Distribution and amplification of an electric field in ferroelectric-dye heterostructures

The distribution of an electric field has been investigated in Langmuir-Blodgett thin-film heterostructures prepared by sequentially transferring layers of the ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene) P(70% VDF 70% ?C30% TrFE) and the azo dye. An alternating-current voltage applied to the electrodes is unevenly distributed across layers of the dye and the copolymer in accordance with a constant induction along the normal to the heterostructure. The electric-field-induced shift of the absorption spectrum of the dye serves as a probe of the electric field and reveals a noticeable amplification of the electric field in the dye layers (by a factor of 2?C7). The electric field in the ferroelectric is directed opposite to the direction of the electric field applied to the electrodes of the heterostructure.     

Properties and the nature of electric-field-induced variations in optical absorption spectra of polymethine dyes in polymer matrices

The effect of a permanent electric field on the absorption spectra of ionic (cationic and anionic) and intraionic symmetric and asymmetric polymethine dyes in poly-N-epoxypropylcarbazole, polyvinylbutyral, and polystyrene polymer films is studied. It is found that the electric field causes an increase in absorption of symmetric dyes in the short-wavelength region and a decrease in their absorption in the long-wavelength region. Asymmetric dyes exhibit opposite properties. These properties are common for these groups of dyes irrespective of their chemical structure and ionic nature. They are retained both in photoconducting polymers and in polymers that are incapable of charge photogeneration. These effects also take place in liquid solutions, although in this case they are much weaker. Various mechanisms of the influence of an electric field on the spectra, such as the electron transfer, Stark and Kerr effects, intermolecular interactions, and dye isomerization, are discussed. It is shown that the spectral effects are caused by the redistribution of the electron density in a chromophore of the dye in the ground state resulting in a change in the probability of vibronic transitions.     

Charge transfer between ZnO crystals and dye layers

The interaction between crystal and adsorbed dye molecules has been studied under well defined conditions by measurements of field effect and spectrally sensitized photoconductivity. The (101̄0) surfaces of n-type ZnO crystals (band gap 3.3 eV) are cleaned in ultrahigh vacuum. A pretreatment with atomic hydrogen produces an accumulation layer. Merocyanine (polymethine) dye molecules are deposited by sublimation in the same vacuum (coverage (1–2000) × 10¹⁴ cm^?2. Optical excitation of the dye causes a sensitized photoconductivity in the ZnO crystal close to the surface. The spectra distribution resembles the absorption spectrum of the dye with a maximum at 2.3 eV. An electric field applied perpendicular to the dye covered surface induces charge carriers in the crystal and changes the surface conductivity (field effect). Additional excitation of the dye by light causes a slow relaxation of the field-induced change of surface conductivity. This relaxation is observed for both signs of the field. Furthermore a memory of the dye covered crystals has been found. It can be programmed by field and light, read out via the surface conductivity and quenched by light. A phenomenological model for relaxation and memory is refined by kinetic equations and by considerations about charge transport within the dye layer. The observations can only be explained by a charge transfer between crystal and dye operating in both directions. From these results the following conclusion is drawn for the mechanism of spectrally sensitized photoconductivity of the present system: An electron transfer between dye molecules and crystal represents the decisive step rather than an energy transfer.     

Solvent and electric field dependence of the photocurrent generation in donor：acceptor blend system

This paper reports that the blend films of poly (2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) and N,N'-bis(1-ethylpropyl)-3,4: 9,10-perylene bis (tetracarboxyl diimide) (EP-PDI) with the weight ratio of 1:2.5 have been prepared by spin-coating from chloroform (CF) and chlorobenzene (CB) solutions respectively. The absorption spectra and the morphology of the blend films show that large crystal-like EP-PDI aggregates are formed in film prepared from CB solution, which corresponds to a new absorption shoulder near 590nm, while there is no shoulder around 590\,nm in the UV--Vis absorption spectra of the blend film from CF solution. The electric-field dependence spectra of the photocurrent generation quantum yield of the film from CB solution shows that at weak electric field the EP-PDI aggregates act as more efficient sensitizers, but at strong electric field the quantum yields become almost invariable over the entire spectral range no matter what the state of EP-PDI, monomer or aggregate. At strong electric field, the photocurrent generation yields of both films from CF and CB solution saturate and their yield spectra become spectrally similar, mentioning that at strong electric field the photoexcitons dissociate efficiently and the free charges are collected by the electrodes almost completely.     

外场作用下蒽分子的激发特性研究

蒽(anthracene)具有良好的热稳定性以及较高的荧光量子产率的优点, 是最早用于研究有机发光器件(organic light-emitting device, OLED)的材料之一. 在本文中, 主要利用量子化学方法研究了不同外电场对蒽分子激发特性的影响规律. 首先采用密度泛函理论(density functional theory, DFT)在6-311G(d, p)基组水平上对蒽分子基态结构进行优化, 基于稳定基态结构, 利用含时密度泛函(time-dependent density functional theory, TDDFT)以及同一基组水平, 计算出蒽分子的前十个激发态的激发能、跃迁偶极矩、振子强度和紫外吸收光谱等数据. 然后以密度泛函B3P86方法优化出的不同外电场下蒽分子基态结构为基础, 使用TDDFT方法研究了不同外电场对蒽分子前线轨道能级和激发特性的影响规律. 结果显示, 无场时蒽分子在紫外区域234.50 nm处有一个较强的吸收峰, 对应基态电子跃迁至第5激发态吸收光子波长; 在外电场作用下, 蒽分子电子由基态跃迁到激发态的各项光谱参数均有显著变化, 加场后蒽分子的吸收光谱发生了红移, 由紫外波段移向了紫外–可见光波段, 与实验值相符合. 分子前线轨道的计算结果也表明蒽分子的最高占据轨道(highest occupied molecular orbital, HOMO)和最低未占据轨道(lowest unoccupied molecular orbital, LUMO)能量差值在不同电场下存在差异. 关键词： 蒽 外电场 激发特性     

Spectral and nonlinear studies of night blue dye

Solid-state dye-doped polymer is an attractive alternative to the conventional liquid dye solution. In this Letter the spectral characteristics and the nonlinear optical properties of the dye night blue are studied. The spectral characteristics of night blue dye doped poly(methylmethacrylate) modified with additive n-butyl acetate (nBA) are studied by recording its absorption and fluorescence spectra and the results are compared with the corresponding liquid mixture. The nonlinear refractive index of the dye in nBA and dye doped polymer film were measured using z-scan technique [S.-B., Mansoor, A.A. Said, T.-H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quantum Electron. 26 (1990) 760], by exciting with He–Ne laser. The results obtained are intercompared. Both the samples of dye night blue show a negative nonlinear refractive index. The origin of optical nonlinearity in the dye may be attributed due to laser-heating induced nonlinear effect.     

Spectra and dissociation properties of Freon 31 under electric field

Freon 31 is a carcinogenic substance. It is of great significance for studying its degradation under external electric field. The bond length, energy, dipole moment, orbital energy lever distribution, infrared spectra, and dissociation properties of Freon 31 molecule under the external field are investigated using the density functional theory on basis set level of B3LYP/6-311++G(d, p). In addition, the effects of electric field on ultraviolet–visible absorption spectra of the molecule are studied with CIS/6-311G++(d, p) method. The results indicate that spectra and dissociated properties under electric field have changed significantly. As the electric field (0–0.04 atomic units) along negative direction of y-axis increases, the bond length of carbon–chlorine bond gradually increases and tends to break; the bond length of carbon–fluorine bond gradually decreases. The energy gap first increases and then decreases changing with electric field. The infrared spectra and ultraviolet–visible absorption spectra exhibit blue shift or bathochromic shift under electric field. Moreover, the potential energy surface of Freon 31 about Carbon–Chlorine bond is scanned by the same basis set with configuration interaction - single excitation (CIS) method. The result shows the barrier of dissociation gradually decreases with the electric field. When the intensity of electric field is equal to 0.04 atomic units, the barrier has disappeared. In addition, the molecule is induced to fragmentation due to carbon–chlorine bond breaking. The results offer important reference to the degradation of contaminants.     

Effects of aggregation of squaryl dye on photogeneration of charges in amorphous molecular semiconductors based on poly-N-epoxypropyl carbazole

Peculiarities of photoconductivity, photoabsorption, and photoluminescence of poly-N-epoxypropyl carbazole films doped with an intraionic dye are investigated. The anomalous temperature dependences of the photoconductivity of the films in the dye absorption region are explained by studying the effect of the dye concentration, temperature, and external electric field on the liberation of captured charges from the traps formed in the films as a result of doping. It is concluded that an increase in the dye concentration enhances its aggregation, intensifies the photogeneration of triplet electron-hole pairs, and narrows the spatial distribution of traps for holes in the vicinity of the dye. The hole charges captured in such traps are connected through the Coulomb interaction with negatively charged fragments of dye molecules. The traps are destroyed through thermal activation.     

Optical transitions of holes in uniaxially compressed germanium

Spontaneous emission and photoconductivity of germanium with gallium impurity are studied for determining the energy spectrum of hole states in this material in which radiation can be induced as a result of transitions of holes between these states. Holes were excited by electric field pulses with a strength up to 12 kV/cm at T = 4.2 K under uniaxial compression of samples up to 12 kbar. It has been found that hole emission spectra for transitions between resonant and local states of the impurity have a structure identical to the photoconductivity and absorption spectra. Transitions from resonance states, which are associated with the heavy hole subband, have not been detected. It has been found that in an electric field lower than 100 V/cm, a compressed crystal emits as a result of transitions of heavy holes. In a strong electric field (1–3 kV/cm), emission is observed in the energy range up to 140 meV, and transitions with emission of TA and LO phonons appear in such a field. The emission spectra under pressures of 0 and 12 kbar differ insignificantly. Hence, it follows that the contributions from heavy and light holes in a strong electric field are indistinguishable.     

Dependence of optical properties of monoclinic MnWO4 on the electric field of incident light

Considering the electric field of incident light along four particular directions [100], [110], [011], and [010], the optical properties of monoclinic MnWO₄ were investigated by the first-principle methods. The calculated electronic structures show that the O 2p states and Mn 3d states dominate the top of the valence bands, while the W 5d and Mn 3d states play a key role in the bottom of the conduction bands. The dielectric function and other optical properties, including absorption coefficient, reflectivity spectra, and energy-loss spectra, were calculated and analyzed. The results predicted the maximum static dielectric function when the electric field of incident light was along the [100] direction; meanwhile the absorption edge was calculated to be consistent with the energy band gap and the values and positions of peaks in absorption coefficient are related with the electric field of light. Otherwise, it is found that the appearance of peaks in the energy-loss spectra is also dependent on the electric field and simultaneously corresponds to the edge of absorption spectra and the peaks' position of reflectivity spectra.     

Intense laser effects on the optical properties of asymmetric GaAs double quantum dots under applied electric field

We investigated the combined effects of a non-resonant intense laser field and a static electric field on the electronic structure and the nonlinear optical properties (absorption, optical rectification) of a GaAs asymmetric double quantum dot under a strong probe field excitation. The calculations were performed within the compact density-matrix formalism under steady state conditions using the effective mass approximation. Our results show that: (i) the electronic structure and optical properties are sensitive to the dressed potential; (ii) under applied electric fields, an increase of the laser intensity induces a redshift of the optical absorption and rectification spectra; (iii) the augment of the electric field strength leads to a blueshift of the spectra; (iv) for high electric fields the optical spectra show a shoulder-like feature, related with the occurrence of an anti-crossing between the two first excited levels.     

Effect of elastomer matrix type on electromechanical response of conductive polypyrrole/elastomer blends

Electrorheological properties of prestine elastomers and polypyrrole/elastomer blends were investigated to identify the most suitable elastomer to be used in electroactive actuator applications. Seven types of elastomer: poly(acrylate) copolymers (AR70, AR71, AR72, and SAR), poly(dimethyl siloxane) (PDMS), poly (styrene butadiene) (SBR), and poly(styrene isoprene styrene) (SIS) were chosen as the candidate dielectric elastomers. For the pure elastomers, the storage modulus responses, ΔG′, amongst the elastomers differ by 6 orders of magnitude at the electric field strength of 2 kV/mm. The storage modulus sensitivity, ΔG′/G′₀, increases with electric field strength and attains maximum values of 97% for SBR, 148% for SAR, 232% for AR70, 13% for PDMS, 69% for AR71, 54% for AR72, and 10% for SIS at the electric field strength of 2 kV/mm. Correlations are identified between the elastomer storage modulus sensitivity and G′₀ and/or electrical conductivity. For the undoped polypyrrole/elastomer blends with the particle concentrations of 1, 2, 3, 4, and 5 vol%, ΔG′ increases linearly with concentration in the absence of electric field but nonlinearly with electric field on, due to the obstruction of the matrix-dipole interaction at small concentrations and the nonuniform polypyrrole particle dispersion in the matrix at high concentrations.     

Modulation spectroscopy on metamorphic InAs quantum dots

The modulation spectroscopy on 1.45 μm metamorphic InAs quantum dots (QDs) with In_0.3Ga_0.7As capping layer grown on GaAs substrate by molecular beam epitaxy (MBE) has been investigated by differential absorption (Δα), electro-reflectance (ER), and photo-reflectance (PR) spectra at different reverse bias. The optical transitions of the ground state, excited states, and wetting layers were identified and discussed. The micro-structure characterization was also analyzed by TEM and AFM. The variation of refractive index spectra (Δn) by calculating Δα spectra through Kramers–Kronig transform is obtained to study the electro-absorption behaviors. Additionally, a simple physical model is proposed to explain the experimental values between the Δn and ΔR spectra performed by two different modulation spectroscopies (Δα and ER). The built-in electric field of metamorphic InAs QDs structure was determined to analyze the Franz–Keldysh Oscillation (FKO) extreme in PR spectra with different bias.     

Emission tuning study of RGB blends. Interaction of two EL polymers and a red dye

A study of light emission in photo- and electroluminescence in blends composed of a blue and a green emitting polymers with a red dye was performed. To gain information about the tunability of the RGB mixture, the blends varied in composition. The polymers were poly(2,7-9,9′-dihexylfluorenediyl) (blue), poly(9,9-di-hexylfluorenediyl divinylene-alt-1,4-phenylenevinylene) (green) and the dye was 4-(dicyanomethylene)-2-methyl-6-(dimethylaminostyryl)-4H-pyrane (red).It was verified that in photoluminescence Forster type energy transfer plays the main role in the emission. In solution a large concentration of the acceptors is needed to attain the transfer radii, whereas in the solid state, the transfer is very sensitive to small variations in the acceptor content, with large changes in emission spectra. The band gaps of the components allowed a cascade energy transfer mechanism. In electroluminescence, apart from the energy transfer mechanism, an important role is played by the trapping of charge carriers, resulting in significant differences between the PL and EL spectra.     

外电场作用下Si_2O分子的激发特性

主要对2种Si2O分子异构体的激发特性进行研究,由计算结果可知,外电场对Si2O分子的激发能,振子强度,跃迁偶极矩及吸收光谱有着显著的影响.无外电场时三角型Si2O(C2v,1A1)分子在可见光区无吸收谱,外电场作用下其在可见光区(407.18—526.93nm)有比较弱的吸收谱.直线型Si-Si-O(C∞v,3Σ-)分子在有无外电场作用时在蓝光和紫光区均有一定的吸收谱,其中比较难得的是在蓝色光区(478.88—488.59nm)呈现较强的吸收谱.     

Theoretical and experimental research on spectral performance and laser induced damage of Brewster's thin film polarizers

Polarizers respectively with broad polarizing region bandwidth, large layer thickness error tolerance and high extinction ratio are designed and prepared. Transmittance spectra of the prepared samples are measured at Brewster's angle, and the results show that different requirements can be fulfilled by optimized designs. Spectral performance of designs with higher layer thickness error tolerance coincides better with the theoretical spectra. Laser induced damage threshold of the prepared samples are evaluated. Electric field distribution, defect, film absorption, and damage morphology are investigated, and the results indicate that electric field distribution in high index layers is the main reason that causes the difference of laser induced damage threshold. For both p polarized and s polarized light, the lower the electric field peak value and the farther the layer, which has the strongest electric field away from air, the higher the laser induced damage threshold.     

Anomalous variation of the exciton Fano-resonance spectra in strongly biased Wannier-Stark ladder

Effects of the Wannier-Stark ladder (WSL) resonance on optical absorption spectra in strongly biased superlattices are theoretically investigated by solving the multichannel scattering problem relevant to the WSL-exciton Fano-resonance. When the bias of an electric field F is applied such that a WSL subband state is energetically aligned with adjacent ones, resulting in strong repulsion (anticrossing) due to Zener resonance, an onset of exciton absorption notably shifts toward the lower energy side. However, just a slight change of F away from the anticrossing leads to a peculiar suppression, lowering the absorption edge. According to a qualitative analytic model, such an anomalous variance is found ascribable to delocalization of WSL subband wave functions across several periods through a mixing of an exciton reduced mass in the region of the potential well with that in the region of the potential barrier.     

Polymer solar cells based on a PEDOT:PSS layer spin-coated under the action of an electric field

In the process of fabrication of polymer photovoltaic(PV) devices,poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) thin film,acting as an anode buffer layer,is spin-coated under the action of an electric field.The PV devices with a PEDOT:PSS layer spin-coated under the action of a static electric field exhibit improved short-circuit current density(J sc) and power conversion efficiency(PCE).The investigation of morphology shows that the appropriate intensity of the electric field can increase the roughness of the surface of the PEDOT:PSS layer,which results in improved contact between the anode and hole transport layer and thus enhances the J sc of the devices.Chemical analysis is also provided by x-ray photoelectron spectroscopy(XPS) spectra.     

Optical probe,local fields,and Lorentz factor in ferroelectrics

An optical probe is suggested that allows measurements of the local field and Lorentz factor (L) in ferroelectric medium. The copolymer poly (vinylidene fluoride/trifluoroethylene) is mixed with Pd-tetraphenylporphyrin (TPP-Pd) that has a very narrow absorption band. Thus, TPP-Pd serves as a molecular optical probe of the local field. During the switching of the electric field lower than the coercive one the factor L of an unpolarized ferroelectric mixture is found to be of about 1/3 that corresponds to the random distribution of molecular dipoles in the ferroelectric. With increasing field, the dipole orientation acquires a lower symmetry and L tends to zero as predicted by lattice sum calculations for vinylidene fluoride. The knowledge of the field dependence of L and the usage of the optical probe makes it possible to measure directly the local and macroscopic fields in the individual elements of various ferroelectric-dielectric heterostructures.     

Multiple switching behaviors of poly(N-isopropylacrylamide) hydrogel with spironaphthoxazine and D-π-A type dye

A multi-switchable poly(NIPAM-co-SPO-co-D-π-A dye) hydrogel with a photochromic spironaphthoxazine and an electron donor-π-conjugated-electron acceptor (D-π-A) type dye was prepared by typical radical copolymerization. The low critical solution temperature (LCST) behavior was investigated by UV–vis spectroscopy, which allows the measurement of the phase transition from 20 to 40 °C in aqueous solution. The fluorescence intensity of the poly(NIPAM-co-SPO-co-D-π-A dye) was temperature-dependent. Reversible modulation of fluorescence intensity was achieved using alternating irradiation with UV and visible light. Prepared polymer hydrogel also exhibited spectra change when not only used Cu²⁺ cation but also an acid unit.