SiO2 nanoparticles doped nematic liquid crystal system: An experimental investigation on optical and dielectric properties

The aim of this work is to investigate the effect of silica (SiO₂) nanoparticles (NPs) on optical and dielectric properties of BBEA nematic liquid crystal (NLC). For optical analysis the photoluminescence (PL) and UV-absorbance experiments have been performed. The doped system is showing enhancement in the intensity of photoluminescence with varying concentration of nanoparticles. A red shift is observed in the emission spectra of NLC doped with silica nanoparticles. The PL emission peak of NLC is observed at 377.3 nm which is shifted to 379.7 nm in the presence of silica nanoparticles. We have also observed the enhancement in the value of UV absorption for silica doped systems in comparison to the pure system. Energy band gap of pure and doped systems has been calculated and it is found that the energy band gap is decreasing with concentration which is a promising result of this study. The dielectric parameters of the pure and doped NLC systems were carried out as a function of frequency and temperature. Different dielectric parameters such as relative permittivity, loss factor and dielectric conductivity have been measured. The pure and silica nanoparticles doped systems has shown decreased value of dielectric permittivity and loss factor at lower frequency region and at higher frequency regions these values became constant. The value of relative permittivity also decreases with concentration. The increased value of a.c. conductivity for doped systems can be utilized in device designing. Moreover, the temperature dependence of the birefringence (Δn) was determined from the transmitted intensity of light for pure and doped systems and the improvement in its value for both composites has been observed. Improved value of birefringence has pronounced applications in optical devices.     

Effect of Ni doping on ferroelectric and dielectric properties of strontium barium niobate crystals

The influence of Ni doping on the ferroelectric and dielectric properties have been examined in Sr_0.61Ba_0.39Nb₂O₆ (SBN:61) relaxor crystals. The dopants introduced into SBN:61 crystals promote the switching process by reducing the value of threshold nucleation field, and thus coercive field. We present real-time studies of domain nucleation and growth processes in doped SBN:61 by the nematic liquid crystal (NLC) decoration technique. The broad phase transition and low-frequency dielectric dispersion that are exhibited by doped SBN:61 samples have a strong link to the configuration of the ferroelectrics microdomains, which in turn is strongly determined by Ni ions concentration.     

Thermally enhanced optical nonlinearity in nematic liquid crystal close to phase transition temperature

This study investigates the beam profile and the liquid crystal (LC) arrangement affected by an optical field on LC thin films at a temperature close to nematic-isotropic phase transition temperature (T_NI). A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal (NLC). An optical field combined with thermal enhancement enhances molecular reorientation and causes additional molecular excitation along the axis of propagation of the beam. The reorientational nonlinearity yields an undulating structure with multi-foci; the length between each pair of foci increases with time, as described.     

Study of dipole dynamics and pre-transitional effects at isotropic to nematic phase transition by low-frequency dielectric relaxation measurements

Thermal microscopy (TM) as well as low-frequency (LF) dielectric relaxation studies are carried out on a nematic liquid crystal (NLC), viz E7 (Merck Ltd, UK). The isotropic to nematic (IN) transition temperatureT _IN determined by TM and LF-dielectric permittivity measurements agrees with the available data. Dielectric loss studies in the frequency region of 5–10?MHz indicate a relaxation (in the kHz region) akin to Debye type off-centered dispersion. The observed nematic relaxation is found to correspond to reorientation (about the short axis) of the nematic dipole to the external field. The temperature variation of the nematic relaxation frequencyf _R is found to follow an Arrhenius shift, with an activation energy of 1.7?eV. Temperature variation of the dielectric strength (Δε = ε _o ? ε_∞) and the distribution parameter α in the nematic phase are discussed. The dynamic response of the nematic dipoles and growth of pre-transitional fluctuations are found to be nonlinear in the vicinity of the IN transition. The value of the exponent α_eff = 0.072 indicates weak growth of transitional fluctuations across the IN transition.     

Specific features of luminescence quenching in a nematic liquid crystal doped with nanoparticles

The change in the intensity of the photoluminescence (PL) spectra of nematic liquid crystal (NLC) composites as a function of the concentration of CdSe/ZnS semiconductor quantum dots (QDs) and TiO₂ and ZrO₂ nanoparticles ~5 nm in diameter has been investigated. It is shown that the PL-quenching intensity in composites with CdSe/ZnS QDs exceeds that in composites with TiO₂ and ZrO₂ nanoparticles. The lowfrequency spectra of these composites with a concentration of 0.1 wt %, recorded in the range of 10²–10³ Hz, and the content of mobile ions in them have been investigated. It is found that the dielectric loss in the composite with CdSe/ZnS QDs is much higher and the content of mobile ions is larger by a factor of 3 than in the composites with TiO₂ and ZrO₂ nanoparticles. It is shown that an increase in the CdSe/ZnS QD concentration in NLC composites leads to an increase in the dielectric loss and a decrease in the PL intensity. Possible mechanisms of the interaction between NLC molecules and CdSe/ZnS QDs are discussed.     

Analysis of birefringence property of three different nematic liquid crystals dispersed with TiO2 nanoparticles

In the present work TiO₂ nanoparticles (NPs) have been dispersed into three different nematic liquid crystals (2020, 1823A and 1550C) in different concentration. The value of the birefringence (Δn) has been calculated by the transmitted intensity method at a 632.8 nm wavelength. NLC 2020 used in the present study is a high birefringent material (Δn = 0.44), NLC 1550C is a low birefringent material (Δn = 0.067) and NLC 1823A is a mid birefringent material (Δn = 0.14). An increased value of birefringence has been found after dispersion of TiO₂ NPs in all three NLCs but this increment depends upon the concentration of the dopant material, temperature range and chemical character of the mixtures. It is suggested that this LC materials can be applicable in making of phase shifters, compensators and many more photonic devices.     

Light-scattering experiments in dye-doped liquid crystals both to determine crystal parameters and to construct consistent neural network empirical physical formulas for scattering amplitudes

The aim of this paper is two-fold. Firstly, static laser light-scattering amplitude measurements in azo-dye doped nematic liquid crystals (NLCs) were made versus scattering angle, temperature and applied bias voltage. Three NLC parameters were determined: the elastic constant ratios K₁₁/K₂₂ by regression, phase transition temperatures, and Freedericksz voltages from the graphs. They were all doping ratio dependent. Secondly, as a novel approach, by a nonlinear universal function approximator layered feedforward neural network (LFNN) we constructed an explicit form of empirical physical formulas (EPFs) for theoretically unknown nonlinear azo-dye doped NLC scattering amplitude functions. Excellent LFNN test set (i.e. yet-to-be measured experimental data) predictions prove that the constructed LFNN-EPPs estimate unknown amplitude functions consistently. The LFFN-EPFs, too, confirmed the doping-ratio dependency. Also, comparing LFNN and regression amplitude fits, the LFNN fits were significantly better. In conclusion, physical laws embedded in the physical data can be consistently extracted by LFNN. One major potential application in the nonlinear optics domain is that these LFNN-EPFs, by differentiation, integration, minimization, etc., can be used to obtain further NLC scattering amplitude related molecular structural physical quantities. This could in turn help us to develop new nonlinear optical materials.     

向列相液晶沿面到垂面排列转变的分子场理论

以摩擦基板间的液晶薄层为研究对象,用分子场理论研究了向列相液晶分子排列转变行为.分子质心固定在简单立方晶格的格点上.液晶由极性分子构成,与基板相接触的一层分子同时受到色散和极性两类表面作用.通过自洽的数值计算,获得3种相图,清楚地展示了摩擦基板间向列相出现的从高温沿面到低温垂面排列的转变;获得实现这类转变所需要的两类表面作用的参数范围.结果表明:基板的摩擦会改变基板表面色散作用,但不会影响基板表面极性作用;表面极性相互作用能引起基板间向列相液晶发生沿面到垂面排列转变.     

Electrical Freedericksz transitions in nematic liquid crystals containing ferroelectric nanoparticles

A new theoretical approach was elaborated to explain the contradictions reported in many papers about the electric threshold for Freedericksz transition in nematic liquid crystal (NLC) with ferroparticles additives. The free energy density of the mixture was estimated and the contributions of the interaction energy of NLC molecules with ferroparticles surface were calculated. Experimental results for 5CB-BaTiO₃ mixture are given.     

Inhomogeneous threshold director reorientation in a planar nematic flexoelectric cell with finite anchoring energy

The dependence of the threshold parameters and the period of the electric-field-induced spatially periodic reorientation of the director in a flexoelectric nematic liquid crystal (NLC) on the anchoring conditions at the surface of a planar NLC cell has been studied. The threshold electric field and the corresponding wave-number of the periodic structure of the director field have been numerically calculated for arbitrary values of the anchoring energy. In the case of strong anchoring, the corresponding analytical expressions are obtained in a single-constant approximation. A decrease in the azimuthal anchoring energy leads to an increase in the intervals of possible values of the flexoelectric parameter ν and the ratio K₂/K₁ of the Frank elastic constants. A decrease in the polar anchoring energy leads to narrowing of these intervals as compared to the case of infinitely strong anchoring at the NLC cell surface.     

Investigation of Nematic Liquid Crystals Doped with Spherical Multiferroic Nanoparticles in the Presence of a Magnetic Field

Recently, multiferroic nanoparticles (MFNPs) have attracted significant attention as dopant in the nematic liquid crystal (NLC) medium. Here, behavior of the NLC molecules and MFNPs in the presence of a magnetic field is theoretically investigated in the framework of continuum theory, based on our knowledge. The electric field of the MFNPs is taken into account in the calculations. Freedericksz transition (FT) threshold field in the presence of spherical nanoparticles is determined. Dependence of the FT threshold to the diameter and volume fraction of the nanoparticles was studied. Also, the NLC director and nanoparticle reorientations for the magnetic field larger than the FT threshold field are studied. It was shown that the FT threshold of NLC is decreased by doping of spherical MFNPs. Migration of the nanoparticles to the central layers of the cell (segregation effect) is investigated.     

Investigation of electronic structure of Nd-doped TiO2 nanoparticles using synchrotron radiation photoelectron spectroscopy

Doping with transition metal ions in TiO₂ has been found effective to modify the electronic structure of TiO₂ nanoparticles. Application of synchrotron radiation photoelectron spectroscopy (SRPES) to Nd-doped TiO₂ nanoparticles revealed that there existed different peak positions and structure with different doping concentration in the valence band spectra. From the onset of valence band spectrum, it was observed that doping Nd ions alters the electronic structure and makes the band gap of TiO₂ narrow.     

Cr和W掺杂的单层MoS2电子结构的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法, 研究了Cr和W掺杂对单层二硫化钼(MoS₂)晶体的电子结构性质的影响. 计算结果表明: 当掺杂浓度较高时, W对MoS₂的能带结构几乎没有影响, 而Cr的掺杂则影响很大, 表现为能带由直接带隙变为间接带隙, 且禁带宽度减小. 通过进一步分析, 得出应力的产生是导致Cr掺杂的MoS₂电子结构变化的最直接的原因.     

Temperature dependent photoluminescence properties of needle-like ZnO nanostructures deposited on carbon nanotubes

The structural and optical properties of the needle-like ZnO nanostructure grown on the carbon nanotubes (ZnO/CNTs) have been studied by scanning electron microscope (SEM), X-ray diffraction (XRD), and photoluminescence (PL) spectra. It can be seen that there is about tens of nanometers in diameter of the single ZnO nanorod from the SEM picture. The XRD analysis shows that the prepared film is of typical wurtzite hexagonal phase without impurity. Temperature dependence of electronic transitions in the ZnO/CNTs has been investigated by PL in detail. The emission features in near band gap at 10 K reveal a redshift trend compared to ZnO single crystal, which is associated with the strong interfacial connection between ZnO and CNTs. Moreover, the intensities of all transitions in near band gap and visible regions decrease with increasing the temperature but increase with the excitation power. It can be concluded that the combined effect from ZnO and CNTs plays an important role in the PL response. The emission variations with the temperature for the ZnO/CNTs are the result of the electron–phonon interaction and the lattice thermal expansion.     

稀掺杂GaNxAs1－x(x≤0.03)薄膜的调制光谱研究

利用室温下压电调制反射光(PzR)谱技术系统测量了N掺杂浓度为0.0%—3%的分子束外延生长GaN_xAs_1-x薄膜，并对图谱中所观察的光学跃迁进行了指认.在GaN_0.005As_0.995和GaN_0.01As_0.99薄膜的PzR谱中观察到此前只在椭圆偏振谱中才看到的N掺杂相关能态E₁+Δ₁+Δ_N.当N掺杂浓度达到 关键词： 压电调制反射光谱(PzR) xAs_1-x薄膜')" href="#">GaN_xAs_1-x薄膜 分子束外延(MBE)     

Anomalous increase of the thermopower and thermoelectric figure of merit in Ga-doped <Emphasis Type="Italic">p</Emphasis>-(Bi<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals

The effect of Ga doping on the temperature dependences (5 K ≤ T ≤ 300 K) of the Seebeck coefficient α, electrical conductivity σ, thermal conductivity coefficient κ, and thermoelectric figure of merit Z of p-(Bi_0.5Sb_0.5)₂Te₃ single crystals has been investigated. It has been shown that, upon Ga doping, the hole concentration decreases, the Seebeck coefficient increases, the electrical conductivity decreases, and the thermoelectric figure of merit increases. The observed variations in the Seebeck coefficient cannot be completely explained by the decrease in the hole concentration and indicate a noticeable variation in the density of states due to the Ga doping.     

First-principles investigation of electronic and optical properties in wurtzite Zn<Subscript>1-x</Subscript>Mg<Subscript>x</Subscript>O

A first-principles study has been performed to evaluate the electronic and optical properties of wurtzite Zn_1-xMg_xO. Substitutional doping is considered with Mg concentrations of x = 0, 0.0625, 0.125, 0.1875 and 0.25, respectively. Mg incorporation can induce band gap widening due to the decrease of Zn 4s states. The imaginary part of the dielectric function shows that the optical transition from band edge emission decreases slightly with increasing Mg contents. The optical band gap also increases from 3.2 to 3.7 eV with increasing Mg contents from 0.0625 to 0.25. The calculated results suggest that relatively high Mg concentration is necessary for effective band gap engineering of wurtzite Zn_1-xMg_xO.     

Effect of carbon nanotubes on the isotropic to nematic and the nematic to smectic- A phase transitions in liquid crystal and carbon nanotubes composites

A high-resolution ac-calorimetric study on the weakly first-order isotropic to nematic (I -N and the continuous nematic to smectic-A (N -SmA phase transitions of the liquid crystal octyl-cyanobiphenyl (8CB) doped with well-dispersed multiwall carbon nanotubes (CNTs) as a function of CNT concentrations is reported. Thermal scans were performed for all samples having CNT weight fraction from f_w \phi_{{w}}^{} = 0.0005 to 0.0060 over a wide temperature range well above and below the two transitions in pure 8CB. Both the I -N and the N -SmA transitions evolve in character and have their transition temperatures qualitatively offset by ∼ 1.10 K lower as compared to that in pure 8CB for all 8CB+CNTs samples. The enthalpy change associated with each phase transition is essentially the same as that of pure 8CB and remains unchanged with increasing f_w \phi_{{w}}^{} . However, there is an evidence that the thermal transport properties of the composites differ from the pure LC upon cooling below a f_w \phi_{{w}}^{} -dependent temperature within the nematic phase. In addition, a new C_p feature is resolved for intermediate f_w \phi_{{w}}^{} samples that appears to be correlated to this onset temperature.     

向列型液晶中简并四波混频相位共轭的理论研究

研究向列型液晶中由光致分子重取向引起的简并四波混频光学相位共轭现象,着重讨论液晶薄膜厚度d、探测光与一泵浦光之间的夹角θ_ω以及光的入射面和液晶指向矢之间的夹角β对相位共轭光产生效率的影响.     

The effect of non-mesogenic biphenyl impurity on the orientational order parameter of 4-cyano-4′-n-octyl-biphenyl (8CB)

The role of non-mesogenic impurities on a mesogen and the consequent alteration of mesogenic properties have always been of great interest in the tailoring of mesogenic behaviour. In the present work the effect of rigid, non-polar, nonmesogenic solute ‘biphenyl’ on the optical properties of the mesogen 4-cyano-4′-n-octyl-biphenyl (8CB) has been reported. The phase transitions and thermal variations of the refractive index, polarisability and order parameter ?P₂? of the 8CB–biphenyl systems of various concentrations have been investigated using a He–Ne laser beam. The results have been compared with ?P₂? of pure 8CB. It has been observed that biphenyl lowers both smecticA–nematic and nematic–isotropic transition temperatures by an amount that depends on impurity concentration.