Determination of the Surface Tilt Angle by Attenuated Total Reflection

Thin SiO₂ layers prepared by oxidizing an organo-silane in an Ar/O₂ rf plasma discharge produce parallel alignment for nematic liquid crystals with essentially zero tilt. In addition, if the rf plasma system is arranged to produce an Ar/O₂ beam, uniform parallel alignment can now be obtained with the director in the substrate plane at an angle perpendicular to the plane of incidence of the rf plasma beam. We have studied the aligning properties of such films using the evanescent wave generated by attenuated total reflection (ATR) on a thin gold film when a surface electromagnetic wave is excited. Liquid crystal cells were constructed on a high index glass prism with a 500 Å gold film and a 200 Å aligning layer. By following the ATR minimum as a function of applied electric field, the tilt at the interface of the liquid crystal and the aligning layer was calculated.     

Growth of YBa2Cu3O7 films with [110] tilt of CuO planes to surface on SrTiO3 crystals

YBa₂Cu₃O₇ films with the CuO plane tilted to the surface have been grown on SrTiO₃ crystalline substrates by pulsed laser deposition. This tilt was obtained by rotating the film lattice with respect to the substrate surface around its [110] axis oriented parallel to the surface. The zero tilt of the CuO plane was implemented at the orientation of the SrTiO₃ crystal surface parallel to the (100) plane. The rotation angles were varied in the range from 0° to 70°. It is found that, being tilted at any angle, the CuO planes of the film remain parallel to one or several crystallographic planes of the (100)-type substrate. In the range of tilt angles from 0° to 49°, the film has a single-crystal structure. Above 49° the film is transformed into a three-domain texture and its surface roughness sharply increases.     

Investigations of the crystal growth of PbMoO4 by the Czochralski method

The thermal conditions at the liquid/solid interface are characterized by temperature measurements in the melt and on the growing crystal with 〈001〉-seed orientation. The thermal boundary layer was determined, from which the relation of effective thermal conductivity of the liquid and solid phase was found to be λ_l,eff/λ_s,eff = 0.25. The liquid/solid interface is extremly convex towards the melt and has a conical shape. When the crystal diameter reaches a certain value the cone was truncated with the formation of a facet in the centre. This typical interface shape is mainly the result of a difference between the effective thermal conductivity of melt and crystal.     

Refractive index and dispersion measurements in prismatic thin film of phasmidic columnar liquid crystals

In this work, we explore the refractive index of the phasmidic columnar liquid crystal for homeotropic oriented samples (column perpendicular to the plates). To measure the ordinary index n_o of the columnar liquid crystal C₁₀, a simple and accurate method is presented using the deflection by reflection of a laser beam (λ = 633nm) through a prismatic phasmidic thin film placed between two glass plates in the homeotropic orientation. For the C₁₀ samples, we found n_o = 1.5512 ± 0.0002 at T = 100°C. We used the same method to determine the isotropic index n_l of the isotropic phase and found n_l = 1.5301 ± 0.0002 at T = 120°C. Using the precision of the same method in polychromatic laser beam, we measured the variation of the ordinary index with the wavelength λ and characterize the dispersion by the known Cauchy's dispersion relationship n_o = A + B/λ². We deduce an estimation of the birefringence for several samples.     

The effect of concentration and molecular weight of polycarbonate on the elastic constants of a nematic liquid crystal solvent

A polarized upconverting luminescence in a novel upconverison nanorods (UCNRs)/liquid crystal network polymer (LCN) composite film is proposed. The UCNRs were fabricated by hydrothermal method, which were further modified to be introduced into LCN precursors with good dispersion. The orientation of UCNRs in LCN precursors was found to be strongly influenced by the configuration of LC molecules which could be modulated by external electric field. The UCNRs/LCN composite films in planar and homeotropic configuration were fabricated by UV photopolymerization. It is demonstrated that upconverting fluorescence emission from the UCNRs/LCN film with planar orientation is polarization-dependent, while the emission from the UCNRs/LCN film with homeotropic alignment is not polarized. These observations are ascribed to the directional arrangement of the UCNRs enabled by the LC molecules alignment and the crystal symmetry of the UCNR matrix. This work paves a different avenue to the construction of the upconversion nanoparticles/polymer composite system with prospective application value in anti-counterfeiting optical films.     

A combination of oscillating slit and oscillating film techniques for the investigation of lattice defects

A combination of oscillating slit and oscillating film techniques is described. This X-ray topographical method allows reflections due to the same places on the crystal surface to be recorded first in form of a topogramme registrated on the oscillating film set in the parallel position to the crystal surface. Second on the stationary film set parallel to the crystal surface in the form of an interference line Kα₁ or Kß outgoing from the investigated area and registrated on the first topogramme at the fixed angular position of the crystal and slit and next, recorded separate on the oscillating film and on the stationary film set in the distance of 50 mm from the crystal (oscillation axis). As an example a serie of photographs corresponding to four types of pattern obtained in the case of a PbSnTe crystal with small angle boundaries are presented.     

Thin thermosensitive film coating

Thin film thermosensitive coatings are obtained, representing liquid crystal microdroplets dispersed in a polymeric matrix. Gelatin is chosen as a polymeric material. The obtaining of microdroplets is carried out by condensation of the solution of liquid crystal in organic solvents at their mixing with gelatin aqueous solution. The obtained coating distinguishes with temperature stability, homogeneity of the colouring, stability of the time indicators.     

Energy criteria for orientation of smectic C* liquid crystals in electrooptic elements

Energy analysis of the types of orientation of smectic C* liquid crystals is performed. Criteria for the type and quality of their orientation in electrooptic elements with surfaces of orientants of different nature are revealed. An energy parameter of the optical quality of planar orientation of a smectic C* liquid crystal is proposed, which determines the contrast ratio of electrooptic elements independent of the type and nature of the smectic C* liquid crystal and the orientant.     

Calculation of the shear stresses in (111)- and (001) semiconductor substrates with film edges

Assuming a film-edge induced stress distribution in an isotropic crystal, the shear stresses in the glide systems of a diamond-type semiconductor substrate are calculated. Quantitative information on magnitude and position of the respective shear stress maxima is obtained in dependence on crystal orientation and technologically relevant film configurations.     

Electric Field Effects on Sensitized Fluorescence Emission From Nematic Liquid Crystal Cells

Remarkable electric field effects were observed with sensitized fluorescence intensities of a perylene derivative (BPDC), when rectangular AC waves (up to 10 V_0-p ) were applied to nematic liquid crystal cells containing a sensitizer (C307 or AHDA) and an acceptor (BPDC) as the guest molecules. Measurements of the fluorescence lifetime of AHDA indicated that the rates of energy transfer from AHDA to BPDC were independent of the electric fields. The electric field effects completely changed the sign when the intensity variation detected at vertical direction was compared with that at the horizontal direction with respect to the liquid crystal cell. The observed electric field effects on the sensitized fluorescence were consequently suggested to come from the orientation control of transition dipole moment of BPDC through the molecular alignment of the liquid crystal. The suggestion was confirmed by the measurement of dichroic absorption of the donor-acceptor systems.     

Tuning of a Dye Laser By a Liquid Crystal†

We describe how a cholesteric liquid crystal device can be used in a laser cavity to induce tuning of the laser line. Tuning of the order of 10Å is obtained by applying low voltages to the liquid crystals film. The problems, the advantages and the feasibility of the method is discussed, reporting several experimental data.     

Two Frequency Addressing of a DTN-Cell

The authors have already reported that a DTN-cell (depolarization in a twisted nematic-cell) had several advantages to a matrix display such as a sharp threshold and wide viewing angle. But if the usual amplitude selection method is applied to a large scale matrix display using the DTN-mode, multiplex capability is limited by the cutoff frequency F_c, of a liquid crystal. Therefore, the authors investigated a two frequency addressing method, that is, simultaneous application of a constant low-frequency voltage and a variable high-frequency voltage. In this method, a liquid crystal with lower-F_c can be used. In addition, some advantages of extremely sharp threshold, high contrast and relatively fast response and recovery can be obtained by using a liquid crystal with large negative dielectric anisotropy. These advantages are useful for a large scale matrix display.     

MgO(100)衬底上生长BaTi2O5薄膜及其性能研究

利用脉冲激光沉积技术,在MgO(100)衬底上生长了BaTi2O5薄膜,探讨了沉积条件(衬底温度和氧分压)对薄膜结构的影响,并对其介电和光学性能进行了研究.结果表明:随衬底温度和氧分压的改变,BaTi2O5薄膜的物相和结晶取向逐渐变化;适宜的脉冲激光沉积工艺为衬底温度950~1000 K、氧分压12.5 Pa,在该条件下获得了b轴方向择优生长的BaTi2O5薄膜;该薄膜具有较高的居里温度(750 K),介电常数达2000,而且在可见光和红外波长范围内具有较高的透过率.     

Molecular orientation and dielectric anisotropy properties of 4-cyano-4′-n-heptylbiphenyl-TiO2 liquid crystal composite

The molecular orientation and the dielectric anisotropy of the nematic liquid crystal (LC) 4-cyano-4′-n-heptylbiphenyl (7CB) and of TiO₂-doped 7CB have been investigated. The dielectric properties of the LCs exhibit a relaxation peak that shifts to lower frequencies with increasing voltages. The relaxation frequencies of 7CB and 7CB/TiO₂ liquid crystals were calculated and found to decrease as the bias voltage increases. This is attributed to molecular reorientation. The dielectric anisotropy of the LCs changes from the positive type to negative type and the static electric permittivity and dielectric anisotropy values were found to be lower for the 7CB/TiO₂ system.     

不同退火温度对钡铁氧体薄膜磁性的影响

M型钡铁氧体(BaFe₁₂O₁₉, BaM)是一种单轴磁晶各向异性的六角晶系硬磁材料,由于其具有很强的各向异性场,因此在自偏置微波器件领域具有广阔的应用前景。本文采用常温射频磁控溅射法在(000l)取向的蓝宝石衬底上沉积了厚度约为130 nm的BaFe₁₂O₁₉非晶薄膜,然后分别在850 ℃、900 ℃、950 ℃、1 000 ℃对其空气退火处理3 h,得到BaM晶体薄膜样品。采用X射线衍射仪对薄膜样品进行物相及晶体生长取向鉴别,采用扫描探针显微镜和扫描电子显微镜对薄膜样品的粗糙度和表面形貌进行测量和观察,采用振动样品磁强计对样品进行了静态磁性能测试。实验结果表明,退火后的薄膜样品的主晶相为BaM,且具有(000l)取向择优生长,其微观组织结构都表现为C轴垂直于膜面的颗粒状结构。退火温度为900 ℃时所得样品的各项性能达到最佳,其表面粗糙度为2.8 nm,矩形比为0.84,饱和磁化强度为247 emu/cm³,矫顽力为1 528 Oe。     

Polarization mechanism of LiNbO3 crystal due to thermoelectric power and current-induced electric fields during growth via micro-pulling-down method

The mechanism of polarization due to thermoelectric power and current-induced electric fields during the growth of LiNbO₃ crystals was studied using a micro-pulling-down method. With no applied electric current, a +c single-domain crystal was grown regardless of the domain orientation of the seed crystal. This +c domain growth was consistent with the direction of the electric field caused by the thermoelectric power in the liquid, despite an opposing electric field in the solid due to the opposite sign of the Seebeck coefficient. Thus, it was the electric field in the liquid that determined the domain structure of the growing crystal. On the other hand, when a current was applied from the melt to the crystal, a −c domain crystal was grown. The electric current required for this domain inversion to occur became larger as the temperature gradient in the solid phase decreased. This shows that the electric field in the solid phase became large enough to induce domain inversion from +c to −c through a combination of the thermoelectric power in the solid phase and current-induced electric field.     

Zur Bestimmung der Gitterkonstanten und der Orientierung nichtkubischer Kristalle mit der Pseudo-Kossel-Technik

Pseudo-Kossel-patterns are very usefull for the study of small monocrystalline regions. This paper deals with a method, which permits the evaluation of transmission patterns without knowledge of the distance between the X-ray source and the film. The center of a Pseudo-Kossel-Diagramm is determined during the procedure of evaluation, and the crystal can be oriented with respect to the film in any desired way. By the method presented here it is possible to determine the reciprocal lattice of cubic, tetragonal, hexagonal and orthorhombic crystals and their orientation. To know the lattice parameters is not necessary. The method permits the estimation of lattice parameters with an inaccuracy λa/a ≈ ± 1 · 10⁻⁴ even for relatively small Bragg-angles. To demonstrate the procedure tetragonal ZnSiP₂ is used.     

Polystyrene Derivative Films for Liquid Crystal Alignment

We studied the alignment of liquid crystal molecules on rubbed, thin polymer films of polystrene and its derivatives. Liquid crystal molecules were aligned perpendicular to the rubbing direction, although the polymer main chains of rubbed films were oriented along the rubbing direction. The alignment of the liquid crystal depended on the choice of side phenyl groups of polystyrene derivatives, and the pretilt angle varied with the type and the position of the substituents that were introduced into the side phenyl groups. Homogeneous alignment for ferroelectric liquid crystal was obtained by using poly(NO₂ styrene), poly(p-OH styrene) and poly(4-vinylpyridine) films, which have relatively high β-dispersion and glass transition temperatures.     

The Smetic Polymorphism and the Phase Transitions in the Liquid Crystal 4, n-Hexyloxyphenyl-4, n′-Decyloxybenzoate

Three different methods — microscopical, depolarized light scattering and conoscopic have been used for the investigation of the polymorphism of 4, n-hexyloxyphenyl-4, n′-decyloxybenzoate (HOPDOB) at homeotropic orientation. A new textural transition is registered during cooling in smectic C phase connected with a change in the tilt (ω) and azimuthal (φ) angles of the molecular director. At cooling we found smectic E phase and by the conoscopic method we determined this phase as tilted smectic type (E_t) with 24° tilt angle. It is also found that B phase is a normal type smectic liquid crystal — B_N.