Carbon nanotube dispersion in nematic liquid crystals: An overview

The aim of present review article is to present a comprehensive and current overview of scientific advancement in liquid crystal and carbon nanotube suspension. Particular attention has been paid to the recent developments and fundamental understanding of carbon nanotube dispersion in nematic liquid crystals. The dispersion and interaction of carbon nanotube in liquid crystal matrices and more elaborately the effect of dispersion on the properties of liquid crystalline materials has been extensively discussed. Recent progress has shown that even a very minute concentration of carbon nanotube in liquid crystals can have a reflective impact on the electrical and optical properties of liquid crystals. Liquid crystals provide a distinctive environment for controlling the alignment of carbon nanotubes whereas carbon nanotubes are important for the enhancement and fine-tuning of liquid crystalline properties. Potential applications of liquid crystal and carbon nanotube suspension are briefly discussed. Conclusion and future perspectives of this rapidly emerging field is provided at the end.     

Ordered materials for organic electronics and photonics

We present a critical review of semiconducting/light emitting, liquid crystalline materials and their use in electronic and photonic devices such as transistors, photovoltaics, OLEDs and lasers. We report that annealing from the mesophase improves the order and packing of organic semiconductors to produce state-of-the-art transistors. We discuss theoretical models which predict how charge transport and light emission is affected by the liquid crystalline phase. Organic photovoltaics and OLEDs require optimization of both charge transport and optical properties and we identify the various trade-offs involved for ordered materials. We report the crosslinking of reactive mesogens to give pixellated full-colour OLEDs and distributed bi-layer photovoltaics. We show how the molecular organization inherent to the mesophase can control the polarization of light-emitting devices and the gain in organic, thin-film lasers and can also provide distributed feedback in chiral nematic mirrorless lasers. We update progress on the surface alignment of liquid crystalline semiconductors to obtain monodomain devices without defects or devices with spatially varying properties. Finally the significance of all of these developments is assessed.     

Recent advances in vacuum assisted self-assembly of cellulose nanocrystals

Cellulose nanocrystals (CNC) iridescent films composed of the chiral nematic liquid crystal phase have attracted much attention owing to their fascinating optical properties. Herein, we summarized and discussed the recent advances in preparing iridescent CNC film via vacuum-assisted self-assembly (VASA) method, which has the advantage to fabricate large area, highly oriented and structurally homogeneous CNC liquid crystal film. The discussed aspects include (i) Factors that influence the self-assembly behavior of CNC nanorods during VASA. (ii) The strategy for modifying the optical, thermal as well as the mechanical properties of VASA assisted CNC iridescent film. (iii) Co-assembling of CNC liquid crystal phase and other functional nanoparticles by VASA technique. In all cases, key issues are highlighted and future developments that we consider as the most relevant are identified. Furthermore, future research activities are suggested in the conclusion.     

平行向列相液晶法-珀滤光片的光学特性分析

采用一种简单、稳定的方法-差分迭代法对平行向列相液晶在外电场作用下指向矢的空间分布进行了模拟计算.在此基础上,利用琼斯矩阵方法对平行向列相液晶法布里-珀罗滤光片的本征偏振态及光学特性进行了研究.结果表明,平行向列相液晶法布里-珀罗滤光片的共振模式是两组互相垂直的线性偏振态,且具有调谐范围宽,偏振敏感等特点.     

Refractive index of nematic liquid crystals in the submillimeter wave region

Large electro-optic effects of liquid-crystal materials are attractive in applications to various optical devices in a wider wavelength region. Fundamental optical properties in the submillimeter wave region, such as refractive indices and transmission losses for some cyanobiphenyl nematic liquid crystals, have been investigated for the first time, to our knowledge, with a submillimeter laser. Refractive indices of the liquid crystal materials for ordinary and extraordinary rays are a little larger than those in the visible region, and a larger birefringence comparable with the visible region can also be obtained. Although the loss level is larger by ~2 orders of magnitude than that of quartz plate, which is an excellent window in the submillimeter wave region, the transmission of the liquid crystal cell is high enough.     

Investigation of the liquid crystal alignment layer: effect on electrical properties

We investigate the electrical behavior of a symmetric liquid crystal (LC) cell: elecrode-silane-LC-silane-electrode. The silane (chlorodimethyloctadecyl-silane) layer induces a homeotropic orientation of the nematic liquid crystal (NLC) molecules. The wettability technique is used to detect the change of the surface energy of the electrode upon cleaning and silane layer deposition. We report on the dynamic impedance measurements of the nematic liquid crystal cell. It is found that the silane alignment layer has a blocking effect on the liquid crystal (LC) cell. We also study the relaxation behavior of the cell which is later assimilated as an electrical equivalent circuit.     

Investigation of the liquid crystal alignment layer: effect on electrical properties

Abstract

We investigate the electrical behavior of a symmetric liquid crystal (LC) cell: elecrode-silane-LC-silane-electrode. The silane (chlorodimethyloctadecyl-silane) layer induces a homeotropic orientation of the nematic liquid crystal (NLC) molecules. The wettability technique is used to detect the change of the surface energy of the electrode upon cleaning and silane layer deposition. We report on the dynamic impedance measurements of the nematic liquid crystal cell. It is found that the silane alignment layer has a blocking effect on the liquid crystal (LC) cell. We also study the relaxation behavior of the cell which is later assimilated as an electrical equivalent circuit.     

Optical and microhardness studies of KDP crystals grown from aqueous solutions with organic additives

Non-linear optical (NLO) materials have acquired new significance with the advent of a large number of devices utilising solid-state laser sources. From this technological point of view, potassium dihydrogen phosphate (KDP) having superior non-linear optical properties has been exploited for variety of applications. KDP crystals were grown from aqueous solutions added with organic additives by the slow cooling method for obtaining better non-linear properties. The influence of the organic additives on the optical and mechanical properties has been studied. It has been observed that the addition of organic additives improves the mechanical strength of the crystal. It is also observed that the addition of organic additives improves the optical transmission percentage of the crystal.     

Investigations of a wedge-shaped nematic liquid crystal cell using Mach–Zehnder interferometer

We report optical interferometric studies of a wedge-shaped nematic liquid crystal cell. Interference fringes were observed when a nematic liquid crystal cell was placed in one of the arms of Mach–Zehnder interferometer. In the case of homogenous gap cell, the fringe contrast remained unaffected on applying voltage. However, in the case of wedge-shaped cell, the fringe contrast was found to degrade under an applied electric (DC) field and it became poorer at higher voltages. At higher voltages the fringe contrast improved where complete switching occurred. The degradation in fringe contrast due to wedge-shaped cell structure might find applications for speckle reduction in future laser-based rear projection displays.     

Elastic modulus of a silicon thin film fabricated by nanotransfer printing

Transfer printing, a promising method for fabricating multi-scale structures on various substrates such as semiconductors and polymers, has been used to fabricate flexible devices with performance superior to that of conventional organic flexible devices. Although thin films might be expected to suffer damage during the transfer printing process, no reports of the degradation of mechanical properties during transfer printing have been published. The change in mechanical properties before and after transfer printing should be evaluated in terms of reliability and design for transfer printing to be successful. We propose a method of fabricating freestanding 200-nm-thick single-crystal silicon (SCS) thin-film specimens using transfer printing in order to investigate the mechanical properties of the transferred SCS thin-film specimens. The fabrication method combines several techniques such as semiconductor manufacturing, liftoff, and transfer printing processes. The core technology in this method is the fabrication of freestanding SCS thin-film structures suspended between two fixed ends. The mechanical properties of the freestanding SCS thin-film structures were measured using a microtensile machine capable of optical strain measurement. The test results provide insight into device design and reliability evaluation of flexible electronics fabricated by nanotransfer printing.     

Analysis of dichroic dye-doped polymer-dispersed liquid crystal materials for display devices

Dichroic polymer-dispersed liquid crystals (DPDLCs) based on nematic liquid crystal materials with azo dye were investigated in detail for the application of display devices. Polarizing optical microscopy, differential scanning calorimeter and electro-optic experiments all have shown that the DPDLC containing low concentration of dyes modifies the basic properties of these materials like optical transmission, threshold voltage, contrast ratio and absorbance factor. A minimum amount of dye needs to be added to the liquid crystal with the polymer matrix for its effective phase separation and to minimize the transmittance in the OFF state and therefore gives rise to an overall improvement in contrast ratio of the devices. Molecular orientation and dynamics in droplet sizes are readily controlled in these DPDLC materials. These findings imply that the value of the threshold electric field E_th is approximately 8 V/μm for pure polymer-dispersed liquid crystal (without dye) where the threshold electric field E_th values are approximately 4.0 V/μm, 2.0 V/μm, 1.7 V/μm 1.0 V/μm, for 0.0625%, 0.125%, 0.25% and 0.5% with azo dye in DPDLCs, respectively. From the results we can also infer that the maximum contrast is approximately 2.55 times the minimum contrast observed in the experiment for DPDLCs. The results show that the DPDLC with proscribed dye concentration will be possibly suitable and promising functional electronic materials for green technology flexible liquid crystal display.     

Liquid Crystals for Charge Transport,Luminescence, and Photonics

Ordered molecular materials are increasingly used in active electronic and photonic organic devices. In this progress report we discuss whether the self‐assembling properties and supramolecular structures of liquid crystals can be tailored to improve such devices. Recent developments in charge‐transporting and luminescent liquid crystals are discussed in the context of material requirements for organic light‐emitting devices, photovoltaics, and thin film transistors. We identify high carrier mobility, polarized emission, and enhanced output‐coupling as the key advantages of nematic and smectic liquid crystals for electroluminescence. The formation of anisotropic polymer networks gives the added benefits of multilayer capability and photopatternability. The anisotropic transport and high carrier mobilities of columnar liquid crystals make them promising candidates for photovoltaics and transistors. We also outline some of the issues in material design and processing that these applications demand. The photonic properties of chiral liquid crystals and their use as mirror‐less lasers are also discussed.     

纤维素纳米晶手性向列液晶在颜色防伪中的应用

目的介绍纤维素纳米晶(Cellulose Nanocrystal,CNC)手性向列液晶的形成原理与特征,以及该结构的调控方法,包括螺距和折射率的调控,最后总结纤维素纳米晶及其衍生材料在颜色防伪中的应用。方法归纳CNC手性向列液晶的形成机理方法,如硫酸水解结合蒸发诱导自主装法以及以CNC为手性模板与其他溶液共混制备法。最后总结国内外CNC手性向列液晶在颜色防伪中的研究现状,并简要讨论纤维素纳米晶在颜色防伪的未来发展趋势。结论纤维素纳米晶,具有绿色环保、来源丰富、合成工艺简单等特点,在一定条件下能形成手性向列液晶结构,具有独特的光学性质并呈现出结构色,可应用于颜色防伪。     

超化学计量比氧化铀晶体的研究进展

氧化铀不仅是重要的核材料, 也是潜在的多功能材料。UO₂晶体具有优异的半导体性能和抗辐照能力, 其禁带宽度(1.3 eV)与Si(1.1 eV)相近, 塞贝克系数是常用热电材料BiTe的4倍, 对太阳光的全吸收使其成为高效的太阳能电池材料, 在半导体、太阳能和热电等领域具有巨大的应用潜力。但是UO₂随着环境变化会出现从缺氧到过氧的价态变化(UO_2±x, x= -0.5~1), 即超化学计量比特性, 给材料制备和性能控制等方面带来很多问题。本文从相图出发, 总结了各种铀氧化物的结构及其稳定性, 重点聚焦UO₂晶体的研究进展。理想化学计量比UO₂被认为是最好的Mott绝缘体, 其电导率是相对稳定的; 超化学计量比氧化铀则具有半导体特性, 其电导率、热导率、扩散系数以及光学性能都与x密切相关。目前, UO₂晶体生长主要采用化学气相输运法(CVT)、冷坩埚法、水热法、升华法、助熔剂法等, 晶体尺寸和质量还不理想, 冷坩埚法和水热法被认为是最有潜力的生长技术。氧化铀单晶生长研究不仅有助于深入了解UO₂材料特性, 也为其在太阳能电池、热电器件以及未来电子学领域的应用提供可能性。     

Electronically controlled surface plasmon dispersion and optical transmission through metallic hole arrays using liquid crystal

The enhanced optical properties of metal films periodically perforated with an array of sub-wavelength size holes have recently been widely studied in the field of surface plasmon optics. The ability to design the optical transmission of such nanostructures, which act as plasmonic crystals, by varying their geometrical parameters gives them great flexibility for numerous applications in photonics, opto-electronics, and sensing. Transforming these passive optical elements into devices that may be actively controlled has presented a new challenge. Here, we report on the realization of an electrically controlled nanostructured optical system based on the unique properties of surface plasmon polaritonic crystals in contact with a liquid crystal (LC) layer. We discuss the effect of LC layer modulation on the surface plasmon dispersion, the related optical transmission and the underlying mechanism. The reported effect may be used to achieve active spectral tuneability and switching in a wide range of applications.     

液晶垂直取向膜材料的研究

叙述了近几年来液晶垂直取向材料领域的发展。对目前常见的几种具有应用前景的垂直取向技术分别给予了关注,同时初步探讨了其各自的取向机理,并展望了该技术在未来几年可能获得的发展。     

Micro-indentation of metallic photonic crystals: experimental and numerical investigations

Photonic crystals (PCs) are synthetic materials that are used to control light propagation. PCs have a frequency bandgap where light is forbidden to propagate. This bandgap is strongly tied to the microstructure of the photonic crystal. Three-dimensional tungsten photonic crystal in a Lincoln-log microstructure has been suggested as a strong alternative filter in photovoltaic cells with significantly high power efficiency. PCs have also been suggested as sensors for submicron damage. Therefore, mechanical characterization of three-dimensional photonic crystals becomes of interest. Here we report on mechanical characterization of tungsten PC using means of micro-indentation. We also present a three-dimensional finite element simulation of the structural response of a Tungsten photonic crystal under micro-indentation load. Stresses developed in the PC can be used to quantify the level of damage in the crystal. We compare our simulation results with the experimental observations of a Vickers and Knoop micro-indentation experiments of tungsten PC. The FE models were proven able to simulate the mechanical response of the PC with a good accuracy. The calibrated FE models can be further used to realize the mechanical behavior of PC under different thermal and mechanical stresses when used as filters in photovoltaic cells or to simulate the effect of damage in PC sensors.     

末端为噻吩基团酯类化合物的合成、表征及液晶性能研究

以噻吩甲醛、对氨基苯甲酸、溴丙烷、对羟基苯甲酸为原料,经过醚化反应、希夫碱反应、DCC/DMAP酯化反应合成了噻吩衍生物液晶。化合物的结构由红外光谱、核磁共振氢谱进行了表征,噻吩衍生物的液晶性能用偏光显微镜进行了研究。结果表明:以噻吩为末端的化合物在升温和降温过程中均表现出很好的向列相液晶性能;特别是在降温过程中表现出较低的结晶点和更宽的液晶相温度范围。     

Active modulation of nanorod plasmons

Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogues of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100% using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogeneous to a twisted nematic phase of the liquid crystal covering the nanorods. With our method it will be possible to actively control optical antennas as well as other plasmonic elements.