Electric Polarization Effects Research Articles

On the EQE-bias characteristics of bottom-illuminated AlGaN-based metal-semiconductor-metal photodetectors with asymmetric electrode geometry

The bias dependence of the external quantum efficiency (EQE) of bottom-illuminated Al0.5Ga0.5N/AlN metal-semiconductor-metal photodetectors shows certain features which are directly related to the device geometry, i.e., the electrode design and the absorber layer thickness. Asymmetric detectors of any absorber thickness between 0.5 μm and 0.1 μm show a reduced EQE in reverse saturation. Furthermore, an enhanced EQE appears below the threshold either in reverse polarity for thick (i.e., 0.5 μm, 0.3 μm) or in forward polarity for thin (0.1 μm) AlGaN absorber layers. In order to understand the underlying physics of these phenomena, measured and simulated data are compared in the framework of a recently developed model for symmetric devices, taking into account defect-related non-radiative recombination and electric polarization effects. The electron capture time for recombination in the AlGaN absorber and the (background) donor density in the AlN buffer layer are the only parameters to be adjusted to reproduce all experimental trends qualitatively.

Flexible PVDF/Cu/PVDF-NaNbO3 photoanode with ferroelectric properties: An efficient tuning of photoelectrochemical water splitting with electric field polarization and piezophototronic effect

Piezophototronic effect and electric field polarization using ferroelectric-semiconductors are recently discovered phenomena for realizing efficient photoelectrochemical (PEC) water splitting. However, conventionally semiconductor materials deposited on rigid and mechanically brittle (Si, FTO and ITO) substrates have been used as photoelectrodes which hinder the scalability of PEC technology. Herein, we introduce a facile and economically feasible approach for the fabrication of flexible and scalable photoelectrodes with ferroelectric properties for efficient PEC water splitting. A flexible PVDF/Cu/PVDF-NaNbO3 photoanode using ferroelectric-semiconducting PVDF-NaNbO3 film on flexible PVDF/Cu substrate is fabricated and an enhancement in the water oxidation performance of ~121% with electric field polarization is demonstrated. A positively poled photoanode exhibits the highest photocurrent (~1.37mA/cm2) compared to as-prepared photoanode (~0.72mA/cm2) and negatively poled photoanode (~0.62mA/cm2). Moreover, due to flexibility and piezoelectric properties of PVDF/Cu/PVDF-NaNbO3 photoanode, a further enhancement in the photocurrent ~26% is obtained using the piezophototronic effect. Our present approach of fabricating flexible ferroelectric-photoelectrodes with tunable PEC properties will pave the way for constructing high performance commercial PEC systems.

The effect of magnetization and electric polarization on the anomalous transport coefficients of a chiral fluid

The effects of finite magnetization and electric polarization on dissipative and non-dissipative (anomalous) transport coefficients of a chiral fluid are studied. First, using the second law of thermodynamics as well as Onsager’s time-reversal symmetry principle, the complete set of dissipative transport coefficients of this medium is derived. It is shown that the properties of the resulting shear and bulk viscosities are mainly affected by the anisotropy induced by external electric and magnetic fields. Then, using the fact that the anomaly induced currents do not contribute to entropy production, the corresponding algebro-differential equations to non-dissipative anomalous transport coefficients are derived in a certain derivative expansion. The solutions of these equations show that, within this approximation, anomalous transport coefficients are, in particular, given in terms of the electric susceptibility of the medium.

The Effect of Electrical Polarization on Electronic Structure in LSM Electrodes: An Operando XAS, RIXS and XES Study

The influence of electrical polarization on Mn in La0.5Sr0.5MnO3±δ electrodes has been investigated by operando High Energy Resolved Fluorescence Detected X-Ray Absorption Near-Edge Structure (HERFD-XANES) spectroscopy, Kβ X-ray Emission Spectroscopy (XES) and Resonant Inelastic X-ray Scattering (RIXS) at the Mn K-edge. The study of polarization induced changes in the electronic properties and structure has been carried out at 500°C in 10–20% O2 with electrical polarization applied in the range from −850 mV to 800 mV. Cathodic polarizations in the range −600 mV to −850 mV induced a shift in the Mn K edge energy towards lower energies. The shift is assigned to a decrease in the average Mn oxidation state, which based on Kβ XES changes from 3.4 at open circuit voltage to 3.2 at −800 mV applied potential. Furthermore, RIXS rendered pronounced changes in the population of the Mn 3d orbitals, due to filling of the Mn d-orbitals during the cathodic polarization. Overall, the study experimentally links the electrical polarization of LSM electrodes to the structural and electronic properties of Mn - these properties are expected to be of major importance for the electrocatalytic performance of LSM electrode towards the oxygen reduction reaction.

マルチフェロイック複合材料の電気磁気特性に及ぼす非軸分極処理の影響

A multiscale simulation is presented for polycrystalline multiferroic composite materials consisting of ferroelectric and ferromagnetic phases. The purpose of this study is to estimate the effect of off-axis electric and magnetic polarization on macroscopic magnetoelectric (ME) coupling constants. The asymptotic homogenization theory was employed for scale-bridging between macrostructure and microstructure. The crystal orientations and material types were randomly allocated into the microstructure. Especially the microstructural crystal orientations were controlled by the combination of off-axis electric and magnetic polarization treatments. Tetragonal perovskite BaTiO3 and hexagonal spinel CoFe2O4 were employed for ferroelectric and ferromagnetic phases, respectively. As the angle between electric and magnetic fields of polarization treatment was changed from 0 to 90 degree, the homogenized physical properties were calculated for the polycrystalline multiferroic composite material. The computation indicated the macroscopic ME constants and piezomagnetic constants vary more widely through off-axis polarization treatment than the other properties. It was confirmed that ME constants can be tailored by the combination between electric and magnetic polarization treatments.

Controlled in Vivo Nacre Formation in Flat Pearls with Hydroxyapatite Bioceramic Nuclei

We carried out in vivo experiments of cultured flat pearl production using disc-like hydroxyapatite (HAp) bioceramics as a nucleus, with and without electric polarization treatment, and evaluated the effects of electric polarization and the quality of HAp materials on flat pearl formation. The thickness of the lamellar structures of the nacreous aragonite layers increased when polarized HAp pellets, instead of nonpolarized HAp pellets, as the nucleus, were used as the nuclei, while the thickness of the columnar structures of the prismatic calcite layers decreased. These phenomena were observed remarkably on the negative potential surface of the polarized HAp pellet. The electric polarization treatment promoted the formation rate of the nacreous lamella structure. It was speculated that electric polarization may control the physiological actions of mantle tissue and regulate the formation of the prismatic calcite and nacreous aragonite layers. The polarization treatment leads to an interference color with ...

Magnetic, dielectric, and magnetoelectric properties in Sr2CoGe2O7

We investigate the magnetoelectric effect in Sr2CoGe2O7, which has a two-dimensional magnetic interaction between Co ions in the ab plane. This compound shows a weak magnetism and field-induced magnetic transition below the temperature of 7 K. It does not exhibit electric polarization under zero magnetic field. However, by applying an external magnetic field, the electric polarization is induced around its Néel temperature and increases with increasing magnetic field. The induced electric polarization in Sr2CoGe2O7 is almost two orders of magnitude smaller than that of Ba2CoGe2O7. This result suggests that the different ionic radius of the strontium and the barium anion plays a key role in determining the property of electric polarization. The origins of electric polarization and magnetoelectric effect are discussed.

Electrical Polarization Effects on the Optical Polarization Properties of AlGaN Ultraviolet Light-Emitting Diodes

In this paper, the electrical polarization effects on the optical polarization properties of Al 0.35 Ga 0.65 N multiple quantum well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) by varying the Al content in AlGaN quantum well (QW) barriers are qualitatively analyzed. Numerical simulation results show that the varied potential barrier height and different polarization-induced electric field resulting from varying the Al content in AlGaN QW barriers have great influence on the transverse electric (TE) and transverse magnetic (TM) emission intensities. Both the TE- and TM-polarized spontaneous emissions from the Al 0.35 Ga 0.65 N QWs decrease with the increase of the Al content in AlGaN QW barriers, which is attributed to the poor carrier confinement resulting from the more serious band bending effect and the reduced effective potential barrier height in the conduction band, especially, the degree of optical polarization increases with the increase of the Al content in AlGaN QW barriers due to the rearrangement of the valence subbands near the Γ-point of the Brillouin zone. It is consequently concluded that the TE and TM spontaneous emissions and the degree of optical polarization are closely related to the Al content in AlGaN QW barriers of the Al 0.35 Ga 0.65 N MQW UV LEDs.

Origin of the Bismuth-Induced Decohesion of Nickel and Copper Grain Boundaries

Ductile metals such as Ni and Cu can become brittle when certain impurities (e.g., Bi) diffuse and segregate into their grain boundaries (GBs). Using first-principles calculations, we investigate the microscopic origin of the Bi-induced loss of cohesion of Ni and Cu GBs. We find that the Bi bilayer interfacial phase is the most stable impurity phase under the Bi-rich condition, while the Bi monolayer phase is a metastable phase regardless of the value of the Bi chemical potential. Our finding is consistent with the recent experimental observation for Ni GBs [Luo et al. Science 333, 1730 (2011)]. The electric polarization effect of the Bi bilayer substantially enhances the strength of the Bi-metal interfacial bonds, stabilizing the bilayer phase over other phases. The Bi-Bi interlayer bonding is significantly weakened in the GBs, leading to a factor of 20 to 50 decrease in the GB cohesion, which has strong implications for the understanding of Bi-induced intergranular fracture of Ni and Cu polycrystals.

Effects of electrical polarization on the opening rate constant of a voltage-gated ion channel

Ion channel gating kinetics can be described using Kramers' diffusion theory of reaction rates between several closed and open states, where transition rates between states depend exponentially on the membrane potential V. It has been suggested that transition rates have a more complex dependence on V at voltage extremes, but this has never been quantified. We measured the rate constant of the last closed to open transition in a voltage-gated ion channel and show that it does not depend exponentially on membrane potential at values of V greater than ≈150 mV. To explain this behavior, we estimate the effects of electrical polarization of the water contained in crevices within the channel protein, using an electrostatic model of the approximate three-dimensional geometry and the nonlinear effects of charges on the polarization of water.

Thermal annealing effect on internal electrical polarization in organic solar cells

This article reports experimental studies on internal electrical polarization effects by using optical absorption and photoexcitation assisted capacitance–voltage (C–V) measurements based on morphological development in the standard P3HT:PCBM solar cell. We observe that morphological development can increase absorption intensity upon thermal annealing. The increase of absorption intensity essentially reflects an enhancement of absorption coefficient of local donor and acceptor structures. We attribute the enhancement of absorption coefficient to the well-known morphological developments: increased polymer crystallinity and molecular aggregations caused by thermal annealing. Furthermore, the enhancement in absorption coefficient indicates stronger electrical polarizations in the P3HT:PCBM film. The C–V studies find that increasing local electrical polarizations can lead to an enhancement on the generation of charge carriers at donor:acceptor interfaces and the transport of generated charge carriers to respective electrode interfaces in the P3HT:PCBM device. Our experimental findings suggest that local electrical polarizations play an important role in the development of photovoltaic processes in organic solar cells.

Ferroelectric substrate effects on the magnetism, magnetotransport, and electroresistance of La0.7Ca0.3MnO3thin films on BaTiO3

La${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$ optimally doped epitaxial films were grown on ferroelectric BaTiO${}_{3}$ substrates. Electronic transport (magnetoresistance and electroresistance) and magnetic properties showed important anomalies in the temperature interval between 60 and 150 K, below the metal-insulator transition. Scanning probe microscopy revealed changes in BaTiO${}_{3}$ surface morphology at those temperatures. La${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$ thickness is a critical factor: 120-\AA{}-thick films showed large anomalies sensitive to electric poling of the BaTiO${}_{3}$, whereas the behavior of 150-\AA{}-thick films is closer to that of the reference La${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$ samples grown on SrTiO${}_{3}$. We propose that, through inhomogenous strain and electric polarization effects, the ferroelectric substrate induces an inhomogenous spin distribution in the magnetic layer. This would imply the coexistence of in-plane and out-of-plane ferromagnetic patches in La${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$, possibly interspersed with antiferromagnetic regions, as it has recently been theoretically predicted. Substrate poling effects are investigated, and a magnetoelectric coupling is demonstrated.

Polarization effect on wettability of bioceramic electrets

Bioceramic electrets are produced by electric polarization. Above all, polarized calcium hydroxyapatite (HA) ceramics are excellent electrets: HA electrets have a strong influence on the osteoblast behaviors. Osteoblasts are susceptible to the surface characteristics of bioceramics and stimulation from outside the cells. The purpose of this report is to introduce the fundamentals of bioceramic electrets such as the effects of electrical polarization on surface characteristics and osteoblastic adhesion. The surface characteristics revealed that electrical polarization had no effect on surface roughness, crystallinity, and constituent elements. According to contact angle measurement, electrically polarized HA, which provides two kinds of surfaces, negatively charged HA (N-surface) and positively charged HA (P-surface), was even more hydrophilic than that of conventional HA. Morphological observations and quantitative analyses revealed that the typical adhered cells had a round shape on the HA but had a spindle or fan-like spreading configuration on the N-surface and the P-surface 1 h after seeding. After 3 h of cultivation, the rate of the number of spread cells and the size of focal adhesions on the HA increased and approached that of the N-surface and P-surface. However, the cell areas positively stained for actin, which indicates the degree of cell spreading, were distinctly larger on the N-surface and P-surface than that on the HA. The number of focal adhesions per cell was also less than that on the N-surface and P-surface.

Intrinsic Metallic and Semiconducting Cubic Boron Nitride Nanofilms

We show by density functional theory calculations with both hybrid and semilocal functionals that cubic boron nitride (111) nanofilms are intrinsically metallic and even turn into semiconductors once the thickness is less than 0.69 nm, which is in sharp contrast to the known insulating nature of boron nitride materials. The exceptional metallic or semiconducting band gap is due to a combined effect of thickness-dependent inbuilt electric polarization and labile near-gap states unique in the polar nanofilms. The band gap and dipole moment of the nanofilms can be further significantly tuned by applying an in-plane strain. These distinguished features of the boron nitride nanofilms are robust to surface passivation and can be enhanced by hybridizing with diamond films, thereby opening an exciting prospect of using the versatile cubic nanofilms in future electronic and piezoelectric devices.

The impact of heat treatment on interactions of contact-poled biphasic calcium phosphates with proteins and cells

A number of studies have reported improved bone integration for calcium phosphate based materials electrically “poled” by an external electric field prior to implantation. In our study we investigated the effects of electrical polarization of a biphasic ceramic composed of 80% hydroxyapatite and 20% β-tricalcium phosphate. As contact poling involves elevated temperatures as a prerequisite for inducing charge, we used two reference types: samples without any heat treatment and poling, and samples with no poling but heat treatment identical to that of the poled samples. All heat-treated samples (poled or unpoled) showed an improved wettability, which was attributed to a reduced hydrocarbon contamination. Heat treatment alone provoked an accelerated spreading of osteoblast-like cells, whereas on poled samples a retarded cell spreading was observed. While proliferation and several differentiation markers were not influenced by either heat treatment or poling, the release of proinflammatory cytokines interleukin-6 and -8 was significantly reduced for all heat-treated samples, irrespective of additional electrical poling. The study demonstrated that the behaviour of cells in contact with poled biphasic ceramics was influenced by two parameters: heating and charge. Our data revealed that heating of the calcium phosphate ceramics had a much more pronounced effect on cell behaviour than charge.

Enhancement of Electrical Conductivity in Plasticized Chitosan Based Membranes

Plasticized natural macromolecules-based polymer electrolyte samples were prepared and characterized. The plasticization of chitosonium acetate with glycerol increased the ionic conductivity value from 3.0 × 10−7 S/cm to 1.1 × 10−5 S/cm. The conductivity temperature relationship of the samples exhibits either VTF or Arrhenius type depending on the glycerol concentration in the sample. The dielectric studies evidencing the relaxation process in the plasticized sample at low frequency region are due to the electric polarization effect. Moreover, the samples were transparent in the Vis region, showed thermal stability up to 160°C and good surface uniformity.

Interface electronic structure between organic semiconductor film and electrode metal probed by photoelectron yield spectroscopy

We present an investigation of the interface between organic semiconductor films and metal substrates (organic/metal interface) using photoelectron yield spectroscopy (PYS) as the probing technique. PYS studies were conducted on the pentacene/Au, copper phthalocyanine (CuPc)/Au, and perfluorinated zinc phthalocyanine (F 16ZnPc)/Au, and the results were compared with literature results obtained using conventional ultraviolet photoemission spectroscopy (UPS). PYS is advantageous for probing the electronic structure of the organic/metal interface because of the relatively long mean free path of photoexcited electrons with very low kinetic energy in PYS, which enables the detection of the photoelectrons from the metal substrate buried deep in the organic film. We demonstrate herein that the use of PYS reduces the significance of the final state effect of the electronic density surrounding the photohole at the organic molecule generated after the photoemission; this effect is known as the electric polarization effect. Although this effect has a significant influence on the results obtained using conventional UPS, the reduced influence of the final state effect in PYS makes it possible to construct an energy level diagram at the organic/metal interface with greater accuracy than can be achieved with UPS. In addition, a novel mechanism of the photoelectron detection for PYS enables us to apply PYS to very thick organic films, and therefore, PYS provides a reliable value of ionization energy for organic films without the influence of the substrate. Because the interface electronic structure has a significant influence on the carrier injection properties of organic devices, the increased reliability of the information obtained by PYS will render it very useful for the improvement of device performance as well for understanding their operation principles.

Correlation between magnetocapacitance effect and polarization flop direction in a slanted magnetic field in multiferroic helimagnets

The relationship between the magnetocapacitance effect and rotation direction of electric polarization (P) in a canted magnetic field has been investigated for multiferroic RMnO3 (R = Tb1−xDyx and Eu0.6Y0.4). We observed a clear correlation between the enhancement of the magnetocapacitance effect and the rotation direction of P in a P-flop transition. These results indicate that the mobility and the stability of the 90° domain wall in a P-flop transition are dominated by its thickness.

Effect of electric field polarization and temperature on the effective permittivity and conductivity of porous anodic aluminium oxide membranes

Porous insulators offer new opportunities for the controlled guest–host synthesis of nanowires for future integrated circuits characterized by low propagation delay, crosstalk and power consumption. We propose a method to estimate the effect of the electric field polarization and temperature on the electrical properties of different types of synthesized porous anodic aluminium oxide membranes. It results that the effective permittivity along the pore axis is generally 20% higher than the one in the orthogonal direction. The type of solution and the voltage level applied during anodization are the main parameters affecting the AAO templates characteristics, i.e. their porosity and chemical content. The values of permittivity of the final material, are typically in the range 2.6–3.2 for large pore diameter membranes including phosphorus element and having a low water content, and in the range 3.5–4 for the ones with smaller pores, and showing sulphur element incorporation. Moreover, the dc conductivity of the different membranes appears to be correlated to the pore density.

Frequency splitting of a multi-layered electric ring resonator

We present experimental results on the multilayering effects of an electric ring resonator. The electromagnetic response of the electric ring resonator is measured via a scattering matrix using a vector network analyzer at the X-band frequency. Structures of the electric ring resonator with up to four layers were tested and analyzed using commercial software. We demonstrate that, in an electric ring resonator, the electric and magnetic dipole polarization effect gives rise to resonance frequency splitting when the cell is multilayered.