Frequency-dependent Measurements Research Articles

Effects of Ce–Dy rare earths co-doping on various features of Ni–Co spinel ferrite microspheres prepared via hydrothermal approach

The effects of Ce–Dy co-doping on the crystal structure, optical, dielectric, magnetic properties, and hyperfine interactions of Ni–Co spinel ferrite microspheres synthesized hydrothermally have been studied. A series of ferrites with the general formula Ni0.5Co0.5CexDyxFe2-2xO4 were synthesized with x values ranging from 0.00 to 0.10. The phase, crystallinity, and morphology of ferrite microspheres were analyzed by X-ray powder diffractometry (XRD), scanning and transmission electron microscopes (SEM and TEM), respectively. The structural analyses of the synthesized ferrite microspheres confirmed their high purity and cubic crystalline phase. The Diffuse reflectance spectroscopic (DRS) measurements were presented to calculate direct optical energy band gaps (Eg) and is found in the range 1.63 eV - 1.84 eV. 57Fe Mossbauer spectroscopy showed that the hyperfine magnetic field of tetrahedral (A) and octahedral (B) sites decreased with the substitution of Dy3+-Ce3+ ions that preferrentially occupy the B site. The impact of the rare-earth content (x) on the magnetic features of the prepared NiCo ferrite microspheres was investigated by analyzing M-H loops, which showed soft ferrimagnetism. The magnetic features illustrate a great impact of the incorporation of Ce3+-Dy3+ ions within the NiCo ferrite structure. The saturation magnetization (Ms), remanence (Mr), and coercivity (Hc) increased gradually with increasing Ce–Dy content. At x = 0.04, Ms, Mr, and Hc attain maximum values of about 31.2 emu/g, 11.5 emu/g, and 512.4 Oe, respectively. The Bohr magneton (nB) and magneto-crystalline anisotropy constant (Keff) were also determined and evaluated with correlation to other magnetic parameters. Further increase in Ce3+-Dy3+ content (i.e., x ≥ 0.06) was found to decrease Ms, Mr, and Hc values. The variations in magnetic parameters (Ms, Mr, and Hc) were largely caused by the surface spins effect, the variations in crystallite/particle size, the distribution of magnetic ions into the different sublattices, the evolutions of magneto-crystalline anisotropy, and the variations in the magnetic moment (nB). The squareness ratios were found to be lower than the predicted theoretical value of 0.5 for various samples, indicating that the prepared Ce–Dy substituted NiCo ferrite microspheres are composed of NPs with single-magnetic domain (SMD). Temperature and frequency-dependent electrical and dielectric measurements have been done to estimate the ac/dc conductivity, dielectric constant, and tangent loss values for all the samples. The ac conductivity measurements confirmed the power-law rules, largely dependent on Ce–Dy content. Impedance analysis stated that the conduction mechanisms in all samples are mainly due to the grains-grain boundaries. The dielectric constant of NiCo ferrite microspheres give rise to normal dielectric distribution, with the frequency depending strongly on the Ce–Dy content. The observed variation in tangential loss with frequency can be attributed to the conduction mechanism in ferrites, like Koop's phenomenological model.

Electrical, kinetic and photoelectrical properties of CuAlMnMg shape memory alloy/n-Si Schottky diode

In this study, the copper-aluminum-manganese-magnesium (Cu–Al–Mn–Mg) shape memory alloy was utilized for the fabricated CuAlMnMg/n-Si/Al structure. The electrical characteristics of the diode were examined by using illumination intensity and frequency dependent current and admittance measurements. Basic electrical parameters such as barrier height (ϕb0), ideality factor (n), and series (Rs) and shunt (Rsh) resistances of the generated diode were obtained from the measured current-voltage (I–V) data using thermionic emission (TE) theory. Illumination impact on the diode parameters indicate that the device displays photoconducting behavior. Furthermore, the ϕb0 value was established from Norde method. There is a good agreement between ϕb0 values acquired from conventional I-V and Norde method. It was observed that the rise in the applied illumination intensity increased the values of Rs and Rsh. In addition, the admittance (Y=G+iωC) measurements were carried out in a wide frequency range. As a result of the experimental measurements, it has been shown that the produced CuAlMnMg/n-Si Schottky diode can be operated in optoelectronic practices, especially as a photodiode.

Pressure-Induced Excitations in the Out-of-Plane Optical Response of the Nodal-Line Semimetal ZrSiS.

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density-functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS.

Effect of DC Bias on Dielectric Properties of NdFeO3

The solid-state synthesis method was used to prepare polycrystalline samples of NdFeO3. The effect of dc bias was observed on the dielectric properties of prepared NdFeO3 samples. Impedance was found to be comprised of three contributions corresponding to grains, grain boundaries, and sample-electrode contact. Temperature and frequency-dependent dielectric measurements revealed that the sample electrode contact and grain boundary responses get modified, while grain response remains unaltered under dc bias. Large dielectric permittivity (of the order of ∼103) in the entire studied temperature range (100 K–450 K) and at high frequencies (∼5 MHz) was found to remain unaffected under dc bias, suggesting that the large dielectric constant is an intrinsic property of NdFeO3.

Temperature and frequency dependent dielectric and conductivity properties of Sr doped LiFe5O8

We report on enhanced dielectric constant and reduction in dielectric loss with Sr doping in LiFe5O8 bulk material. LiSrxFe5-xO8 (x = 0, 0.025, 0.05) samples are seen to crystallize in ordered α- LiFe5O8 phase. Dielectric constant is observed to increase from 2 × 104 to 3 × 106 and loss decreases with the doping of Sr at Fe site. Temperature dependent DC conductivity follows the Arrhenius relationship and the activation energy associated with the conduction process is seen to be 0.42, 0.41 and 0.52 eV for the LiSrxFe5-xO8 (x = 0, 0.025, 0.05) samples. Temperature and wide frequency dependent dielectric measurements indicate that the Sr doped LiFe5O8 samples exhibit relaxation behavior and can be employed for high frequency applications.

(Invited) Compact Model for Short-Channel Organic Thin-Film Transistors with Extension for Non-Quasistatic Circuit Simulation and Variability Analysis

We present a compact model for organic thin-film transistors (TFTs) that covers both below-threshold and above-threshold operation with a single charge-based current expression [1]. The effect of hopping transport is included by a power-law mobility model. Contact resistances for staggered and coplanar device structures (Fig. 1) are considered, which can be nonlinear in the case of work-function mismatch between contacts and organic semiconductor [2]. Short-channel effects in submicron transistors, such as threshold-voltage roll-off and drain-induced barrier lowering (DIBL), are included [3].From the expression for the accumulated channel charge, a closed-form model for the drain-current variability due to carrier-number and correlated mobility fluctuations has been derived, relating these statistical variations to the trap density in the channel [4].Furthermore, by applying a partitioning scheme, charges are explicitly attributed to the source and drain contacts, allowing the derivation of a charge-based capacitance model valid for quasi-static operation [5]. Charges in the gate-to-contact overlap regions have been included.Utilizing this device model in a transmission-line approach, a circuit simulation including non-quasistatic effects is possible [6]. In the circuit-simulation netlist, the TFT is replaced by a macro model consisting of a finite number of n single transistors, whereby each single transistor represents a section of the intrinsic channel capacitance and a segment of the intrinsic channel resistance. This model is able to capture the charging and discharging of the channel capacitance of each segment through the channel resistance of the adjacent transistors. In this way, the frequency dependence of the node-to-node capacitances of the full device is obtained.The compact model has been implemented in the hardware description language Verilog-A and verified using results of measurements performed on organic p-channel TFTs fabricated on flexible polyethylene naphthalate (PEN) substrates by stencil lithography (Figs. 2, 3). The TFTs consist of 25-nm-thick aluminum gate electrodes, a 5.3-nm-thick hybrid AlOx/SAM gate dielectric, 30-nm-thick gold (Au) source and drain contacts coated with a pentafluorobenzenethiol (PFBT) monolayer, and a 25-nm-thick vacuum-deposited layer of the small-molecule organic semiconductor DPh-DNTT [7,8]. The transmission-line approach has been verified with good accuracy by comparison with frequency-dependent admittance measurements and numerical simulations of the transient switching behavior of organic TFTs. For a transient analysis, the optimum number of segments required to achieve good agreement between the results from the model and those from TCAD simulations is approximately n=6.In conclusion, the model presented here is fully capable of providing accurate results in dc, small-signal ac, and transient circuit simulations, including short-channel effects. Furthermore, the same set of equations allows an estimation of the drain-current variability due to charge trapping. Acknowledgements: This project was funded by the German Federal Ministry of Education and Research ("SOMOFLEX", No. 13FH015IX6), German Research Foundation (DFG) under Grant KL 1042/9-2 (SPP FFlexCom), and EU H2020 RISE ("DOMINO", No. 645760). We acknowledge AdMOS GmbH, Germany for support.

AlN/GaN Superlattice Channel HEMTs on Silicon Substrate

High-electron-mobility transistor (HEMT) with AlN/GaN superlattice (SL) channel has been demonstrated on a silicon substrate. High OFF-state breakdown voltage ${V}_{\text {BR}}$ of 670 V (gate–drain spacing ${L}_{\text {GD}} = 8\,\,\mu \text{m}$ ) along with a maximum output current of 196 mA/mm, a channel electron total mobility of 507 cm2/ $\text{V}\cdot \text{s}$ , and an ON/ OFF ratio of over 107 was achieved in this novel HEMT. For the HEMT with ${L}_{\text {GD}} = 22\,\,\mu \text{m}$ , a high ${V}_{\text {BR}}$ of 1700 V was obtained with substrate floated. The influence of ${L}_{\text {GD}}$ , the gate–source voltage ${V}_{\text {GS}}$ , the isolated pattern, and substrate grounded and floated type was discussed to analyze the breakdown characteristics of HEMT. We investigated the trap states in the AlN/GaN SL channel of HEMTs by frequency-dependent capacitance and conductance measurements. A trap state density of $7.4\times 10^{{12}}$ – $1.2\times 10^{{13}}$ cm−2eV−1 is located at ${E}_{\text {T}}$ in a range of 0.29–0.33 eV of the main channel, while the trap state density in the parasitic channel decreases from $3.9\times 10^{{11}}$ cm−2eV−1 at an energy of 0.27 eV to $1.1\times 10^{{11}}$ cm−2eV−1 at an energy of 0.38 eV. The fabricated AlN/GaN SL channel HEMTs in this work reveal a great step toward the realization of the SL channel HEMTs on cost-effective silicon substrate and provide a novel technology for AlGaN multichannel devices to obtain high output current.

Thermo-kinetic modelling of the giant Snoek effect in carbon-supersaturated iron

Snoek relaxation in interstitial bcc solid solutions is the origin of the Snoek peak in frequency-dependent and temperature-dependent internal friction measurements. When applied to low-carbon steels, the internal friction profiles show a linear dependence of the peak height with carbon content. However, recent Monte Carlo simulations of frequency-dependent internal friction exhibited a non-linear giant peak height at high carbon contents. To investigate this effect in temperature-dependent internal friction, we developed a thermo-kinetic mean-field theory of the Snoek relaxation phenomenon. By taking into account the collective behaviour of the interstitial atoms, our theory predicts (i) a non-linear dependence of the peak height with composition when approaching the order–disorder transition of the alloy; (ii) a shift in the temperature of the peak; (iii) a composition-dependent activation enthalpy of the relaxation time. The theory is exemplified by the case of carbon-supersaturated iron.

Shear waves and sound attenuation in underwater waveguides.

In addition to dissipation of acoustic energy in the seabed, bottom-interacting normal modes are attenuated by radiation of shear waves into soft sediments, where shear speed is small compared to the sound speed in water. The shear-wave contribution and the dissipation have distinct frequency dependencies, and their relative magnitude affects the observed frequency dependence of mode attenuation. Previous studies suggested that the shear-wave contribution to the attenuation is proportional to the cube of the small ratio of the shear and sound speeds. Here, coupling of compressional and shear waves in layered soft sediments is analyzed. Besides the well-known, third-order contribution to the attenuation due to shear-wave generation at the water-sediment interface, a stronger, first-order, contribution is found to occur due to compressional-to-shear wave conversion at interfaces within the sediment. First-order effects of weak shear on mode travel times are also identified. Stratification of the sediment density and interference of shear waves reflected within the seabed control the frequency dependence of the shear-wave contribution to sound attenuation. With the shear-wave contribution being larger than previously estimated, its effect on the experimentally measured frequency dependence of the sound dissipation may need to be re-assessed.

Global Skewness and Coherence for Hypersonic Shock-Wave/Boundary-Layer Interactions with Pressure-Sensitive Paint

The global surface pressure was measured on a 7° half-angle circular cone/flare model at a nominally zero angle of attack using pressure-sensitive paint (PSP). These experiments were conducted to illustrate fast PSP’s usefulness and effectiveness at measuring the unsteady structures inherent to hypersonic shock-wave/boundary-layer interactions (SWBLIs). Mean and fluctuating surface pressure was measured with a temperature-corrected, high-frequency-response (≈10 kHz) anodized-aluminum pressure-sensitive paint (AA-PSP) allowing for novel, global calculations of skewness and coherence. These analyses complement traditional SWBLI data-reduction methodologies by providing high-spatial-resolution measurements of the mean and fluctuating locations of the shock feet, as well as the frequency-dependent measure of the relationship between characteristic flow features. The skewness indicated the mean locations of the separation and reattachment shock feet as well as their fluctuations over the course of the test. The coherence indicated that the separation and reattachment shock feet fluctuate about their mean location at the same frequency as one another, but 180 degrees out of phase. This results in a large-scale ‘breathing motion’ of the separated region characteristic of large separation bubbles. These experimental findings validate the usefulness of AA-PSP, and associated data-reduction methodologies, to provide global physical insights of unsteady SWBLI surface behavior in the hypersonic flow regime. Similar methodologies can be incorporated in future experiments to investigate complex and novel SWBLIs.

Room temperature memristive switching in nano-patterned LaAlO3/SrTiO3 wires with laterally defined gates

We present room temperature memristive switching in a nano-patterned LaAlO3/SrTiO3 wire with laterally defined gates in proximity to the wire. Closed bias voltage sweeps show pinched hysteresis loops with zero bias resistance values of up to Ron = 8 MΩ and Roff = 1.2 GΩ for the on and off state, respectively. The maximum Roff/Ron ratio is 150. Frequency dependent measurements show a cutoff frequency of around 10 Hz, and the alteration of set point voltages enables us to precisely set and control the resistance off-on ratio. We explain the memristive switching by charge localization on the laterally defined gates, which couple capacitively to the wire and enhance or decrease the resistance dependent on the amount of transferred charges. Our finding enables the realization of geometry-based memristive switching devices, which make use of the form-dependent wire-gate capacitance.

Structural, Dielectric and Electric Modulus Studies of MnFe2O4/(MWCNTs)x Nanocomposites

Nanocomposites of multi-walled carbon nanotubes (MWCNTs) and manganese ferrites (MnFe2O4) nanoparticles (i.e., MnFe2O4/(MWCNTs)x; x = 0 ~ 20 wt.% nanocomposites) were synthesized using two-step synthesis process. Initially, MnFe2O4 nanoparticles were prepared by co-precipitation method and, at second stage, different weight percentages (wt.%) of MWCNTs were added and mixed by ultra-sonication assisted method to get the desired MnFe2O4/(MWCNTs)x nanocomposites. X-ray diffraction (XRD) technique was used to investigate the crystal structure and phase purity of MnFe2O4/(MWCNTs)x nanocomposites. The accumulation of MnFe2O4 nanoparticles on the surface of MWCNTs was examined via scanning electron microscopy (SEM). The elemental composition of these nanocomposites was determined with the aid of energy-dispersive x-ray (EDX) spectroscopy, and Fourier transform infrared spectroscopy (FTIR) was employed for the estimation of different vibrational modes. The frequency-dependent dielectric measurements were conducted at room temperature with the help of LCR meter. The improved dielectric and modulus properties of MnFe2O4 nanoparticles with increasing MWCNTs contents signified the importance of these MnFe2O4/(MWCNTs)x nanocomposites for different energy storage applications.

An Interpretable Deep Learning Model for Automatic Sound Classification

Deep learning models have improved cutting-edge technologies in many research areas, but their black-box structure makes it difficult to understand their inner workings and the rationale behind their predictions. This may lead to unintended effects, such as being susceptible to adversarial attacks or the reinforcement of biases. There is still a lack of research in the audio domain, despite the increasing interest in developing deep learning models that provide explanations of their decisions. To reduce this gap, we propose a novel interpretable deep learning model for automatic sound classification, which explains its predictions based on the similarity of the input to a set of learned prototypes in a latent space. We leverage domain knowledge by designing a frequency-dependent similarity measure and by considering different time-frequency resolutions in the feature space. The proposed model achieves results that are comparable to that of the state-of-the-art methods in three different sound classification tasks involving speech, music, and environmental audio. In addition, we present two automatic methods to prune the proposed model that exploit its interpretability. Our system is open source and it is accompanied by a web application for the manual editing of the model, which allows for a human-in-the-loop debugging approach.

Dynamic heterogeneity, cooperative motion, and Johari-Goldstein [Formula: see text]-relaxation in a metallic glass-forming material exhibiting a fragile-to-strong transition.

We investigate the Johari-Goldstein (JG) [Formula: see text]-relaxation process in a model metallic glass-forming (GF) material ([Formula: see text]), previously studied extensively by both frequency-dependent mechanical measurements and simulation studies devoted to equilibrium properties, by molecular dynamics simulations based on validated and optimized interatomic potentials with the primary aim of better understanding the nature of this universal relaxation process from a dynamic heterogeneity (DH) perspective. The present relatively low temperature and long-time simulations reveal a direct correspondence between the JG [Formula: see text]-relaxation time [Formula: see text] and the lifetime of the mobile particle clusters [Formula: see text], defined as in previous DH studies, a relationship dual to the corresponding previously observed relationship between the [Formula: see text]-relaxation time [Formula: see text] and the lifetime of immobile particle clusters [Formula: see text]. Moreover, we find that the average diffusion coefficient D nearly coincides with [Formula: see text] of the smaller atomic species (Al) and that the 'hopping time' associated with D coincides with [Formula: see text] to within numerical uncertainty, both trends being in accord with experimental studies. This indicates that the JG [Formula: see text]-relaxation is dominated by the smaller atomic species and the observation of a direct relation between this relaxation process and rate of molecular diffusion in GF materials at low temperatures where the JG [Formula: see text]-relaxation becomes the prevalent mode of structural relaxation. As an unanticipated aspect of our study, we find that [Formula: see text] exhibits fragile-to-strong (FS) glass formation, as found in many other metallic GF liquids, but this fact does not greatly alter the geometrical nature of DH in this material and the relation of DH to dynamical properties. On the other hand, the temperature dependence of the DH and dynamical properties, such as the structural relaxation time, can be significantly altered from 'ordinary' GF liquids.

Understanding the sodium cation conductivity of human epileptic brain tissue.

Transient and frequency-dependent conductivity measurements on excised brain-tissue lesions from epilepsy patients indicate that sodium cations are the predominant charge carriers. The transient conductivity ultimately vanishes as ions encounter blockages. The initial and final values of the transient conductivity correspond to the high-frequency and low-frequency limits of the frequency-dependent conductivity, respectively. Carrier dynamics determines the conductivity between these limits. Typically, the conductivity rises monotonically with increasing frequency. By contrast, when pathology examinations found exceptionally disorganized excised tissue, the conductivity falls with increasing frequency as it approaches its high-frequency limit. To analyze these measurements, excised tissues are modeled as mixtures of “normal” tissue within which sodium cations can diffuse and “abnormal” tissue within which sodium cations are trapped. The decrease in the conductivity with increasing frequency indicates the predominance of trapping. The high-frequency conductivity decreases as the rate with which carriers are liberated from traps decreases. A relatively low conductivity results when most sodium cations remain trapped in “abnormal” brain tissue, while few move within “normal” brain tissue. Thus, the high densities of sodium nuclei observed by 23Na-MRI in epilepsy patients’ lesions are consistent with the low densities of diffusing sodium cations inferred from conductivity measurements of excised lesions.

Stress- and frequency-dependent properties of relaxor-like sodium bismuth titanate

Despite the importance of $({\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2})\mathrm{Ti}{\mathrm{O}}_{3}$ as an end member in lead-free ferroelectrics and as an oxide ion conductor, the relaxor/ferroelectric nature remains unclear. In order to understand the relaxor-like behavior, frequency-dependent macroscopic mechanical measurements of polycrystalline $({\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2})\mathrm{Ti}{\mathrm{O}}_{3}$ were performed as a function of poling state, revealing the role of a potential field-induced long-range ferroelectric order on the nonlinear hysteretic stress-strain behavior. The mechanical measurements showed an increase in remanent strain and decrease in coercive stress with electrical poling, consistent with previous studies of relaxors. Electrical poling and mechanical texturing were found to influence the frequency dispersion of the relative permittivity, highlighting the potentially relaxor-like response. Further, the relative permittivity showed a directional dependence with respect to the previously applied electrical and mechanical fields. These data are discussed in conjunction with ex situ stress- and electric-field-dependent piezoresponse force microscopy measurements that revealed a clear ferroelectric domain switching through the application of a sufficiently high electric field, but no change of the domain configuration for uniaxial compressive stresses up to \ensuremath{-}750 MPa. The in situ stress-dependent crystal structure, which was characterized using synchrotron x-ray diffraction, however, indicates stress-induced ferroelastic domain switching as the primary hysteretic process.

Frequency- and magnetic-field-dependent properties of ordered magnetic nanoparticle arrangements

We present investigations of the frequency and magnetic field dependent properties of ordered magnetic nanoparticles (MNPs) arrangements consisting of magnetite (${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$) nanoparticles with an average diameter of 20 nm by employing micro Brillouin light scattering microscopy. We utilized electron beam lithography to prepare hexagonally arranged, circularly shaped MNP assemblies consisting of a single layer of MNPs using a variant of the Langmuir-Blodgett technique. By comparing the results with nonstructured, layered superlattices of MNPs, further insight into the influence of size and geometry of the arrangement on the collective magnetic properties is obtained. We show that at low static external field strengths, two signals occur in frequency dependent measurements for both nonstructured and structured assemblies. Increasing the static external field strength results in a sharpening of the main signal, while the satellite signal decreases in its intensity and increases in its linewidth. The occurrence of multiple signals at low external field strengths is also confirmed by sweeping the static external field and keeping the excitation frequency constant. Furthermore, two-dimensional spatial mapping of the resonances reveals that the main and the satellite signal originate from the center and the edge, respectively, of a single circular MNP assembly. Micromagnetic simulations confirm these assignments and the dependence of the two signals on the static external field strength, justifying an interpretation of the observed characteristics in terms of different local environments of MNPs forming the MNP assembly.

Determination of defect states and surface photovoltage in PTB7:PC71BM based bulk heterojunction solar cells

Defects formed during the device processing acts as recombination centers and shows adverse effects on the photovoltaic performance of the organic solar cells (OSCs). In this manuscript, a study on the effect of defect states on the characteristics of bulk heterojunction (BHJ) solar cells, consisted of unannealed or thermally annealed active layer (ActL) of PTB7:PC71BM blend, have been performed by using the illumination intensity-dependent current-voltage and frequency dependent impedance spectroscopy measurements. Our results indicate that the photovoltaic performance of devices based on annealed ActL is superior than the bench dried ActL due to thermal annealing induced improvement in the thin film morphology. The measured low-frequency capacitance-voltage (C–V) characteristics under varying illumination intensities show strong dependency on the photoexcitation however, the change in the C–V characteristics is found to be quite different for unannealed and annealed devices. The C–V characteristics are analyzed using the drift-diffusion model to extract the effective built-in voltage (Vbi) and the surface photovoltage as a function of illumination intensity. The capacitance-frequency characteristics under illumination reveal that the change in the capacitance is associated with the photo-induced carrier occupation in the intermixture donor-acceptor phases, which act as the defect states in such devices. Further, it is modeled to quantify the defect states and to demonstrate the importance of thermal annealing for improving the OSCs device performance.

Dipolar order in an amphidynamic crystalline metal-organic framework through reorienting linkers.

Amphidynamic crystals, which possess crystallinity and support dynamic behaviours, are very well suited to the exploration of emergent phenomena that result from the coupling on the dynamic moieties. Here, dipolar rotors have been embedded in a crystalline metal-organic framework. The material consists of Zn(II) nodes and two types of ditopic bicyclo[2.2.2]octane-based linkers-one that coordinates to the Zn clusters through two 1,4-aza moieties, and a difluoro-functionalized derivative (the dipolar rotor) that coordinates through linked 1,4-dicarboxylate groups instead. Upon cooling, these linkers collectively order as a result of correlated dipole-dipole interactions. Variable-temperature, frequency-dependent dielectric measurements revealed a transition temperature Tc = 100 K, when a rapidly rotating, dipole-disordered, paraelectric phase transformed into an ordered, antiferroelectric one in which the dipole moments of the rotating linkers largely cancelled each other. Monte Carlo simulations on a two-dimensional rotary lattice showed a ground state with an Ising symmetry and the effects of dipole-lattice and dipole-dipole interactions.

Electro‐optical performances of nanostructured SrTiOx films: The effect of plasma power, Ar/O2 ratio and annealing

AbstractThe present work evaluates the effects of plasma power and oxygen mixing ratios (OMRs) on structural, morphological, optical, and electrical properties of strontium titanate SrTiOx (STO) thin films. STO thin films were grown by magnetron sputtering, and later thermal annealing at 700°C for 1 h was applied to improve film properties. X‐ray diffraction analysis indicated that as‐deposited films have amorphous microstructure independent of deposition conditions. The films deposited at higher OMR values and later annealed also showed amorphous structure while the films deposited at lower OMR value and annealed have nanocrystallinity. In addition, all as‐deposited films were highly transparent (~80%–85%) in the visible spectrum and exhibited well‐defined main absorption edge, while the annealing improved transparency (90%) within the same spectrum. The calculated direct and indirect optical band gaps for films were in the range of 3.60‐4.30 eV as a function of deposition conditions. The refractive index of the films increased with OMRs and the postdeposition annealing. The frequency dependent capacitance measurements at 100 kHz were performed to obtain film dielectric constant values. High dielectric constant values reaching up to 100 were obtained. All STO samples exhibited more than 2.5 μC/cm2 charge storage capacity and low dielectric loss (less than 0.07 at 100 kHz). The leakage current density was relatively low (3 × 10−8Acm−2 at +0.8 V) indicating that STO films are promising for future dynamic random access memory applications.