Advancement trends in dye-doped polymer dispersed liquid crystals—a survey review

At present there is significant interest in the doping of various types of dyes into polymer dispersed liquid crystals (PDLCs) in display films. Till to date, various types of dyes have been employed to improve the electro-optical properties of PDLC films in various situations by different research groups. Some of the advantageous features of dye-doped PDLC predict that such dye-doping into the PDLC films resolve the scattering effect, improve the films absorbance, enhance the degree of alignment, stability, and the electro-optical properties of the film such as reflectance, contrast ratio and better transmittance. The current article contains a short review on the present and past studies conducted on the dye-doped polymers dispersed liquid crystals films. The fabrication techniques and their various applications in the different display areas are reviewed, along with the new key findings are discussed.     

Single‐layered microscale linear‐gradient PDLC material for electro‐optics

We report on single‐layered optical material of linear‐gradient microscale polymer‐dispersed liquid crystal (PDLC). E7/NOA65 composite films formed by pulsed UV laser photopolymerization‐induced phase separation exhibit two morphology types, namely a bipolar and a hybrid alignment of liquid crystal droplets. The specific structural properties of the produced PDLC layers, such as the droplet shape uniformity and alignment, as well as the droplet size control through the film thickness, facilitate the efficient control on the electro‐optical (EO) response, thus being of practical interest for EO device applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of flexible heterojunction based on ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/pentacene/Al

The organic solar cell based on poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS)/pentacene (Pc) was fabricated using a flexible polyethyleneterephthalate (PET) substrate coated with conductive ITO layer and aluminum contact film formed on a Pc layer by electron beam deposition. The performed investigations of current–voltage characteristics indicate that the devices operate like Schottky diodes. Under illumination of an ITO/PEDOT:PSS/Pc/Al multilayer structure the photovoltaic effect is measured with open circuit voltages up to 0.5 V, short circuit current of 0.6 μA and fill factor 0.2. The spectral-photocurrent excitation profile covers a full range of visible light (380–700 nm).     

Poly(3,4-ethylenedioxythiophene)/carbon-based nanocomposite for gas sensing

Abstract

In this study, we have manufactured sensory films based on poly(3,4-ethylenedioxythiophene)/carbon nanotubes/reduced graphene oxide nanocomposite. The charge transport in obtained films was analyzed using impedance spectroscopy and temperature dependence of conductivity in the 90–325?K temperature range. The activation mechanism of charge transfer was established and the activation energy of conductivity was determined. The percolation effect in the nanocomposite was found with increasing the content of carbon nanoparticles. A decrease in resistance and an increase in the capacity of hybrid films due to the adsorption of water and ammonia molecules were revealed. Found features of the charge transport processes can expand the prospects of application of nanocomposite films for sensor electronics.     

Dielectric relaxation and electro-optical switching behavior of nematic liquid crystal dispersed in poly(methyl methacrylate)

Polymer dispersed liquid crystal composite films were prepared from poly(methyl methacrylate) and nematic liquid crystal E44 by solvent induced phase separation method. In the present investigation we report effect of liquid crystal concentration on the electro-optical and dielectric properties of the composite films. The results were interpreted in terms of phase separation of liquid crystal and polymer, shape and size of liquid crystal droplet, interfacial charge layer effect, liquid crystal loading and miscibility of liquid crystal in the polymer matrix. The miscibility between two phases at interface was investigated by employing Fourier‐Transform Infrared Spectroscopy and differential scanning calorimetry. Morphological study showed that liquid crystal phase is embedded in a spongy poly(methyl methacrylate) matrix and homogeneous distribution increased with increasing E44 content. Electro optical behavior of these composite films under the condition of an externally applied AC electric field (0–200 Vp-p, 50–1000 Hz) and wide range of temperature was determined using He–Ne laser (wave length 632.8 nm) as a light source. It was found that Poly(methyl methacrylate)/E44 (30/70) wt.% composite has more significant properties than the other concentrations. The performance of all composites showed variations with respect to applied voltage as well as temperatures. Dielectric measurement of polymer dispersed liquid crystals has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24 °C to 100 °C. The Maxwell–Wagner effect due to interfacial charge accumulation between boundaries of liquid crystal droplets and surrounding of polymer matrix has been observed.     

A non-template hydrothermal route to uniform 3D macroporous films with switchable optical properties

Here we report the direct synthesis of long-range uniform 3D macroporous film on the surface of glass via a non-template strategy by reacting silica with a mixture solution of HCl and H₂O₂ under hydrothermal conditions. The size of macropores can be tuned from 1 μm to 7 μm by changing the hydrothermal temperature. Followed by the impregnation or release of water, the 3D porous films showed a high-contrast optical switchable property. The maximum optical transmittance modulation of these 3D porous films was up to 50% over the visible and near IR wavelength range. Furthermore, liquid-induced optical switching device were designed as a sandwich structure. The average transmittance of the device was varied between 62.3% (from 350 to 1100 nm) at the transparent state and 13.2% (from 350 to 1100 nm) at the opaque state, respectively. Such a wide modulation range of optical transmittance is suitable for application of this porous film to energy saving smart windows.     

The structural transformation and properties of spin-on poly(silsesquioxane) films by thermal curing

In this study, the structural transformation and properties of five commercially available poly(silsesquioxanes) by thermal curing were investigated, including poly(hydrogen silsesquioxanes) (HSQ and T12), and poly(methylsilsesquioxanes) (MSQ, T7 and T9). These materials with a different cage/network ratio and side groups (Si-H and Si-CH₃). The FTIR spectra show that the poly(silsesquioxane) films have different contents of the Si-O-Si cage and network structures, which significantly affects the refractive index and dielectric constant. The shifting of the Si-O-Si network band in the FTIR spectra can be correlated with their molecular structures. The refractive indices and dielectric constants of the studied poly(silsesquioxane) films increase with increasing the Si-O-Si network content. The retention of the Si-H or Si-CH₃ side group suggests the existence of the cage structures in the poly(silsesquioxane) films. The Si-O-Si cage structure results in a larger free volume than the Si-O-Si network structure in the poly(silsesquioxane) films and thus reduces the refractive index and dielectric constant. It is supported by the porosity result. The order of the refractive index in the studied poly(silsesquioxanes) films is T12>HSQ for the Si-H side group and T7>T9>MSQ with the Si-CH₃ side group, which can be correlated with the Si-O-Si network content. The poly(silsesquioxane) film with the Si-CH₃ side group has a lower refractive index than the Si-H side group at the same Si-O-Si network content, which is probably due to the steric hindrance effect of the CH₃ group.     

Preparation of mesoporous oxide films via block copolymer templating

Mesoporous silica films have been synthesized by a modified sol-gel method in the presence of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers as structure-directing agents. Influence of synthetic conditions on the formation of mesoporous silica films has been investigated. Porosity of samples can be controlled by block copolymer concentration and aging time of precursor solutions, while the block length of block copolymers affects mesoporous structures. Post-deposition chemical treatment using vaporized ammonia makes it possible to control and stabilize the mesoporous structures. Cooperation interaction mechanism can be considered as an acceptable model for the structural formation in the synthesis of mesoporous silica films.     

Electrical properties of AsxSe1−x (x ≤ 0.05) Mott-barriers

We have experimentally measured the current-voltage and capacitance-voltage characteristics of Au/amorphous As_xSe_1 − x (x ≤ 0.05)/Zr trilayer structures at temperatures from 4 to 295 K. The observed capacitance of structures with an amorphous As_xSe_1 − x (a-As_xSe_1 − x) thickness of ~ 0.4 to ~ 2.8 μm does not significantly change over the entire range of applied bias (− 5 V to 5 V), indicating that the a-As_xSe_1 − x films are fully depleted and thus the structures are Mott barriers. The current-voltage (I-V) characteristics of the a-As_0.03Se_0.97 device at low (< 3000 V/cm) to moderate fields (3000 V/cm-10000 V/cm) follow the predictions of trap limited space charge conduction theory, as they exhibit Ohmic behavior at low fields and trap limited space charge current at moderate fields. According to the trap limited space charge current model of Lampert, the a-As_0.03Se_0.97 film has an effective hole mobility, Θμ (with Θ < 1), of ~ 5 × 10^− 7 cm²/V-sec at 295 K. This value is similar to, but consistently lower than previously reported mobilities inferred from time of flight measurements. The current at high fields (> 10⁴ V/cm) increases rapidly with applied field as a result of carrier emission from localized states and is consistent with transport by the Poole-Frenkel mechanism. A permanent transition to a high conductance state (~ 10^− 3 S) is observed after exposure to very high electric fields (~ 4 × 10⁵V/cm).     

Dependence of RF power on the phase transformation for boron nitride films deposited on graphite at room temperature

Cubic boron nitride (cBN) thick films deposited on mainly c-axis-oriented graphite substrate at room temperature and zero bias by radio frequency (RF) magnetron sputtering were studied. In the growth process, RF power plays a key role in determining the content of cubic phase in films, while the conventional substrate heating and biasing have been neglected. With increase in RF power, the dominated content of films converts from explosion boron nitride (eBN) to cBN. The transformation mechanism has been discussed. The unique structural properties of the “soft” graphite are favorable to propose simple conditions for growing “hard” cBN films. Furthermore, the optical band gap of BN films having ∼90% cubic phase is of ∼5.8 eV obtained from ultraviolet–visible optical transmission measurement. The electron field emission examination shows that cBN film on graphite has a high emission current density of 2.8×10⁻⁵ A/cm² at an applied field of ∼30 V/μm.     

(111)取向0.7PMN-0.3PT薄膜机电耦合性能

铁电薄膜由于其优异物理性能,而被广泛应用于微电子、光电子、微机电领域。在铁电薄膜理论研究方面,热力学理论可以有效地预测铁电薄膜的相结构、极化特性和机电性能等,且已在(001)取向铁电薄膜的研究中取得了较好的应用,而对于(111)取向铁电薄膜的研究报道非常少。因此,本文通过对序参量坐标转换的方法,构建了(111)取向薄膜的热力学势能函数及其机电性能计算方法。基于此,研究了(111)取向0.7PMN-0.3PT铁电薄膜的相结构及其机电性能。研究结果表明,(111)取向0.7PMN-0.3PT铁电薄膜的相结构主要存在沿晶轴方向三个极化可互换的对称相:顺电相PE、菱方相R和单斜相M_A(或M_B)。在应变和外电场的调控下,(111)取向0.7PMN-0.3PT薄膜展现出优良的机电性能,在R和M_A相变点处,介电常数ε11、ε22、ε33和面外压电系数d33取得了极大值。在外电场E₃分别为0、50 kV/cm、100 kV/cm和200 kV/cm时,面外介电常数ε33的峰值分别为4 382、2 646、2 102和1 600,面外压电系数d33的峰值分别为303.8 pm/V、241.9 pm/V、219.7 pm/V和195.1 pm/V。应变和外电场能够较好地调控薄膜的机电耦合性能,可为优异机电耦合性能的器件制备提供参考。     

Fine-patterning on glass substrates by the sol-gel method

A novel process has been developed to form fine patterns on glass substrates. The process consists of the following steps: (1) coating of gel film on a substrate, (2) patterning on the gel film, and (3) heat-treatment. The key point in this process is the control of hardening of the gel films by the addition of organic polymers such as polyethylene glycol (PEG). The addition of PEG increased the viscosity of the coating solutions and delayed the gelation time, but had little effect on the optical properties of the resultant heat-treated glass films. The newly developed process is expected to be applicable to the formation of the optical devices such as diffraction gratings and pre-groovesof optical memory disks.     

无电镀沉积于硅衬底上铜纳米晶的场发射性能

采用无电镀沉积技术在经过机械抛光的单晶硅衬底上沉积了铜纳米晶.利用X射线衍射数据,估算出所沉积铜纳米晶的平均粒径大约为40nm.对120s无电镀沉积样品的场发射测试表明,该样品的开启场强为～5.5V/μm,在场强达到9.26V/μm时的场发射电流密度可达到62.5μA/cm2.对相应的沉积过程和场发射机理进行了分析.结果表明,无电镀沉积技术有可能成为制备具有较好场发射性能的金属/硅冷阴极的一种可供选择的方法.     

Dielectric properties of water in amorphous mixtures of polymers and other glass forming materials

We report on the dielectric and calorimetric results of water solutions of poly(vinyl methyl ether) (PVME), poly(vinyl pyrrolidone) (PVP), pentaethylene glycol (5EG) and tri-poly(propylene glycol) (3PG) for a temperature range of 130 K up to crystallization. For 3PG and 5EG two relaxation processes (Process I and II) were observed for all the hydration levels whereas for PVME and PVP a single peak was detected (Process II). We will show that the temperature dependence of the relaxation time of Process II, related to water molecules dynamics, presents a crossover at a temperature about the calorimetric glass transition of the whole system. We show that this crossover could be interpreted in terms of confinement effects due to the frozen-in of the whole solution. Finally, in spite of the fact that water molecules are surrounded by different environments, the results are very similar for the all the investigated systems, which suggest a general behavior for confined water. From this finding, we have obtained an intrinsic value of the activation energy for water dynamics in confining environments.     

HREM investigations of intermediate ZnO and ZnO/Y–ZrO2 layers in Y–ZrO2 bicrystals and ZnO films grown on Y–ZrO2 bicrystal substrates

The structure of yttrium-stabilized ZrO₂ (YSZ) bicrystals with ZnO and ZnO/YSZ/ZnO/YSZ/ZnO intermediate layers, as well as ZnO films grown on YSZ bicrystal (1 1 0)/90° substrates, has been investigated by means of high-resolution electron microscopy (HREM) and microanalysis. All bicrystals were produced by the solid-phase intergrowth (SPI) method. The internal ZnO film in the bicrystal formed at the SPI temperature of 1400°C consisted of domains with two symmetrical orientations: , and , . A bicrystal with a ZnO/YSZ/ZnO/YSZ/ZnO internal film was formed at the temperature of 1200°C. There was no mixing of ZnO and YSZ films and no traces of any solid-phase reactions were observed. Grains in all internal ZnO films and ZnO films grown on the bicrystal substrates had numerous stacking faults. It was found that SPI does not influence the density and structure of these defects. Orientational relationships between YSZ and ZnO in all samples were determined. The ZnO films grown on (1 1 0)/90° bicrystal substrates inherited the grain boundary (GB) from the substrate. Its structure and geometry is determined by four variants of ZnO grain growth.     

Variation of crystallization mechanisms in flash-lamp-irradiated amorphous silicon films

Flash lamp annealing (FLA) can form polycrystalline silicon (poly-Si) films with various microstructures depending on the thickness of precursor amorphous Si (a-Si) films due to the variation of crystallization mechanisms. Intermittent explosive crystallization (EC) takes place in precursor a-Si films thicker than approximately 2 μm, and the periodicity of microstructure formed resulting from the intermittent EC is independent of the thickness of a-Si films if their thickness is 2 μm or greater. In addition to the intermittent EC, continuous EC and homogeneous solid-phase crystallization (SPC) also occur in thinner films. These crystallization mechanisms are governed by the ignition of EC at Si film edges and the homogeneous heating of interior a-Si. The results obtained in this study could be applied to control the microstructures of flash-lamp-crystallized poly-Si films.     

Electrical properties of photoanodically generated thin oxide films on n-GaN

The characteristics of photoelectrochemically (PEC) generated gallium oxide films on n-GaN using an 0.002 M KOH electrolyte are described. The chemical composition of the resistive layers was analyzed by Auger electron spectroscopy. The DC and HF characteristics of Al/Ti/PEC-Ga₂O₃ (gallium sesquioxide)/GaN structures were studied with current–voltage and capacitance–voltage measurements, respectively. Under reverse bias we found extremely low leakage currents (<10⁻⁸ Acm⁻² at −15 V) and a very low interface state density; high-temperature operation (up to 166°C tested) motivates the integration of the described dielectric layer forming technique into GaN based device process schemes. Our method may also be employed as gate recess technology.     

Growth of germanium sulfide by hot wire chemical vapor deposition for nonvolatile memory applications

The growth of amorphous germanium sulfide (Ge–S) thin films using the hot wire chemical vapor deposition method has been performed at deposition temperatures in the range of 22–450 °C and pressures between 100 and 1800 Pa. Tetraallylgermanium and propylene sulfide were used as precursors for germanium and sulfur, respectively. The growth rate varies in the range of 1 and 100 nm/min and increases with increasing pressure and decreasing temperature. However, only the films deposited with lower growth rate exhibit conformal filling and good step coverage that could be observed at a growth rate of approximately 20 nm/min. Higher temperatures yield higher Ge content in the Ge–S films. In addition, the typical resistive switching behavior (three or four orders of magnitude) indicated that those films are suitable for nonvolatile memory applications.     

Effects of thermal conversion on the metal/polymer interface of layer-by-layer PPV/DBS films

In this paper, the thermal conversion effects on the metal/polymer interface of poly(p-phenylene vinylene) (PPV) films were investigated. The substrates studied were: aluminum, indium–tin oxide, gold and glass (BK7). Layer-by-layer PPV films were processed from poly(xylylidene tetrahydrothiophenium chloride) and dodecylbenzenesulfonate with 5 and 20 layers. The thermal conversion treatment was performed at 110 °C and 230 °C. The films were investigated through emission spectra. Selectively, the emission occurred in large PPV segments and it showed a significant dependence on the film/metal interface. It was clear that this synthesis process reduced the metal degradation in the interface. The Huang–Rhys factor was estimated to demonstrate this effect.     

Synthesis and electrochromic behavior of lithium-doped WO3 films

The electrochromic properties of amorphous peroxotungsten films prepared via the sol–gel route were determined. By means of the addition of LiCF₃SO₃ to the precursor solutions, films with different electrochemical properties were obtained. The electrochromic properties of the films were investigated using in situ spectroelectrochemical method and impedance measurements in propylene carbonate (PC) with 0.1 M LiClO₄ as electrolyte. During the cathodic polarization at −0.7 V, a blue color was observed with a reversible variation between 25% and 80% of the transmittance in the visible and near infrared range in the 5 mol% doped film. It reached a maximum cathodic charge density of 8 mC/cm². The bleaching process in the lithium-doped films was complete, whereas in the films without Li⁺, a residual color was observed. The impedance response of the WO₃ films showed the influence of the doping level on the electronic conduction and ionic transport of Li⁺ through the films.