Director orientation of a ferroelectric liquid crystal on substrates with rubbing treatment: The effect of surface anchoring strength

Abstract

One of the most important problems with ferroelectric liquid crystals is obtaining homogeneous as well as bistable alignment, not only to study their physical characteristics but also for their application to optical devices. In this connection it was predicted that the formation of homogeneous alignment requires strong surface anchoring, whereas bistability requires weak anchoring. We have therefore developed a method to determine the surface anchoring strength, and have tried to clarify whether there is a suitable anchoring range. It was found that A/K ₂₂ (A is the surface anchoring and K ₂₂ is the twist elastic constant of liquid crystal) of 4 × 10^?2 to 2 μm^?1 satisfies the contradictory requirements for homogeneous as well as bistable alignments for the material studied, and the bookshelf structure is successfully formed with this condition.     

The formula of anchoring energy for a nematic liquid crystal

The interface energy for a nematic liquid crystal (NLC) is considered as the sum of potential energy between LC molecules and molecules of the substrate surface, and a formula for anchoring energy is derived by elementary principles. The anchoring energy for a NLC should have two terms, the first term is the same as the Rapini-Papoular expression, the second is related to the normal of interface and resultes from the biaxial property of a NLC induced by interface. Hence there are two anchoring coefficients, W₁ and W₂. We demonstrate that W₁ is equal to the tilt angle strength A_θ, and W₂ corresponds to the difference between A_θ and the azimuthal strength A_ϕ. Thus A_θ-A_ϕ is due to the biaxial property of the NLC near the interface. Applying this formula to the twisted NLC cell, we discuss the threshold and saturation field, as well as the maximal tilt angel θ_m with respect to A_θ/A_ϕ. Previously proposed formulae are discussed from our point view.     

Thermodynamics of (1,4-difluorobenzene + an n-alkane) and of (hexafluorobenzene + an n-alkane)

Excess molar enthalpies H^E and excess molar volumes V^E have been measured, as a function of mole fraction x₁, at 298.15 K and atmospheric pressure for the five liquid mixtures (x₁1,4-C₆H₄F₂ + x₂n-C_lH_2l+2), l = 7, 8, 10, 12 and 16. In addition, H^E and excess molar heat capacities C_P^E at constant pressure have been determined for the two liquid mixtures (x₁C₆F₆ + x₂n-C_lH_2l+2), l = 7 and 14, at the same temperature and pressure. The instruments used were flow microcalorimeters of the Picker design (the H^E version was equipped with separators) and a vibrating-tube densimeter, respectively.

The excess enthalpies of the five difluorobenzene mixtures are all positive and quite large; they increase with increasing chain length l of the n-alkane from H^E(x₁ = 0.5)/(J mol⁻¹) = 1050 for l = 7 to 1359 for l = 16. The corresponding excess volumes V^E are all positive and also increase with increasing l: V^E(x₁ = 0.5)/(cm³ mol⁻¹) = 0.650 for l = 7 and 1.080 for l = 16. Interestingly, the excess enthalphies of the corresponding mixtures with hexafluorobenzene are only about 5% larger, whereas the excess volumes of (x₁C₆F₆ + x₂n-C_lH_2l+2) are roughly twice as large as those of their counterparts in the series containing 1,4-C₆H₄F₂. Specifically, at 298.15 K H^E(x₁ = 0.5)/(J mol⁻¹) = 1119 for (x₁C₆F₆ + x₂n-C₇H₁₆) and 1324 for (x₁C₆F₆ + x₂n-C₁₄H₃₀), and for the same mixtures V^E(x₁ = 0.5)/(cm³ mol⁻¹) = 1.882 and 2.093, respectively. The excess heat capacities for both systems are negative and of about the same magnitude as the excess heat capacities of mixtures of fluorobenzene with the same n-alkanes (Roux et al., 1984): C_P^E(x₁ = 0.5)/(J K⁻¹ mol⁻¹) = −1.18 for (x₁C₆F₆ + x₂n-C₇H₁₆), and −2.25 for (x₁C₆F₆ + x₂n-C₁₄H₃₀). The curve C_P^E vs. (x₁ for x₁C₆F₆ + x₂n-C₁₄H₃₀) shows a sort of “hump” for x₁ 0.5, which is presumed to indicate emerging W-shape composition dependence at lower temperatures.     

The stability diagram of a nematic liquid crystal confined to a cylindrical cavity

Different nematic structures confined to a long cylindrical cavity with homeotropic surface anchoring are studied using a numerical minimization of the free energy of the uniaxial nematic liquid crystal. The stability of escaped radial structures and planar polar structures (with and without line defects) is analysed in terms of the ratio of elastic constants K₂₄/K₁₁, K₃₃/K₁₁, anchoring strength and external magnetic field applied perpendicular to the symmetry axis of the cylinder. We draw the analogy between the stability diagram of the cylindrical structures and structures in a spherical droplet. In particular, a simple way extracting the value of the saddle-splay elastic constant K₂₄ from the stability studies is discussed.     

The hydrophobic acyl-chain effect in the lipid domains appearance through phospholipid bilayers

An intermolecular interaction model for selective association processes of double-chain phospholipids in bilayer lipid membranes has been proposed, analysed and solved numerically. A large variety of binary mixtures of asymmetrical double-chain phospholipids with the cross-sectional areas of the polar headgroups a₁ = 40 Ų (the first component) and a₂ = 60 Ų (the second component) have been investigated. Changing the hydrophobic acyl-chain lengths of both mixture components, we found in all cases that the self-association probability (the association of like-pairs of phospholipids) of the first component in parallel alignment of the electric dipole moments of the polar headgroups is higher than the cross-association probability (the association of cross-pairs of phospholipids) and the self-association probability of the second component. This result is in good agreement with the experimental evidence that where the cross-sectional area of the polar headgroups matches the hydrocarbon chain-packing cross-sectional area (a 2Ξ 40 Ų), lipids possess a high tendency to aggregate into well packed bilayer structures with the acyl-chains oriented perpendicularly to the bilayer plane. Our theoretical data confirm that the double-chain phospholipids may associate themselves into anti-parallel alignment of the polar headgroups (P′₂₂) as well. The hydrophobic acyl-chain effect of phospholipids may modulate the distribution of lipid domains within bilayers that have a large variety of functional roles in cellular metabolism.     

Influence of K24 on the structure of nematic liquid crystal droplets

A phenomenological free energy is used to describe the stable ordering of nematic liquid crystals confined to supramicron spherical cavities. In particular the effects of the saddle splay elastic constant, K₂₄, on the equilibrium structures and phase diagram of droplets with homeotropic surface anchoring are discussed. Some structures are illustrated by the corresponding simulated polarization microscope textures. Possibilities for an experimental determination of the saddle-splay elastic constant and surface anchoring strength by studying the radial-axial structural transition in such droplets are analysed. It is shown that the K₂₄ term in the elastic free energy stabilizes a deformed droplet structure even in the limit of the zero anchoring strength.     

范德华复合物Kr-C2H2的势能面和振转光谱的理论研究

采用[CCSD（T）]-F12方法和aug-cc-pVTZ基组,同时引入中心键函数（3s3p2d1f1g）构建了Kr-C₂H₂体系的高精度四维势能面.在构建势能面时考虑了分子间的振动方式及C₂H₂单体内的ν₁对称伸缩和ν₃反对称伸缩振动.将计算得到的四维势能面在Q₁方向和Q₃方向分别做积分得到C₂H₂单体分别处于振动基态和（ν₁,ν₃）=（1,1）激发态的平均势能面.计算结果表明,这2个平均势能面均存在2个等价的T型全局极小值和2个等价线性极小值.全局极小值的几何构型位于R=0.41 nm,θ=65.6°/114.4°,势阱深度为151.88 cm^-1.对径向部分采用离散变量表象法（DVR）,角度部分采用有限基组表象法（FBR）,并结合Lanczos循环算法计算了Kr-C₂H₂的振转能级和束缚态.计算结果表明,复合物在（ν₁,ν₃）=（1,1）区域的带心位移为-1.48 cm^-1,表现为红移,与实验值-1.38 cm^-1很接近;计算得到的红外跃迁频率也与实验值相吻合,说明得到的从头算势能面具有高精度.     

Étude des isomères (CO)Fe(PF3)4

Assignments of a certain number of vibrational bands of (CO)Fe(PF₃)₄ are made with reference to the C_3v and C_2v isomers. A study of the Raman and spectra at low temperature revealed that in the solid state only one isomer exists. Furthermore, a comparative study of liquid (CO)Fe(PF₃)₄ and HCo(PF₃)₄ of molecular symmetry C_3v was made. These two approaches made it possible to determine the difference in enthalpy between the two isomers of the iron complex (+0-52±0.05 kcal.mole⁻¹) in the liquid state, as well as to verify the assignments proposed previously. The greater stability of the C_2v isomer is discussed.     

Strongly non-linear optical ferroelectric liquid crystals for frequency doubling

From our research for novel non-linear optical (NLO) materials for frequency doublers and optical modulators we report on new ferroelectric liquid crystals, which for the first time, exhibit second order NLO coefficients (for example d₂₂ = 5 pm V^-1, which are comparable to those of state of the art inorganic NLO materials. The novel compounds contain 5-amino-2-nitrophenyl groups attached close to the chiral centres. The switching behaviour of the new compounds, their spontaneous polarization, as well as their frequency doubling of Nd:YAG laser pulses in the S*_c and in the glass state, are reported. Moreover their waveguiding properties are presented.     

Flow induced bistable anchoring switching in nematic liquid crystals

A hydrodynamically controlled switching of orientations of a nematic bistable surface anchoring is observed. A viscous torque moves surface lines between domains corresponding to bistable anchoring. An observed threshold for this viscous torque is attributed to relative spatial inhomogeneities of surface energy (∼10^-2).     

NMR line-width, self-diffusion and relaxation studies of neopentanol in liquid and solid phases

The translational and rotational dynamics of the liquid and disordered (solid I) phases of neopentanol are investigated using high-field multinuclear NMR. The extensive line-narrowing of the ¹H resonances for solid I is ascribed to the onset of translational diffusion whereas the line-narrowing of the deuteron and carbon-13 signals is dominated by molecular reorientations. The activation energy of the molecular self-diffusion is 34 and 71 kJ mol⁻¹ for the liquid and solid I phases, respectively. The self-diffusion coefficient of solid I is 3.2 × 10⁻¹³ m² s⁻¹ at the melting point.

A thorough analysis of the multinuclear T₁ data is presented. The activation energy of the overall tumbling motion in the liquid and solid I phases, obtained from the hydroxyl deuteron T₁ data, is 36 and 52 kJ mol⁻¹, respectively. The internal reorientations have a profound effect on the spin-lattice relaxation times of the methyl and methylene groups by reducing the effective correlation time by an order of magnitude relative to the overall tumbling motion in solid I. The long correlation time (22 and 58 ps of liquid and solid neopentanol at the melting point) and high activation energy suggest that the overall tumbling motion in the liquid and disordered phases involves hydrogen-bonded aggregates.     

OL负载Cu催化剂及其催化氧化CO及乙酸乙酯性能

采用氧化还原法合成了层状锰氧化物(OL),并以OL为载体采用离子交换法制备了不同Cu负载量的Cu_x/OL催化剂。利用X射线衍射(XRD)、电子扫描电镜(SEM)、N_2吸附/脱附、H_2~-程序升温还原(H_2-TPR)、TG(热重)、X射线光电子能谱(XPS)、O_2-程序升温脱附(O_2-TPD)等技术对所制催化剂进行结构和织构表征,并对其催化氧化CO及乙酸乙酯活性进行了评价。结果表明,OL具有典型的层状锰氧化物结构,适量掺杂Cu对OL的结构和织构影响不大,但Cu的掺杂明显影响Cu_x/OL的还原性、氧移动性及催化剂表面Cu~(2+)/CuO、(Mn~(2+)+Mn~(3+))/Mn~(4+)和Oads/Olatt的比例。Cu_x/OL的催化性能与以上因素密切相关。在Cu_x/OL样品中,Cu_5/OL催化剂具有最佳的催化活性(CO催化氧化,T_(50)=70°C和T_(90)=100°C;乙酸乙酯催化氧化T50=160°C,T90=200°C)。同时,Cu_5/OL催化剂具有最佳的还原性能、氧移动性能和最多的Cu~(2+)、(Mn~(2+)+Mn~(3+))和表面吸附氧浓度。Cu_x/OL催化性能与铜锰之间相互作用、还原性和氧移动性能密切相关。     

Rates of the reactions CN + H2CO and NCO + H2CO in the temperature range 294–769 K

The rate coefficients of the reactions: (1) CN + H₂CO → products and (2) NCO + H₂CO → products in the temperature range 294–769 K have been determined by means of the laser photolysis-laser induced fluorescence technique. Our measurements show that reaction (1) is rapid: k₁(294 K) = (1.64 ± 0.25) x 10⁻¹¹ cm³ molecule⁻¹ s⁻¹; the Arrhenius relation was determined as k₁ = (6.7 ± 1.0) x 10⁻¹¹ exp[(−412 ± 20)/T] cm³ molecule⁻¹ s⁻¹. Reaction (2) is approximately a tenth as rapid as reaction (1) and the temperature dependence of k₂ does not conform to the Arrhenius form: k₂ = 4.62 x 10⁻¹⁷T^1.71 exp(198/T) cm³ molecule⁻¹ s⁻¹. Our values are in reasonable agreement with the only reported measurement of k₁; the rate coefficients for reaction (2) have not been previously reported.     

Atomic and molecular absorption spectra of indium in air-acetylene flame

Absorption spectra of both atomic and molecular species in the air-acetylene flame, which are produced when the aqueous solutions of indium dissolved in HNO₃, HF, HCl, HBr and HI were aspirated into the flame, have been investigated in the u.v. region. Numerous atomic absorption lines of indium have been observed in the absorption spectra. Most of these lines were previously listed only as emission lines. Those atomic lines have been ascribed to the electronic transitions from the ground states of 5p ²P_1/2⁰ and 5p ²P_3/2⁰ to the excited states such as ls ²S_1/2, md ²D_3/2, nd ²D_5/2, 4p^{2 4}P_1/2, 4p^{2 4}P_3/2 and 4p^{2 4}P_5/2, respectively, where 13 l 6 and 14 m,n 5. The molecular absorption bands for InF, InCl and InBr in the airacetylene flame have been also observed near 234 nm, 267 nm and 282 nm, respectively, as the electronic transition of ¹Σ⁺ → ¹Π₁ Those absorption bands show fine structures due to the molecular vibrations. The spectral parameters for the molecular vibrations have been obtained from the simulations of the observed spectra. The molecular absorption band for InI was not observed because of the decomposition of the molecule in the flame. In addition, the molecular absorption band for InO has been observed near 273 nm and those for NO near 205 and 215 nm.     

Kinetics and mechanism of the reaction between silver(III) and thiourea in aqueous alkaline media

The tetrahydroxoargentate(III) ion, Ag(OH)₄⁻, is rapidly reduced by thiourea (tu) in accordance with the three term rate law RATE = {k₁+(k₂+k₃[OH⁻])[tu]}[Ag^III] where k₁ = 1.08 s⁻¹, k₂ = 1.46 x 10³ M⁻¹ s⁻¹, and k₃ = 2.02 x 10³ M⁻² s⁻¹. The k₁ path occurs via the rate-determining aquation of Ag(OH)₄⁻ while the other two paths involve axial attack of thiourea on silver. The higher values of k₂ and k₃ compared to the ethylenediamine reaction, which obeys the same rate law, is a reflection of the greater nucleophilicity of tu.

Following the redox reaction, solutions become brown in a reaction that obeys pseudo-first-order kinetics. Similar behaviour is observed when tu is replaced by Na₂S or thio-acetamide and when Ag^I reacts with any of these sulphur containing compounds. We attribute this process to the Ag^I promoted formation of sulphide species which eventually precipitate as Ag₂S.     

Kinetics and mechanisms of the redox reaction of hexaaquotitanium(III) and the ethylenediamine tetraacetato titanium(III) complex by aqueous solutions of bromine

At 25°C, I = 1.0 M (CF₃SO₃⁻Li⁺+CF₃SO₃H), [H⁺] = 0.034–0.274 M and λ = 453 nm, the rate equation for the oxidation of Ti(H₂O), ₆³⁺ by bromine was found to be: −d/[Br₂]T/dt=kK/[Br₂][Ti^III]/[H⁺]+K+kK/[Br₃⁻][Ti^III]/[H⁺+K, where k = 9.2 × 10⁻³ M ⁻¹ s ⁻¹ and K = 4.5 × 10⁻³ M. At [H⁺] = 1.0 M, [Br⁻] = 0.05–0.4 M, the apparent second-order rate constant decreases as [Br⁻] increases.

The pH-dependence of the oxidation of Ti^III-edta by bromine is interpreted in terms of the change in identity of the Ti^III-edta species as the pH of the reaction medium changes. The second-order rate constants were fitted using a non-linear least-square computer program with (1/k₀^edta)² weighting into an equation of the form: k₀^edta =k₁+k₂K₁[H⁺]⁻¹+k₃K₁K₂[H⁺]^−2/1+K₁[H⁺[H⁺⁻¹+K₁K₂[H⁺]⁻², with K₁ and K₂ fixed as earlier determined at 9.55 × 10⁻³ and 2.29 × 10⁻⁹ M, respectively, for the oxidation of bromine. k₁=k₂=(3.1±0.32)×10³M⁻¹s⁻¹ k₃=(2.3±0.45)×10⁶N⁻¹s⁻¹.

It is proposed that these electron transfer reactions proceed by univalent changes with the production of Br₂^.− as a transient intermediate. An outer-sphere mechanism is proposed for these reactions. The homonuclear exchange rate for Ti^III-edta+Ti^IV-edta is estimated at 32 M⁻¹ s⁻¹.     

碰撞能对K(2S)+HF(X1Σ+)→KF(X1Σ+)+H(2S)反应的影响

基于Sayós等构建的基态势能面, 运用准经典轨线方法对原子-分子反应K(²S)+HF(X¹∑⁺)→KF(X¹∑⁺)+ H(²S)的立体动力学性质进行了研究. 计算了该反应的极化微分反应截面、 两矢量k?j'相关分布函数P(θ_r)、 三矢量k?k'?j'相关分布函数P(?_r)和空间分布函数P(θ_r,?_r). 结果表明, 产物KF的转动角动量j′不仅在垂直于反应物相对速度矢量k的方向上有强烈的取向效应, 而且还定向于y轴的负方向, 转动角动量j′敏感地依赖于碰撞能.     

On the vibrational spectra and conformational stability of 1,1-dimethylhydrazine from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

Variable temperature (−105 to −150 °C) studies of the infrared spectra (3500–400 cm⁻¹) of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, in liquid krypton have been carried out. No convincing spectral evidence could be found for the trans conformer which is expected to be at least 600 cm⁻¹ less stable than the gauche form. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from MP2/6-31G(d) ab initio calculations. The predicted infrared and Raman spectra are compared to the experimental ones. The adjusted r₀ parameters from MP2/6-311+G(d,p) calculations are compared to those reported from an electron diffraction study. The energy differences between the gauche and trans conformers have been obtained from MP2 ab initio calculations as well as from density functional theory by the B3LYP method calculations from a variety of basis sets. All of these calculations indicate an energy difference of 650–900 cm⁻¹ with the B3LYP calculations predicted the larger values. The potential function governing the conformational interchange has been predicting from both types of calculations and comparisons have been made. The barrier to internal rotation by the independent rotor model of the inner methyl group is predicted to have a value of 1812 cm⁻¹ and that of the outer one of 1662 cm⁻¹ from ab initio MP2/6-31G(d) calculations. These values agree well with the experimentally determined values of 1852±16 and 1558±12 cm⁻¹, respectively, from a fit of the torsional transitions with the coupled rotor model. For the coupled rotor model the predicted V′₃₃ (sin 3τ₀ sin 3τ₁ term) value which ranged from 190 to 232 cm⁻¹ is in reasonable agreement with the experimental value of 268±3 cm⁻¹ but the predicted V₃₃ (cos 3τ₀ cos 3τ₁ term) value of −73 to −139 cm⁻¹ is 25% smaller and of the opposite sign of the experimental value of 333±22 cm⁻¹. These theoretical and spectroscopy results are compared to similar quantities of some corresponding molecules.     

Induction effects on IR-predissociation spectra of (SF6)2, (SiF4)2 and (SiH4)2

The observed difference in transition strength for (SF₆)₂, (SiF₄)₂ and (SiH₄)₂ IR-predissociation spectra is explained by induction effects (μ₀₁²/R₁₂⁶) which have to be included in the interaction Hamiltonian in addition to the dominant dipole-dipole term (μ₀₁²/R₁₂³).     

单层，少层和块状WS2薄膜中声子位移随温度的变化

Two-dimensional transition metal disulfides (TMDs) have recently attracted significant research attention due to their rich physical and chemical properties. Graphene has also been studied intensively due to its high electron mobility of ~200000 cm²·V⁻¹·s⁻¹. Since there is no band gap, it is difficult for a graphene-based device to achieve high current on/off ratio. For TMDs, such as MoS₂, MoSe₂, WSe₂, and WS₂, the band gaps of these materials can be adjusted according to the number of layers. Since TMD has the advantage of suppressing source-drain tunneling current in an ultra-short transistor and offering superior immunity to short-channel effects, it is also attractive for use as a channel material in Si complementary metal oxide semiconductor (CMOS) devices larger than 22 nm. Among them, MoS₂ in single-layer and multi-layer films have been intensively researched for many years. MoS₂-based field effect transistors (FETs) with excellent electrical properties have been reported. WS₂ has lower in-plane electronic mass than MoS₂, MoSe₂, and MoTe₂, and therefore has potential for higher carrier mobility or higher output current for WS₂-based FETs. Experimental research on WS₂ is limited compared to MoS₂, and more work is needed to further exploit the full potential of WS₂-based FETs. Therefore, the electron-phonon interaction and vibration properties of WS₂ used in nano-electronic applications and FETs must be investigated. To this end, mono-layer (1L), few-layer (FL), and bulk WS₂ films were prepared using mechanical exfoliation from a WS₂ crystal. 3M scotch-tape was used for transferring the WS₂ films. Detailed temperature-dependent Raman study on 1L, FL, and bulk WS₂ films has been conducted using a 514-nm excitation laser. Raman spectroscopy, as an effective and non-destructive approach for phonon vibration study, has been used to evaluate TMDs. The Raman spectra reveal much useful information on the test sample in terms of peak position and spectral shape change. With the film thickness increasing to bulk, the A_1g(Γ) and E_2g¹(Γ) modes show blue-shift and red-shift, respectively, with respect to 1L WS₂. Moreover, when the dominant Raman vibration modes swaps between E_2g¹(Γ) and A_1g(Γ), the "cross-over" temperature was identified for 1L, FL, and bulk WS₂ films. WS₂ shows smaller frequency change Δ between the E_2g¹(Γ) and A_1g(Γ) modes than MoS₂, with varying film thickness. The temperature coefficient of the Raman peak position was one magnitude lower for WS₂ than MoS₂, implying that WS₂ has better thermal stability than MoS₂. The results of this systematic study provide a physical guidance for WS₂-based device design.