Edge Onset Research Articles

Electronic band structure and optical properties of BGaAsBi/GaAs using 16 band kp Hamiltonian

The numerical simulations of optical and electronic properties of BGaAsBi/GaAs quantum wells (QWs) are obtained by diagonalizing the 16-band kp Hamiltonian with distinct boron and bismuth compositions that exhibit multifaceted potential in 1.3–1.5 μm. It has been observed that under strain-relaxed conditions for a doping concentration of 12.5 % (B, Bi), the anticipated values of spin-orbit (SO) coupling energy (ΔSO) and the bandgap (Eg) are 302 meV and 0.952 eV, respectively. Additionally, strain interaction on band structure decreases the band gap from (Eg = 0.92eV–0.85eV), increasing dopant concentration from 8 % to 12.5 %. The consequence of incorporating both Bi and B impurity is a reduction in electron effective mass of BGaAsBi by ∼1.3 times concerning the host GaAs, enhancing the optical properties of BGaAsBi/GaAs quantum-confined heterostructures. Moreover, an increase in well-width introduced a red shift and reduced the peak amplitude of the gain spectra. In addition, for solar cell application, the onset of the fundamental absorption edge is depicted at an energy of about 0.96 eV. For laser applications, the optimum threshold current density and quality factor of BGaAsBi/GaAs quantum well achieved at room temperature are 1089 A/cm2 and 3653, respectively, at well width (w = 2.0 nm), cavity length (L = 0.5 mm), and average thickness of the active region (d = 45 nm). The variation in power density is studied with injected current density, while the quality factor is calculated with well width. The outcomes could be beneficial for future use in optical devices.

Statistical investigations of profile error impact on flow and performance of a low-pressure turbine cascade

Profile error impacts on turbomachinery flow and blade performance have been attracting widespread attention. In the study, the characteristics of profile error of about one thousand real low-pressure turbine blades are extracted. Sensitivities of total pressure loss coefficient (ζ), outflow angle (β), and Zweifel lift coefficient (zw) of the blade to the basis modes of profile error are calculated. Flow solutions of the blades considering specified basis modes with high sensitivities illustrate that profile error contributes much to the variations of transition onset and flow acceleration on the suction side and flow mixing intensity in the wake. Uncertainty quantification of performance changes is then implemented by the method of moment (MM) using second-order sensitivities. With only 5% computational cost of that by Monte Carlo simulation (MCS), the MM-based statistical results are close to MCS ones with maximum relative error not exceeding 1.07%. The statistical results reveal that the variations of both β and zw are linearly dependent, whereas the variation of ζ is nonlinearly dependent on profile error. As the variation range of profile error increases, the standard deviation and skewness increase, indicating that the performance is more dispersive and the nonlinear dependence of ζ on profile error is intensified. Finally, the MCS flow fields are analyzed. Statistical shear stress near the leading edge and transition onset, statistical boundary layer momentum thickness near transition onset, statistical intermittency near transition onset, and statistical entropy in the wake are more considerable. The impact mechanisms of profile error on turbine flow and performance changes are demonstrated.

Imaging of the electronic bonding of diamond at pressures up to 2 million atmospheres

Diamond shows unprecedented hardness. Because hardness is a measure of resistance of chemical bonds in a material to external indentation, the electronic bonding nature of diamond beyond several million atmospheres is key to understanding the origin of hardness. However, probing the electronic structures of diamond at such extreme pressure has not been experimentally possible. The measurements on the inelastic x-ray scattering spectra for diamond up to 2 million atmospheres provide data on the evolution of its electronic structures under compression. The mapping of the observed electronic density of states allows us to obtain a two-dimensional image of the bonding transitions of diamond undergoing deformation. The spectral change near edge onset is minor beyond a million atmospheres, while its electronic structure displays marked pressure-induced electron delocalization. Such electronic responses indicate that diamond's external rigidity is supported by its ability to reconcile internal stress, providing insights into the origins of hardness in materials.

Lindblad master equation approach to the topological phase transition in the disordered Su-Schrieffer-Heeger model

We use the Lindblad equation method to investigate the onset of a mobility edge and the topological phase transition in the disordered SSH chain connected to two external baths in the large bias limit. From the scaling properties of the nonequilibrium stationary current flowing across the system, we recover the localization/delocalization in the disordered chain. To probe the topological phase transition in the presence of disorder, we use the even-odd differential occupancy as a means to discriminate topologically trivial from topologically nontrival phases in the out-of-equilibrium system. Eventually, we argue how to generalize our method to other systems undergoing a topological phase transition in the presence of disorder.

Preliminary Results of a Structural Health Monitoring System Application for Real-Time Debonding Detection on a Full-Scale Composite Spar.

The present paper reports the outcomes of activities concerning a real-time SHM system for debonding flaw detection based on ground testing of an aircraft structural component as a basis for condition-based maintenance. In this application, a damage detection method unrelated to structural or load models is investigated. In the reported application, the system is applied for real-time detection of two flaws, kissing bond type, artificially deployed over a full-scale composite spar under the action of external bending loads. The proposed algorithm, local high-edge onset (LHEO), detects damage as an edge onset in both the space and time domains, correlating current strain levels to next strain levels within a sliding inner product proportional to the sensor step and the acquisition time interval, respectively. Real-time implementation can run on a consumer-grade computer. The SHM algorithm was written in Matlab and compiled as a Python module, then called from a multiprocess wrapper code with separate operations for data reception and data elaboration. The proposed SHM system is made of FBG arrays, an interrogator, an in-house SHM code, an original decoding software (SW) for real-time implementation of multiple SHM algorithms and a continuous interface with an external operator.

Effects of Multiple Local Environments on Electron Energy Loss Spectra of Epitaxial Perovskite Interfaces.

The role of local chemical environments in the electron energy loss spectra of complex multiferroic oxides was studied using computational and experimental techniques. The evolution of the O K-edge across an interface between bismuth ferrite (BFO) and lanthanum strontium manganate (LSMO) was considered through spectral averaging over crystallographically equivalent positions to capture the periodicity of the local O environments. Computational techniques were used to investigate the contribution of individual atomic environments to the overall spectrum, and the role of doping and strain was considered. Chemical variation, even at the low level, was found to have a major impact on the spectral features, whereas strain only induced a small chemical shift to the edge onset energy. Through a combination of these methods, it was possible to explain experimentally observed effects such as spectral flattening near the interface as the combination of spectral responses from multiple local atomic environments.

Surface Core Hole Electron Energy-Loss Fine Structure in MgO: Experiment and Theory

Abstract Core holes are an important contributing factor to the core-loss fine structure in electron energy-loss spectroscopy (EELS). While there has been much work on bulk materials, less is known about core hole screening in more complex dielectric environments, such as free surfaces or interfaces between two different materials, even though the latter is frequently encountered in high spatial resolution EELS analysis. In this work, experimental O K- and Mg L3,2-EELS edges from the free surfaces of a MgO cube are measured using scanning transmission electron microscopy (STEM). The free surface O K-spectrum shows extra intensity at the edge onset compared with the “bulk” spectrum. Core hole screening near a free surface is modeled using electrodynamic theory. It is shown that much of the extra intensity is due to reduced core hole screening, while genuine surface states make a smaller contribution to the fine structure. The low energy Mg L3,2-edge, however, does not show any significant change at a free surface. This is because the measurement is less surface sensitive due to strong delocalization.

Diaphite-structured nanodiamonds with six- and twelve-fold symmetries

Nanodiamonds (ND) with 1-5 nm dimensions found in meteorites or produced by chemical vapour deposition (CVD) and detonation synthesis are typically described in terms of an sp3-bonded carbon network. However, ultra-high-resolution transmission electron microscopy (uHRTEM) combined with density functional theory (DFT) modelling leads to a different structural interpretation. uHRTEM imaging and nanodiffraction studies of many NDs show six-fold symmetry features whose identity has long been controversial. We also observe diffraction patterns with twelve equally-spaced and symmetrically but unequally arranged reflections, indicating structures with crystallographically-forbidden ideal and distorted twelve-fold symmetry. Structural models based on our DFT calculations lead to an interpretation of these unusual features found throughout the meteoritic and CVD samples in terms of sp3 domains arranged around and coherently bonded to graphitic domains embedded within the diamond matrix. The bonding at the sp2-sp3 interface can explain the unusual features observed in electron energy-loss spectra (EELS) below the onset of the main diamond C1s core-loss edge leading to predictions of low-dimensional conductivity behaviour. The presence of sp2- as well as sp3-bonded regions allows us to interpret previously unexplained features of the Raman spectra and EELS data of ND materials.

Impaired neural response to speech edges in dyslexia

Speech comprehension has been proposed to critically rely on oscillatory cortical tracking, that is, phase alignment of neural oscillations to the slow temporal modulations (envelope) of speech. Speech-brain entrainment is readjusted over time as transient events (edges) in speech lead to speech-brain phase realignment. Auditory behavioral research suggests that phonological deficits in dyslexia are linked to difficulty in discriminating speech edges. Importantly, research to date has not specifically examined neural responses to speech edges in dyslexia. In the present study, we used MEG to record brain activity from normal and dyslexic readers while they listened to speech. We computed phase locking values (PLVs) to evaluate phase entrainment between neural oscillations and the speech envelope time-locked to edge onsets. In both groups, we observed that edge onsets induced phase resets in the auditory oscillations tracking speech, thereby enhancing their entrainment to speech. Importantly, dyslexic readers showed weaker PLVs compared to normal readers in left auditory regions from ~.15 sec to ~.65 sec after edge onset. Our results indicate that the neural mechanism that adapts cortical entrainment to the speech envelope is impaired in dyslexia. These findings here are consistent with the temporal sampling theory of developmental dyslexia.

Oxide-Free Three-Dimensional Germanium/Silicon Core-Shell Metalattice Made by High-Pressure Confined Chemical Vapor Deposition.

Metalattices are crystalline arrays of uniform particles in which the period of the crystal is close to some characteristic physical length scale of the material. Here, we explore the synthesis and properties of a germanium metalattice in which the ∼70 nm periodicity of a silica colloidal crystal template is close to the ∼24 nm Bohr exciton radius of the nanocrystalline Ge replica. The problem of Ge surface oxidation can be significant when exploring quantum confinement effects or designing electronically coupled nanostructures because of the high surface area to volume ratio at the nanoscale. To eliminate surface oxidation, we developed a core-shell synthesis in which the Ge metalattice is protected by an oxide-free Si interfacial layer, and we explore its properties by transmission electron microscopy (TEM), Raman spectroscopy, and electron energy loss spectroscopy (EELS). The interstices of a colloidal crystal film grown from 69 nm diameter spherical silica particles were filled with polycrystalline Ge by high-pressure confined chemical vapor deposition (HPcCVD) from GeH4. After the SiO2 template was etched away with aqueous HF, the Ge replica was uniformly coated with an amorphous Si shell by HPcCVD as confirmed by TEM-EDS (energy-dispersive X-ray spectroscopy) and Raman spectroscopy. Formation of the shell prevents oxidation of the Ge core within the detection limit of XPS. The electronic properties of the core-shell structure were studied by accessing the Ge 3d edge onset using STEM-EELS. A blue shift in the edge onset with decreasing size of Ge sites in the metalattices suggests quantum confinement of the Ge core. The degree of quantum confinement of the Ge core depends on the void sizes in the template, which is tunable by using silica particles of varying size. The edge onset also shows a shift to higher energy near the shell in comparison with the Ge core. This shift along with the observation of Ge-Si vibrational modes in the Raman spectrum indicate interdiffusion of Ge and Si. Both the size of the voids in the template and core-shell interdiffusion of Si and Ge can in principle be tuned to modify the electronic properties of the Ge metalattice.

Temperature-dependent change of the fractal dimension of Cu dendrites on Cu(111)

We investigate the shape of monatomic high Cu islands on a Cu(111) surface by variable-temperature scanning tunneling microscopy between 110 K and 240 K. Low temperature dendrites evolve towards more compact shapes at increasing temperature; finally reaching the equilibrium shape of a hexagon with rounded corners. Time-lapsed imaging at increasing temperature reveals the onset of shape change to be at ≈170 K, corresponding to the onset of edge and corner diffusion of atoms along the island’s borders. Despite a substantial variation for individual islands at each temperature, the mean fractal dimension increases monotonously between 170 K up to 240 K, from the smallest to the largest values feasible for islands grown on surfaces.

Quantification of non-bridging oxygens in silicates using X-ray Raman scattering

Detecting and quantifying non-bridging oxygen (NBO) atoms is of particular interest for understanding the physical properties of melts or compressed materials, and requires an unequivocal spectral signature usable during in situ measurements. In this paper, we evidence a low-energy feature of NBO in lithium silicate crystals using X-ray Raman scattering (XRS) spectroscopy around the energy losses of the oxygen K-edge. A specific peak at 534–535 eV in the edge onset is unequivocally attributed to the presence of NBO. Its intensity is used to quantify NBO in lithium silicate glasses. A similar feature at low energy has also been evidenced in Na2SiO3 and MgSiO3, generalizing the method to other alkali and alkali-earth silicates. This non-destructive method of NBO quantification, which is based on an X-ray inelastic scattering technique, can be extended to other spectroscopies such as electron-energy loss spectroscopy, and soft X-ray absorption spectroscopy at the oxygen K edge.

Direct Observation of Nonlinear Coupling between Pedestal Modes Leading to the Onset of Edge Localized Modes.

Prior to eruptive events such as edge localized modes (ELMs), quasicoherent fluctuations, referred to as pedestal modes, are observed in the edge of fusion devices. We report on the investigations of nonlinear coupling between these modes during quasistationary inter-ELM phases leading to the ELM onset. Three dominant modes, with density and magnetic signatures, are identified as key players in the triggering mechanism of certain classes of ELMs. We demonstrate that one of these modes is amplified by the two others through three wave interactions. The amplified mode is radially shifted relative to the other two modes towards the last-closed flux surface as the ELM event approaches. Our results suggest that nonlinear coupling of pedestal modes, associated with radial distortions pushing out of the pedestal, is a possible mechanism for the triggering of low frequency ELMs relevant for future fusion devices.

Electron beam induced changes in optical properties of glassy As35S65 chalcogenide thin films studied by imaging ellipsometry

Spectroscopic imaging ellipsometry was used to study 50 μm × 50 μm squares recorded into glassy As35S65 thin film by electron beam with different exposure doses. Optical constants (refractive index n and extinction coefficient k) of electron exposed areas were determined from imaging ellipsometric data recorded over a spectral range 390–800 nm. Complex dielectric permittivity of each of electron exposed square (glassy As35S65 with different electron exposure dose) was parametrized by a Tauc-Lorentz oscillator. Gradient optical model with the refractive index changing linearly within the layer thickness was used.Evaluation of parameters of Tauc-Lorentz oscillator revealed decrease of energy bandgap, increase of the refractive index in whole recorded spectra (more pronounced at the top of the film) and increase of extinction coefficient in the part of spectra where the material absorbs with the increasing electron exposure dose used for exposure of the squares.The decrease in local disorder with increasing electron exposure dose is quantified from the short wavelength absorption edge onset using a Mott-Davis model. The dependence of the parameters of Wemple-DiDomenico model for the transparent part of the optical constants spectra on electron exposure dose is shown.Thin film with recorded image was subsequently etched in amine based solution in aprotic solvent. While native film was fully etched off the remaining height of the exposed areas is increasing with the increasing electron exposure dose used.Knowledge of the changes of optical properties and etching rates induced by electron beam of different exposure doses can be used for fabricating of optical elements by high resolution electron beam lithography.

Synthesis and characterization of perovskite FAPbBr3−x I x thin films for solar cells

FAPbI3, FAPbBr3, and FAPbBr3−x I x perovskite thin films were produced in a single step from a solution containing a mixture of FAI, PbI2, FABr, and PbBr2 (FA = formamidinium). FAPbBr3−x I x perovskite thin films were deposited onto ITO-coated glass substrates by spin coating. X-ray diffraction analyses confirmed that these thin-film perovskites crystallize in the cubic phase (Pm-3 m) for all composition range 0 ≤ x ≤ 3. Mixed lead perovskites showed a high absorbance in the UV–Vis range. The optical band gap was estimated from spectral absorbance measurements. It was found that the onset of the absorption edge for FAPbBr3–x I x thin films ranges between 1.47 and 2.20 eV for x = 0 and x = 3, respectively. Photoluminescence emission energies for mixed halide perovskites were also dependent on their composition and presented intermediate values from 810.4 nm for FAPbI3 to 547.3 nm for FAPbBr3.

Spectroscopic ellipsometry characterization of spin-coated Ge25S75 chalcogenide thin films

Abstract Spectroscopic ellipsometry study on spin-coated non-toxic Ge25S75 thin films annealed at different temperatures were conducted. Multi sample analysis with two sets of samples spin-coated onto soda-lime glass and onto silicon wafers was utilized. Optical constants (refractive index n and extinction coefficient k) of these films were determined from ellipsometric data recorded over a wide spectral range (0.05–6 eV). Different parametrization of Ge25S75 complex dielectric permittivity which consists of a Tauc-Lorentz or Cody-Lorentz oscillator describing the short wavelength absorption edge, a Lorentz or Gauss oscillators describing phonon absorption or optically active absorption of alkyl ammonium germanium salts in the middle infrared part of spectra is discussed. Using a Mott-Davis model, the decrease in local disorder with increasing annealing temperature is quantified from the short wavelength absorption edge onset. Using the Wemple-DiDomenico single oscillator model for the transparent part of the optical constants spectra, a decrease in the centroid distance of the valence and conduction bands with increasing annealing temperature is shown and increase in intensity of the inter-band optical transition due to annealing temperature occurs. Intensity of absorption near 3000 cm−1 could be used as alternative method to evaluation of quality of prepared films.

Spectroscopic ellipsometry characterization of ZnO:Sn thin films with various Sn composition deposited by remote-plasma reactive sputtering

ZnO:Sn thin films were deposited onto thermally oxidized silicon substrates using a remote plasma reactive sputtering. Their optical constants (refractive index n and extinction coefficient k) were determined from ellipsometric data recorded over a wide spectral range (0.05–6eV). Parametrization of ZnO:Sn complex dielectric permittivity consists of a parameterized semiconductor oscillator function describing the short wavelength absorption edge, a Drude oscillator describing free carrier absorption in near-infrared part of spectra and a Lorentz oscillator describing the long wavelength absorption edge and intra-band absorption in the ultra-violet part of the spectra. Using a Mott-Davis model, the increase in local disorder with increasing Sn doping is quantified from the short wavelength absorption edge onset. Using the Wemple-DiDomenico single oscillator model for the transparent part of the optical constants spectra, an increase in the centroid distance of the valence and conduction bands with increasing Sn doping is shown and only slight increase in intensity of the inter-band optical transition due to Sn doping occurs. The Drude model applied in the near-infrared part of the spectra revealed the free carrier concentration and mobility of ZnO:Sn. Results show that the range of transparency of prepared ZnO:Sn layers is not dramatically affected by Sn doping whereas electrical conductivity could be controlled by Sn doping. Refractive index in the transparent part is comparable with amorphous Indium Gallium Zinc Oxide allowing utilization of prepared ZnO:Sn layers as an indium-free alternative.

MAPbI2.9-xBrxCl0.1 hybrid halide perovskites: Shedding light on the effect of chloride and bromide ions on structural and photoluminescence properties

The optical and structural properties of CH3NH3PbI3 can be adjusted by introducing other extrinsic ions such as chloride and bromide. In this work, mixed bromide iodide lead perovskites with a 10% fraction of chloride were prepared from methylamine, lead nitrate and the corresponding hydro acid (X=I, Br, Cl). The effect of bromide and chloride incorporation on different properties of perovskite thin film was investigated. The Pawley fit method indicates the formation of the iodide halide MAPbI3 Pm-3m cubic phase for x=0 and the tetragonal P4/mmm phase for x≥0.3. All deposited films showed a strong absorbance in the UV–vis range. The band gap values were estimated from absorbance measurements. It was found that the onset of the absorption edge for MAPbI2.9-xBrxCl0.1 thin film perovskites ranges between 1.60 and 1.80eV. Moreover, it was found that both Cl and Br affect the PL emission of the mixed halide lead perovskite, the MAPbI2.9−xBrxCl0.1 films displayed intermediate values from 730nm (MAPbI2.2Br0.7Cl0.1) to 770nm (MAPbI2.6Br0.3Cl0.1).

Investigation of phase separation in InGaN alloys by plasmon loss spectroscopy in a TEM

ABSTRACTPhase separation of InxGa1-xN alloys into Ga-rich and In-rich regions was observed by a number of research groups for samples grown with high indium content, x. Due to the radiation sensitivity of InGaN to beam damage by fast electrons, high-resolution imaging in transmission electron microscopy (TEM) or core-loss electron energy-loss spectroscopy (EELS) may lead to erroneous results. Low-loss EELS can yield spectra of the plasmon loss regions at much lower electron fluxes. Unfortunately, due to their delayed edge onset, the low energetic core losses of Ga and In partially overlap with the plasmon peaks, all of which shift with indium content.Here we demonstrate a method to quantify phase separation in InGaN thin films from the low-loss region in EELS by simultaneously fitting both plasmon and core losses over the energy range of 13-30eV. Phase separation is shown to lead to a broadening of the plasmon peak and the overlapping core losses, resulting in an unreliable determination of the indium concentration from analyzing the plasmon peak position alone if phase separation is present. For x=0.3 and x=0.59, the relative contributions of the binary compounds are negligibly small and indicate random alloys. For xnom.=0.62 we observed strong broadening, indicating phase separation.

Role of the chemical substitution on the structural and luminescence properties of the mixed halide perovskite thin MAPbI3 − xBrx (0 ≤ x ≤ 1) films

Mixed bromide iodide lead perovskites were prepared from methylamine, lead nitrate and the corresponding hydroX acid (X=I, Br), they were then deposited as thin films on ITO substrate by the spin coating process. X-ray diffraction analyses indicated the formation of a tetragonal phase I4/mcm up to x=0.4 and a cubic perovskite with space group Pm3m in the composition range of 0.6≤x≤1. Mixed lead perovskites showed a high absorbance in the UV–vis range. The band gap energy of thin films were estimated from absorbance spectral measurements, it was found that the onset of the absorption edge for MAPbI3−xBrx (x<1) thin films is ranging between 1.58 to 1.72eV. Photoluminescence emission energies for mixed halide perovskites presented intermediate values from 781nm (MAPbI3) to 545nm (MAPbBr3).