Preparation of poly (N-vinylcarbazole)-co-poly(2-(dimethylamino)ethyl methacrylate) based hydrogen bonded side-chain liquid crystal copolymer

In this study, new side chain liquid crystalline copolymers were prepared from N-vinyl carbazole (NVC) and 2-(Dimethylamino)ethyl methacrylate) (DMAEM) as a hydrogen bond acceptor copolymer and 8-(4-cyanobiphenyl-4′-oxy)octan-1-ol (LC8) by molecular self-assembly processes via hydrogen bond formation between nitrogen of (DMAEM) and hydroxyl group of the LC8. The formation of H bond was confirmed by using FTIR spectroscopy. The liquid crystalline behavior of the copolymers and homopolymer of the (DMAEM) was investigated using a differential scanning calorimeter (DSC) and polarized optical microscopy. The dielectric relaxation properties of H-bonded Side Chain LC Copolymers (HB-LCP) doped 8-(4-cyanobiphenyl-4′-oxy)octan-1-ol (LC8) and pure LC8 liquid crystals have been investigated by the dielectric spectroscopy (DS) method. The dielectric behavior of the LCs shows a dielectric relaxation process. The relaxation frequency of the LCs was changed by the addition of HB-PLC. It is evaluated that the dielectric strength and relaxation properties of LC8 and LC8/HB-PLC LCs can be controlled by 1% HB-PLC dopant.     

Inside Front Cover: Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters (Adv. Funct. Mater. 2/2008)

On p. 212, Torsten Hegmann and co‐workers describe nematic liquid crystals (N‐LCs) confined in planar liquid crystal cells after doping with small quantities of gold nanoclusters. These give rise to a dual alignment mode and electro‐optic response (Freedericksz transition). By fine‐tuning of experimental conditions, N‐LCs doped with gold nanoclusters can be electrically reoriented and aligned either like N‐LCs with a positive dielectric anisotropy (used in twisted nematic displays) in a planar cell or alternatively as N‐LCs with a negative dielectric anisotropy (used in large LCD TVs based on the vertical alignment mode). We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.     

Tunable Frequency Selective Surfaces and Negative-Zero-Positive Index Metamaterials Based on Liquid Crystals

We utilize the properties of aligned nematic liquid crystal (LC) cells in the design of: (i) a new type of metamaterial whose index of refraction is tunable from negative, through zero, to positive values and (ii) micron-scale metallodielectric and all-dielectric tunable frequency selective surfaces (FSSs). The metamaterial is constructed by randomly doping a liquid crystal substrate with coated dielectric (non-magnetic) spheres and can be utilized over a large spectral range. FSSs with a liquid crystal superstrate are synthesized using conventional and genetic algorithm methods to exhibit broadband tunable filter characteristics at mid-infrared (mid-IR) wavelengths. These LC tunable FSS structures can be used to develop a new class of infrared/optical switches for terahertz applications.     

含氟三环NCS液晶材料的介电性能研究

为了探究液晶材料的介电性能,本文研究了4PPTGS和4PUTGS两种含氟三环NCS类液晶材料的介电各向异性和介电损耗。首先用精密LCR表(Agilent E4980A)测量液晶盒的电容并用双盒模型和液晶盒电容模型得到4PPTGS和4PUTGS两种液晶材料的平行和垂直介电常数,再由电压-电容特性曲线得到它们的阈值电压,并进一步探讨了介电各向异性和阈值电压对温度的依耐性;然后,在20 Hz~10kHz范围内研究了外加电压频率对液晶材料介电损耗的影响,两种液晶材料在1kHz左右都存在介电损耗峰值,为了减小器件的功耗和提升器件的质量,液晶材料应选择在介电损耗小的频率下工作;最后,通过对平行和垂直排列向列相盒中液晶材料在不同电压下介电损耗的测试与分析,介电损耗的变化是由于在外加电场下液晶分子固有偶极矩的取向极化引起的,介电损耗值的大小与液晶分子的排列状态密切相关。此项研究对提升液晶材料在应用中的介电性能具有一定的指导意义。     

低损耗高介电常数CCTO陶瓷的制备与性能研究

为了降低CaCu3Ti4O12(CCTO)陶瓷材料的介质损耗,采用传统固相反应法制备了组分为CaCu3–yZry/2Ti4O12(CCZTO)的陶瓷样品。研究了ZrO2掺杂对CCTO陶瓷性能的影响。结果表明:所制CCZTO陶瓷样品在维持了CCTO陶瓷材料介电常数大、低频介电常数随频率和温度变化小的优点的同时,介质损耗大幅降低;其介电常数和介质损耗的指标满足美国电子工业协会EIAZ5U标准,而温度系数αc性能指标优于EIAX7A标准所规定的±55×10–6/℃,是一种综合性能技术指标优良的新型高介电常数陶瓷材料。     

掺偶氮染料聚甲基丙烯酸甲酯的介电特性研究

制备了具有不同偶氮生色团掺杂浓度的主客式聚甲基丙烯酸甲酯（PMMA）样品。利用介电测量的方法，观测了样品在532nm光照射下的介电常数和介电损耗的变化。发现随着生色团浓度的增加，样品的介电常数增大；而生色团的掺人对于聚甲基丙烯酸甲酯样品的介电损耗则主要表现在对β弛豫的影响。浓度一定的生色团样品的介电常数随着激发光功率的增加而增大；样品的高频介电损耗随着激发光功率的增大而减小，但低频直流电导损耗则随着激发光功率的增加而增大。利用生色团在光致异构反应和取向运动过程中与聚甲基丙烯酸甲酯基体的相互作用，定性地解释了上述实验现象。     

等离子体处理过的基板上液晶平行取向的两种模式

我们提出了一种用于液晶盒的新取向工艺,可以得到0或非0的预倾角。这项工艺是用定向的等离子体流倾斜辐照基板。我们用一个阳极层发射器(anodelagerthruster)作为等离子体辐射源,以产生层状的等离子体流。它适合于处理大面积基板,可以处理有机(聚合物)和无机(玻璃,ITO)层。等离子体流辐照可以得到两种类型的液晶取向:(1)最可及(择优)取向轴位于离子束方向和基板法线组成的入射平面上;(2)最可及取向轴垂直于入射面。随着照射总剂量的增加,取向方向可以从类型(1)向类型(2)转变。在第一种取向模式中,可以通过改变工艺参数,如入射角、离子流密度和离子能等来改变预倾角。第二种取向模式的特征是预倾角为0。第一种模式的方位锚泊能系数相对较弱(W=10-3 Erg/cm2),而第二种类型锚泊能很强(W>10-1 Erg/cm2),与摩擦聚合物基板相当。两种模式的取向特征可以用来产生满足所需参数的取向,和构图(pattern)液晶盒基板。这种工艺方法可以克服传统摩擦工艺的某些缺点。     

碳纳米管超晶格结构能隙的第一性原理研究

本文采用基于密度泛函理论的CASTEP模块研究了B/N共掺(5,5)碳纳米管环超晶格的电子结构。形成能计算结果为负值表明,B/N原子对共掺碳纳米管环具有稳定存在的可能性。能带结构和态密度结果表明,B/N原子对的掺入使得(5,5)金属型碳纳米管能隙打开,导电性质向半导体转变。当管径在合理的变化范围内,纯碳纳米管的能隙宽度强烈敏感于管径的变化,而B/N共掺碳纳米管环结构的能隙值随管径的变化较小,这就降低了碳纳米管电子器件的制备要求。对新型结构施加变形作用,压缩变形使得B/N共掺碳纳米管环的能带宽度增大,这相当于提高了碳纳米管的掺杂体积浓度;拉伸变形作用下所得结论恰恰相反。实现控制碳纳米管超晶格结构的导电性能,对纳米管电子器件的应用具有重要意义。     

Energy transport simulation for graded HBT's: Importance of settingadequate values for transport parameters

An energy transport simulation method for graded AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is presented in which Al composition-, doping density-, and energy-dependences of transport parameters are considered. For several representative Al composition and doping densities, parameters such as electron mobility, energy relaxation time, and upper-valley fraction are evaluated as a function of electron energy by a Monte Carlo method. For the other Al composition, these are determined by a linear interpolation method. Calculated cutoff frequency characteristics and electron velocity profiles are compared with those obtained by using more simplified approaches, demonstrating the importance of giving adequate transport parameters, particularly in analyzing graded band-gap base HBTs     

Improved room-temperature thermoelectric characteristics in F4TCNQ-doped CNT yarn/P3HT composite by controlled doping

High room-temperature thermoelectric performance is important for low-grade waste heat power generation as there are plenty of heat thrown away uselessly in our daily life, most of which are below 100 °C. However, most of the thermoelectric materials are limited to high temperature application. In this work, room-temperature thermoelectric power factor of carbon nanotube (CNT) yarn is improved by controlled doping, which is achieved by making composite with poly 3-hexylthiophene −2, 5-diyl (P3HT) followed by doping with 2, 3, 5, 6-tetrafluo-7, 7, 8, 8-tetracyanoquinodimethane (F4TCNQ). The temperature-dependent Seebeck coefficient based on power–law model suggests that P3HT shifts the Fermi energy of CNT yarn towards the valence band edge, and reduces the ionic scattering and carrier relaxation time. As a result, the Seebeck coefficient is increased while the variation of Seebeck coefficient with temperature is reduced, and hence, the room-temperature thermoelectric power factor is improved. With controlled doping, the power factor of CNT yarn/P3HT composite reaches to 1640–2160 μW m⁻¹K⁻² at the temperature range of 25–100 °C, which is higher than that of CNT yarn alone.     

异质栅纳米碳管场效应管线性掺杂效应的研究

The effects of linear doping profile near the source and drain contacts on the switching and high- frequency characteristics for conventional single-material-gate CNTFET （C-CNTFET） and hetero-material-gate CNTFET （HMG-CNTFET） have been theoretically investigated by using a quantum kinetic model. This model is based on two-dimensional non-equilibrium Green＇s functions （NEGF） solved self-consistently with Poisson＇s equations. The simulation results show that at a CNT channel length of 20 nm with chirality （7, 0）, the intrinsic cutoff frequency of C-CNTFETs reaches up to a few THz. In addition, a comparison study has been performed between C-and HMG-CNTFETs. For the C-CNTFET, results reveal that a longer linear doping length can improve the cutoff frequency and switching speed. However, it has the reverse effect on on/off current ratios. To improve the on/off current ratios performance of CNTFETs and overcome short-channel effects （SCEs） in high-performance device applications, a novel CNTFET structure with a combination of an HMG and linear doping profile has been proposed. It is demonstrated that the HMG structure design with an optimized linear doping length has improved high-frequency and switching performances as compared to C-CNTFETs. The simulation study may be useful for understanding and optimizing high-performance of CNTFETs and assessing the reliability of CNTFETs for prospective applications.     

Cobalt–titanium multilayer thin films: Effect of thickness of titanium spacer layer on impedance properties

We investigated the impedance parameters of cobalt–titanium (Co–Ti) multilayer thin films deposited on native oxidized Si (100) substrate under ultra-high vacuum (4×10⁻⁸ mbar) by magnetron sputtering at room temperature. Electrical properties of Co/Ti/Co multilayer films were analyzed depending on the thickness of Ti spacer layer with the impedance spectroscopy as a function of frequency. Co/Ti multilayer films exhibited dielectric relaxation in both real and imaginary part of dielectric constants at the kilohertz frequency region and piezoelectric properties at the megahertz frequency region. We determined that the fabricated multilayer films have complex and super imposed type behavior when DC conductivity is used at lower frequency, resonance event and relaxation properties.     

Unprecedented Dual Alignment Mode and Freedericksz Transition in Planar Nematic Liquid Crystal Cells Doped with Gold Nanoclusters

We demonstrate that alkylthiol‐capped gold nanoclusters doped into nematic liquid crystals (N‐LCs) with positive dielectric anisotropy give rise to an unprecedented dual alignment mode and electro‐optical response, which has a potential impact on current liquid crystal (LC) display technologies and N‐LC optical‐biosensor design. By fine‐tuning experimental conditions (temperature, electric field, and alignment), N‐LCs doped with gold nanoclusters can be aligned and electrically reoriented either like N‐LCs with a positive dielectric anisotropy in a planar cell or, alternatively, as N‐LCs with a negative dielectric anisotropy in a homeotropic cell, both at lower threshold voltages than the pure N‐LC.     

采用碳纳米管导电薄膜作为OLED的阳极

采用碳纳米管(CNT)替代ITO作为OLED阳极可以 解决ITO薄膜存在的可弯曲性能差,可靠性低等缺 点,使得柔性显示成为可能。本文采用混合型CNT导电薄膜作为阳极,探讨了CNT薄膜的制备 工艺、掺 杂方式及表面修饰等因素对绿光OLED性能的影响。实验结果表明,P型掺杂对CNT薄 膜的导电性能影响 有限;而PEDOT修饰层可以很好的提高CNT导电薄膜的平整度;此外,采用“十字交叉 ”的阳极形状有助于降低 阳极拐角处毛刺。通过优化器件各参数,制备的PET/CNTs/PEDOT/NPB/ALq₃/LiF/Al绿光OL ED发光效率达 到了195 cd/m²,结果表明采用混合型CNT作为OLED阳极是可行的。     

Stepped Transformers for Partially-Filled Transmission Lines (Comments)

The use of a lumped equivalent circuit for the transverse section of a waveguide has been used to determine the cutoff frequency of ridged guide. In essence, the transverse section of the guide is equated to an LC tank and the resonant frequency of the tank is determined by the usual formula. Sullivan and Parkes have extended this method to the analysis of a ridged guide partially loaded with dielectric. They account for the additional capacitance introduced by the dielectric and include this capacitance as part of the LC tank to be analyzed for the resonant frequency. While the simplified lumped-network equivalents are always desirable in analyzing microwave networks, we should be wary of overgeneralizing them. In the case of dielectric slab loading in a waveguide, the location of the added capacitance as well as its maguitude has an effect in determining the cutoff frequency. (This can also be said of the discontinuity capacitance at the edge of a ridge.) Note how a dielectric slab when centered in a rectangular waveguide will lower the cutoff frequency far more than when flush with the sidewall. This is not accounted for in the LC tank equivalent. The effect of each increment of capacitance due to each lamina of dielectric will be largely determined by the distance from the lamina to the short-circuit walls. Therefore, the author believes that the case of a partially dielectric-loaded guide can only be genuinely analyzed by the transverse resonance method or by some other method which accounts for the distributed parameters involved.     

The effect on the impedance characteristics of the metal oxides (Al2o3 and Zno) doping into polyaniline

In this study, the doping metal oxides; ZnO and Al₂O_3, (MO for short) into poly (aniline) (PANI) how that affects the dielectric properties have been investigated by the impedance analysis technique. PANI, PANI-Al₂O₃ and PANI-ZnO were synthesized by chemical oxidative polymerization. Both FTIR and SEM are used to characterize the structure and morphologies of these composites. Dielectric properties of PANI and PANI–MO composites have been performed in the frequency range 100 Hz −1 MHz. It was seen that the values of the dielectric constant and impedance increase with doping Al₂O₃ and ZnO into PANI. Absorption coefficient (α) and relaxation times (τ) parameters were calculated and it is revealed that the relaxation mechanism changed significantly by the doping of metal oxide into PANI. Moreover, the conductivity properties of PANI-MO composites were performed and it is seen that “s” parameter value correlated Barrier Hoping (CBH) Mechanism.     

Room‐Temperature Dielectric Switchable Nanocomposites

Room‐temperature switchable dielectric materials are of interest for many applications, including solar energy storage, smart switches, automatic filters, and next‐generation sensors. Here, a temperature‐triggered dielectric switchable nanocomposite by dispersing octadecylamine‐grafted multiwalled carbon nanotubes (ODA‐MWCNTs, for short) into hexadecane is reported. The composite has low permittivity at molten state and high permittivity at frozen state, and the permittivity switch is triggered around 18 °C. The highest permittivity contrast ratio reaches 106.4 at 2.0% CNT volume fraction. The composite shows frequency‐sensitive and temperature‐ramping‐rate‐sensitive properties. Further investigation indicates that the permittivity switch is caused by the change of the ODA‐MWCNT percolating networks during phase transition.     

Plasma Modification of Fluoropolymers for Aligning Liquid Crystals

In this paper we describe methods to control liquid crystal (LC) alignment using plasma discharge on ferroelectric fluoropolymers. Two different plasma modification techniques were investigated: corona discharge and RF plasma in Ar gas. Corona discharge is a proven technique known to reorient the dipoles in poly (vinylidene fluoride) and its copolymers resulting in a strong remnant polarization. The polarization was patterned resulting in preferential LC alignment in selected regions. RF plasma in Ar gas defluorinates the polymer surface leading to planar alignment of positive dielectric anisotropy LCs. The defluorination of the alignment layer also causes low voltage switching of the LC.     

取向层厚度及介电常数对TN-LCD影响的研究

在具有高驱动电压的液晶显示器中,为了对液晶器件起到保护作用,以采用增加取向层厚度的方法来解决某些显示器件由于驱动电压过高导致的问题。当取向层厚度不可忽略时,随着取向层厚度增加会导致器件驱动电压升高。本文利用扭曲向列相液晶显示器结构,通过模拟和实验分析了取向层厚度对LCD的影响以及不同介电常数的取向层对LCD的影响。结果表明:当取向层的介电常数大于20时,能够有效降低TN-LCD驱动电压。当其介电常数大于500时,其厚度变化对驱动电压的影响变得很小。本文结果对降低高驱动电压液晶器件的驱动电压有重要的指导性意义。     

Dielectric spectroscopy of metal nanoparticle doped liquid crystal displays exhibiting frequency modulation response

Twisted nematic liquid crystal displays (TN-LCDs), doped with the nanoparticles of metal, such as Pd, Ag, or Ag-Pd, which are protected with ligand molecules, such as nematic liquid crystal, exhibit a frequency modulation (FM) electro-optical (EO) response with short response time of milliseconds (ms) or sub-ms order together with the ordinary rms voltage response. These devices are called FM/AM-TN-LCDs; they are distinct from the ordinary LCDs featured by the amplitude modulation (AM) response. The phenomena of the FM/AM LCDs may be attributed to the dielectric dispersion of a heterogeneous dielectric medium known as the Maxwell-Wagner effect. It is experimentally shown that the frequency range spreads from several tens hertz to several tens kilohertz and the spectrum is more or less centered about the dielectric relaxation frequency. We formulated a theory based on an equivalent circuit model to evaluate the dielectric relaxation frequency and the dielectric strengths; and we succeeded in explaining the dependence of the dielectric relaxation frequency on the concentration of nanoparticles and the their dielectric and electrical properties, whereas conventional theories based on electromagnetic theory are unable to explain this concentration dependence. This paper reports on the experimental results of the EO effects and the dielectric spectroscopy including the dielectric relaxation times and the dielectric strengths of nematic liquid crystal, 5CB (4-pentyl-4'-cyanobiphenyl), doped with the metal nanoparticles of I'd alone and Ag-Pd composite; and discusses how the observed dielectric relaxation frequency or dielectric relaxation time depend on the concentration of the doped nanoparticles and also their electrical and dielectric properties.