Centrosymmetrical layer

Lee et al. [392, 393] devised a self-assembly process which makes use of 1,10-decane-b/s-phosphonic acid and ZrOCl2. After an initial step, shown in Figure 6.7, the surface is exposed alternately to aqueous solutions of these two reagents. The surfaces were washed between reactions. Figure 6.8 shows the film thickness as measured by ellipsometry as a function of the number of layers. It is evident that the process is successful, but it would not be suitable for the formation of non-centrosymmetric layers (but see [395] below). One obvious problem is... 

Fig. 46 Examples of polyphilic rod-like compounds and possible modes of self-assembly of compound 174 (a) segregated non-centrosymmetric layer which would lead to longitudinal ferroelectricity if adjacent layers would organize with the same direction and (b) non-segregated centrosymmetric layer [297]...

Zvyagin BB, Rabotnov VT, Sidorenko OV, Kotel nikov, DD (1985) Unique mica built of non-centrosymmetrical layers. Izvestia Akad Nauk SSSR (ser Geol) 5 121-124 (inRussian)... 

Figure 1. Pyrophyllite, reference model of dioctahedral smectites centrosymmetrical layer, (a) Projection on the biperiodic plane of the lattice (ab). (b) Projection on a plane perpendicular to the b axis. Only those atoms situated between the planes x and x of the projection (a) are represented. Arrows A indicate the centers of the hexagonal cavities of the surface of the layer. Arrows M and B indicate eventual localization of negative charges created by isomorphous replacements. Af—octahedral charges (montmorillonite) tetrahedral charges (beidellite).

Calculations (Merino and Oberlin [1967]) show that the first type of diagram is characteristic of the noncentrosymmetric layer (Figure 2, symmetry group cl ml plane), whereas the second (extinction of spots 11, iT) is characteristic of the centrosymmetric layer (Figure 1, symmetry group c2, mm plane). Thus, the structure of nontronite is centrosymmetric, whereas that of montmorillonite is not. 

The nonmesogenic compound CB2 is described here, because it shows a reversible distortive solid-solid phase transition at 290.8 K (transition enthalpy 0.9 kj/mol) from the centrosymmetric low temperature phase I to the noncentrosymmetric high temperature phase II. The crystal structures of both solid phases I and II are very similar [45] as demonstrated in Fig. 2. The molecules are arranged in layers. The distances between the cyano groups of adjacent molecules are 3.50 A Ncyano-Ncyano and 3.35 A Ncyano-C ano for phase I and 3.55 A Ncyano-Ncyano and 3.43 A Ncyano-Ccyano for phase II. In the two... 

In contrast to sulfurous acid, H2S03, which is not known as pure substance, the respective selenium compound can be obtained in crystalline form. H2Se03 crystallizes with the non-centrosymmetric space group P2 2,2] and contains pyramidal H2Se03 molecules.54 Within the molecules the bond lengths Se-O and SeOH differ by about 10 pm and the molecules are connected to puckered layers by strong hydrogen bonds. 

ABC triblock copolymers have recently proven to be useful in constructing the so-called three-layer, onion, or core-shell-corona micelles, as described in Sect. 7.2. These micelles are characterized by a centrosymmetric structure and a micellar core with two different concentric compartments. Noncentrosymmetric structures from ABC triblock copolymers blended with AC diblocks have, however, been reported in bulk by Goldacker et al. [290]. 

This approach may also be applied to racemic bilayers built up from homo-chiral Langmuir-Blodgett monolayers. By measuring the two-dimensional diffraction pattern from such a bilayer it is possible to deduce the molecular chirality of each of the two monolayers in the order they were inserted to construct the bilayer. This approach can be extended to multilayers. Thus, in principle, we close the circle started in Section IV-G-1. It is possible to assign the absolute configuration of chiral molecules in centrosymmetric crystals provided that one can construct the crystal (in this case the multilayer) by adding homochiral layers one by one. 

Stable Z-type LB films have been prepared, however, by the alternative deposition of two different monolayers (A and B) in a head-to-head ABAB arrangement [142]. Two different monolayers can be layered a number of different ways (AABBAA, AABAAB, ABBABB, etc.) and there are a myriad of ways to arrange three or more different monolayers in LB films. These types of LB films have no plane of symmetry (i.e. they are non-centrosymmetric) and manifest non-linear optical behavior [108,143]. Schematics of some of the different types of LB films are illustrated in Fig. 11. 

A number of special optical techniques such as the study of second harmonic generation by irradiation of non-centrosymmetric systems by high intensity laser light will be discussed in relation to particular materials and problems. However, one optical technique having a general applicability, namely ellipsometry, must be discussed here. It is one of the best techniques available to determine the thickness of a thin organic film. Such determinations are important as they allow one to have an independent check on the number of layers deposited, given that the thickness of one layer has been determined by X-ray diffraction. 

Attempts to produce pyroelectric devices have been less numerous than those aimed at producing second harmonic generators. Clearly a pyroelectric device must be non-centrosymmetric in character and must thus be formed either from an alternate layer structure or from a Z layer structure. 

Figure 5.17. The two molecules discussed in [299]. These molecules can be deposited by the LB technique in alternate layers and form a non-centrosymmetric structure which can be used to generate a second harmonic in the visible spectrum. It is believed that the single hydrocarbon chain of one material interdigitates with the double chain of the other material, thus forming a stable structure.

Figure 3.1 shows a simplified picture of an interface. It consists of a multilayer geometry where the surface layer of thickness d lies between two centrosymmetric media (1 and 2) which have two different linear dielectric constants e, and e2, respectively. When a monochromatic plane wave at frequency co is incident from medium 1, it induces a nonlinear source polarization in the surface layer and in the bulk of medium 2. This source polarization then radiates, and harmonic waves at 2 to emanate from the boundary in both the reflected and transmitted directions. In this model, medium 1 is assumed to be linear. 

In this model, the structural symmetry of the boundary region is reflected in the form and magnitude of the tensor elements of the surface nonlinear susceptibility, xf and the bulk anisotropic susceptibility, . For 1.06/tm excitation, the penetration depth of E(co) is about 100 A. The surface electric dipole contribution is thought to arise from the first 10 A. The electric quadrupole allowed contribution to E(2to) from the decaying incident field is attenuated by e-2 relative to the surface dipole contribution. Consequently, the symmetry of the SH response should reflect the symmetry of at least the first two topmost layers. For a perfectly terminated (111) surface, the observed symmetry should be reduced from the 6 m symmetry of the topmost layer to 3 m symmetry as additional layers are included. This is consistent with the observations for the centrosymmetric Si(lll) surface response shown in Fig. 3.2 [67, 68]. 

The 3P (ccp) structure is centrosymmetric, and there are three equivalent packing directions along the cubic diagonals. The six neighbors of an atom in any close-packed layer for 2P (hep) or 3P (ccp) are at the corners of a centrosymmetric hexagon. For hep, the six neighbors in... 

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