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Low power, high sensitivity, and selectivity chemiresistive gas sensors are in urgent demand for hydrogen sulfide (H2S) detection to protect human health and the world's ecosystem. In this study, multiwalled carbon nanotubes (MWCNTs) and copper oxide (CuO) submicrometer size particles' compositions were utilized to fabricate low-temperature H2S gas sensors, which were prepared using a screen-printing technique on inter-digited patterned SiO2/Si substrates. The heterostructure of MWCNT@CuO was confirmed through high-resolution transmission electron microscopy analysis and x-ray diffraction patterns. The x-ray photoelectron spectroscopy analysis reveals the chemical states, binding energies, and oxygen vacancy (Ov). Brunauer–Emmett–Teller analysis of nitrogen physisorption analysis was conducted on the samples to analyze sensor surface areas and pore size distribution. The as-fabricated MWCNT@CuO sensor shows a relative response (ΔR/R%) of 73% toward 10 ppm H2S at 50 °C temperature in a selective manner, which is 1.6 times higher than that of devices based on bare CuO. The MWCNT@CuO interface modifies the morphology and also constructs a p–p heterojunction. This leads to the reforming of the band structure and results in a low resistance of the matrix, as well as a high chemisorbed oxygen content. The use of metal oxide semiconductors with MWCNTs offers a promising approach for the development of high-performance gas sensors that are energy-efficient. [ABSTRACT FROM AUTHOR]

Successful synthesis of novel high entropy ceramic (HEC) for ultra-high temperature application classes, namely, borides, carbides, and nitrides, has been experiencing a bottleneck in having a suitable design and successful synthesis strategy. Producing high-entropy ultra-high-temperature ceramics from their oxides offers a major processing benefit, while employing a design approach using machine learning enhances the efficiency of the formation of single-phase HECs. In this regard, we propose a generalized strategy to generate a semi-synthetic database for each of these classes using literature data and atomic environment mapping-based structure plots, which can further be used to build machine learning models. The imbalance of the dataset was addressed using adaptive synthetic sampling and the edited nearest neighbors technique. The trained models are able to accurately predict over 90% of the single-phase chemistry for each of the classes. Furthermore, a few compositions representing these classes were successfully synthesized from the corresponding oxide mixture to validate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

The primary objective of the present investigation is to elucidate the operative micromechanisms influencing the strain rate sensitivity and activation volume in (FeCrNi)99Si1 and FeMnNi medium-entropy alloys. Room-temperature nanoindentation experiments at different loading rates were performed to study the evolution of the strain rate sensitivity and activation volume in (FeCrNi)99Si1 and FeMnNi medium-entropy alloys. The (FeCrNi)99Si1 samples were subjected to plane strain deformation by rolling at 77 and 300 K to study the effect of temperature on the strain rate sensitivity, while the FeMnNi and (FeCrNi)99Si1 samples were subjected to simple shear deformation by high-pressure torsion at 300 K to examine the effect of strains. Contrary to the well-documented trend observed in fcc metals and alloys, where the strain rate sensitivity typically increases with decreasing grain size, the present study reveals a distinct behavior for the current alloys. Similarly, these alloys are characterized by extremely low activation volumes of a few tens of b3 compared to 100–1000 b3 for conventional fcc metals and alloys in the microcrystalline grain size regime. Unlike conventional fcc metals and alloys, there is an insignificant change in the activation volume of the current high-/medium-entropy alloy (H/MEA) with decreasing grain size from the microcrystalline to nanocrystalline regime. The unique evolution of strain rate sensitivity and activation volume in H/MEAs is explained in terms of the evolution of distinct dislocation structures as well as synergistic operation of additional mechanisms such as twinning, phase transformation from fcc to hcp phases, cluster strengthening, and short-range ordering due to the aperiodic energy landscape existing in MEAs. [ABSTRACT FROM AUTHOR]

The present work describes the preparation of a new series of chalcogenide glasses in an As14Sb26S(60−x) (AgI)x system intending to explore its thermal, structural, optical, mechanical, and electrical properties. The differential scanning calorimetry results of the studied glasses show the sharp decrease in glass transition temperature (Tg) with the successive incremental inclusion of AgI in the composition, implying the structural changes in the glass network. A thorough Raman analysis corroborates the occurrence of changes in the glass network due to the formation of AsI3 units and Ag–S–As bonds with increasing AgI content. Also, structural changes can be reflected with the change in the optical bandgap (Eg) that was calculated using Tauc equations where it was found that Eg is in harmony with the observed structural variations of glasses. The studied glasses possess a transmittance window (∼0.68–12 μm) with transmittance above 60% in the mid-infrared region. These structural changes are closely related to the significant enhancement of conductivity of the present glasses from 10−8 to 10−6 S/cm at 373 K with a decrease in activation energies. Impedance spectra for the glass with highest AgI revealed the presence of two different relaxation processes. AC conductivity data followed an Arrhenius behavior as well as Jonscher's power law. The present work provides insights into glass network modifications due to silver iodide inclusion and its role in the enhancement of conductivity. [ABSTRACT FROM AUTHOR]

The role of curvature effects, in the thermal-field (TF) emission of electrons from nanotips, has been investigated. It is found that for field emitter tips having tip radius of curvature R a < 50 nm, the error on using the planar tunneling potential to evaluate the current density is significant compared to the curvature-corrected potential. Furthermore, the error is found to be strongly temperature dependent at low apex fields, whereas at high fields, it is only moderate. For emitter tips having tip radius R a ≥ 5 nm, a unified expression for current density applicable for field, thermal-field, and thermionic emission is obtained. In the analysis, a single linearization point corresponding to the peak of the normal energy distribution of electrons is chosen, about which the Gamow exponent is expanded. The temperature-dependent curvature-corrected current density equation is numerically validated against a benchmark that performs the current density integral with the Kemble form of the Wentzel–Kramers–Brillouin transmission coefficient up to the peak of the tunneling potential, while above the barrier, the Morse transmission coefficient is used. The agreement is found to be good. It is inferred that the curvature plays a significant role in field and thermal-field emission, whereas in thermionic emission, it is not as important. In the limit where the apex radius of curvature R a → ∞ , it is able to retrieve the Murphy–Good equation as well as the Richardson–Laue–Dushman equation in their respective limits. [ABSTRACT FROM AUTHOR]

We report a large cryogenic magnetocaloric effect stemming from an unconventional borderline first-order magnetic phase transition with negligibly small thermomagnetic hysteresis in a rare-earth-based intermetallic compound PrNdIn. The sample exhibits maximum magnetic field-induced entropy change as large as −10 J/Kg K (for 20 kOe magnetic field change) near the boiling point of oxygen. Magnetocaloric properties of PrNdIn are comparable to those of other known potential magnetocaloric materials with operating temperatures ranging between 50 and 125 K. The magnetic properties of the present sample are qualitatively reminiscent of those of the binary Pr2In and Nd2In, including the emergence of a second low-temperature anomaly in the temperature dependence of magnetization. [ABSTRACT FROM AUTHOR]

A new diamond anvil cell experimental approach has been implemented at the European x-ray Free Electron Laser, combining pulsed laser heating with MHz x-ray diffraction. Here, we use this setup to determine liquidus temperatures under extreme conditions, based on the determination of time-resolved crystallization. The focus is on a Fe-Si-O ternary system, relevant for planetary cores. This time-resolved diagnostic is complemented by a finite-element model, reproducing temporal temperature profiles measured experimentally using streaked optical pyrometry. This model calculates the temperature and strain fields by including (i) pressure and temperature dependencies of material properties, and (ii) the heat-induced thermal stress, including feedback effect on material parameter variations. Making our model more realistic, these improvements are critical as they give 7000 K temperature differences compared to previous models. Laser intensities are determined by seeking minimal deviation between measured and modeled temperatures. Combining models and streak optical pyrometry data extends temperature determination below detection limit. The presented approach can be used to infer the liquidus temperature by the appearance of SiO 2 diffraction spots. In addition, temperatures obtained by the model agree with crystallization temperatures reported for Fe–Si alloys. Our model reproduces the planetary relevant experimental conditions, providing temperature, pressure, and volume conditions. Those predictions are then used to determine liquidus temperatures at experimental timescales where chemical migration is limited. This synergy of novel time-resolved experiments and finite-element modeling pushes further the interpretation capabilities in diamond anvil cell experiments. [ABSTRACT FROM AUTHOR]

Over the last few decades, research thrust in the growth and engineering of high quality atomically smooth epitaxial thin films have increased tremendously, displaying unprecedented quantum correlated phenomena and ample potential for oxitronics. Regardless of the significant progress made thus far, the growth of high quality defect-free atomically smooth epitaxial conducting perovskite oxide thin films continues to be a very active area of research, as several intrinsic and/or extrinsic factors affect the film quality and consequent electronic properties. Here, we made efforts to summarize the progress made with focus on the growth of atomically smooth electronic quality epitaxial thin films of several highly conducting ABO3 perovskites with low resistivity (high conductivity). Concomitantly, optimization of growth parameters (kinetics and thermodynamics of the material) and the film–substrate interaction seems to play a crucial role in epitaxy quality. Highly conducting epitaxial oxide thin films are pivotal in bridging the gap between oxides and the practicalities in integrating them for electronics. [ABSTRACT FROM AUTHOR]

In this paper, we show the direct correlation between the suppression of the polaronic oxygen vacancy defect (Vo) density and gas sensor response of 1 at. % Mo-doped V2O5 nanowires (MVONWs). Doping 1 at. % Mo5+ leads to substitution at the V5+ site in V2O5 nanowires (VONWs) and, therefore, reduction in Vo defects. This, in turn, affects the charge carrier hopping sites and, subsequently, enhances the sensor response at lower temperatures (<320 °C). The Mo5+ dopants lead to the lowering of Fermi energy (EF) toward valence band maxima due to the reduced Vo donor density. The polaron suppression is confirmed with the activation energy of polaron hopping, increasing from 195 to 385 meV in VONWs and MVONWs. As a result, the response to ethanol gas enhanced as the depletion width is widened for the given cross section of the nanowires. This may lead to a large depletion controlled cross-sectional area and, therefore, better sensitivity. At about 350 °C, VONWs show a change in the slope of resistance vs temperature (MIT), which is not observed in the case of MVONWs. This is attributed to the presence of the enhanced non-stoichiometry of V ion resulting in metallic behavior and accompanied by a sudden rise in the sensor response at this temperature. Moreover, the absence of MIT may be attributed to the lack of such a sudden rise in the response in MVONWs. [ABSTRACT FROM AUTHOR]

Lattice distortion is considered to be one of the four core effects in a multicomponent high-entropy alloy. However, their effect is least understood from experiment and theoretical standpoints. The present investigation revealed a unique way to understand this effect by combining experiments with density functional theory (DFT) calculations. A small amount of Al and Si were carefully added to the whole-solute matrix of Cantor alloys. The different-sized atomic species introduces a huge lattice distortion in the matrix, leading to a simultaneous improvement in yield strength (YS), ultimate tensile strength (UTS), and percent elongation. An extensive DFT simulation indicates that a lattice distortion is prominent in an Al-containing alloy, whereas Si does not induce a lattice distortion. However, Si leads to severe interlayer lattice distortion, caused by the displacement of Si, during twinning. This leads to the improvement of YS, UTS, and ductility. Lattice distortion and its variants play significant effects on the mechanical properties of high-entropy alloys (HEAs) in terms of local lattice distortion, providing an uneven energy landscape for the movement of line defects or interlayer distortion. The inherent nature of local lattice distortion in HEAs leads to the wavy or tortuous dislocation, unlike a straight dislocation in conventional alloys. The movement of the wavy type of dislocation through a distorted or defective lattice requires large stress, resulting in a pronounced effect on solid solution strengthening. This local lattice distortion also dictates the degree of the interlayer distance distortion in the vicinity of atoms, leading to an increase or decrease in stable stacking fault energy that decides the deformation mode via slip or twinning. [ABSTRACT FROM AUTHOR]

The curvature-corrected field emission current density, obtained by linearizing at or below the Fermi energy, is investigated. Two special cases, corresponding to the peak of normal energy distribution and mean normal energy, are considered. It is found that the current density evaluated using the mean normal energy results in errors in the net emission current below 3% for apex radius of curvature R a ≥ 5 nm and for apex fields E a in the range of 3–10 V/nm for an emitter having a work function of ϕ = 4.5 eV. An analytical expression for the net field emission current is also obtained for local parabolic tips using the generalized cosine law. The errors are found to be below 6% for R a ≥ 5 nm over an identical range of apex field strengths. The benchmark current is obtained by numerically integrating the current density over the emitter surface and the current density itself computed by integrating over the energy states using the exact Gamow factor and the Kemble form for the WKB transmission coefficient. The analytical expression results in a remarkable speed-up in the computation of the net emission current and is especially useful for large area field emitters having tens of thousands of emission sites. [ABSTRACT FROM AUTHOR]

Here, glancing angle deposition is employed to synthesize the undoped GeO2 and Mg-doped (0.4 and 0.8 at. %) GeO2 nanowires (NWs) on a Si substrate. The microscopic images show the formation of the NW-like morphology of the grown materials. The gradual decrease in the average ratio of length to diameter depicts the worsening of the formation of NWs with the incorporation of Mg into the GeO2 host lattice. This also affects the crystallinity characteristics of the materials, which have been demonstrated from the selected area electron diffraction (SAED) pattern of the materials. The polycrystallinity nature of undoped GeO2 NWs changes to amorphous due to the introduction of Mg, which has been confirmed from both the obtained SAED and x-ray diffraction patterns of the samples. The presence of Mg was confirmed from the obtained broad bands at 473 and 437 cm−1 in the Fourier transmission spectrum of the doped samples. The increasing conductance with the temperature of Au/undoped GeO2 devices can be explained by the thermionic emission process, whereas the Mg-GeO2 device shows an overall decrease in conductance with increasing temperature. We have ascribed the origin of this abnormal conductance as the positive temperature coefficient of resistance, which is one of the first reports, due to the generation of random grain boundaries and enormous electron trapping at the Au/Mg-GeO2 NW junction. Furthermore, the undoped GeO2 NW device shows good temperature-dependent conductivity as well as stability compared to the doped one. [ABSTRACT FROM AUTHOR]

Boron carbide (B 4 C) has been well studied both theoretically and experimentally in its bulk form due to its exceptional hardness and use as a high-temperature thermoelectric. However, the properties of its two-dimensional nanosheets are not well established. In this paper, using van der Waals-corrected density-functional theory simulations, we show that bulk B 4 C can be cleaved along different directions to form B 4 C nanosheets with low formation energies. We find that there is minimal dependence of formation energies on cleavage planes and surface terminations, even though the bulk is not van der Waals layered. This anomalous stability of B 4 C nanosheets is found to be a result of surface reconstructions that are unique to B-rich systems. While the density of states of the bulk B 4 C indicate that it is a semiconductor, the B 4 C nanosheets are found to be predominantly metallic. We attribute this metallic behavior to a redistribution of charges on the surface bonds of the films. The Seebeck coefficients of the B 4 C films remain comparable to those of the bulk and are nearly constant as a function of temperature. Our results provide guidance for experimental synthesis efforts and future application of B 4 C nanosheets in nanoelectronic and thermoelectric applications. [ABSTRACT FROM AUTHOR]

Superconducting weak link (WL), acting as a Josephson junction (JJ), is one of the widely used elements in superconductor science and quantum circuits. A hysteretic JJ with robust switching between its superconducting and resistive state is an excellent candidate for single-photon detection. However, the ubiquitous fluctuations in the junction strongly influence the stability of the states and, thus, the transition from one to the other. Here, we present an experimental study of switching statistics of critical and retrapping currents of a JJ based on niobium WL in its hysteretic regime. The mean lifetimes of the two metastable states, namely, the zero-voltage superconducting state and finite-voltage resistive state, are estimated from the distributions. Further, close to the hysteresis crossover temperature, observed telegraphic noise in the time domain due to random switching between the states provides their lifetimes directly. We present a thermal model introducing a double-well (bistable) feature with an intriguing quantity with respect to the devices' temperature states. The effects of temperature fluctuations on the stability of the states are shown. We discuss our results toward further improvement of the efficiency of superconducting WL or nanowire single-photon detectors. [ABSTRACT FROM AUTHOR]

The factors that contribute to the accuracy of the cold field emission current within the contemporary frameworks are investigated. It is found that so long as the net current is evaluated using an expression for the local current density obtained by linearizing the Gamow factor, the primary source of error is the choice of the energy at which the Taylor expansion is done, but not as much on the choice of the method used to arrive at the approximate Gamow factor. A suitable choice of linearization energy and the implementation of the Kemble correction allow the restriction of errors to below 3% across a wide range of local fields. [ABSTRACT FROM AUTHOR]

We have investigated the in-plane and out-of-plane magnetostriction (λ) and strain sensitivity (d λ / dH) of the polycrystalline ZnFe2O4 (ZFO) film on an Si(100) substrate at room temperature using the optical cantilever beam magnetometer. A remarkable enhancement in magnetostriction (129.34%) and strain sensitivity (218.49%) is obtained in the out-of-plane configuration in comparison with the same in the in-plane configuration. The film possesses a high positive magnetostriction (strain sensitivity) of 325.67 ± 0.42 ppm (4.65 × 10 − 9 A − 1 m) and 746.92 ± 1.18 ppm (14.81 × 10 − 9 A − 1 m) for in-plane and out-of-plane geometry, respectively, at room temperature. The huge enhancement in magnetostriction and strain sensitivity is ascribed to the shape anisotropy of the ZFO/Si composite in the out-of-plane configuration, and thus, the out-of-plane configuration would be highly potential in designing magnetic actuators, magnetic memory devices, and bio-medical devices at room temperature. [ABSTRACT FROM AUTHOR]

Block copolymer (BCP) self-assembled nanostructures as a template in conjunction with a low-cost inorganic material deposition method can be a practical solution for many applications in the fields of microelectronics, optoelectronics, and plasmonics. Here, we demonstrate the fabrication of TiO2 nanodot films using polystyrene-b-polymethylmethacrylate (PS-b-PMMA) BCP as a template and a simple solution dipping process for TiO2 deposition. For this purpose, we prepared BCP templates using two different methods, namely, the selective deposition method and the masked deposition method. In the selective deposition method, as-grown self-assembled cylinder forming PS-b-PMMA was used as a template and in the masked deposition method, PMMA was etched out selectively from PS-b-PMMA nanostructured films. The scanning electron microscopy results show the average diameter of TiO2 nanodots grown by the selective deposition method is smaller compared to the masked deposition method, whereas the inter-nanodot distance is similar for both deposition methods. X-ray diffraction and photoluminescence confirm the formation of TiO2 in samples deposited by these two methods. The smaller nanodot size for the selective deposition method can be attributed to the limited interaction of the Ti precursor used here with the PMMA copolymer active functional groups. Therefore, in addition to being advantageous due to less processing steps, the selective deposition method can be used for the fabrication of lower dimensional nanostructures by identifying proper precursors and polymers and by controlling the interaction parameters. Our results will be useful for exploring interactions of other polymers with inorganic material precursors and thereby fabricating different nanostructures with desired morphologies using a simple and cost-effective dipping method. [ABSTRACT FROM AUTHOR]

We demonstrate an Nb 2 O 5 / β - Ga 2 O 3 metal–insulator–semiconductor (MIS) hetero-junction diode with Nb 2 O 5 as the high-k dielectric insulator for more efficient electric field management resulting in enhanced breakdown characteristics compared to a β - Ga 2 O 3 Schottky barrier diode. Nb 2 O 5 dielectric films were grown using atomic layer deposition and exhibited a high dielectric constant of 50. The high dielectric constant resulted in a 5 × lower electric field at the metal/dielectric interface in the MIS diode compared to the metal/ β - Ga 2 O 3 interface in the Schottky barrier diode. With good electron conduction in forward bias enabled by the negative conduction band offset of Nb 2 O 5 with respect to β - Ga 2 O 3 , the MIS design led to a 3 × improvement in the reverse blocking voltage with a slight trade-off in the specific on-resistance. Overall, a 3.3 × increase in the power figure of merit was observed (3.25 MW / cm 2 for the Schottky diode and 10.8 MW / cm 2 for the MIS diode). A detailed analysis of the energy band line-up and the forward and reverse current transport mechanisms are also presented using analytical modeling and 2D technology computer-aided design simulations. [ABSTRACT FROM AUTHOR]

Optimization of materials' performance for specific applications often requires balancing multiple aspects of materials' functionality. Even for the cases where a generative physical model of material behavior is known and reliable, this often requires search over multidimensional function space to identify low-dimensional manifold corresponding to the required Pareto front. Here, we introduce the multi-objective Bayesian optimization (MOBO) workflow for the ferroelectric/antiferroelectric performance optimization for memory and energy storage applications based on the numerical solution of the Ginzburg–Landau equation with electrochemical or semiconducting boundary conditions. MOBO is a low computational cost optimization tool for expensive multi-objective functions, where we update posterior surrogate Gaussian process models from prior evaluations and then select future evaluations from maximizing an acquisition function. Using the parameters for a prototype bulk antiferroelectric (PbZrO3), we first develop a physics-driven decision tree of target functions from the loop structures. We further develop a physics-driven MOBO architecture to explore multidimensional parameter space and build Pareto-frontiers by maximizing two target functions jointly—energy storage and loss. This approach allows for rapid initial materials and device parameter selection for a given application and can be further expanded toward the active experiment setting. The associated notebooks provide both the tutorial on MOBO and allow us to reproduce the reported analyses and apply them to other systems (https://github.com/arpanbiswas52/MOBO_AFI_Supplements). [ABSTRACT FROM AUTHOR]

Field-emission studies incorporating the effect of space charge reveal that for planar emitters, the steady-state field E P , after initial transients, settles down to a value lower than the vacuum field E L. The ratio ϑ = E P / E L is a measure of the severity of space-charge effect, with ϑ = 0 being the most severe and ϑ ≃ 1 denoting the lack of significant effect. While E L can be determined from a single numerical evaluation of the Laplace equation, E P is largely an unknown quantity whose value can be approximately found using physical models or can be determined "exactly" by particle-in-cell or molecular dynamics codes. We propose here a simple model that applies to planar as well as curved emitters based on an application of Gauss's law. The model is then refined using simple approximations for the magnitude of the anode field and the spread of the beam when it reaches the anode. The predictions are compared with existing molecular dynamics results for the planar case and particle-in-cell simulation results using PASUPAT for curved emitters. In both cases, the agreement is good. The method may also be applied to large area field emitters if the individual enhancement factors are known, for instance, using the hybrid model [D. Biswas, J. Vac. Sci. Technol. B 38, 063201 (2020)]. [ABSTRACT FROM AUTHOR]

Any degree of surface roughness could play a significant role in determining the optical properties of ultra-thin films required for epsilon-near-zero (ENZ) applications. In this report, we have provided a systematic analysis of the evolution of an ENZ mode with increasing surface roughness values and established both experimentally and theoretically that roughness acts as a supporting mechanism for achieving a strong ENZ plasmon resonance response in randomly rough indium tin oxide thin films. For pulsed laser deposited indium tin oxide thin films, ENZ plasmon-mediated absorption is enhanced monotonically with the increasing surface roughness. A value of 99.75%, depicting near-perfect absorption, at a wavelength of 1335 nm for the incidence angle of 50 ° is demonstrated experimentally via Kretschmann–Raether configuration for the film with the highest surface roughness. A modified transfer matrix method based on the anisotropic Bruggemann effective medium approximation is being used to effectively simulate the experimental spectra, and based on this analysis, an even higher absorption is predicted at lower angles outside the experimentally viable domain. Such a high value of absorption just above the ENZ wavelength is due to the strong electric field enhancement inside the film layer, while in terms of absorption loss, surface roughness leads the way and contributes immensely toward the occurrence of perfect absorption in the collective media. Modification of the ENZ mode dispersion in the presence of a surface roughness layer is also discussed, and observed perfect absorption is recognized as the outcome of the crossover between the internal damping and radiation damping terms. [ABSTRACT FROM AUTHOR]

Colloidal suspension of free standing silicon/silicon oxide core/shell nanoparticles has been synthesized using a mechanochemical top-down approach. Quasi-mono-dispersed core size distribution of synthesized nanoparticles has been confirmed using different structural, morphological, and optoelectronic characterizations. Raman, continuous wave photoluminescence and time-resolved photoluminescence studies have been performed on synthesized colloidal nanoparticles in ethanol medium. Asymmetric broadening of the Raman peak (red shifted with respect to that of bulk silicon) has been observed. Intensities and positions of photoluminescence emission peaks are prominently dependent on excitation photon energy. Moreover, the photoluminescence decay time varies from sub-nanoseconds to tens of nanoseconds. The decay time also exhibits a strong dependence on the excitation wavelength, while the emission wavelength is kept unaltered. The abovementioned observations indicate the slow relaxation of photo-excited carriers in silicon quantum dots. This particular phenomenon takes place due to phonon mode discretization, which is further responsible for the radiative recombination of hot-carriers and consequent strong visible emission. The enhancement of hot-carrier lifetime in colloidal silicon quantum dots is the key requirement for the active material of the hot-carrier solar cell. Runny texture of the synthesized material inhibits practical device implementation; therefore, the synthesized nanoparticles have been embedded in the silica xerogel matrix. The impression of phonon mode confinement, in silicon quantum dots embedded in a hard matrix, has been observed, resulting in increased hot-carrier lifetime. The enhanced hot-carrier lifetime can lead to the realization of a silicon-based active material for the hot-carrier solar cell. [ABSTRACT FROM AUTHOR]

A simple expression for the Gamow factor is obtained using a second-order curvature-corrected tunneling potential. Our results show that it approximates accurately the "exact-WKB" transmission coefficient obtained by numerically integrating over the tunneling region to obtain the Gamow factor. The average difference in current density using the respective transmission coefficients is about 1.5 % , across a range of work functions ϕ ∈ [ 3 − 5.5 ] eV, Fermi energy E F ∈ [ 5 − 10 ] eV, local electric fields E l ∈ [ 3 − 9 ] V/nm, and radius of curvature R ≥ 5 nm. An easy-to-use correction factor λ P is also provided to approximately map the "exact-WKB" current density to the "exact" current density in terms of E F / ϕ. The average error on using λ P is found to be around 3.5 %. This is a vast improvement over the average error of 15 % when λ P = 1. Finally, an analytical expression for the curvature-corrected current density is obtained using the Gamow factor. It is found to compare well with the "exact-WKB" current density even at small values of local electric field and the radius of curvature. [ABSTRACT FROM AUTHOR]

Proper coupling between structural and magnetic transitions is critical for the emergence and control of magnetocaloric effects in solids. We examine the influence of minor substitutional doping (replacing Mn by Cr and Al by Sn) and interstitial doping with B on the magnetic, structural, and magnetocaloric properties of recently discovered Mn0.5Fe0.5NiSi0.94Al0.06 alloy exhibiting a giant magnetocaloric effect near room temperature. We demonstrate that magnetocaloric properties of the base compound can be controlled and, in some cases, improved by chemical substitutions. First-principles computations elucidate how small changes in the composition affect properties in this family of compounds and, thus, provide useful guidance for the selection of suitable doping elements for such materials. The magnetic-field-induced entropy change measured for Mn0.5Fe0.5NiSi0.94Al0.06B0.005 is −22 J/kg K near room temperature for the applied magnetic field of 2 T, and it is among the highest known values for this class of materials. [ABSTRACT FROM AUTHOR]

Field emission tips with an apex radius of curvature below 100 nm are not adequately described by the standard theoretical models based on the Fowler–Nordheim and Murphy–Good formalisms. This is due to the breakdown of the "constant electric field" assumption within the tunneling region leading to substantial errors in current predictions. A uniformly applicable curvature-corrected field emission theory requires that the tunneling potential is approximately universal irrespective of the emitter shape. Using the line charge model, it is established analytically that smooth generic emitter tips approximately follow this universal trend when the anode is far away. This is verified using COMSOL for various emitter shapes including the locally non-parabolic "hemisphere on a cylindrical post." It is also found numerically that the curvature-corrected tunneling potential provides an adequate approximation when the anode is in close proximity as well as in the presence of other emitters. [ABSTRACT FROM AUTHOR]

Ternary dielectrics with varying composition formed by alloying two binary oxides can enable tunable optical and electrical properties for advanced technological applications. Atomic layer deposition (ALD) gives precise control over ternary dielectric composition through the ability to finely tune the precursor pulsing ratio. This work presents ALD development of Si-rich SixTi1−xO2 dielectrics with varying composition (x), along with spectroscopic and electrical characterization of their properties. Stoichiometry of the SixTi1−xO2 films was determined using X-ray photoelectron spectroscopy. Their composition-dependent refractive index, energy bandgap, and reflectance show promise for diverse optical applications ranging from anti-reflective coatings in photovoltaics to optical waveguides. This work also reports a first comparative study of SixTi1−xO2 films prepared by thermal (T-) and plasma-enhanced (PE-) ALD with varying Si composition and deposition temperatures. Deposition rates of 0.67–0.92 Å/cycle were obtained for SixTi1−xO2 films with x = 0.5–0.91 deposited using PE-ALD at 250 °C, which were higher than that of T-ALD at 200 °C (0.42–0.05 Å/cycle). PE-ALD also exhibited a high deposition rate of 0.81 Å/cycle for SixTi1−xO2 film with x = 0.91 at a low growth temperature of 150 °C. The PE-ALD Si-rich silica–titania films show substantially lower (100×) leakage current densities than the thermally deposited films, along with higher breakdown fields for decreasing deposition temperature. A dielectric constant as low as ∼5 was achieved for PE-ALD SixTi1−xO2 films with high Si (x = 0.91) content. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a proof-of-concept oxygen sensor based on the thermoelectric principle using polycrystalline GdBaCo2O5+δ, where 0.45 < δ < 0.55 (GBCO). The lattice oxygen in layered double perovskite oxides is highly susceptible to the ambient oxygen partial pressure. The as-synthesized GBCO sample processed in ambient conditions shows a pure orthorhombic phase (Pmmm space group) and a δ-value close to 0.5 as confirmed by x-ray diffraction Rietveld refinement. The x-ray photoelectron spectroscopy (XPS) shows a significant Co3+ oxidation state in non-octahedral sites in addition to Co3+ as well as Co4+ in octahedral sites. The insulator-to-metal transition (MIT) is observed at 340 K as seen from resistivity and Seebeck coefficient. The Seebeck coefficient shows a large change of 10–12 μV/K with a time constant of ∼20 s at 300 K, when the gas ambience is changed from 100% oxygen to nitrogen and vice versa. The diffusion of oxygen in the GdOδ planes leads to the hole doping, which is a dominant factor for a large change observed in the Seebeck coefficient. This is also evident from the higher fraction of oxidized Co4+ as seen from XPS measurements. The interfacial grain boundary in addition to the oxygen diffusion contributes to the change in Seebeck. The change in Seebeck coefficient is minimal in the metallic state due to an insignificant increase in the carrier concentration, but the response is fairly well and reproducible for stoichiometry δ = 0.5 ± 0.05 below MIT. This principle shall be of significant importance in designing oxygen sensors operational at room as well as cryogenic temperatures. [ABSTRACT FROM AUTHOR]

The lifetime of GaAs photocathodes in polarized electron guns is limited due to the delicate activation layer. An atomically clean and smooth GaAs surface is needed to deposit a robust activation layer, such as Cs 2 Te , with longer lifetime compared to traditional (Cs,O) activation. A previous experiment with Cs 2 Te activation on GaAs used heat cleaning temperatures around 400 ° C to avoid an increase in surface roughness [Bae et al., Appl. Phys. Lett. 112, 154101 (2018)]. High-temperature heat cleaning around 580 ° C , which results in a relatively contamination-free surface, could be one possible way to improve quantum efficiency. However, one should be cautious about surface roughness degradation during high-temperature heat cleaning. In this paper, we report results of surface roughness measurements on native, heat cleaned, and (Cs,O) activated GaAs photocathodes under vacuum. The results, measured by ultrahigh vacuum scanning tunneling microscopy, show that the surface roughness improves as the heat cleaning temperature is increased, by at least a factor of three for 580 ° C heat cleaning, compared to the native sample. Activation with (Cs,O) is shown to increase surface roughness by a factor of four compared to a 580 ° C heat cleaned sample. This confirms that high-temperature heat cleaning can be useful for depositing good quality robust activation layers on GaAs. We also report chemical analysis for each step of preparation for p-doped GaAs photocathodes using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and ultraviolet photoelectron spectroscopy. Our results indicate that the (Cs,O) activation layer forms a sandwich structure consists of Cs and oxygen. We found no formation of any specific compound such as Cs 2 O or Cs 11 O 3. [ABSTRACT FROM AUTHOR]

Thin films of the manganite (La 1 − y Pr y) 1 − x Ca x MnO 3 exhibit dynamic phase coexistence with micrometer scale, fluid-like ferromagnetic metallic (FMM) regions interspersed in a charge-order insulating background. It has been previously reported that a uniform electric field realigns the fluid-like FMM regions due to a phenomenon similar to dielectrophoresis. Here, we report that non-uniform electric fields have a stronger effect on the FMM regions as expected from the dielectrophoresis model. The dynamic percolation of the FMM regions is observed over a wider range of temperatures compared to the results in a uniform field. Additionally, in a non-uniform electric field, the time required for dynamic percolation along the magnetic hard axis (t B ) decreased with increasing applied voltage (V A ) as a power law, V A − δ , with δ ≈ 5 while δ < 2 for a uniform electric field. Our results in a non-uniform electric field provide strong evidence in favor of the dielectrophoresis model and a unique method for manipulating micrometer-sized ferromagnetic regions using electric fields. [ABSTRACT FROM AUTHOR]

Disorders present at the interfaces drive solid state amorphization (SSA) and recrystallization in Ni/Ti multilayers. In this communication, the effect of disorders present in the bulk Ni layers is studied on SSA and recrystallization. The disorder in the Ni layers is created by sputtering Ni layers of the Ni/Ti multilayers under the mixed ambience of argon and nitrogen. It has been observed recently that the neutron optical performances of Ni/Ti graded multilayers or supermirrors can be improved by sputtering of Ni layers under the mixed ambience of argon and nitrogen. In this work, Ni/Ti periodic multilayers deposited under pure argon ambience and under mixed ambience of argon and nitrogen have been annealed under vacuum at 200 °C, 400 °C, and 600 °C for 2 h. All the annealed multilayer samples along with as-deposited multilayers are characterized by x-ray diffraction and grazing incidence x-ray reflectivity measurements in specular and non-specular modes. Atomic diffusion within the layers and across interfaces has been studied as a function of annealing temperature for both sets of multilayers. It is found that the interstitial nitrogen atoms present in the Ni layers play a crucial role in the diffusivity at room temperature as well as high temperature and thus influence the SSA and recrystallization phenomena. [ABSTRACT FROM AUTHOR]

This study investigates the ability to locally ignite metallized propellants via microwave absorption. Metallized energetic composite films incorporating high mass loadings of aluminum and titanium nanoparticle fuels within a polyvinylidene fluoride (PVDF) polymer matrix were constructed by direct-write additive manufacturing (3D printing). Simulations of power absorption for both Ti and Al nanoparticles reveal that the passivating shell composition likely plays a significant role in the observed ignition phenomenon. Various architectures of interest were constructed for predictable microwave ignition and propellant propagation. It was found that, although aluminum nanoparticles and composites do not ignite via exposure to microwaves, titanium nanoparticles can be used as efficient reactive microwave susceptors enabling a localized ignition source. This approach enables various architectures of previously studied high energy Al/PVDF systems to be fabricated and outfitted with a microwave-sensitive titanium composite in strategic locations as a means of remote ignition for aluminum systems. [ABSTRACT FROM AUTHOR]

In this study, aluminum, a p-block element, is substituted at the Cu(1) site, and its effect on the structural and thermoelectric properties of tetrahedrite Cu12−xAlxSb4S13 (x = 0.1, 0.25, 0.5, and 0.75) was investigated. The samples were prepared via solid-state synthesis followed by induction hot pressing. The theoretical calculations, using density functional theory (DFT), showed that the Al substitution results in lowering the band degeneracy near the Fermi level (EF) with EF moving towards the bandgap, indicating effective compensation of holes. The projected density of states (PDOS) revealed almost negligible hybridization of Al states with Cu 3d and S 3p states near EF, thus resulting in relatively low DOS near EF. The electrical resistivity and Seebeck coefficient increased with increasing Al content due to the compensation of holes and reduction of the charge carrier concentration. However, the Seebeck coefficient values were relatively low due to a low DOS near EF, as indicated by the DFT calculations. Although the electronic thermal conductivity (κe) decreased with increasing Al concentration, the magnitudes of the total thermal conductivity (κT) could not be reduced significantly. As a result, a maximum zT of 0.6 at 673 K was obtained for Cu11.9Al0.1Sb4S13. Based on the current study and previously reported results, the paper demonstrates how the phase stability and transport properties of the tetrahedrite are affected significantly by the nature of the substituent at the Cu(1) tetrahedral site. [ABSTRACT FROM AUTHOR]

Resonant excitation and manipulation of complex interactions among two or more resonances in high-index dielectric nanostructures provide great opportunities for engineering novel optical phenomena and applications. However, difficulties often arise when interpreting the observed spectra because of the overlap of the broad resonances contributed by many factors such as particle size, shape, and background index. Therefore, selective excitation of resonances that spectrally overlap with each other provides a gateway towards an improved understanding of the complex interactions. Here, we demonstrate selective excitation and enhancement of multipolar resonances of silicon nanospheres using cylindrical vector beams (CVBs) with different diameters of nanospheres and numerical apertures (NAs) of the excitations. By combining single particle spectroscopy and electrodynamic simulations, we show that the radially polarized beam can selectively excite the electric multipoles, whereas the azimuthally polarized beam can selectively excite the magnetic multipoles even though multipolar resonances are convoluted together due to their spectral overlap. Moreover, focusing the CVBs with high NA can lead to a dominant longitudinal polarization of the electric or magnetic field. We show that the enhanced longitudinal polarization with increasing NA of the radially and azimuthally polarized beams can selectively enhance the electric and magnetic multipolar resonances, respectively. Our approach can be used as a spectroscopy tool to enhance and identify multipolar resonances leading to a better understanding of light-matter interactions in other dielectric nanostructures as well as serve as a first step toward excitation of dark mode and Fano resonances in dielectric oligomers by breaking the symmetry of the nanostructures. [ABSTRACT FROM AUTHOR]

The influence of magnetic field cycling through the first-order magnetostructural transformation on the magnetic and magnetocaloric properties, as well as hysteresis of polycrystalline Gd5Si2Ge2, has been studied using magnetometry. The cycling has a minimal effect on the magnetic field-induced entropy change and the phase transformation temperature of the material. On the other hand, magnetic hysteresis decreases by 30% after approximately ten cycles and remains low unless the sample is moved far into the paramagnetic regime. Factors playing a role in the history dependence of hysteresis have been discussed. [ABSTRACT FROM AUTHOR]

The pseudoperovskite BiMnO3 is a multiferroic candidate, but missing details of materials properties are impeding potential technological applications. BiMnO3 has a centrosymmetric structure that precludes ferroelectricity in bulk stoichiometric samples, while some films have reported ferroelectricity along with a decreased magnetic response. This puzzle motivated a study of one such film, deposited by pulsed laser deposition onto SrTiO3. Probes utilized include microscopy, diffraction, reflectometry, and X-ray absorption. These experiments in the context of the existing literature show an anomalous unit-cell volume and a (magnetic) depth profile. Then, the resulting inhomogeneous deficiency of Bi and Mn metals may stabilize a multiphase system that explains the decreased magnetism. Film nanostructure and strain effects are also considered. [ABSTRACT FROM AUTHOR]

Al x In y Ga (1 − x − y) N / GaN heterostructures were grown on 4-in. p -type Si wafers to investigate the effect of In composition in the quaternary nitride layer on the electrical performance of Al 2 O 3 / AlInGaN / GaN -based normally-ON metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). From the comparative study of the electrical measurements, it was observed that the transport properties of the devices were relatively poor in the presence of higher In composition in the quaternary-N layer. The deterioration of the electrical characteristics of MIS-HEMTs originated from the formation of deep pits on the AlInGaN epilayer surface caused by the segregation of In atoms during epitaxial growth. However, the formation of such pits was reduced for the quaternary epilayer with lower In content and exhibited better transport performance. A maximum current density (I d , m a x ) of 780 mA/mm with a specific ON-resistance of 0.71 m Ω cm 2 was observed for the device fabricated on the wafer with an In composition of 9% in the AlInGaN epilayer. We have achieved a high breakdown voltage of 793 V with a device with the gate-to-drain distance (L g d ) of 20 μ m under the off-state condition. [ABSTRACT FROM AUTHOR]

We propose a novel heterostructure system consisting of compounds with chemical formula A 2 Mo 3 O 8 (A, B: Zn, Mg, Cd) that can host a two-dimensional electron/hole gas (2DEG/2DHG). The formation of a 2DEG/2DHG in these heterostructures, which have a low interfacial strain, is driven by a polarization discontinuity at the interface. The sheet carrier densities and charge localization in these heterostructures are comparable to other well-known heterostructures that host 2DEG opening up the possibility of a wide variety of applications. [ABSTRACT FROM AUTHOR]

Predictive modeling of materials requires accurately parameterized constitutive models. Parameterizing models that describe dynamic strength and plasticity require experimentally probing materials in a variety of strain rate regimes. Some experimental protocols (e.g., plate impact) probe the constitutive response of a material using indirect measures such as free surface velocimetry. Manual efforts to parameterize constitutive models using indirect experimental measures often lead to non-unique optimizations without quantification of parameter uncertainty. This study uses a Bayesian statistical approach to find model parameters and to quantify the uncertainty of the resulting parameters. The technique is demonstrated by parameterizing the Johnson-Cook strength model for aluminum alloy 5083 by coupling hydrocode simulations and velocimetry measurements of a series of plate impact experiments. Simulation inputs and outputs are used to calibrate an emulator that mimics the outputs of the computationally intensive simulations. Varying the amount of experimental data available for emulator calibration showed clear differences in the degree of uncertainty and uniqueness of the resulting optimized Johnson-Cook parameters for Al-5083. The results of the optimization provided a numerical evaluation of the degree of confidence in model parameters and model performance. Given an understanding of the physical effects of certain model parameters, individual parameter uncertainty can be leveraged to quickly identify gaps in the physical domains covered by completed experiments. [ABSTRACT FROM AUTHOR]

The electronic structures of SrRu 1 − x Ir x O 3 films (0 ≤ x ≤ 1) have been investigated by employing polarization-dependent soft X-ray absorption spectroscopy. In SrIrO 3 , the coherent quasi-particle peak at the Fermi level ( E F ) is clearly observed, reflecting the spin-orbit-coupled correlated metallic nature of SrIrO 3. X-ray linear dichroism signals for x = 0 and x = 1 are negligibly weak, demonstrating the pseudo-cubic structure for SrRuO 3 and the strong spin-orbit coupling in SrIrO 3. In contrast, finite linear dichroism is observed for the intermediate x regime, indicating that the narrow d x z ↓ and d y z ↓ bands close to E F are occupied less than the wide d x y ↓ band due to the enhanced in-plane hopping. [ABSTRACT FROM AUTHOR]