Ferroelectric Films

The preparation and properties of ferroelectric films are reviewed. Specific attention is directed to ferroelectric films prepared by wet chemical methods. Emphasis is placed on the microstructure and properties of SBT films for memory applications, and their dependence on chemistry and processing, as well as on the effects of chemistry and processing on the properties of piezoelectric and pyroelectric films. Emphasis is placed on recent results obtained in our laboratories, including modeling of the ferroelectric behavior of films and experimental results in each of these areas. Also considered is the use of ferroelectric films in memory, piezoelectric and pyroelectric devices of various types, with specific note made of the recently-announced use of pyroelectric sensors in automobiles.     

Quantum chemical calculations for polyvinylidene fluoride with substitution defects

The MNDO calculation method and molecular cluster model are used to study the influence of substitution defects formed by alkali metal (Li, Na, K) and halogen (Cl, I) atoms on the saturation polarization and forbidden gap of polyvinylidene fluoride (CH₂2CF₂) a pyroelectric with pronounced photoelectric properties. Densities of state are calculated for both the defectless pyroelectric and for the sample with substitution defects. The calculated energy characteristics are in good agreement with the available experimental data. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 468-473, May–June, 2000.     

Measurement of the pyroelectric response and of the thermal diffusivity of microtomized sections of 'single crystalline' ferroelectric liquid crystalline elastomers

The pyroelectric response and the thermal properties of microtomized sections of 'single-crystalline' ferroelectric liquid crystalline elastomers (FLCE) have been investigated. The microtomized sections were manufactured in the thickness range from 20 to 100 mum. The roughness of the surfaces due to the cutting process was investigated with an alpha-stepper and found to be in the range of about 5 mum. The thermal diffusivity of the elastomer at different temperatures was determined by placing the sample on a pyroelectric substrate and measuring the pyroelectric signal of the substrate under appropriate conditions. From the pyroelectric current spectra, information about the spatial distribution of the polarization could be deduced. It was found to be constant except in a range also about 5 mum near the surface. From the combination of this result and that of the alpha-stepper measurements, the conclusion is drawn that the molecular orientation at the surface is destroyed due to the cutting process. The magnitude of the spontaneous polarization was calculated from the pyroelectric measurements at different temperatures, making use of the thermal diffusivity values.     

Measurement of the pyroelectric response and of the thermal diffusivity of microtomized sections of 'single crystalline' ferroelectric liquid crystalline elastomers

The pyroelectric response and the thermal properties of microtomized sections of 'single-crystalline' ferroelectric liquid crystalline elastomers (FLCE) have been investigated. The microtomized sections were manufactured in the thickness range from 20 to 100 mum. The roughness of the surfaces due to the cutting process was investigated with an alpha-stepper and found to be in the range of about 5 mum. The thermal diffusivity of the elastomer at different temperatures was determined by placing the sample on a pyroelectric substrate and measuring the pyroelectric signal of the substrate under appropriate conditions. From the pyroelectric current spectra, information about the spatial distribution of the polarization could be deduced. It was found to be constant except in a range also about 5 mum near the surface. From the combination of this result and that of the alpha-stepper measurements, the conclusion is drawn that the molecular orientation at the surface is destroyed due to the cutting process. The magnitude of the spontaneous polarization was calculated from the pyroelectric measurements at different temperatures, making use of the thermal diffusivity values.     

Polymer based interfaces as bioinspired 'smart skins'

This work reports on already achieved results and ongoing research on the development of complex interfaces between humans and external environment, based on organic synthetic materials and used as smart 'artificial skins'. They are conceived as wearable and flexible systems with multifunctional characteristics. Their features are designed to mimic or augment a broad-spectrum of properties shown by biological skins of humans and/or animals. The discussion is here limited to those properties whose mimicry/augmentation is achievable with currently available technologies based on polymers and oligomers. Such properties include tactile sensing, thermal sensing/regulation, environmental energy harvesting, chromatic mimetism, ultra-violet protection, adhesion and surface mediation of mobility. Accordingly, bioinspired devices and structures, proposed as suitable functional analogous of natural architectures, are analysed. They consist of organic piezoelectric sensors, thermoelectric and pyroelectric sensors and generators, photoelectric generators, thermal and ultra-violet protection systems, electro-, photo- and thermo-chromic devices, as well as structures for improved adhesion and reduced fluid-dynamic friction.     

Evaluation of the solid state dipole moment and pyroelectric coefficient of phosphangulene by multipolar modeling of X-ray structure factors

The electron density distribution of the molecular pyroelectric material phosphangulene has been studied by multipolar modeling of X-ray diffraction data. The "in-crystal" molecular dipole moment has been evaluated to 4.7 D corresponding to a 42% dipole moment enhancement compared with the dipole moment measured in a chloroform solution. It is substantiated that the estimated standard deviation of the dipole moment is about 0.8 D. The standard uncertainty (s.u.) of the derived dipole moment has been derived by splitting the dataset into three independent data-sets. A novel method for obtaining pyroelectric coefficients has been introduced by combining the derived dipole moment with temperature-dependent measurements of the unit cell volume. The derived pyroelectric coefficient of 3.8(7) x 10-6 Cm 2K-1 is in very good agreement with the measured pyroelectric coefficient of p = 3 +/- 1 x 10-6 Cm-2 K-1. This method for obtaining the pyroelectric coefficient uses information from the X-ray diffraction experiment alone and can be applied to much smaller crystals than traditional methods.     

From Synthesis to Applications: Copper Calcium Titanate (CCTO) and its Magnetic and Photocatalytic Properties

Investigations focusing on electrical energy storage capacitors especially the dielectric ceramic capacitors for high energy storage density are attracting more and more attention in the recent years. Ceramic capacitors possess a faster charge-discharge rate and improved mechanical and thermal properties compared with other energy storage devices such as batteries. The challenge is to obtain ceramic capacitors with outstanding mechanical, thermal and storage properties over large temperature and frequencies ranges. ABO₃ as a type of perovskites showed a strong piezoelectric, dielectric, pyroelectric, and electro-optic properties useful as energy storage and environmental devices. CaCu₃Ti₄O₁₂ (CCTO) perovskite with cubic lattice (Im3 symmetry) was discovered to have a colossal dielectric constant (10⁴) that is stable over a wide range of frequencies (10 Hz–1 MHz) and temperature independence (100–300 K). The origin of this high dielectric constant is not fully established, specially because it is the same for single crystal and thin films. In this review, the history of CCTO will be introduced. The synthesis and the sintering approaches, the dopant elements used as well as the applications of CCTO will be reported. In addition to dielectrical properties useful to energy storage devices; CCTO could serve as photocatalytic materials with a very good performance in visible light.     

Electron-beam evaporated silicon as a top contact for molecular electronic device fabrication

This paper discusses the electronic properties of molecular devices made using covalently bonded molecular layers on carbon surfaces with evaporated silicon top contacts. The Cu "top contact" of previously reported carbon/molecule/Cu devices was replaced with e-beam deposited Si in order to avoid Cu oxidation or electromigration, and provide further insight into electron transport mechanisms. The fabrication and characterization of the devices is detailed, including a spectroscopic assessment of the molecular layer integrity after top contact deposition. The electronic, optical, and structural properties of the evaporated Si films are assessed in order to determine the optical gap, work function, and film structure, and show that the electron beam evaporated Si films are amorphous and have suitable conductivity for molecular junction fabrication. The electronic characteristics of Si top contact molecular junctions made using different molecular layer structures and thicknesses are used to evaluate electron transport in these devices. Finally, carbon/molecule/silicon devices are compared to analogous carbon/molecule/metal junctions and the possible factors that control the conductance of molecular devices with differing contact materials are discussed.     

The Influence of Porosity on the Electromechanical and Pyroelectric Properties of Ca-Modified PbTiO3 Thin Films

The properties of Pb_1-xCa_xTiO₃ (x = 0–30) thin films for electromechanical and pyroelectric applications can be further improved if porous, low-dielectric constant layers are being used. The electric field dependent strain, piezoelectric coefficient, pyroelectric coefficient and the pyroelectric figure of merit were evaluated as a function of the Ca-content and relative density of the thin films. The heating-rate during the final anneal was observed to be the controlling parameter for the evolution of either dense or porous microstructures. Both 2-methoxyethanol and 1,3-propanediol based solution precursors were used for spin-coating platinized Si₃N₄/SiO₂/Si wafers. Microstructure-property relationships and electrical properties concerning the domain mobility were examined.     

Thermal and dielectric properties of LiKSO4 and LiCsSO4

Measurements of the thermal and dielectric properties of single crystals of the double sulfates LiKSO₄ and LiCsSO₄ are reported. Uncertainty in the determination of the space group of LiCsSO₄ is resolved on the basis of optical second harmonic generation and pyroelectric measurements; the correct space group is Pcmn. Measurements of the pyroelectric coefficient, dielectric constant, and specific heat of LiKSO₄ permit an assessment of the suitability of this material for use in pyroelectric detectors.     

Advanced polymers for electrooptics

Polymers have been in use for a long time as passive materials for components in electronics and electrooptics. They combine the possibility of easy processing with an infinite potential of functionalization. They can be conductors or semiconductors, ferroelectrics and can exhibit interesting properties such as photoconductivity, piezo, pyroelectricity or nonlinear optical properties. As a consequence they are now used for merging active components in the field of electrooptics such as displays, sensors or modulators for optical signal treatment. This paper describes the properties and applications of some recent polymers in the field of electroptics. In the first part of this paper the properties of amorphous copolymers with a pending group with large hyperpolarizability are described. These amorphous copolymers show high optical nonlinear coefficients after poling under an electric field. We have used these copolymers for the manufacture of an electrooptic modulator working at 1.3 μm at a frequency of 1 GHz. The second part deals with ferroelectric polymers and their pyroelectric properties. The realization and performance of an IR pyroelectric sensor using copolymers of polyvinylidene fluoride–ethylene trifluride are described. Some new results concerning polymersdispersed liquid crystals are also described.     

偏氟乙烯和四氟乙烯共聚反应

本文用过氧化二碳酸二异丙酯为引发剂,丙酮为分子量调节剂,对偏氟乙烯-四氟乙烯共聚反应进行了研究,并制得了不同组成,不同分子量的样品,用色谱分析方法分析在共聚反应过程中单体组成的变化,方便地确定了偏氟乙烯-四氟乙烯共聚反应中的“恒比”组成范围,并能控制共聚物组成的均匀性,丙酮有效地调节了共聚物的分子量,对共聚物的性能测定表明,它们有很好的机械性能和热稳定性,在“恒比”组成范围的共聚物的热电系数有一个极大值,它对应于熔点的极小值。     

Thermal behavior of ferroelectric Polyamide 11 in relation to pyroelectric properties

The pyroelectric properties of oriented thin films of ferroelectric Polyamide 11 have been studied in the temperature range of −100°C up to +140°C. The temperature dependence of the experimental pyroelectric coefficient has been analyzed. Three changes of slope of the pyroelectric coefficient are observed at −20, +50, and +100°C. The origin of the lower temperature event has not yet been defined. The upper transition is attributed to chain movements in crystalline regions, and more precisely, to a crystalline phase transition. The intermediate event is close to the glass transition temperature T_g observed by DSC. It is attributed to the manifestation of the glass transition. Below T_g, the variations of the pyroelectric coefficient are very small. For higher temperatures, it increases rapidly, attesting to a major contribution of secondary pyroelectricity and dimensional effects above T_g. The breaking of hydrogen bonds occurring at the glass transition temperature observed on DSC thermograms does not affect pyroelectric properties. Pyroelectric properties are mildly reduced after annealing at temperatures up to +140°C. A comparative study of oriented ferroelectric films prepared by quenching from the melt and nonoriented slowly cooled samples has been carried out by means of DSC. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 715–723, 1999     

Effect of crystallochemical parameters of a piezophase on piezoelectric and pyroelectric properties of a polymer-piezoceramic composite

The piezoelectric and pyroelectric properties of the composites based on HDPE and lead-zirconate-titanate piezoceramics are shown to depend on the crystallochemical parameters of the piezophase. As the electroconductivity of cations and covalence of the B″″-O bonds decrease owing to enhanced adsorption interactions between polymer phases and the piezoceramic, the concentration of charge localization sites increases and the mobility of polymer chains at the interface decreases. As a result, piezoelectric and pyroelectric coefficients tend to increase.     

The Contribution of Pyroelectricity of AgI Crystals to Ice Nucleation

The pyroelectricity of AgI crystals strongly affects the icing temperature of super‐cooled water, as disentangled from that of epitaxy. This deduction was achieved by the design of polar crystalline ceramic pellets of AgI, with experimentally determined sense of polarity. These pellets are suitable for measuring both their pyroelectric properties as well as the icing temperature of super‐cooled water, separately on each of the expressed Ag⁺ and I^? hemihedral surfaces. The positive pyroelectric charge at the silver‐enriched side elevates the icing temperature, whereas the negative charge at the iodide side decreases that temperature. Moreover, the effect of pyroelectric charge remains dominant despite the presence of contaminants on both the silver and the iodide‐enriched surfaces. Consequently an electrochemical process for ice nucleation is suggested, which might be of relevance for understanding the role played by electric charges in heterogeneous icing processes in general.     

Anisotropic networks, elastomers and gels

Anisotropic networks, elastomers and gels exhibit piezoelectric, pyroelectric, ferroelectric and NLO properties of potential interest for use communication and processing technologies. The formation, properties and applications of such anisotropic, mainly liquid crystalline, networks are described. If some of the molecules in a liquid mixture contain at least two reactive groups which can be either photochemically or thermally polymerized, then crosslinked, anisotropic networks, elastomers and gels can be produced. Solid macroscopically aligned elastomers or networks can be formed as required beforehand or simultaneously by orientation of the sample. Anisotropic gels consist of a solid anisotropic network and non-covalently bonded, but strongly oriented domains of low molar mass liquid crystals. Anisotropic networks, elastomers preformed amorphous or liquid crystalline polymers incorporating additional reactive groups, which can be macroscopically oriented in the additional crosslinking reactions. Reversible networks, elastomers and gels can be prepared either non-covalently or covalently by thermally side group polymers and low molar mass molecules, liquid crystalline properties in the pure state. in many electro-optic devices for optical and gels can be prepared from liquid crystalline state and then fixed by reversible linkages between, for example, neither of which necessarily exhibit     

Photoswitching Molecular Junctions: Platforms and Electrical Properties

Remarkable advances in technology have enabled the manipulation of individual molecules and the creation of molecular electronic devices utilizing single and ensemble molecules. Maturing the field of molecular electronics has led to the development of functional molecular devices, especially photoswitching or photochromic molecular junctions, which switch electronic properties under external light irradiation. This review introduces and summarizes the platforms for investigating the charge transport in single and ensemble photoswitching molecular junctions as well as the electronic properties of diverse photoswitching molecules such as diarylethene, azobenzene, dihydropyrene, and spiropyran. Furthermore, the article discusses the remaining challenges and the direction for moving forward in this area for future photoswitching molecular devices.     

Molecular-scale electronics: From device fabrication to functionality

Device fabrication and functionality are two crucial aspects in molecular-scale electronics. Recent advancesin this field, including fabrication and application of nanogap electrodes, self-assembled monolayers and their functional devicesarehighlighted in this review paper.     

Optimization of the process for synthesis of a ceramics based on lead ferroniobate

Microstructure, phase transitions, and electrical properties of a ceramics based on lead ferroniobate were considered. The possibility of optimizing the process for synthesis of this ceramics by using a glass-forming additive of lithium carbonate in synthesis via an intermediate phase with a columbite structure in order to provide steadily high pyroelectric properties was analyzed.     

Analytical techniques for characterization of organic molecular assemblies in molecular electronics devices

The analytical techniques used for the physical characterization of organic molecular electronic-based devices are surveyed and discussed. These protocols include methods that are used to probe molecular assemblies such as single wavelength ellipsometry, water contact angle goniometry, cyclic voltammetry, infrared spectroscopy, and X-ray photoelectron spectroscopy, and methods used to measure charge transport properties of devices such as scanning tunneling microscopy, and inelastic electron tunneling spectroscopy. Examples from our laboratory and the literature are given for each of these analytical techniques.