Search

More than a decade of active electrocaloric (EC) material research has produced several EC materials that exhibit a giant electrocaloric effect (ECE) at high electric fields, which is assured by direct measurement. These EC materials have enabled the demonstration of EC cooling devices, which exhibit temperature lifts of more than 10 K. These research and development efforts have revealed the critical importance of electrical breakdown, which is common in all electric and dielectric materials and devices under high voltages and electric fields. In general, the electric field for reliable device operation of dielectrics has to be less than 25% of the typical electric breakdown strength. To realize EC cooling devices with competitive performance requires advanced EC materials that generate large ECE (ΔT > 5 K) under these low electric fields. Double-bond (DB) defect modified P(VDF-TrFE-CFE) relaxor polymers, as reported recently, generate large ECE under low electric fields without any fatigue effects even after 1 × 106 field cycles. These relaxor ferroelectrics promise to meet the application challenge. A closely coupled experimental and theoretical study of EC materials will undoubtedly lead to advanced EC materials that generate large ECE at low electric fields beyond the DB modified relaxor polymers. This will result in practical and high-performance EC coolers, which are environmentally benign, compressor-free, and highly efficient. [ABSTRACT FROM AUTHOR]

Under ultraviolet (UV) excitation with photon energy larger than 4.5 eV, a charge-state conversion from negatively to neutrally charged nitrogen-vacancy (NV− to NV0) centers in diamond samples is realized. The UV-induced charge-state conversion is found to strongly depend on the N concentration in the sample and the irradiation fluence of the electron beam. For the samples with high N concentrations, low-fluence (2.5 × 1017 cm−2) 10-MeV electron beam irradiation usually leads to UV-induced charge-state conversion efficiency higher than that of the samples irradiated with high fluences (≥2.5 × 1018 cm−2). For the samples with a few ppm N, however, the charge-state conversion efficiency is relatively low in the cases of irradiation fluences in this work. Meanwhile, UV-induced NV0 luminescence exhibits temperature dependence different from that of visible-light excited NV0 or NV− centers; that is, the photoluminescence intensity does not reach saturation at temperatures lower than 135 K but decreases with the decrease in temperature. Based on the photoluminescence excitation spectra of NV0 centers in the UV region, the UV-induced charge-state conversion is suggested to involve free-hole generation, diffusion, and the capture by ground-state NV− centers. [ABSTRACT FROM AUTHOR]

The current-induced magnetization switching (CIMS) was successfully observed in epitaxial L10-FePt/CrxPt1−x (0 ≤ x ≤ 1) heterostructures grown by molecular beam epitaxy with large perpendicular magnetic anisotropy. With increasing Cr content, the critical switching current density (Jc) in FePt/CrxPt1−x heterostructures exhibited a decreasing trend, where it was greatly reduced by 69% in FePt/Cr (3d) films compared to FePt/Pt (5d) films with strong spin–orbit coupling. Furthermore, the same switching polarities were observed for all FePt/CrxPt1−x samples, indicating that the orbital Hall effect played a dominant role in CIMS for FePt/Cr films because of opposite spin Hall angles for Cr and Pt. Our results will put forward the applications of L10-FePt in collaboration with the orbital Hall effect from 3d metals in current-controlled magnetic random access memory and neuromorphic computing. [ABSTRACT FROM AUTHOR]

The effect of tuning the depolarization field in (001)-oriented ultrathin epitaxial PbZr0.4Ti0.6O3/SrTiO3/PbZr0.4Ti0.6O3 ferroelectric heterostructures is investigated. The thickness of the dielectric spacer (SrTiO3) is maintained constant at 2 unit cells. The ferroelectric layer thickness in the heterostructure (each PbZr0.4Ti0.6O3) layer varied from 8 to 15 nm is exploited as the parameter to tune the depolarization field. Piezoresponse force microscopy reveals a domain evolution from continuous labyrinthine domains to individual nanoscale bubble domains under the influence of an increasing depolarization field. A statistical analysis of the domain features (i.e., domain wall length and domain fraction) reveals that this change in domain morphology also affects the wall roughness and its associated disorder. The local coercive voltage obtained using switching spectroscopy piezoresponse force microscopy finds the 15 nm film to have the lowest coercive voltage. This is attributed to both a strain-induced increase in tetragonality and the depolarization field-induced changes in the domain morphology. [ABSTRACT FROM AUTHOR]

We investigate the magnetic and ferroelectric behaviors of 5d Ir4+ substituted Mn1−xIrxWO4+ δ samples. The electric polarization is enhanced to a certain extent due to the Ir substitution of Mn. Furthermore, the non-polar AF1 phase is partially suppressed by the ferroelectric AF2 phase, resulting in the coexistence of the two phases below TAF1. It is suggested that the lattice distortion, the suppressed magnetic moment, and the increased Dzyaloshinskii–Moriya interaction are the key reasons for the enhanced polarization and the stabilized ferroelectric AF2 phase. [ABSTRACT FROM AUTHOR]

By solving the continuity equation of excitons under steady excitation, a theoretical model for the excitonic luminescence of semiconductors was developed taking into account the exciton diffusion and surface recombination. The theoretical model was used to analyze the photoluminescence (PL) spectra of ZnO obtained from the bulk single-crystal samples with and without surface passivation, showing that the nonradiative recombination on the surface is an important channel of losing excitons, thus substantially reducing the PL quantum efficiency of excitons at room temperature. In addition, the surface recombination was found to have impacts on the fine structure of excitonic luminescence at low temperature. Using the theoretical model, the diffusion length of excitons at room temperature was estimated and found to be different from sample to sample, strongly depending on the sample processing. The theoretical model was demonstrated to be capable of accurately fitting the temperature-dependent PL intensity of passivated samples and showed that the exciton diffusion has significant impacts on the dynamics of excitonic luminescence at high temperature. [ABSTRACT FROM AUTHOR]

In this study, polycrystalline SnSe was synthesized via a rapid, cost-effective, and large-scale synthesis route. The obtained SnSe powders were pressed into pellets via spark plasma sintering (SPS) at different temperatures. Powder X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM) were used to characterize the crystal structures and morphology of the SnSe samples. The XRD results indicate that the orientation factors increase monotonously with the increase of sintering temperature. The FESEM images show that sintering temperatures have no obvious influence on the particle size. Positron annihilation measurements indicate that vacancy defects exist in all the sintered SnSe samples, and they recover gradually with increasing sintering temperatures. These vacancy defects are responsible for the lower lattice thermal conductivity in samples sintered at lower temperatures. The electrical conductivity, power factor, thermal conductivity, and figure of merit ZT show nearly the same variation trend, which increases initially with the increasing sintering temperature up to 550 ° C then decreases with further increase of the sintering temperature, which is possibly due to slight oxidation of SnSe. A maximum ZT value of ∼ 0.47 at 430 ° C was achieved for the 550 ° C sintered sample, which is higher than those reported for undoped polycrystalline SnSe around this temperature. Thus, we provide a simple, energy-saving, and effective method to synthesize polycrystalline SnSe in large quantities, and SPS is an effective method to optimize thermoelectric performance. [ABSTRACT FROM AUTHOR]

The magneto-Seebeck (MS) effect, which is also called magneto-thermo-power, was observed in Co/Cu/Co and NiFe/Cu/Co spin valves. Their Seebeck coefficients in the parallel state were larger than those in the antiparallel state, and the MS ratio defined as (SAP -SP)/SP could reach –9% in our case. The MS effect originated not only from trivial giant magnetoresistance but also from spin current generated due to spin-polarized thermoelectric conductivity of ferromagnetic materials and subsequent modulation of the spin current by different spin configurations in spin valves. A simple Mott two-channel model reproduced a –11% MS effect for the Co/Cu/Co spin valves, qualitatively consistent with our observations. The MS effect could be applied for simultaneously sensing the temperature gradient and the magnetic field and also be possibly applied to determine spin polarization of thermoelectric conductivity and the Seebeck coefficient of ferromagnetic thin films. [ABSTRACT FROM AUTHOR]

The voltage noise characteristic and sensitivity of magnetic tunnel junction sensors are crucial for ultralow field detection. In this work, we used a soft magnetic material electrode Co70.5Fe4.5Si15B10 as a sensing layer to improve the sensitivity. Then, a bias field along the easy axis of a free layer was applied to improve the linearity and manipulate the sensitivity of magnetic tunnel junction sensors. More importantly, random telegraph noise was suppressed by the bias field, resulting in hysteresis-free performance. The highest sensitivity of 3.9%/Oe and the best field detectivity of 4.5nT/√Hz at 10 Hz without hysteresis have been achieved. The sensors showed excellent performance with CoFeSiB electrodes, indicating that it is an effective way to improve the performance of sensors by introducing the bias field. [ABSTRACT FROM AUTHOR]

Osmium (Os) is a hexagonal-close-packed metal with a non-spherical Fermi surface that seriously deviates from the assumption in the Mayadas-Shatzkes electrical-resistivity model (MS model) for the size effects of polycrystalline films of metals due to electron scattering by grain boundaries. In this work, we studied the resistivity of the Os films with different thicknesses as a function of temperature in the range of 20 to 296 K. The electron scattering by the surface was found to be unimportant in the contributions to the size effects of resistivity of Os films with a sufficient thickness. Based on the first-principles calculations, an analytical equation was suggested for correction to the MS model and used for fitting the temperature-dependent resistivity of the Os films. The results show that correction to the MS model is necessary and the residual resistivity caused by the defects and impurities cannot be neglected. In addition, the inhomogeneity of resistivity in the direction perpendicular to the film surface was discussed under an assumption of parallel circuits. [ABSTRACT FROM AUTHOR]

Electric transport coefficients such as carrier type, density, and mobility are the important physical parameters in designing functional devices. In this work, we report the study on the electric transport coefficients of the highly epitaxial LaBaCo2O5+δ (LBCO) films, which were discussed as a function of electric conductivity for the first time and compared with the results calculated by the theory for mixed conduction. The mobility in the LBCO films was determined to be ∼0.85 and ∼40 cm²/V s for holes and electrons, respectively, and the density of p-type carriers strongly depends on the oxygen deficiency. Solid evidence is presented to demonstrate that the oxygen deficiency cannot make LBCO materials changed from p- to n-type. The n-type conduction observed in experiment is a counterfeit phenomenon caused by the deficiency in Hall measurement, rather than a realistic transition induced by oxygen deficiency. In addition, the temperature-dependent conductivity was discussed using the differential coefficients, which might be useful in the study of the samples with magnetic transition. [ABSTRACT FROM AUTHOR]

Research describing the characteristics of hydrogenated amorphous silicon carbide films is presented. In particular the effect of varying negative radio frequency bias voltage at the substrate is investigated.

Microwave power effects on emission characteristics of hydrogenated amorphous silicon carbide were studied.

Bias-assisted hot filament chemical vapor deposition was used to produce turbostratic boron carbonitride films.

X-ray photoelectron spectroscopy has been used in studying molybdenum-containing carbon films. The latter have been fabricated using an electron cyclotron resonance chemical vapor deposition. Results indicate that the atomic fraction of Mo in the films could be modified by altering the CH4/Ar gas flow ratio. For instance, the atomic fraction can be transformed from 15.11% to 0.32% by changing the ratio from 4/20 to 20/10.

The structural development of a fabricated lead zirconate titanate thin films on platinized silicon upon annealing was described by transmission electron microscopy and x-ray diffraction. An increase in the amount of a transient intermetallic phase Pt3Pb with annealing time, which decays after reaching the maximum, was also found. The Avrami equation was used to simulate the kinetic process of growth and decay.

A study was conducted to analyze the low temperature crystallization of pyroelectrical lead zirconate titanate thin films prepared by a sol-gel technique. The films were characterized by x-ray diffraction and transmission electron microscopy. Results indicated the presence of a metastable Pt3Pb intermetallic phase. Its formation correlated with the drying and annealing temperature, film thickness and annealing time.

A study is conducted on the effects of pressure on hydrogenated amorphous carbon (a-C:H) thin films deposited using electron cyclotron resonance chemical vapor deposition. Results show that increased deposition pressure increased the hardness, sp3 fraction and optical gap of the films. This indicates that plasma chemistry strongly influences the deposition process and properties of the film.

In this work, a synaptic transistor based on the indium gallium zinc oxide (IGZO)-aluminum oxide (Al2O3) thin film structure, which uses ultraviolet (UV) light pulses as the pre-synaptic stimulus, has been demonstrated. The synaptic transistor exhibits the behavior of synaptic plasticity like the paired-pulse facilitation. In addition, it also shows the brain's memory behaviors including the transition from short-term memory to long-term memory and the Ebbinghaus forgetting curve. The synapse-like behavior and memory behaviors of the transistor are due to the trapping and detrapping processes of the holes, which are generated by the UV pulses, at the IGZO/Al2O3 interface and/or in the Al2O3 layer. [ABSTRACT FROM AUTHOR]

Extended X-ray-absorption fine-structure and laser-induced mass analyses of the local structure of porous silicon samples reveal that Si-O and Si-Si bonding features are present in the X-ray spectra and that the characteristic peak corresponds to the different silicon hydroxides. Depth profiling X-ray absorption fine-structure analysis reveals roughness of the porous silicon surface in the nanometer order. The fine-structure spectra also reveal a characteristic feature whose strength rises with Si-Si peak intensity.

Superparamagnetic behavior of ultrathin CoFeB film grown on epitaxial BiFeO3 (BFO) thin film was investigated by using anomalous Hall effect (AHE) at different temperatures. A transition from superparamagnetism to blocked state was observed for CoFeB grown directly on SrTiO3 (STO) substrate, but absent for CoFeB grown on BFO from 30K to 300 K. By fitting normalized AHE data with Langevin equation, average grain sizes of the superparamagnetic CoFeB grown on STO and BFO were estimated. Considering the relationship between volume of nanoparticle and blocking temperature (TB), TB of CoFeB grown on BFO was above 30K which was not observed experimentally, indicating that in addition to surface roughness or oxidation, interfacial exchange coupling also played a role in the suppression of TB. By taking the interfacial exchange coupling between BFO and superparamagnetic CoFeB as an effective field, Bloch equations were introduced to study the relaxation process of superparamagnetic CoFeB. This suppression of TB was proposed to be attributed to a cooperative effect of interfacial exchange coupling as well as roughness or oxidation between BFO and superparamagnetic CoFeB. [ABSTRACT FROM AUTHOR]

High energy density polymer materials are desirable for a broad range of modern power electronic systems. Here, we report the development of a new class of polymer dielectrics based on polyurea and polythiourea, which possess high thermal stability. By increasing the dipole density, the dielectric constant of meta-phenylene polyurea and methylene polythiourea can be increased to 5.7, compared with aromatic polyurea and aromatic polythiourea, which have a dielectric constant in the range of 4.1-4.3. The random dipoles with high dipolar moment and amorphous structure of these polyurea and polythiourea based polymers provide strong scattering to the charge carriers, resulting in low losses even at high electric fields. Consequently, this new class of polymers exhibit a linear dielectric response to the highest field measured (>700 MV/m) with a high breakdown strength, achieving high energy density (>13 J/cm3) with high efficiency (>90%). [ABSTRACT FROM AUTHOR]

Temperature dependence of the magnetoelectric (ME) coefficient was studied for FeBSiC/Pb(Zr, Ti)O3(PZT)/FeBSiC laminates over temperature range of -35~85 °C. The variation in ME coefficient as a function of temperature was found to be significantly dependent on the volume fraction of FeBSiC layers and the lateral dimension of the laminate composites. Our results show that minimum number of FeBSiC layer with smaller lateral dimension can provide more stable ME coefficient in wide temperature range, due to the competition of the effects of temperature influenced piezoelectricity, magnetostriction, and mechanical properties of the constituents and the interfacial bonding layer. This finding indicate that it is beneficial to promote miniaturization of the ME composite based devices in view of temperature stability. [ABSTRACT FROM AUTHOR]

The phosphors Ca3(PO4)2:Tm3+ by co-doping Ce3+ have been synthesized by conventional high-temperature solid-state reaction method. Their spectroscopic properties in the UV-VIS-NIR range have been investigated. The first 5d crystal field level location and stokes shift have been determined from the UV excitation and emission spectra of Ca3(PO4)2:Ce3+. The three-photon NIR quantum-cutting luminescence of Tm3+ assigned to the electronic transitions of 1G4→3H4, 3H4→3F4 and 3F4→3H6 is observed, whether in Ca3(PO4)2:Tm3+ or Ca3(PO4)2:Tm3+,Ce3+. The energy transfer from Ce3+ to Tm3+ takes place with energy-transfer efficiency up to 34.5% for the Ca3(PO4)2:Tm3+,Ce3+. A cross relaxation scheme using the 5d states of Ce3+ and f-f transition of Tm3+ is proposed. The mechanism is revealed from energy level and decay measurements. The results show that the broadband absorption of Ce3+ sensitizer not only extends the spectrum conversion in UV region but also greatly enhances the photoluminescence intensities of the three-photon quantum cutting luminescence of Tm3+ doped Ca3(PO4)2. [ABSTRACT FROM AUTHOR]

The optical conductivity (σ1) of SrTiO3 for various vacancies has been systematically studied using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry. For cation (Ti) vacancies, σ1 shows large spectral weight transfer over a wide range of energy from as high as 35 eV to as low as 0.5 eV and the presence of mid-gap states, suggesting that strong correlations play an important role. Meanwhile, for anion (O) vacancies, σ1 shows changes from 7.4 eV up to 35 eV. [ABSTRACT FROM AUTHOR]

We apply an implicit particle-in-cell Monte-Carlo (PIC-MC) method to study a radio-frequency argon microdischarge at steady state in the glow discharge limit, in which the microdischarge is sustained by secondary electron emission from the electrodes. The plasma density, electron energy distribution function (EEDF), and electron temperature are calculated in a wide range of operating conditions, including driving voltage, microdischarge gap, and pressure. Also, the effect of gap size scaling (in the range of 50-1000 μm) on the plasma sustaining voltage and peak electron density at atmospheric pressure is examined, which has not been explored before. In our simulations, three different EEDFs, i.e., a so-called three temperature hybrid mode, a two temperature α mode, and a two temperature γ mode distribution, are identified at different gaps and voltages. The maximum sustaining voltage to avoid a transition from the glow mode to an arc is predicted, as well as the minimum sustaining voltage for a steady glow discharge. Our calculations elucidate that secondary electrons play an essential role in sustaining the discharge, and as a result the relationship between breakdown voltage and gap spacing is far away fromthe Paschen law at atmospheric pressure. [ABSTRACT FROM AUTHOR]

We report the influence of blending the poly(vinylidene fluoride-trifluoroethylene- chlorofluoroethylene) terpolymer [P(VDF-TrFE-CFE), a member of the relaxor polymer family that exhibits fast response speeds, giant electrostriction, high electric energy density, and large electrocaloric effect] with the ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer [P(VDF-TrFE)] on its dielectric response. Although both components form separate crystalline phases, at low copolymer content, the P(VDF-TrFE-CFE)/P(VDF-TrFE) blends entirely exhibit a relaxorlike linear dielectric response, since the interfacial couplings to the bulky defects in the terpolymer convert the normal ferroelectric copolymer into a relaxor. On the other hand, the linear and particularly nonlinear dielectric experiments, i.e., temperature dependences of the second and the third harmonic dielectric response, clearly evidence that in blends with 20-50 wt. % of P(VDF-TrFE), the ferroelectric and relaxor states coexist. The nonlinear dielectric response further reveals the onset of ferroelectric behavior also in blends with low copolymer amount, due to a high VDF content in the terpolymer, which increases the ferroelectric interactions: While in relaxor polymers with lower VDF content, the third order nonlinear dielectric constant, in accordance with the theoretical predictions, exhibits solely positive values, here it changes sign even in the pure P(VDF-TrFE-CFE). [ABSTRACT FROM AUTHOR]

An efficient three-photon near-infrared (NIR) quantum cutting (QC) is reported in Tm3+ singly doped YVO4 polycrystalline phosphors, where an optimized content of Tm3+ is determined to be 1.0 mol. %. Upon the absorption of a visible photon around 473 nm, three NIR photons emitting at 1180, 1479, and 1800 nm can be obtained efficiently by the sequential three-step radiative transitions of Tm3+. The underlying mechanisms are analyzed in terms of the steady and dynamic fluorescence spectra measurements. Internal quantum yield is calculated to be 161.8% as a theoretical value when luminescence quenching due to defect species can be overcome. In addition, the broadband ultraviolet (UV)-excited [VO4]3- can strongly sensitize the 1G4 level of Tm3+ in the wavelength range likely from 250 to 360 nm, greatly increasing the UV photo-response and NIR fluorescent intensity of Tm3+. The further development of this broadband three-photon NIR QC material would explore the new route to improve the photo-response of novel photoelectronic devices, particularly in 250-360 nm. [ABSTRACT FROM AUTHOR]

The spintronics devices based on electric field is a key challenge today and the exchange bias effect is the basic structure used in these devices. We have studied the electric field control of annealed Co/CoO exchange bias system fabricated on the (011)-Pb(Mg1/3Nb2/3)O3-PbTiO3 piezoelectric substrate. The Co/CoO heterostructure was cooled down from above the Neel temperature (TN = 291 K) down to 15 K in the presence of electric and magnetic fields; the sample was cooled every time for each electric field measurement in order to avoid the training effect. The exchange bias effect was found to increase by 44% at the highest electric field value of 8 kV/cm measured at 15 K. The electric field tuning of exchange bias is attributed to the different magnetization reversal processes for left and right branches of magnetic hysteresis loop. Our results reinforce the possibility for the use of electric field as a tool to control the future spintronics devices. [ABSTRACT FROM AUTHOR]

In this study, the influences of reflow time and strain rate on interfacial fracture behaviors of the Sn-4Ag/Cu solder joints were investigated through in-situ observation. The interfacial microstructures of the solder joints reflowed for different times were first observed by scanning electronic microscope and measuring laser confocal microscope. Then tensile tests of the solder joints were conducted using in-situ tensile stage at different strain rates, and the interfacial fracture processes were in-situ observed. The observation results reveal that the thickness and surface roughness of the interfacial Cu6Sn5 layers increase linearly with increasing square root of the reflow time. Due to the serious strain concentration, fractures of the solder joints occur around the solder/Cu6Sn5 interface. The solder joints reflowed for a short time usually fracture inside the solder close to the joint interface, while the long-time reflowed solder joints are more apt to fracture in the interfacial Cu6Sn5 layer, because the stress applied on the Cu6Sn5 grains increases with increasing reflow time and grain size of Cu6Sn5. The solder joints reflowed for moderate time have the highest fracture resistance. At higher strain rate, the solder can apply a higher stress on Cu6Sn5 layer prior to its fracture, making the latter more apt to fracture. [ABSTRACT FROM AUTHOR]

Sub-monolayer quantities of Mg are introduced in multilayer stacked ZnMgTe quantum dots (QDs) embedded in ZnSe barriers in order to reduce the hole confinement energy by controlling the bandgaps and band-offsets of ZnTe/ZnSe system having type-II band alignment. The photoluminescence (PL) emission from such ZnMgTe/ZnSe QD structure is found to be a broad band centered at 2.35 eV. The higher energy side of the PL band shows a larger blue-shift with increasing excitation intensity and a faster life-time decay due to a greater contribution of the emission from the smaller size dots and the isoelectronic bound excitons. It is found that the characteristic decay time of the PL evolves along the band with a value of 129 ns at 2.18 eV to 19 ns at 2.53 eV. The temperature dependent PL emission is controlled by two thermally activated processes: ionization of electrons away from QD state to the barrier (EA1 ∼ 3 meV) by breaking the type-II excitons and thermal escape of the holes from the ground state to the barrier (EA2 ∼ 114-116 meV). We propose a modified band diagram and energy levels for this ZnMgTe/ZnSe multilayer QD system by determining the composition of Mg inside the QDs and solving the 1-D Schrodinger's equation and show that Mg incorporation lowers the hole activation energy via modification of the valence band offset without changing the barrier significantly. [ABSTRACT FROM AUTHOR]

Electrically induced behavior was investigated and compared in the non-stretched and uniaxially stretched poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE) terpolymer-a member of the P(VDF-TrFE)-based relaxor polymers family that exhibits fast response speeds, giant electrostriction, high electric energy density, and large electrocaloric effect. Although the temperature dependence of the low-field complex dielectric constant is almost identical in the non-stretched and stretched samples, the dc bias electric field via higher nonlinear contribution more heavily alters the dielectric response of the non-stretched terpolymer. The polarization response and, particularly, the induced electrostrictive strain are, on the other hand, much higher in the more-oriented stretched films. The changes in polar-correlation range induced by film stretching also strongly influence the directly measured electrocaloric response, which shows more pronounced temperature dependence in the stretched terpolymer. These results suggest that electrically induced properties of relaxor polymer films can be tailored by controlling the preparation conditions. [ABSTRACT FROM AUTHOR]

We report the structural properties and spatial ordering of multilayer ZnMgTe quantum dots (QDs) embedded in ZnSe, where sub-monolayer quantities of Mg were introduced periodically during growth in order to reduce the valence band offset of ZnTe QDs. The periodicity, period dispersion, individual layer thickness, and the composition of the multilayer structures were determined by comparing the experimental high resolution x-ray diffraction (HRXRD) spectra to simulated ones for the allowed (004) and quasi-forbidden (002) reflections in combination with transmission electron microscopy (TEM) results. Secondary ion mass spectroscopy (SIMS) profiles confirmed the incorporation of Mg inside the QD layers, and the HRXRD analysis revealed that there is approximately 32% Mg in the ZnMgTe QDs. The presence of Mg contributes to higher scattering intensity of the HRXRD, leading to the observation of higher order superlattice peaks in both the (004) and (002) reflections. The distribution of scattered intensity in the reciprocal space map (RSM) shows that the diffuse scattered intensity is elongated along the qx axis, indicating a vertical correlation of the dots, which is found to be less defined for the sample with larger periodicity. The diffuse scattered intensity is also found to be weakly correlated along the qz direction indicating a weak lateral correlation of the dots. [ABSTRACT FROM AUTHOR]

Hexagonal Ba(Ti1-xFex)O3-δ (x = 1/6, 1/3) ceramics treated with post annealing are specifically synthesized to explore the origin mechanism of the unusual ferromagnetism in the doped system. X-ray diffraction refinements and transmission electron microscope experiments reveal that their structures are incommensurately modulated owing to simultaneous oxygen vacancies at both O1 and O2 sites. Consequently, coexisting weak ferrielectricity and weak ferromagnetism are presented at room temperature. Analysis on their leakage current plot reveals that their conduction follows trap-filled limit model. In combination with the magnetism studies on 5 mol% Fe-doped crystals [Phys. Rev. B 83, 144407 (2011)], the reduction of ferromagnetism with an increase of conductivity suggests that dynamic exchanges of trapped electrons among the bound magnetic polarons attribute to the intrinsic ferromagnetism. [ABSTRACT FROM AUTHOR]

Poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] 90/10 wt% copolymers were fabricated by extrusion-blown and hot press-quench methods. The electric field induced phase transition was observed starting from 79 MV/m, and completing at 216 MV/m, in these copolymers. The transition was manifested by a change from a double hysteresis loop to a single hysteresis loop and supported by the x-ray diffraction measurement. Linear relationships between the magnetoelectric coupling coefficient and the in situ poling electric field, the remanent polarization and the field, were observed at electric fields less than 300 MV/m. A magnetoelectric coupling coefficient as large as 12 V/cm·Oe was obtained. [ABSTRACT FROM AUTHOR]

A theoretical model for the electrocaloric effect (ECE) in relaxor ferroelectrics is presented. By solving a self-consistent relation for the ECE temperature change ΔT and minimizing numerically the mean field free energy for relaxors, the field and temperature dependence of ΔT is calculated. The corresponding harmonic Landau coefficient a=a(T), which differs from the ferroelectric case by always being positive, is derived from the spherical random bond-random field model, and the fourth-order coefficient b is treated as a phenomenological parameter, which can be either positive or negative. For b<0, a line of field-induced first-order relaxor-to-ferroelectric phase transitions exists in relaxors, which terminates at a liquid-vapor type critical point ECP,TCP. The critical behavior close to ECP,TCP is analyzed. It is shown that near the first-order phase transition a temperature or field interval or gap formally appears, where ΔT cannot be found. However, domain formation in the coexistence range should restore the continuous behavior of the ECE observed in real systems. Finally, it is shown that the ECE responsivity R1=ΔT/E reaches a maximum near the critical point, in agreement with recent experiments. [ABSTRACT FROM AUTHOR]

The electrocaloric effect (ECE), i.e., the conversion of electric energy into heat, is of great importance for application in new generation cooling or heating devices that would be friendlier to the environment. Here, utilizing direct measurements of the ECE change of the temperature ΔT via a high resolution calorimeter, we study the ECE as a function of the magnitude of the electric-field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems. Relatively large ΔT of ∼2 to 3 K were obtained at modest fields of 90 kV/cm, even in the case of ceramic materials. The effective responsivity ΔT/E as a function of the electric field shows a characteristic peak near the critical point, which demonstrates the importance of proximity to the critical point for the enhancement of the electrocaloric effect. Experimental results are in good agreement with the theoretical calculations based on the spherical random-bond random-field model. [ABSTRACT FROM AUTHOR]

In this study, the shear fracture behaviors of the Cu6Sn5 grains at the Sn-4Ag/Cu interface were studied by indentation test; the fracture mechanisms and shear strength were characterized through analyzing the indentation curves and fracture morphologies. Experimental results reveal that shear fractures of the Cu6Sn5 grains occur at the foundation or the center portion, depending on their size and shape, and there are bursts on the force-depth curves corresponding to the fracture. The shear fracture strength of the Cu6Sn5 grains fractured at their center portion is close to the shear strength of the Cu6Sn5 intermetallic compounds, and its average value is about 670 MPa. [ABSTRACT FROM AUTHOR]

C60 nanotubes with diameters of smaller than 500 nm are fabricated by a modified liquid-liquid interfacial precipitation method. In situ angle dispersive synchrotron x ray diffraction and Raman scattering under pressures have been employed to study the structure evolution of the C60 nanotubes. The experimental results indicate that there is a pressure induced irreversible amorphization at 40.1 GPa. An isostructural phase transition occurs in the pressure range of 9.29-12.2 GPa, which is probably relative to the changes in the bonding type of C60 nanotubes. [ABSTRACT FROM AUTHOR]

Bi3+-Ln3+ (Ln = Dy, Er, Ho, Eu, and Sm) co-doped YVO4 phosphors are proposed as UV-absorbing luminescent converter candidate to enhance the power conversion efficiency and photochemical stability of dye-sensitized solar cells (DSSCs). The phosphors can efficiently convert UV photons in a broad range from 250 to 400 nm into visible emissions, which can be absorbed by DSSCs. Efficient broadband down-conversion UV light into near-infrared emission around 1000 nm was achieved in the YVO4:Bi3+,Yb3+ phosphors. The energy transfer from V5+-Bi3+ charge-transfer state to Yb3+ was shown to be a cooperative down-conversion type by the luminescence spectra, energy transfer efficiency, and luminescence decay curves. The YVO4:Bi3+,Yb3+ phosphors are promising for boosting the efficiency of crystalline silicon solar cells by down-converting the UV part of the solar spectrum to near-infrared photons with a twofold increase in the photon number. This research may open up promising new perspectives for designing novel luminescent materials for photovoltaic cells with high efficiency. [ABSTRACT FROM AUTHOR]

The effect of strain on the current carrying capacity of ZnSe nanowire has been studied by in situ transmission electron microscopy (TEM). Under TEM inspection the strain can be created at the selected position in a single ZnSe nanowire by the compressive stress applied along its axial direction using a movable probe electrode. The induced strain is controllable in the magnitude of curvature of the ZnSe nanowire bent by careful manipulation of the movable probe electrode. In situ current-induced Joule heating has confirmed that the strained segment in a single ZnSe nanowire exhibited better ability than the unstrained segments against Joule heating. Consequently, the current carrying capacity of the ZnSe nanowire can be effectively enhanced by intentionally created strain. The experimental results have also proved that a significant increase of the electrical conductance and the thermal resistance can be achieved simultaneously in a single nanowire by the intentionally designed and created strain. [ABSTRACT FROM AUTHOR]

We present a scheme to measure the thermal conductivity of individual nanowires using temperature-dependent and microphotoluminescence spectroscopy. The CdS nanowires suspended over trenches in silicon substrate were excited with a focused laser beam in confocal configuration. The photoluminescence at the laser spot gave a measure of the local temperature rise determined by the excitation laser intensity and the thermal conductance of the nanowires. By obtaining the temperature gradient across the nanowire and the length of the suspended segment, the thermal conductivity of CdS nanowires with diameters in the range of 200-400 nm was extracted to be in the range of 4.9-6.2 W/m K. [ABSTRACT FROM AUTHOR]

Tm3+ and Ho3+ codoped transparent glass ceramic (GC) containing CaF2 nanocrystals were fabricated by melt-quenching and subsequent thermal treatment. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of CaF2 nanocrystals among the glass matrix. Energy-dispersive x-ray spectroscopy results evidenced the incorporation of Tm3+ and Ho3+ into the CaF2 nanocrystals. Judd-Ofelt parameters were calculated based on the absorption spectra, the smaller Ω2 and larger Ω6 imply that Tm3+ and Ho3+ ions has entered into a symmetrical ionic crystal field. 2.02 μm emission spectral of the GC samples were recorded at room temperature with an excitation of 808 nm laser diode. The enhancement of the emission at 2.02 μm in the GC samples could be attributed to more efficient cross relaxation process of Tm3+ : 3H4+Tm3+ : 3H6 →2Tm3+ : 3H4, and energy transfer from Tm3+ to Ho3+ benefited from the incorporation of rare earth ions into CaF2 nanocrystals. [ABSTRACT FROM AUTHOR]

An efficient near-infrared (NIR) quantum-cutting (QC) by converting broadband ultraviolet (UV) into NIR via cooperative down-conversion (CD) has been demonstrated in Gd2O3:Bi3+,Yb3+ phosphors. Upon excitation of UV photon varying from 320–390 nm, NIR emissions has been obtained from transitions of the transition-metal Bi3+:3P1 level to the rare-earths Yb3+:2F5/2 level. The authors have analyzed the measured luminescence spectra and decay lifetimes and proposed a mechanism to rationalize the CD effect. Application of the broadband NIR-QC phosphors might greatly enhance response of silicon-based solar cells by means of down-conversion of UV part of the solar spectrum to NIR photons with a twofold increase in the photon number. [ABSTRACT FROM AUTHOR]

Magnetic and transport properties of stoichiometric Mn3In compound are extensively studied by means of dc magnetization, ac susceptibility, magnetic relaxation, and electrical resistivity measurements. It is found that Mn3In is a ferrimagnet with Curie temperature of 80 K and that a metallic spin-glass-like (SGL) state exists below 45 K, which is indicated by the difference between zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves, invariable FC moments, large remanence effect, magnetic relaxation, and electrical resistivity abnormality. Moreover, the SGL state in Mn3In exhibits some anomalous behaviors, such as no cusp in the ZFC curve at the frozen temperature Tf, a remarkable high-field irreversibility, and a high-field relaxation process. The presence of the SGL state in the Mn3In compound without doping or substitution is discussed in terms of magnetic cluster mode. [ABSTRACT FROM AUTHOR]

A near-infrared quantum cutting phosphor GdBO3:Tb3+,Yb3+ shows poor optical absorption in the ultraviolet-visible excitation spectral range and weak near-infrared luminescence in the 900–1100 nm range, which strongly limit its practical application in silicon-based solar cells, due to the inefficient excitation of the Tb3+ ions for their intra-4f forbidden transitions. Herein, Ce3+ ion has been codoped as a sensitizer in GdBO3:Tb3+,Yb3+, thus enlarging the absorption linewidth and increasing the absorption cross-sections in the ultraviolet-visible region. The presence of Ce3+ results in an enhancement in Yb3+ near-infrared emission intensity by a factor of ∼30, which makes these phosphors more attractive as luminescent downconversion materials for enhanced silicon-based solar cell performance. The relevant luminescence and energy transfer mechanisms involved have also been investigated. The present results demonstrate that the 4f-5d luminescence of Ce3+ may be used to sensitize the Tb3+–Yb3+ downconversion process. [ABSTRACT FROM AUTHOR]