Cells dynamic scattering

RHEED intensities cannot be explained using the kinematic theory. Dynamical scattering models of RHEED intensities are being developed. With them one will be able to obtain positions of the surface atoms within the surface unit cell. At this writing, such modeling has been done primarily for LEED. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Electron dynamic scattering must be considered for the interpretation of experimental diffraction intensities because of the strong electron interaction with matter for a crystal of more than 10 nm thick. For a perfect crystal with a relatively small unit cell, the Bloch wave method is the preferred way to calculate dynamic electron diffraction intensities and exit-wave functions because of its flexibility and accuracy. The multi-slice method or other similar methods are best in case of diffraction from crystals containing defects. A recent description of the multislice method can be found in [8]. 

It has recently been shown that organic photoconductor-liquid crystal sandwich cells can in theory act as dynamic scattering devices 164> and the technical possibilities ought to be tested. In this context, it should be noted that dyes can be used in two-layer photocondensers (consisting e.g. of phthalocyanine and a ferroelectric ceramic), which are very sensitive to light and have a response time of lO-4 to 10-3 Sec 165). 

LEED does not only reveal the relative periodicities of the adsorbate mesh with respect to the substrate lattice. Applying dynamical scattering theory, i.e., modeling the scattering intensity of diffracted beams versus electron energy (so-called I-V curves), allows determination of absolute positions of atoms on the surface [20]. Unfortunately, the complexity of the method limits the number of atoms per unit cell and makes it applicable only to atomic or small-molecule lattices. 

This accumulation of charge also sets up the flow pattern shown in Figure 10.16. If the electric field is not too strong, these convection cells are quite stable (Williams domains). At larger electric fields, however, the flow becomes turbulent, with a complex, fluctuating director configuration (dynamic scattering). 

The threshold conditions and optical appearance of the anisotropic modulated structure strongly depend on the cell thickness. Experiment shows that in thin cells (approximately 10 /xm or less) and, in particular, with well-purified nematic liquid crystals (electrical conductivity ca. 10 ohm cm ), the Kapustin-Williams domains (and the dynamic scattering of light) do not actually appear. Either longitudinal Vistin domains [5, 8,... 

As the thickness decreases the threshold voltage increases very sharply. This can be seen in Fig. 5.14, where the calculated values for Uth are given as a function of the electrical conductivity for typical values of s, = 4.7 and ) = 10 cm s , and the cell thickness is a parameter. Figure 5.14 also explains why the Kapustin-Williams domains and the dynamic scattering of light are not observed in relatively thick samples with a small electrical conductivity of 10 -10 ohm cm . ... 

The application of a high-frequency electric field to the electrodes induces a transition from the turbid focal-conic texture to the transparent planar texture. This effect is used to erase the light-scattering state obtained after the application of a constant or low-frequency electric field to the cell in the dynamic scattering storage effect described in Section 6.3. 

In dynamic scattering displays the turn-on delay is the time required for the hydrodynamic instability to build up. Experiments show that the hydrodynamic instability first occurs when charge carriers have covered a distance corresponding to the layer thickness of the liquid crystal cell. The turn-off characteristic of dynamic scattering devices is determined by two factors ... 

A Sussmann found that the useful lifetime of a d.c. driven cell is inversely proportional to the current density in the cell [83]. On the other hand the electrical conductivity of the liquid crystal determines the contrast of a display based on an electrohydrodynamic effect such as dynamic scattering. The conductivity can therefore not be decreased at will in favor of longer display lifetime. For optimum contrast the conductivity is about 10 (i2 cm)" to 10 (f2 cm)" ... 

The specific electrical conductivity of pure Azoxy-compounds lies between 10 (12 cm)" and 10 ( 2 cm) h Various dopants are added to the Azoxy-compound to influence the conductivity and orientation. The effect of these additives on the variation with time of the electrical conductivity, of the switching times, and of contrast can be measured. It has been found that after roughly 500 hours of operation for instance the cell conductance when measuring the d.c. current flowing through the cell decreases by one to two powers of ten. As subsequent a.c. measurements revealed, this phenomenon results from the formation of double layers with lower electrical conductivity near the electrodes. These double layers disturb the ion injection from the electrode. As a result of low current densities the dynamic scattering disappears almost entirely. 

Dielectric anisotropy is an additive molar property. Thus, a small amount of PEBAB [Ae 10] (about 10-15 mol %) dissolved in MBBA [Ac —0.2] will provide a material suitable for twisted nematic devices. The threshold will, of course, be higher for this mixture than for a pure positive one such as 4-pentyl-4 -cyanobiphenyl, where the dielectric anisotropy is much higher. There are other influences on the threshold voltage for liquid crystal cells, principally the materials elastic constants and, in the case of dynamic scattering, material viscosity. The response times also are dependent upon elastic constants, viscosity, and dielectric anisotropy. These factors are discussed at length in a review by Goodman." ... 

Fig. 10—Diffraction patterns for a perpendiculariy oriented cell (a) V = 10 Vpp, very light diffuse scattering (b) V = 13 Vpp, diffraction rings due to domain formation (c) V = 16 Vpp, strong diffuse scattering characteristic of dynamic scattering. The black square in each picture is a piece of tape placed on the screen.

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