Ferroelectric phase transition in Langmuir-Blodgett films of copper phthalocyanine

A ferroelectric phase transition is observed in Langmuir-Blodgett films prepared from substituted copper phthalocyanine molecules. The linear and nonlinear dielectric properties of the films and the switching of their spontaneous polarization are investigated in the temperature range of the phase transition. The observed features can be explained by the Landau-Ginzburg model of a first-order phase transition. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 625–631 (10 November 1999)     

Polarization switching kinetics of ferroelectric nanostructures of vinylidene fluoride-trifluoroethylene copolymer

The polarization switching kinetics of ferroelectric polymer Langmuir-Blodgett films and copolymer nanocrystals (nanomesas) was investigated using piezoresponse force microscopy. The nanocrystals were prepared by self-organization from Langmuir-Blodgett films of a 70% vinylidene fluoride and 30% trifluoroethylene copolymer. The polarization switching time exhibits an exponential dependence on reciprocal voltage and decreases with temperature being consistent with the nucleation switching dynamics.     

Dielectric dispersion in Langmuir-Blodgett ferroelectric polymer films

The dynamic permittivity of vinylidene fluoride-trifluoroethylene (70:30) copolymer films is measured over wide ranges of frequencies and temperatures. It is revealed that 15-to 45-nm-thick Langmuir-Blodgett films of the vinylidene fluoride-trifluoroethylene copolymer undergo a Debye-type dielectric relaxation. The relaxation time τ increases as the temperature increases and the ferroelectric phase transition is approached. This effect is interpreted as a result of critical slowing down. The influence of gamma irradiation on the dielectric response of Langmuir-Blodgett films consisting of 10 and 30 monolayers is investigated.     

Manifestation of a ferroelectric phase transition in ultrathin films of polyvinylidene fluoride

Temperature dependences of the dielectric properties of ultrathin polyvinylidene fluoride films prepared using the Langmuir-Blodgett method were studied by linear and nonlinear dielectric spectroscopy. It is shown that ultrathin Langmuir films of polyvinylidene fluoride exhibit a manifestation of a first-order ferroelectric phase transition, which can be assigned to the interaction between the spontaneous polarization and the surfaces bounding the film. As the film thickness increases, the effect of the surfaces decreases and the ferroelectric phase transition shifts to high temperatures to vanish altogether when the temperature region of the transition rises above the melting point.     

红外光谱研究聚硅氧烷侧链手性液晶及其单体的单层LB膜

一种聚硅氧烷侧链手性液晶P及其单体M的单层LB膜分别在11mN/m和8mN/m的表面压力下,从水汽界面成功地转移到在镀金玻璃片和氟化钙基片上。π-A曲线表明P及其单体M在水汽界面上单层膜的崩溃压力分别是35和11mN/m。红外透过和反射吸收光谱表明两者的生色团部分及烷基链部分倾斜排列在固体基片上,但其倾斜角不同。M比P有更大的倾斜角。同时,不论P或M其烷基链的轴向与芳香核部分的长轴方向并不重合。P的烷基链比M的烷基链更有序。     

聚(偏氟乙烯-三氟乙烯)纳米薄膜极化反转与疲劳特性

采用旋涂法制备了厚度为140 nm的聚(偏氟乙烯-三氟乙烯)[P(VDF-TrFE)]纳米薄膜, 研究了不同退火温度以及环境相对湿度对薄膜的极化反转和疲劳性能的影响. 运用X射线衍射仪、扫描电子显微镜和傅里叶变换红外光谱仪等测试技术对薄膜的微结构进行了表征. 实验结果表明, 通过不同温度的退火处理, P(VDF-TrFE)铁电薄膜的结晶度随着退火温度的升高而不断提高, 并且一定的温度范围内的退火处理可以提高薄膜的极化性能; 此外, P(VDF-TrFE) 铁电薄膜性能还表现出一定的环境湿度的敏感特性, 这与薄膜的物理性能和结构特点密切相关; P(VDF-TrFE)铁电薄膜在不同的环境湿度条件下 表现出较好的电学特性, 其漏电流均保持在10 ^-7A/cm² 的较低水平. 本工作揭示了再退火过程对薄膜的极化反转速度和疲劳恢复特性的影响, 并结合薄膜二次疲劳结果, 探讨了薄膜可逆的内部疲劳恢复特性机理.     

General features of the intrinsic ferroelectric coercive field

The value of the intrinsic ferroelectric coercive field is obtained independently, from general energy considerations and from the predictions of several models of the ferroelectric state. All predictions yield a value of the order of the depolarization field, which is equal to the spontaneous polarization divided by the dielectric permittivity, and are consistent with the recent measurements of the intrinsic ferroelectric coercive field in ultrathin Langmuir-Blodgett films of copolymers of polyvinylidene fluoride with trifluoroethylene. Prior studies succeeded only in measuring the much smaller extrinsic coercive fields, which are limited by nucleation processes and domain motion.     

1/<Emphasis Type="Italic">E</Emphasis><Superscript>2</Superscript> Law: Solution of the Unsolved Problem in the Physics of Ferroelectrics

An unusual frequency dependence of a coercive field observed in polycrystalline ferroelectric films of Pb(Zr_{1 ? x}Ti_x)O₃ solid solutions in a wide frequency range has been explained within the model of surface sources emitting thin ferroelectric domains with reversed polarization. This model makes it possible to interpret experimental data as the manifestation of the coercivity paradox in polycrystalline films and predicts the existence of the limiting frequency for switching of domains in agreement with the experiment. The proposed mechanism of polarization switching explains the observed temperature dependence of the activation field E_a ∝ T^?1/2. Furthermore, it predicts an increase in the activation energy for the nucleation of domains with an increase in the size of the source, indicating that the coercive field increases with the degradation of small     

Measurement of landau coefficients in ferroelectrics using nonlinear dielectric spectroscopy

A method of measuring the phenomenological Landau-Ginzburg coefficients in an expansion of the free energy of ferroelectrics near the phase transition is proposed. This method is based on simultaneously recording five Fourier components of the current response to a sinusoidal voltage as a function of temperature. An analysis is made of a specific application of the method to Langmuir-Blodgett ultrathin ferroelectric films.     

Intrinsic ferroelectric coercive field

The Landau-Ginzburg theory of ferroelectricity predicts the intrinsic coercive field for polarization reversal, but the observed extrinsic coercive field is always much smaller as a result of nucleation, dynamic processes not covered by the static theory. We have realized the intrinsic coercive field for the first time, in two-dimensional Langmuir-Blodgett polymer films as thin as 1 nm. The measured coercive field is in good agreement with the theoretical intrinsic value, exhibits the expected dependence on temperature, and does not depend on thickness below 15 nm.     

Effect of strontium addition on ferroelectric phase transition of PZT thin films prepared by chemical route

The dependence of ferroelectric phase transition temperature as a function of strontium substitution in lead titanate zirconate thin films (referred here as PSZT) on platinum-coated silicon substrates was investigated. The dielectric study reveals that the material undergoes a diffuse type ferroelectric phase transition that depends on the substitution of Sr for Pb in PZT. At 100 kHz, the phase transition temperatures were 633, 613 and 516 K for PSZT10, PSZT20 and PSZT30 thin films, respectively. On the other hand, the results showed that the dependence of the dielectric constant upon the frequency does not reveal any relaxor behavior. The diffusivity increases with increasing Sr contents in the studied composition range. The experimental data obtained from measurements of the dielectric constant as a function of temperature and frequencies showed a classical behavior of ferroelectric phase transition in PSZT thin films, rather than a relaxor ferroelectric phase transition. The transition temperature decreases with increasing Sr contents due to the decrease in grain size, lattice decrease and local structural disorder.     

Broadband dielectric dispersion in ferroelectric P(VDF-TrFE) copolymer films

Ferroelectric polyvinylidenefluoride-trifluoroethylene copolymer films with different thicknesses are prepared by a solvent-cast technique, by spin coating, and by a horizontal Langmuir–Blodgett technique respectively. The frequency dependent dielectric permittivity of these films is investigated with varying sample thickness and varying temperature in the ferroelectric as well as in the paraelectric phase. A dielectric relaxation according to a Vogel–Tamman–Fulcher law of the relaxation times is found in all samples. However, the relaxation times extracted from the dielectric permittivity in the frequency range are not consistent with the relaxation times determined from the temperature range. An explanation for this behavior is given by a temperature dependent distribution of relaxation times. Additionally, in thin samples a second relaxation with a weak anomalous temperature dependence, i.e. an increasing relaxation time with increasing temperature, is observed at high frequencies. Detailed investigations show that this behavior can be attributed to an electrode effect.     

新型花菁染料Langmuir-Blodgett膜的椭偏光谱研究

利用椭偏光谱法(SE)对Y型花菁染料LB膜在紫外—可见光范围内(λ=2755—8266nm)的光学特性进行了表征,同时得到了该薄膜的光学常量(复介电常数、消光系数、吸收系数、反射系数、折射率等).讨论了LB薄膜对不同频率光的较高吸收特性及其成膜结构之间的关系,对其物理机理给予了解释.此外,采用洛伦兹振子模型对所得的光学参量进行了理论拟合,结果发现：其理论拟合与实验数据符合得非常好. 关键词： Langmuir-Blodgett膜 椭圆偏振光谱法 洛伦兹振子模型     

Temperature-dependent ferroelectric and dielectric properties of Bi3.25La0.75Ti3O12 thin films

To investigate temperature-dependent ferroelectric and dielectric properties of ferroelectric films, Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films were prepared on Pt-coated silicon substrates by pulsed laser deposition. The ferroelectric and dielectric behaviors have been studied in a wide temperature range from 80 K to room temperature. The saturated polarization (P_sat) decreases with decreasing temperature and decreasing electric field, whereas remnant polarization (P_r) shows a more complex temperature dependence. These results, which can be well explained based on a temperature-dependent charged defects-domain wall interaction model, might be helpful for further understanding the domain switching behavior. Based on these results, an alternative way to investigate temperature-dependent ferroelectric fatigue is proposed and experimentally carried out. The measured fatigue rate is found to be linearly dependent on temperature, consistent with the report on Pb(Zr,Ti)O₃ films. Temperature-dependent dielectric measurements of the films further confirm the above explanation.     

A polarizability model for the ferroelectric mode in semiconducting SrTiO3

The temperature dependence of the ferroelectric mode frequency in SrTiO₃ with oxygen vacancies and free electrons, is quantitatively explained on the basis of a microscopic model for ferroelectric soft modes.     

Study of the polarization profile in film ferroelectrics by the thermal square wave method

The possibility of using the thermal square wave method at single frequency for the analysis of the polarization distribution over the depth of a thin-film ferroelectric has been considered. The temperature distribution in a ferroelectric film-substrate system has been calculated. The method has been tested using tin thiohypodiphosphate films on aluminum substrates.     

由Pb过量引起的镧钛酸铅铁电薄膜性能异常的研究

采用金属有机物热分解法制备了不同La含量和不同Pb过量的镧钛酸铅铁电薄膜．在研究铁电性时发现：薄膜样品在极化达到饱和之前，电滞回线会出现束腰现象，束腰现象只与薄膜中的Pb过量有关．随Pb过量的增加和La含量的减少，镧钛酸铅薄膜电滞回线的束腰程度加剧．在C-V特性测试中发现：在Pb过量的样品中，C-V曲线会出现异常的四峰现象，随偏压升高或频率增大，异常程度逐渐减弱甚至会转变成正常的双峰．进一步研究表明：这两种实验现象是统一的，可以用铁电薄膜电畴的钉扎效应解释．由于Pb过量使正常的钙钛矿薄膜中出现了多余的Pb 关键词：     

Anisotropy of dipole-dipole interactions in ferroelectric langmuir films of poly(vinylidene fluoride-trifluoroethylene)

The Lorentz tensor components L _j for poly(vinylidene fluoride-trifluoroethylene, 70/30) Langmuir-Blodgett films in the ferroelectric phase are determined experimentally. The results indicate that intralayer dipole-dipole interaction between the segments of the polymer is stronger than interlayer interaction. This conclusion agrees with the absence of the critical film thickness below which ferroelectricity would cease to exist.     

Surface ferroelectric phase transition in multilayer polymer Langmuir films

The ferroelectric phase transition on the free surface of a polymer ferroelectric Langmuir-Blodgett film was studied by the optical second harmonic generation (SHG) technique. A hysteresis in the temperature dependence of the SHG intensity observed for a multilayer film of a poly(vinylidene fluoride)-trifluoroethylene copolymer in the vicinity of T≈15°C is a manifestation of the first-order ferroelectric phase transition in the topmost surface monolayer of the film.     

LB薄膜中四新戊氧基酞菁锌和四壬基酞菁铜分子聚集体的吸收光谱的温度特性

本文制备了四新戊氧基酞菁锌(Tetra-neopentoxy phthalocyanine zine)(TNPPeZn)和四壬基酞菁铜(Tetra-nonyl phthalocyanine copper)(TNPeCu)两种酞菁衍生物的Langmuir-Blodgett(LB)薄膜.通过测量10～473K温度下的吸收光谱,研究了两种薄膜的分子聚集状态.TNPPeZn的LB薄膜中,存在着分子单体和分子二聚体,在吸收光谱中分别表现为680nm和620nm的吸收峰.随着温度的升高,分子单体逐渐转变为分子二聚体,这个过程是不可逆的.TNPeCu的LB薄膜中,除了分子单体和分子二聚体以外,还有吸收为740nm的分子J聚集体存在.随着温度的变化,J聚集体发生可逆变化.