The formation of Langmuir-Blodgett films

It will be obvious that, when the substrate is much smaller than the trough and the surface viscosity is appreciable, there will exist a complicated flow pattern round the substrate which may cause difficulties. Various ingenious devices designed to overcome this problem have been 

Most semiconductor surfaces are reactive and form oxide surfaces in air or water and thus suffer from the same faults as metals in this respect. This is not true of the layer lattice materials which form valence bonds only in the layer plane. Freshly cleaved surfaces of pyrolytic graphite or molybdenum disulphide are thus promising materials, though they are only available in small areas. In principle it might be possible to form LB layers on these materials by epitaxy but very little progress has so far been made in this direction. 

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Figure 3.3. Forms of isotherm which often appear, (a) This is a typical isotherm observed with an impure long-chain carboxylic acid. (b) Isotherms such as this usually correspond to substantial changes of the conformation of the material as the pressure,, is increased. Such materials usually do not lend themselves to the formation of Langmuir-Blodgett films, (c) This form of isotherm occurs when the material under study tends to form islands before compression.

Figure 28 Sketch of the formation of Langmuir Blodgett films using rod-like polymers.

The formation of Langmuir-Blodgett films by mixing 3T or 4T with arachidic acid in the molar ratio 1 2 failed, in contrast to longer oligothiophenes (see next sections) [27]. 

As reported for a5T (see Section 3.3), the formation of Langmuir-Blodgett films by mixing a6T with arachidic acid was possible [27]. 

A separate section is devoted to phthalocyanines and porphyrins as discotic liquid crystals. The nonlinear optical effects as well as photoconductivity and the possibility of the formation of Langmuir-Blodgett-films of phthalocyanines and derivatives is discussed at some length. Several practical applications of these unconventional materials are mentioned. 

Besides self-assembly on the surface of metals, another example of self-assembly is the formation of Langmuir-Blodgett monolayers on the surface or at the interface of liquids.47,48 One approach to molecular-based electronic devices that has made use of self-assembled Langmuir-Blodgett films is the crossbar-based defect-tolerant approach to molecular computing 49 Rotaxane molecules that depend on physical dislocations within the molecule to produce switching functionality have been designed such that they will... 

One rotaxane molecule is shown in Figure 5.9. This molecule is not expected to demonstrate bistable switching, but is a candidate for a molecular switch that can be configured one time. Note that both ends of the molecule are non-polar hydrocarbon moieties while the center of the molecule is polar. Molecules that have both a non-polar and a polar end, for easier formation of Langmuir-Blodgett films, have been synthesized and tested in devices.51... 

Other monolayer-forming techniques such as the use of Langmuir-Blodgett films to immobilise antibodies have been reported [40]. The method of formation of the layers is very simplistic but their practical apphcations are limited. 

We describe here the design and synthesis of functionalized ligands and their complexation into functionalized grid-like coordination arrays, as well as some first results on the formation of Langmuir-Blodgett (LB) films. 

Liquori et al. [23] first discovered that isotactic and syndiotactic PMMA chains form a crystalline stereocomplex. A number of authors have since studied this phenomenon [24]. Buter et al. [25,26] reported the formation of an in situ complex during stereospecific replica polymerization of methyl methacrylate in the presence of preformed isotactic or syndiotactic PMMA. Hatada et al. [24] reported a detailed study of the complex formation, using highly stereoregular PMMA polymers with narrow molecular weight distribution. The effect of tacticity on the characteristics of Langmuir-Blodgett films of PMMA and the stereocomplex between isotactic and syndiotactic PMMA in such monolayers at the air-water interface have been reported in a series of papers by Brinkhuis and Schouten [27,27a]. Similar to this system, Hatada et al. [28] reported stereocomplex formation in solution and in the bulk between isotactic polymers of / -(+)- and S-(—)-a-methylbenzyl methacrylates. 

Currently, these molecular systems are studies as SAMs on a metal surface. An additional method of preparing ordered monolayers of molecular devices is the use of Langmuir-Blodgett (LB) films. Therefore, a compound with hydrophilic and hydrophobic subunits with the central nitroaniline core similar to 70 was synthesized as in Scheme 3.40. n-Hexylbenzene was easily brominated on neutral alumina and coupled to TMSA followed by silyl removal and coupling to the nitroacetanilide core intermediate, 68, to afford 88. The methyl ester, intermediate 90, was synthesized by the coupling of methyl 4-ethynylbenzoate (89) to 88. The amine was unmasked and the methyl ester was saponified with lithium hydroxide to afford molecular scale device 91. Compound 91 is suitable for the formation of a LB film due to its hydrophilic carboxylic acid end-group and the hydrophobic n-hexyl end-group. 

Daniel M F, Lettington O C and Small M 1983 Investigation into the Langmuir-Blodgett film formation ability of amphiphiles with oyano head groups Thin Solid Films 99 61-9... 

The area of colloids, surfactants, and fluid interfaces is large in scope. It encompasses all fluid-fluid and fluid-solid systems in which interfacial properties play a dominant role in determining the behavior of the overall system. Such systems are often characterized by large surface-to-volume ratios (e.g., thin films, sols, and foams) and by the formation of macroscopic assembhes of molecules (e.g., colloids, micelles, vesicles, and Langmuir-Blodgett films). The peculiar properties of the interfaces in such media give rise to these otherwise unlikely (and often inherently unstable) structures. 

In addition to the preparation of Langmuir and Langmuir-Blodgett films, the use of self-assembly techniques also plays an important role in the formation of particle films. Both physisorption, as, for example, electrostatic adsorption of charged particles from colloidal solution, and chemisorption onto a substrate have been investigated. In Section V.A, electrostatic adsorption will be reviewed chemisorption is the subject of Section V.B. 

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