Poled Films Research Articles

Improving Durability of Dye-Based Polarizing Films Using Novel Reactive Dyes as Dichroic Materials.

Iodine is commonly used as a dichroic material in polarizing films, while dichroic dyes are employed when high heat resistance is necessary. Direct dyes, which can be applied to poly(vinyl alcohol) (PVA) in an acidic environment, are the most popular; however, their hydrogen bonding interaction with the PVA chain can weaken in high-humidity conditions, leading to a potential change in color value or polarization property. Reactive dyes offer a promising alternative for use in high-humidity environments. In this study, five novel reactive dyes were synthesized and used to prepare dye-based polarizing films. The dichroic ratios, order parameters, and transition moments of the reactive dyes were calculated and compared to those of corresponding direct dyes. Molecular orbital calculations indicated minimal effects on the optical anisotropy of the polarizing films due to the transition moments of the reactive dyes remaining close to their molecular axes. As a result, the dichroic ratios of the polarizing films were mainly dependent on the orientation of the dyes towards their stretching direction, and showed a similar behavior compared to direct dyes. Under high-temperature and high-humidity conditions, the polarizing film using reactive dyes showed superior durability compared to the direct dye-based film.

Regulation of electrical properties via ferroelectric polarization for high performance Sb2Te3 thermoelectric thin films

Sb2Te3 based thermoelectric thin films play a crucial role in energy conversion, whose thermoelectric performance highly depends on electrical properties. Herein, we proposed a new electrical properties regulation approach by ferroelectric polarization, which extracted polarization charges and achieved an in-situ modulation of the carrier concentration in Sb2Te3 thermoelectric thin films. In the positively polarized (Pr+) state, the power factor (PF) of Sb2Te3 thin films reached 14.16 μW cm−1 K−2, 19.0 % higher than the unpolarized one. The corresponding Sb2Te3 thin film thermoelectric generator (TEG) demonstrated a high output power density (ωout) of 2.22 mW cm−2 in the Pr+ state at ΔT = 42 K, demonstrating the promising potential for low-grade heat harvesting.

A study on the optimal incident angle of sub-wavelength grating polarization beam splitter

One problem faced by the sub-wavelength grating polarization beam splitter is the lack of a generally-used formula able to conduct accurate calculation of the incident angle. With that background, by the design method of optical film polarization beam splitter, this work realizes an precise calculation under the condition of arbitrary grating constants by transforming the splitter into an optical-film-structure model. According to tests, the incident angles of grating calculated by the method proposed here are consistent with those obtained from the FDTD simulation, illustrating the reliability of applying the polarization beam splitting design method of optical thin films to polarization beam splitting gratings.

A polar transition of spontaneous orientation polarization in organic amorphous thin films

Certain organic molecules autonomously form dipole orientation during a thin-film formation, leading to spontaneous orientational polarization (SOP) even in an amorphous film. Although the polarity of SOP can be controllable by molecular design itself, we demonstrate that a guest–host system also provides a unique tool for controlling SOP. Here, we report that the transition of SOP polarity occurs depending on the concentration of a polar guest molecule in a non-polar matrix. While a 2,8-bis(diphenylphosphoryl)dibenzo-[b,d]thiophene (PPT) film shows negative SOP in the neat film, the polarity could be changed to positive with a concentration increase of the non-polar molecule.

Wake-Up Free Ultrathin Ferroelectric Hf0.5Zr0.5O2 Films.

The development of the new generation of non-volatile high-density ferroelectric memory requires the utilization of ultrathin ferroelectric films. The most promising candidates are polycrystalline-doped HfO2 films because of their perfect compatibility with silicon technology and excellent ferroelectric properties. However, the remanent polarization of HfO2 films is known to degrade when their thickness is reduced to a few nanometers. One of the reasons for this phenomenon is the wake-up effect, which is more pronounced in the thinner the film. For the ultrathin HfO2 films, it can be so long-lasting that degradation occurs even before the wake-up procedure is completed. In this work, an approach to suppress the wake-up in ultrathin Hf0.5Zr0.5O2 films is elucidated. By engineering internal built-in fields in an as-prepared structure, a stable ferroelectricity without a wake-up effect is induced in 4.5 nm thick Hf0.5Zr0.5O2 film. By analysis of the functional characteristics of ferroelectric structures with a different pattern of internal built-in fields and their comparison with the results of in situ piezoresponse force microscopy and synchrotron X-ray micro-diffraction, the important role of built-in fields in ferroelectricity of ultrathin Hf0.5Zr0.5O2 films as well as the origin of stable ferroelectric properties is revealed.

Remarkable heat conduction mediated by non-equilibrium phonon polaritons.

Surface waves can lead to intriguing transport phenomena. In particular, surface phonon polaritons (SPhPs), which result from coupling between infrared light and optical phonons, have been predicted to contribute to heat conduction along polar thin films and nanowires1. However, experimental efforts so far suggest only very limited SPhP contributions2-5. Through systematic measurements of thermal transport along the same 3C-SiC nanowires with and without a gold coating on the end(s) that serves to launch SPhPs, here we show that thermally excited SPhPs can substantially enhance the thermal conductivity of the uncoated portion of these wires. The extracted pre-decay SPhP thermal conductance is more than two orders of magnitude higher than the Landauer limit predicted on the basis of equilibrium Bose-Einstein distributions. We attribute the notable SPhP conductance to the efficient launching of non-equilibrium SPhPs from the gold-coated portion into the uncoated SiC nanowires, which is strongly supported by the observation that the SPhP-mediated thermal conductivity is proportional to the length of the gold coating(s). The reported discoveries open the door for modulating energy transport in solids by introducing SPhPs, which can effectively counteract the classical size effect in many technologically important films and improve the design of solid-state devices.

Utilizing Linear Polymers to Optimize Remanent Polarization and Construct Multilayer Interfaces to Obtain High-Efficient Poly(vinylidene fluoride)-Based Energy-Storage Composites.

Polyvinylidene fluoride (PVDF) has been widely studied as a ferroelectric polymer for energy dielectric applications. However, high-polarization PVDF has a low-efficiency issue, owing to high residual polarization. This study introduces highly insulating, low-loss linear polycarbonate (PC) into PVDF-based dielectrics. The PC layer optimizes the remanent polarization (Dr) of PVDF and maintains it within a small range, thus achieving a high charge-discharge efficiency. The multilayer structural design of PVDF-based dielectrics adjusts the interlayer electric field distribution. We have thoroughly studied the influence of the number and proportion of PC layers on the polarization and breakdown of the multilayer films as well as achieved collaborative regulation of dual parameters. Our results indicate that three layers of PC-PVDF-PC (CPC) films containing a large proportion of PC can polarize under high electric fields and maintain excellent charge-discharge efficiency, achieving an energy density and efficiency of 11.48 J/cm3 and 92.4%, respectively, under 610 kV/mm. The PVDF-based dielectrics prepared in this work are all organic films, and their flexibility and foldability are conducive to the preparation of flexible devices.

Ultrahigh Bulk Photovoltaic Effect Responsivity in Thin Films: Unexpected Behavior in a Classic Ferroelectric Material

The bulk photovoltaic effect (BPE) has drawn considerable attention due to its ability to generate photovoltages above the bandgap and reports of highly enhanced photovoltaic current when using nanoscale absorbers or nanoscale electrodes, which, however, do not lend themselves to practical, scalable implementation. Herein, it is shown that a strikingly high BPE photoresponse can be achieved in an ordinary thin‐film configuration merely by tuning fundamental ferroelectric properties. Nonmonotonic dependence of the responsivity (RSC) on the ferroelectric polarization is observed and at the optimal value of the film polarization, a more than three orders of magnitude increase in the RSC from the bulk BaTiO3 value is obtained, reaching RSC close to 10−2 A W−1, the highest value reported to date for the archetypical ferroelectric BaTiO3 films. Results challenge the applicability of standard first‐principles‐based descriptions of BPE to thin films and the inherent weakness of BPE in ferroelectric thin films.

Combinatorial optimization for high spin polarization in Heusler alloy composition-spread thin films by anisotropic magnetoresistance effect

Half-metallic Heusler alloys are promising candidates for spintronic applications due to their high spin polarization. However, the spin polarization strongly depends on the atomic composition, which is time-consuming to optimize from various compositional combinations. Here, we demonstrate a high-throughput compositional optimization method for high spin polarization in Co2(Mn, Fe)Ge Heusler alloys by combining composition-spread films and anisotropic magnetoresistance (AMR) measurement. Two types of composition-spread films of polycrystalline Co2(Mn1−xFex)Ge and (Co2Mn0.5Fe0.5)1−yGey are fabricated on SiO2/Si substrates by combinatorial sputtering deposition, followed by post-annealing. The compositional dependence of AMR shows the largest negative AMR ratio of −0.13% and the smallest temperature dependence of the resistance change of AMR for y = 0.25 in the (Co2Mn0.5Fe0.5)1−yGey composition-spread film, suggesting the highest spin polarization and the closest nature to the ideal half-metal at this composition ratio. To verify this, we also develop a new technique to measure the compositional dependence of spin polarization by measuring the spin accumulation signals of nonlocal spin-valve devices fabricated on the composition-spread films and observe the highest spin polarization of 82% for y = 0.24. This confirms a clear qualitative correlation between the large negative AMR ratio and high spin polarization. Our combinatorial method using the composition-spread films and the AMR measurement proves to be a facile way for optimizing the fabrication conditions of half-metallic Heusler alloys with high spin polarization.

Effect of swelling degree on structural evolution and orientation behavior of Poly(vinyl alcohol) films during stretching

For the preparation of polarizing film, the initial condensed structure of the PVA film is the key factor to determine the subsequent process conditions such as dyeing and stretching of PVA film. In the present work, the difference of the initial condensed structure of Poly (vinyl alcohol) (PVA) films with different swelling degrees and their influence on the orientation behavior and morphological structure evolution during stretching were investigated in detail. The results showed that the PVA film with a higher swelling degree possessed a lower content of microfibril structure at the same strain and the higher critical strain εs, revealing that the undissolved lamellae could act as the precursor for the formation of oriented microfibrillar structures. Furthermore, in order to investigate the influence of different swelling degrees on the lamellar orientation and the formation of microfiber structure during the stretching of PVA films, the long period of lamellae (Lp), the period of nanofibrils (Lf), and the Full Width at Half Maxima (FWHM) of Lf were calculated. It was found that the Lp and the Lf of the microfibrillar structure in the equatorial and meridional directions were larger and the FWHM of Lf was smaller for PVA films with higher swelling degree, indicating that the PVA film with a higher swelling degree could form a more relaxed and uniform periodically folded microfibrillar network, resulting in a higher light transmission in the visible region. Therefore, on the premise of ensuring the mechanical strength, the PVA film with a relatively higher swelling degree can be used to obtain commercial polarizing films with excellent light transmittance.

Highly Efficient and Stable Binary Cross-Linkable/ Self-Assembled Organic Nonlinear Optical Molecular Glasses.

The development of electro-optical materials with high chromophore loading levels that possess ultrahigh electro-optic coefficients and high long term alignment stability is a challenging topic. Anthracene-maleimide Diels-Alder (DA) reaction and π-π interaction of Anthracene-pentafluorobenzene and benzene-pentafluorobenzene are developed for making highly efficient binary cross-linkable/self-assembled dendritic chromophores FZL1-FZL4. A covalently or non-covalently cross-linked network is formed by DA reaction or π-π interaction after electric field poling orientation, which greatly improves the long-term alignment stability of the materials. An electro-optic coefficient up to 266pmV-1 and glass transition temperature as high as 178°C are achieved in cross-linked film FZL1/FZL2, and 272-308pmV-1 is achieved for self-assembled films FZL1/FZL4 and FZL3/FZL4 due to high chromophore density (3.09-4.02×1020 moleculescm-3 ). Long-term alignment stability tests show that after heating at 85°C for over 500h, 99.73% of the initial r33 value is maintained for poled crosslinked electro-optic films 1:1 FZL1/FZL2. The poled self-assembled electro-optic films 1:1 FZL1/FZL4 and 1:1 FZL3/FZL4 can still maintain more than 97.11% and 98.23%, respectively, of the original electro-optic coefficient after being placed at room temperature for 500h. The excellent electro-optic coefficient and stability of the material indicate the practical application prospects of organic electro-optic materials.

Energy storage and multiferroic properties of La-doped epitaxial BiFeO3 thin films according to La doping concentration

La-doped epitaxial BiFeO3 (LBFO) thin films with La doping concentrations of 0, 10, 20, 25, and 30 % were deposited by pulsed laser deposition on Nb-doped single-crystal SrTiO3 substrates. The LBFO thin films doped with a La doping concentration of 10 % showed the best ferroelectric properties due to a large c/a ratio of about 1.04. As the concentration of La increased, the ferroelectric polarizations of the LBFO films decreased while the ferromagnetic properties steadily improved. In particular, the LBFO thin films tended to increase energy storage efficiency as the concentration of La increased, and the LBFO thin films with a La doping concentration of 30 % showed a high energy storage density of about 30.7 J/cm3.

Ferroelectricity in Epitaxial Perovskite Oxide Bi2WO6 Films with One-Unit-Cell Thickness.

Retaining ferroelectricity in ultrathin films or nanostructures is crucial for miniaturizing ferroelectric devices, but it is a challenging task due to intrinsic depolarization and size effects. In this study, we have shown that it is possible to stably maintain in-plane polarization in an extremely thin, one-unit-cell thick epitaxial Bi2WO6 film. The use of a perfectly lattice-matched NdGaO3 (110) substrate for the Bi2WO6 film minimizes strain and enhances stability. We attribute the residual polarization in this ultrathin film to the crystal stability of the Bi-O octahedral framework against structural distortions. Our findings suggest that ferroelectricity can surpass the critical thickness limit through proper strain engineering, and the Bi2WO6/NdGaO3 (110) system presents a potential platform for designing low-energy consumption, nonvolatile ferroelectric memories.

Flexoelectric and piezoelectric effects in micro- and nanocellulose films

In this work, we evaluated the flexoelectric and piezoelectric contributions to the overall macroscopic polarization in cellulose films. To this end, the flexoelectric μ31 and transverse effective piezoelectric e31,f coefficients of cellulose films were determined using cantilever beam bending. The experiments were based on theoretical developments allowing to separate the flexoelectric from the piezoelectric contribution, represented by an effective flexoelectric coefficient, μeff, depending on both e31,f and μ31. Five free-standing and stainless steel/cellulose bilayer films were prepared from cellulose showing different morphologies and surface charge degrees: two almost neutral cellulose microfibers (CMF) and three (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-oxidized cellulose micro- (TCMF) and nanofibers (TCNF) bearing negative charged groups on the surface.The dielectric properties of the films indicated a low dielectric constant for unmodified CMF, and a huge increase for TEMPO-oxidized samples, which were up to 9 times higher than poly(vinylidene fluoride)-based polymers. TEMPO-oxidized cellulose films exhibited the largest flexoelectric coefficients (almost 7 times higher than those of synthetic polymer dielectrics), which evidenced that the presence of polar groups and surface charge boosted both flexoelectricity and piezoelectricity in unpoled cellulose films. These findings pave the way towards sustainable cellulose-based curvature sensors with large effective flexoelectric coefficients, without the need of preliminary energy consuming poling step.

Generation of out-of-plane polarized spin current by spin swapping

The generation of spin currents and their application to the manipulation of magnetic states is fundamental to spintronics. Of particular interest are chiral antiferromagnets that exhibit properties typical of ferromagnetic materials even though they have negligible magnetization. Here, we report the generation of a robust spin current with both in-plane and out-of-plane spin polarization in epitaxial thin films of the chiral antiferromagnet Mn3Sn in proximity to permalloy thin layers. By employing temperature-dependent spin-torque ferromagnetic resonance, we find that the chiral antiferromagnetic structure of Mn3Sn is responsible for an in-plane polarized spin current that is generated from the interior of the Mn3Sn layer and whose temperature dependence follows that of this layer’s antiferromagnetic order. On the other hand, the out-of-plane polarized spin current is unrelated to the chiral antiferromagnetic structure and is instead the result of scattering from the Mn3Sn/permalloy interface. We substantiate the later conclusion by performing studies with several other non-magnetic metals all of which are found to exhibit out-of-plane polarized spin currents arising from the spin swapping effect.

Tuning surface spin polarization of CoFeB by boron diffusion detected by spin resolved photoemission

Abstract Investigation on the spin polarization of the magnetic CoFeB thin film is of practical importance in spintronic applications. Here, using a direct characterization technique of spinresolved photoemission spectroscopy, the surface spin polarization of amorphous Co40Fe40B20 thin films with different annealing temperatures from 100 ℃ to 500 ℃ prepared by magnetron sputtering is obtained. After high annealing temperature, a quasi-semiconductor state is gradually formed at the CoFeB surface due to the boron diffusion. While the global magnetization remains almost constant, the secondary electrons’ spin polarization, average valence band spin polarization and the spin polarization at Fermi level from spin-resolved photoemission spectroscopy show a general trend of decreasing with the increasing of the annealing temperature above 100 ℃. These distinct surface properties are attributed to the enhanced Fe-B bonding due to the boron segregation upon surface after annealing as confirmed by x-ray photoelectron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy. Our findings provide insight into the surface spin-resolved electronic structure of the CoFeB thin films, which should be important for the development of high-performance magnetic random-access memory.

Energetics and electronic structure of bilayer Janus WSSe

Employing density functional theory along with the effective screening medium method, we investigated the energetics and electronic structure of bilayer Janus WSSe in terms of their interlayer stacking arrangement. Through the orbital hybridization between chalcogen atoms at interfaces, the energetics are sensitive to the interlayer stacking orientation and interface atomic arrangements. This interface atomic arrangement creates the unique electronic structure of bilayer Janus WSSe determined by the dipole moment arrangement of the constituent WSSe layers. The net polarity of thin films of Janus transition-metal dichalcogenides is a simple superposition of the dipole moments of the constituent layers.

Single-shot quantitative differential phase contrast imaging combined with programmable polarization multiplexing illumination.

We propose a single-shot quantitative differential phase contrast method with polarization multiplexing illumination. In the illumination module of our system, a programmable LED array is divided into four quadrants and covered with polarizing films of four different polarization angles. We use a polarization camera with polarizers before the pixels in the imaging module. By matching the polarization angle between the polarizing films over the custom LED array and the polarizers in the camera, two sets of asymmetric illumination acquisition images can be calculated from a single-shot acquisition image. Combined with the phase transfer function, we can calculate the quantitative phase of the sample. We present the design, implementation, and experimental image data demonstrating the ability of our method to obtain quantitative phase images of a phase resolution target, as well as Hela cells.

PVDF/6H-SiC composite fiber films with enhanced piezoelectric performance by interfacial engineering for diversified applications

Piezoelectric silicon carbide (SiC) has been quite attractive due to its superior chemical and physical properties as well as wide potential applications. However, the inherent brittleness and unsatisfactory piezoelectric response of piezoelectric semiconductors remain the major obstacles to their diversified applications. Here, flexible multifunctional PVDF/6H-SiC composite fiber films are fabricated and utilized to assemble both piezoelectric nanogenerators (PENGs) and stress/temperature/light sensors. The open circuit voltage (Voc) and the density of short circuit current (Isc) of the PENG based on the PVDF/5 wt% 6H-SiC composite fiber films reach 28.94 V and 0.24 µA cm–2, showing a significant improvement of 240% and 300% compared with that based on the pure PVDF films. The effect of 6H-SiC nanoparticles (NPs) on inducing interfacial polarization and stress concentration in composite fiber films is proved by first-principles calculation and finite element analysis. The stress/temperature/light sensors based on the composite fiber film also show high sensitivity to the corresponding stimuli. This study shows that the PVDF/6H-SiC composite fiber film is a promising candidate for assembling high-performance energy harvesters and diverse sensors.

Fabrication of non-volatile memory transistor by charge compensation of interfacial ionic polarization of a ferroelectric gate dielectric

Lithium niobate (LiNbO3) is a popular lead free perovskite ferroelectric materials, but its implementation as a gate dielectric for developing a ferroelectric thin film transistor (FeTFT) has not been investigated much due to its low band gap. Low band gap introduces high gate leakage current that results in very low retention time of the device. In this report a solution processed Li5AlO4/LiNbO3/Li5AlO4 stacked layer has been used as a gate dielectric of a metal oxide thin film transistor which reduces gate leakage current by four orders of magnitude with respect to the single layer LiNbO3 gate dielectric device. Besides, due to the ionic polarization of Li5AlO4 thin film that originated from the mobile Li+, charge compensation of the depolarization field has been nearly removed. By reducing these factors, it becomes possible to fabricate metal oxide based FeTFT that has retention time of 7 h which is ∼200 times higher with respect to the LiNbO3 only FeTFT. The ratio of On and Off state of FeTFT is ∼103 which retains ∼ 95% after 7 h of operation. The carrier mobility, ON/OFF ratio and subthreshold swing (SS) of this FeTFT are 2.9 cm2 V − 1 s − 1,7.2 × 104 and 328 mV.decade−1 respectively.