of vanadium oxide phases

The same elements can form different compounds with widely different physical properties. Synthesis of a single-phase material is commonly achieved by controlling experimental conditions. Synthesizing materials that incorporate multiple specific spatially distributed chemical phases is often challenging, especially if different phases must be organized into well-defined spatial patterns. Here, we present an efficient solid reaction laser annealing (SRLA) approach to directly write regions of different local chemical compositions. We demonstrate the practical utility of our approach by locally writing microscale patterns of distinct chemical phases in vanadium oxide thin films. Specifically, we achieved the controlled local recrystallization of a uniform V2O3 matrix into VO2, V3O5, and V4O7 regions exhibiting sharp 1st- and 2nd-order metal–insulator phase transitions over a wide range of critical temperatures, i.e., a characteristic feature of select vanadium oxides that is extremely sensitive to even minute structural or compositional imperfections. We utilized the local chemical phase writing to pattern spiking oscillators with distinct electrical behavior directly in the thin film sample without employing elaborate lithography fabrication. Our laser tuning local chemical composition opens a pathway to synthesize a wide range of artificially micropatterned composite materials, with precision and control unattainable in conventional material synthesis methods.Graphical

Advanced Study of Structural and Optical Characteristics of Nanocrystalline Coomassie Brilliant Blue G-250 for Optoelectronic Device Optimization

This study explores the structural and optical properties of Coomassie Brilliant Blue G-250 (CBBG) using X-ray diffraction (XRD) and spectroscopic techniques. XRD analysis of both powder and thin film samples reveals distinct structural features: the powder exhibits a polycrystalline structure with strong diffraction peaks corresponding to cubic lattice planes, while the thin film shows a mix of crystalline orientations within an amorphous matrix. The Williamson-Hall relation is applied to determine average crystallite sizes, yielding 85.5nm for the powder and 71.8nm for the thin film, indicating their nanocrystalline nature. Microstrain analysis also reveals compressive strain within the thin film. These structural insights are critical for understanding CBBG's mechanical and optical properties, vital for biochemical and optoelectronic applications. Density functional theory (DFT) calculations with the B3LYP/6-311++G(d,p) basis set were used to optimize the molecular geometry and analyze HOMO-LUMO energies and reactive sites via the MEP map. CBBG shows higher hyperpolarizability than urea, indicating its potential for nonlinear optical applications. Miller indices further support its optoelectronic optimization. The real (ε1) and imaginary (ε2) parts of the dielectric constant were analyzed, revealing CBBG thin films exhibit strong polarization and charge displacement, influenced by structural disorder. Additionally, the volume and surface energy loss functions (VELF and SELF) highlight significant internal energy dissipation, emphasizing CBBG's potential for optoelectronic devices. The complex optical conductivity analysis reveals key absorption and dispersion behaviors essential for advanced photonic and electronic device design.

Edge-Detected 4DSTEM - effective low-dose diffraction data acquisition method for nanopowder samples in a SEM instrument

The appearance of direct electron detectors marked a new era for electron diffraction. Their high sensitivity and low noise opens the possibility to extend electron diffraction from transmission electron microscopes (TEM) to lower energies such as those found in commercial scanning electron microscopes (SEM). The lower acceleration voltage does however put constraints on the maximum sample thickness and it is a-priori unclear how useful such a diffraction setup could be. On the other hand, nanoparticles are increasingly appearing in consumer products and could form an attractive class of naturally thin samples to investigate with this setup. In this work we present such a diffraction setup and discuss methods to effectively collect and process diffraction data from dispersed crystalline nanoparticles in a commercial SEM instrument. We discuss ways to drastically reduce acquisition time while at the same time lowering beam damage and contamination issues as well as providing significant data reduction leading to fast processing and modest data storage needs. These approaches are also amenable to TEM and could be especially useful in the case of beam-sensitive objects.

Numerical Simulation and Experimental Analysis of Mare’s Milk Sublimation Drying

This study presents a combined numerical and experimental analysis of heat and mass transfer in mare’s milk during vacuum sublimation drying, which is a process essential for producing high-quality powdered products. The numerical model was developed using two-dimensional simulations and validated against experimental data obtained for varying sample thicknesses (3 mm, 5 mm, and 7 mm). Results demonstrated a strong agreement between the model and experimental temperature data, with a coefficient of determination (R2) of 95% during the sublimation process. The findings revealed that thinner samples (3 mm) exhibited a 20% faster drying rate than thicker samples (7 mm), highlighting the critical role of sample thickness in sublimation dynamics. Additionally, the effects of heat flux distribution and mass loss due to sublimation were analyzed to understand the drying dynamics. This study highlights the importance of optimizing process parameters such as chamber pressure, shelf temperature, and sample thickness to enhance drying efficiency and reduce processing time. The findings provide valuable insights for scaling vacuum sublimation drying of mare’s milk for industrial applications.

Investigation and predictive modeling of the optical behavior of chalcogenide thin film using different artificial neural network techniques

The Te72Ge24As4 samples were recently created in our laboratory in bulk form using the traditional melt-quench method. For its optical characterization. The studied thin film samples have been created using physical vapor deposition. By selecting the 400 nm to 2500 nm spectral range of wavelength, the spectral of the experimental transmission T(λ) and reflectance R(λ) for the studied film samples have been employed to examine optical characteristics. First, we have determined the extinction coefficient (k) and refraction index (n) indices and their spectral distribution of them. Using Tauc's theory, we then computed the optical band gap Eopt. Urbach energy Er is determined from the linear dependence of photon energy on the absorption coefficient which was taken as an indicator to identify the disorder degree in the films. The additional variables, like the dissipation and quality factors, the dielectric constant in complex form, optical, thermal, and electrical conductivity, and volume/surface energy were measured. A comprehensive analysis and predictive modeling using various artificial neural networks (ANNs) techniques were applied to examine the optical behavior of the film samples studied. Materials made of chalcogenide are well-known for having special optical properties, making them appropriate for applications in photonics and optoelectronics. We employed multiple architectures, including Feedforward Neural Networks (FNN) and Recurrent Neural Networks (RNN), to model the extinction coefficient (k) and the refractive index (n) of these films using experimental data. The performance of each model was evaluated using metrics such as mean squared error MSE and correlation coefficients R2. The optical parameters relevant to absorbance, refractive indices, and dielectric coefficients are computed rely on the modeling results and compared with those computed based on experimental measurements. Results demonstrate that FNN and RNN effectively capture the complex relationships between the optical parameters and exhibit small error rates. FFN shows superior accuracy in prediction. That highlights the potential of ANN techniques for advancing the understanding of chalcogenide materials and their applications in modern technology.

Ferroelectricity and piezoelectric energy harvesting of an A3M2X9-type 0D bromobismuthate hybrid with a bulky organic quaternary amine.

Organic-inorganic hybrid ferroelectric compounds of the halobismuthate family have emerged as a focal point of research owing to their reduced toxicity and distinctive optical characteristics. This study presents a novel ammonium hybrid perovskite, [BPMBDMA]·[Bi2Br9], which exhibits both ferro- and piezoelectric properties and crystallizes in the polar noncentrosymmetric Pca21 space group. The nonlinear optical (NLO) activity of [BPMBDMA]·[Bi2Br9] was corroborated through second harmonic generation measurements evidencing its noncentrosymmetric structure, which was further substantiated by piezoresponse force microscopy analyses. Ferroelectric P-E hysteresis loop investigations conducted on a thin film sample of [BPMBDMA]·[Bi2Br9] revealed a saturation polarization (Ps) as much as 11.30 μC cm-2 at ambient temperature. To explore the piezoelectric energy harvesting capabilities of [BPMBDMA]·[Bi2Br9], composite materials were fabricated using polylactic acid (PLA) as a matrix. Notably, a device comprising 10 wt% [BPMBDMA]·[Bi2Br9] in PLA demonstrated a remarkable output voltage of 24.6 V and a peak power density of 13.65 μW cm-2. The practical applicability of this device's output performance was further evaluated through a capacitor charging experiment, wherein a 10 μF capacitor was charged within 160 seconds.

Comparative electron diffraction analysis of strain relaxation in AlxGa1-xN materials in the microelectronics industry: 4D-STEM approach vs. TEM-based N-PED solution.

Owing to its high spatial resolution and its high sensitivity to chemical element detection, transmission electron microscopy (TEM) technique enables to address high-level materials characterization of advanced technologies in the microelectronics field. TEM instruments fitted with various techniques are well-suited for assessing the local structural and chemical order of specific details. Among these techniques, 4D-STEM is suitable to estimate the strain distribution of a large field of view. This study tends to discuss the viability of using two existing commercial solutions with a low-convergence angle 4D-STEM technique and TEM-based Nanobeam Precession Electron Diffraction (N-PED) methods for strain analysis in an industrial context. In such a framework, strain measurements intend to point out the extent of defects and thus reveal a trend of the stress field, rather than to precisely estimate the absolute values of the deformations. Strain distribution maps have been obtained for AlGaN/GaN HEMT devices using two transmission electron diffraction analysis methods. The performances of 4D-STEM and Nanobeam Precession Electron Diffraction (N-PED) solution for strain mapping have been compared for both a relatively thin (≈ 55 nm) and a thick (≈ 150 nm) TEM cross section specimen. The strain maps obtained with both methods have shown comparable results for a thin sample, with the ability to characterize the deformation induced by a 1 nm-thick layer of AlN spacer grown between the AlGaN barrier and GaN channel forming the 2DEG of the HEMT device. The results presented here also illustrate the limitation of both commercial solutions in the case of a thick sample.

STUDY OF THE DIFFUSION LENGTH OF NONEQUILIBRIUM CHARGE CARRIERS IN CADMIUM TELLURIDE BASE LAYERS

Thin film samples of cadmium telluride were obtained using thermal vacuum evaporation to study the diffusion length of minority charge carriers. Initial state analysis was conducted through X-ray structural studies of cadmium telluride condensed on glass substrates. The analysis revealed two diffraction reflections corresponding to the crystallographic planes (111) and (333). Calculations showed that the sizes of coherent scattering regions were 93 nm and 56 nm, while microstrain values were determined as 11.6×10−3 and 4.8×10−3, respectively, for these planes. Electron diffraction studies identified the zone axis of the formed film structures as [110]. Additionally, twinning and forming a layered substructure within the cadmium telluride thin films were observed. Microscopic surface studies provided insights into the surface relief, measured at approximately 130 nm, and the polycrystalline grain size, determined to be 1 micrometer. The method of spectral dependence of small-signal surface photovoltage was employed to investigate the electrophysical properties of cadmium telluride thin films. The diffusion length of minority charge carriers was calculated to be 0.45 micrometers, which is half the size of the grain dimensions. Furthermore, the transmittance coefficient of cadmium telluride was examined in the wavelength range of 800–900 nm. The optical bandgap energy was 1.4 eV, slightly lower than the 1.5 eV observed for monocrystalline cadmium telluride. In conclusion, cadmium telluride film structures produced by thermal vacuum evaporation exhibit electrophysical and electrical parameters inferior to monocrystalline cadmium telluride. These characteristics, particularly the low diffusion length of minority carriers and short electrical signal decay times make such film structures suitable for use in devices requiring long-term stability under the influence of time, extreme temperatures, electric fields, and radiation.

Novel calibration method for fine soil electrical resistivity based on van der Pauw configuration

Abstract Soil electrical resistivity (ER) serves as an important parameter for indirectly monitoring various physical properties. The Miller box, based on Ohm’s Law, is commonly employed to measure resistivity. However, its probes tend to insert destructively into the soil sample. Van der Pauw (vdP) method is a classical, non-destructive technique used for measuring the ER of thin samples. Applying the vdP method to soil, which is a non-flat sample, presents certain challenges. In this paper, a novel soil resistivity calibration method (vdP box) is developed based on the vdP method, with consideration given to thickness extension. The relative resistivity error between the VDP box and Miller box for fine soil is less than 1% with various diameters and moisture content, which verified the accuracy and stability of the new device. This device achieves simple, fast, and non-destructive testing of soil resistivity and provides a novel method to study other properties of soil indirectly through resistivity.

3D Printed Carbon Nanotube/Phenolic Composites for Thermal Dissipation and Electromagnetic Interference Shielding.

Here we demonstrate direct ink write (DIW) additive manufacturing of carbon nanotube (CNT)/phenolic composites with heat dissipation and excellent electromagnetic interference (EMI) shielding capabilities without curing-induced deformation. Such polymer composites are valuable for protecting electronic devices from overheating and electromagnetic interference. CNTs were used as a multifunctional nanofiller to improve electrical and thermal conductivity, printability, stability during curing, and EMI shielding performance of CNT/phenolic composites. Different CNT loadings, curing conditions, substrate types, and sample sizes were explored to minimize the negative effects of the byproducts released from the cross-linking reactions of phenolic on the printed shape integrity. At a CNT loading of 10 wt %, a slow curing cycle enables us to cure printed thin-walled CNT/phenolic composites with highly dense structures; such structures are impossible without a filler. Moreover, the electrical conductivity of the printed 10 wt % CNT/phenolic composites increased by orders of magnitude due to CNT percolation, while an improvement of 92% in thermal conductivity was achieved over the neat phenolic. EMI shielding effectiveness of the printed CNT/phenolic composites reaches 41.6 dB at the same CNT loading, offering a shielding efficiency of 99.99%. The results indicate that high CNT loading, a slow curing cycle, flexible substrates, and one thin sample dimension are the key factors to produce high-performance 3D-printed CNT/phenolic composites to address the overheating and EMI issues of modern electronic devices.

Optical and Geometrical Properties from Terahertz Time-Domain Spectroscopy Data.

Terahertz waves are nondestructive and non-ionizing to synthetic and natural materials, including polymeric and biological materials. As a result, terahertz-based spectroscopy has emerged as a suitable technique to uncover fundamental molecular mechanisms and material properties in this electromagnetic spectrum regime. In terahertz time-domain spectroscopy (THz-TDS), the material's optical properties are resolved using the raw time-domain signals collected from the sample and air reference data depending on accurate prior knowledge of the sample geometry. Alternatively, different spectral analysis algorithms can extract the complex index of refraction of optically thick or optically thin samples without specific thickness knowledge. A THz-TDS signal without apparent Fabry-Pérot oscillations is commonly associated with optically thin samples, whereas the terahertz signal of optically thick samples exhibits distinct Fabry-Pérot oscillations. While several extraction algorithms have been reported a priori, the steps from reducing the time-domain signal to calculating the complex index of refraction and resolving the correct thickness can be daunting and intimidating while obscuring important steps. Therefore, the objective is to decipher, demystify, and demonstrate the extraction algorithms for Fabry-Pérot-absent and -present terahertz signals for various polymers with different molecular structure classifications and nonlinear optical crystal zinc telluride. The experimental results were in good agreement with previously published values while elucidating the contributions of the molecular structure to the stability of the algorithms. Finally, the necessary condition for manifesting Fabry-Pérot oscillations was delineated.

Study of modulation in complex refractive indices induced by ultrafast relativistic electrons using infrared and THz probe pulses

Objective.Achieving ultra-precise temporal resolution in ionizing radiation detection is essential, particularly in positron emission tomography, where precise timing enhances signal-to-noise ratios and may enable reconstruction-less imaging. A promising approach involves utilizing ultrafast modulation of the complex refractive index, where sending probe pulses to the detection crystals will result in changes in picoseconds (ps), and thus a sub-10 ps coincidence time resolution can be realized. Towards this goal, here, we aim to first measure the ps changes in probe pulses using an ionizing radiation source with high time resolution. Approach.We used relativistic, ultrafast electrons to induce complex refractive index and use probe pulses in the near-infrared (800 nm) and terahertz (THz, 300µm) regimes to test the hypothesized wavelength-squared increase in absorption coefficient in the Drude free-carrier absorption model. We measured BGO, ZnSe, BaF2, ZnS, PBG, and PWO with 1 mm thickness to control the deposited energy of the 3 MeV electrons, simulating ionization energy of the 511 keV photons.Main results.Both with the 800 nm and THz probe pulses, transmission decreased across most samples, indicating the free carrier absorption, with an induced signal change of 11% in BaF2, but without the predicted Drude modulation increase. To understand this discrepancy, we simulated ionization tracks and examined the geometry of the free carrier distribution, attributing the mismatch in THz modulations to the sub-wavelength diameter of trajectories, despite the lengths reaching 500µm to 1 mm. Additionally, thin samples truncated the final segments of the ionization tracks, and the measured initial segments have larger inter-inelastic collision distances due to lower stopping power (dE/dx) for high-energy electrons, exacerbating diffraction-limited resolution.Significance.Our work offers insights into ultrafast radiation detection using complex refractive index modulation and highlights critical considerations in sample preparation, probe wavelength, and probe-charge carrier coupling scenarios.

Your Clean Graphene is Still Not Clean

AbstractResearchers working with thin samples, such as monolayer graphene, are consistently struggling against contamination. Indeed, the problem of hydrocarbon contamination is known from the earliest days of electron microscopy and efforts to reduce this problem are ubiquitous to almost all high‐vacuum experiments. Accurate knowledge of the behavior of such contamination is essential for electron beam (e‐beam) based atomic fabrication, where it is aspired to select and control matter on an atom‐by‐atom basis. Here, the vexing question of hydrocarbon contamination on graphene is taken up. Image intensity is used to directly reveal the presence of diffusing hydrocarbons on ostensibly clean graphene. These diffusing hydrocarbons are previously inferred but not directly observed. Surprising dynamic variations of the concentration of these hydrocarbons impels questions about their origin. Here, some possible explanations are presented and some tentative conclusions are drawn. This work updates the conceptual model of “clean graphene” and offers refinements to the description of e‐beam induced hydrocarbon deposition.

Spatially-Resolved EELS Analysis of Surface Species on Metallic Lithium for the Development of Next Generation Li-Batteries.

Metallic Li is considered one of the most attractive anode candidates for solid-state battery technology as well as for the next generation Li-ions batteries [1,2] due to its ultrahigh theoretical specific capacity (3860 mAhg−1)[3]. However, several technical issues hinder its application as anode material. The widespread commercial usage of Li metallic sheet will necessitate a much better understanding of its microstructure and chemical nature.Metallic lithium is highly sensitive to hydrogen, oxygen, nitrogen, and carbon dioxide, which are the major components of wet air, and are likely to form rapidly LiOH, Li2O, Li2O2, Li3N, and Li2CO3 species [4] when exposed to air and/or moisture. It has been shown that this surface passivation layer has a direct impact on the anode interface resistance in solid state batteries [5]. Because of the nature of Li, very few works have looked directly at the surface chemical structure [6]. XPS and SIMS are normally used to study the nature and thickness of this passivation layer. However, these techniques do not allow to directly observe the nature and thickness of this layer. Direct observation, using TEM, would thus be much better.Because of the very high surface reactivity and the low hardness of Li, it is very difficult to prepare thin sample from a precise Li sheet for direct TEM observations and chemical analysis of the material structure and surface chemical nature. In this presentation we will show the direct HRTEM study of the surface chemical nature of metallic Li sheets. We have used a Hitachi air-protection cryogenic FIB holder to prepare thin Li samples and study the surface chemistry using Spatially-Resolved EELS (SR-EELS). The thin samples were prepared from different Li sheets using FIB (NB5000 from Hitachi) with the sample temperature at ~ -90 °C. The samples were transferred under vacuum using the same holder into a Hitachi HF-3300 cold FEG E-TEM for EELS analysis at low temperature without air contact.SR-EELS was done using a high-resolution GIF system (Gatan Quantum ER) for the rapid acquisition of the Li-K-edge, O-K-edge and C-K-edge respectively. Using this technique, it is possible to directly observe the different chemical variation in the passivation layer and its variation from sample to sample.[1] R. May, K. J. Fritzsching, D. Livitz, S. R. Denny, and L. E. Marbella ACS Energy Letters 6 (4), 1162-1169 (2021)[2] Y. Zhang, T.-T. Zuo, J. Popovic, K. Lim, Y.-X. Yin, J. Maier, Y.-G. Guo, Materials Today 33 , 56-74 (2020)[3] X.-B. Cheng, C.-Z. Zhao, Y.-X. Yao, H. Liu, Q. Zhang, Chem 5, 74-96 (2019)[4] D. Jeppson, J. Ballif, W. Yuan and B. Chou, 1978. Lithium literature review: Lithium’s properties and interactions. Richland, Washington: Hanford Engineering Development Lab[5] S.-K. Otto, T. Fuchs, Y. Moryson, C. Lerch, B. Mogwitz, J. Sann, J. Janek and A. Henss, ACS Appl. Energy Mater. 4, 12798−12807 (2021).[6] N. Brodusch, K. Zaghib and R. Gauvin, Micros. Res. & Tech. 78, 30-39 (2015)

Thickness dependence of the second magnetization peak effect in Ba0.6K0.4Fe2As2 single crystals

The second magnetization peak (SMP) effect has been observed widely in many type-II superconductors, but the reason remains elusive. This effect manifests an enhanced critical current density with magnetic field and should be very useful for applications. By measuring the magnetization of optimally doped Ba0.6K0.4Fe2As2 single crystals with different thickness, it is found the SMP effect exists in thick samples and gradually becomes invisible when the sample thickness is reduced to the scale of micrometer. Detailed investigation on the vortex dynamics on samples with different thickness clearly show that there is a common behavior of vortex dynamics in the low field region, which may be characterized by the Bragg glass like elastic vortex motion. This feature holds on in the whole field region for the thin samples, while it turns into the SMP effect for thicker samples when the field is increased. The results suggest that the SMP effect may be induced by the entanglement of the vortex system, and the absence of the SMP effect in thin samples is attributed to the cutoff of the entangled vortex length along c-axis.

Room Temperature Fluorination/Defluorination of FeFx Positive Electrode for All-Solid-State Fluoride Ion Batteries

IntroductionA fluoride battery is a new concept battery that uses fluoride ions as charge carriers. It is attracting attention as an innovative battery that surpasses lithium-ion batteries due to its high theoretical energy density. Furthermore, by making an all-solid-state battery using a solid electrolyte, it is possible to construct a kinetically advantageous battery. Iron fluoride (FeF3) is a positive electrode material with a high theoretical capacity of 712 mAh g-1. A charge/discharge capacity exceeding 500 mAh g-1 at 160 °C has been reported for a composite electrode using FeF3 powder.1 The main future challenge for FeFx positive electrode is to improve charge/discharge characteristics at low temperatures and to clarify the fluorination/defluorination mechanism. However, the currently available solid electrolytes have low conductivity, causing an increase in ohmic polarization of the composite electrode and making it difficult to evaluate the FeFx positive electrode at low temperatures. In this study, FeFx thin film electrodes were fabricated, in which the effect of ohmic polarization can be suppressed by reducing the current. Using the thin film samples, the charge/discharge characteristics and electrode reaction mechanism of FeFx positive electrodes at room temperature were investigated in detail.ExperimentalOn a LaF3 single crystal substrate (solid electrolyte), a 10 nm thick FeF3 layer and C layer were sequentially deposited as a positive electrode and a current collector, respectively, by sputtering. A PbF2 layer and a Pb foil were laminated on the opposite side of the substrate as a counter electrode. The fabricated FeFx all-solid-state thin film battery was placed in a cell that could be evacuated and heated, and then charged/discharged at the desired temperature and current rate (1C was defined 712 mAh g-1, the theoretical capacity of FeF3). The discharge/charge cutoff-voltage was set to -1.5/1.5 V or -2/3 V (vs. PbF2/Pb). The FeFx electrode reaction was electrochemically analyzed by steady state polarization and AC impedance measurements. The FeFx thin film structure was investigated by SEM and XPS. The electronic state, coordination structure, and crystal structure of the FeFx during charging/discharging were analyzed by operando XAFS/XRD using synchrotron radiation (SPring-8/BL28XU).Result and DiscussionThe experimental results are summarized below.(1) The reversible capacities at 25 °C and 0.1C were 380 and 629 mAh g-1 (53% and 88% of the theoretical capacity) for the voltage ranges of -1.5/1.5 V and -2/3 V, respectively. No capacity degradation was observed up to 30 cycles in the cycle test at 25 °C and 0.1C.(2) The electrode reaction current was proportional to the exponent of the overpotential. This relationship is known as Tafel behavior and indicates that the fluorination/defluorination of the FeFx positive electrode is rate-limited by the charge transfer process.(3) During discharging/charging, reversible changes in Fe valences (Fe3+, Fe2+, Fe0), atomic bonds (Fe–F, Fe–Fe), and crystal structure (FeF3, FeF2, Fe) were observed. The reaction capacity determined from the change in average Fe valence and the reversible capacity measured electrochemically were almost equal.(4) These results indicate that the FeFx positive electrode can be charged/discharged (fluorinated/defluorinated) at room temperature with high capacities close to the theoretical value, at practical current rates, and with excellent cyclability.AcknowledgementThis work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under RISING3 project (JPNP21006), Japan.

Evaluation of Band Structure of Single Crystalline Si-Rich SiSn thin Film

IntroductionSilicon Tin (Si1-x Sn x ) is supposed to transfer from the indirect bandgap to the direct bandgap by adding Sn with Si. Hence, it is expected to be applied to the next-generation optical devices such as Si photonics due to its high affinity with Si devices [1, 2]. The crossover point from the indirect bandgap to the direct bandgap has been estimated by various simulations. However, there is a large variation in the Sn fraction from approximately 25% [3] to 48.9% [4]. Therefore, it is essential to verify the bandgap energy and optical properties of Si1-x Sn x from the viewpoint of photonics application.In this study, we evaluated the band structure of single crystalline Si1-x Sn x films by Photoluminescence (PL) spectroscopy and Spectroscopic Ellipsometry (SE).Experimental methodTable 1 summarizes the specifications of Si1-x Sn x thin film samples used in this experiment. Si1-x Sn x thin films were grown on Si substrates by Molecular Beam Epitaxy (MBE) [5] or sputtering. Si1-x Sn x thin films (Sn fraction: 10 and 15%) were also grown on Si1-y Ge y buffer layer/ Si by MBE [2]. Epitaxial growth was confirmed by X-ray diffraction 2D reciprocal lattice space mapping (XRD-2DRSM) in all samples. Sn fraction as well as in-plane biaxial and out-of-plane strain were estimated by XRD-2DRSM. The Si thin film deposited on a Si substrate by sputtering was prepared as the reference.For PL measurements, a He-Cd laser (wavelength: 325 nm, excitation power: 15 mW) was used at the sample stage temperature of 10 K. For SE measurements, the xenon lamp with a wavelength range of 200-1600 nm was used as an excitation source, with the incident angle of 70 degrees.Results and DiscussionFigure 1 shows PL spectra of Si and Si1-x Sn x at the sample stage temperature of 10 K. Band-to-band luminescence spectra derived from Si1-x Sn x were observed from the single-crystal Si1-x Sn x (x=0.034, 0.060 and 0.10) samples which shifted toward the lower energy side with increasing Sn fraction. We calculated the indirect bandgap energy with the Transverse-Optical-phonon-assist energy and the band-to-band luminescence confirmed by PL measurements. As a result, the indirect bandgap energy of Si thin film was 1.157 eV, while the indirect bandgap energy of the other Si1-x Sn x were slightly lower around 1.151 eV. The indirect bandgap of Si1-x Sn x comparing among the three samples, Si0.966Sn0.034, Si0.94Sn0.06, and SiSn0.90Sn0.10/ SiGe buffer shifted slightly toward the lower energy side with increasing Sn fraction. Thus, a slight reduction in the indirect bandgap energy occurred with increasing Sn fraction.Figure 2 shows the imaginary part (ε2) of the complex dielectric function of (a) Si1-x Sn x (x=0.022, 0.034, 0.052, 0.060 and 0.079) and (b) Si substrate and Si thin film measured by SE. The origins of the sharp peaks are direct band transition, which are known as the critical points (CP). The optical transition energy E'0 at the Γ point and the optical transition energy E1 at the Λ point are approximately 3.4 eV in Si substrate. Hence, the origin of the peak at approximately 3.4 eV in Fig. 2 (a) is optical transitions at the Γ and Λ points. The optical transition energy indicates that they similarly shifted to the lower energy side with increasing Sn fraction. This suggests that the direct bandgap energy of Si1-x Sn x decreased. In addition, the peaks were observed at 2.8 and 2.0 eV. However, these may be due to factors other than Sn atoms addition, such as epitaxial related defects, since these peaks were also observed for Si thin film in Fig.2 (b).Figure 3 shows the direct bandgap energy of Si1-x Sn x and Si substrate from SE measurements. We found that the amount of change in the direct bandgap energy from SE results (more than 29 meV) is much larger than those of the indirect bandgap energy from PL results (less than 8 meV). Therefore, it is considered that the bandgap type approaches the direct bandgap with increasing Sn fraction. The crossover point predicted by our results is estimated to be about 47.2% of Sn fraction. This is close to the predicted value simulated by the density functional theory and the first-principles calculations [4] among various simulated values. In conclusion, we have clarified the correlation between the band structure and Sn fraction in single crystalline Si1-x Sn x .[1] M. Kurosawa et al., Appl. Phys. Lett. 106, 171908 (2015).[2] K. Fujimoto et al., Appl. Phys. Express. 16, 045501 (2023).[3] J. Tolle et al., Appl. Phys. Lett. 89, 231924 (2006).[4] Y. Nagae et al., Jpn. J. Appl. Phys. 56, 04CR10 (2017).[5] R. Yokogawa et al., ECS Trans. 98, 291 (2020). Figure 1

Space Charge Effects on the Reaction Kinetics of Metal Exsolution and the Coalescence of Exsolved Nanoparticles

Metal exsolution reactions can be driven in fuel electrode materials under the reducing operation conditions of solid oxide cells. The process enables the synthesis of nanostructured catalysts based on the release of a fraction of reducible dopants from a perovskite host to its’ surface and the subsequent nucleation of finely dispersed oxide-supported metal nanoparticles. The performance of such catalysts strongly depends on the nanoparticle characteristics, such as the nanoparticle size and nanoparticle density.Consequently, dynamic structural and chemical changes of catalyst materials under operation conditions, oftentimes particularly affecting the catalyst surface i.e. the electrochemical interface, play a crucial role for the activity and stability of electrocatalysts. It is essential to understand the mechanistic processes that govern the material response to improve the lifetime of energy conversion devices such as water splitting catalysts or solid oxide cells.We employ epitaxial thin films with atomically defined surface morphologies to study the exsolution kinetics with respect to the mass transport of Ni dopants from the bulk to the perovskite surface. In addition, we investigate the subsequent growth and coalescence behavior of the exsolved nanoparticles during thermal reduction of the thin film samples. For this purpose, different approaches of defect-engineering are explored to investigate the role of donor-type and acceptor-type defect chemistry as well as the role of dislocations for exsolution reactions.We demonstrate that the electrostatic interactions of exsolution-active dopants with the surface potential that is correlated to the inherent surface space charge region of perovskite oxides determines the kinetics of metal exsolution in our material system. Moreover, we reveal that defects that form under the reducing reaction conditions at the surface of the perovskite support can accelerate nanoparticle clustering and hence degradation of metal exsolution catalysts. In addition, we show that dislocations may serve as preferential nucleation sites for nanoparticles during metal exsolution. Based on our findings, we derive strategies for the control of the metal exsolution dynamics and for the stabilization of exsolved nanoparticles.

Determination of E crit Using 1-D Pit Approach and Its Relationship to the Repassivation Potential of 3-D Pits

Localized corrosion is a major concern in industry, as it can compromise components made from corrosion-resistant alloys, ultimately leading to failures, reduction in the lifetime of equipment, downtime in production, and safety issues. Understanding the mechanisms underlying pitting corrosion is paramount to minimizing this form of attack and developing mitigation approaches. Cyclic potentiodynamic polarization (CPP) is a common electrochemical test used to evaluate the susceptibility of materials to pitting corrosion. An important parameter that can be extracted from this technique is the repassivation potential (E rp), which has been used as a measure of resistance to localized corrosion in long-term applications and also for assessing SCC susceptibility. Another interesting approach for evaluating the kinetics of localized corrosion is the 1-dimensional (1-D) pit test, which consists of a thin wire sample embedded in epoxy resin. This setup generates a real pit environment at the material surface as the wire dissolves and provides additional parameters that cannot be directly extracted from CPP curves.In this study, 1-D pit tests were used to evaluate the corrosion behavior of SS316L at different temperatures and to identify the critical conditions for repassivation. The potential was scanned downward from the mass-transport limit region at different potential scans rates () for pits with different depths to allow for precise determination of the repassivation potential and the critical concentration, C crit, associated with pit growth stability at each temperature. Tailored values of were needed because was found to affect the shape of the downward polarization curves of 1-D pits of varying depth. For small pit depths, a scan rate of 20 mV/s is suitable for measuring E rp, which is associated with a sharp decrease in current density caused by passive film formation at the bottom of the pit. For deeper pits, however, lower scan rates are needed to allow enough time for dilution of the electrolyte inside the pit cavity, providing an accurate measurement of E rp from the sharp drop in current.The critical pit bottom potential at repassivation, E crit, can be determined from E rp by correcting for the ohmic potential drop caused by current flow in the pit. Values of E crit measured using the same deviate from the expected logarithmic dependence of E crit on pit depth [1] for depths higher than 100 mm. However, if a tailored is used, i.e. scan rates that decrease with increasing pit depth, then accurate E crit values can be obtained as indicated by the logarithmic relationship over a 10x variation in pit depth. The E crit obtained from 1-D pit tests combined with the equations of mass transport for different pit geometries enable the prediction of E rp for hemispherical 3-D pits in bulk samples, which is approximately the pit geometry commonly seen after cyclic potentiodynamic polarization (CPP) tests. A good correlation was found between E crit from 1-D pit experiments and E crit from CPP curves. The effect of pit cover was also taken into consideration in the ohmic potential drop calculations for bulk 3-D pits. This methodology provides predictions for repassivation potential, enhances the understanding of the variability in E rp values obtained from the CPP technique, and improves the reliability of this parameter.This work was supported by the US Office of Naval Research under Grant No. N00014-23-1-2202.Reference Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part III. A Unifying Set of Principal Parameters and Criteria for Pit Stabilization and Salt Film Formation, Journal of The Electrochemical Society, 165 (2018) C762-C770.

Low cost switching circuit for van der Pauw resistivity and Hall measurements at various temperatures

The electrical characterization of materials, particularly superconductors, semiconductors, and those undergoing metal-insulator transitions (MITs), relies significantly on resistivity and Hall measurements as a function of temperature. The van der Pauw four-point probe method is commonly used for such measurements, which involves resistance measurements for different circuit configurations connected to sample contacts. However, repeating these measurements at different temperatures is challenging and time consuming. This study introduces a novel approach utilizing a switching circuit controlled by an Arduino device and a LabVIEW program to automate resistance measurements for different electrical configurations. The effectiveness of this setup was demonstrated by testing V2O3 thin film samples deposited on Al2O3 (001) substrates using DC magnetron sputtering. The MIT temperature of the sample was 115 K during heating and 100 K during cooling. The sample exhibited p-type charge carriers with a Hall coefficient of 1.3 ± 0.1 x 10−4 cm3/C and Hall mobility of 1.7 x 10−1 cm2/V∙s, which is consistent with findings from other studies employing commercial equipment.