Multilayer structures dependence

The quantity and quality of the deposited monolayer on a solid support is measured by a so-called transfer ratio, tr. This is defined as the ratio between the decrease in monolayer area during a deposition stroke, Al, and the area of the substrate, As. For ideal transfer, the magnitude of tr is equal to 1. Depending on the behavior of the molecule, the solid substrate can be dipped through the film until the desired thickness of the film is achieved. Different kinds of LB multilayers can be produced and/or obtained by successive deposition of monolayers on the same substrate (see Figure 4.11). The most common one is the Y-type multilayer, which is produced when the monolayer deposits on the solid substrate in both up and down directions. When the monolayer deposits only in the up or down direction, the multilayer structure is called either Z-type or X-type. Intermediate structures are sometimes observed for some LB multilayers, and they are often referred to be XY-type multilayers. 

Tubes and blown Aims can be produced as multilayer structures by employing multiple extruders and coextrusion manifolds and dies. Figure 12.44 is a schematic representative of a conventional and new spiral coextrusion die. The designs can be used for both blown-film and blown-molding parison dies. In the extrusion of tubes, such as rigid PVC or PE pipe, the extrudate passes over a water-cooled mandrel and enters a cold-water bath whose length depends on the tube thickness the tube leaves the bath well below its Tm (if it is crystalline) or Tg (if it is amorphous) and is sectioned to the desired lengths. 

In the Fangmuir-Blodgett technique, amphiphilic monolayers, formed at a liquid-air interface, are transferred to a solid substrate by horizontal or vertical transfer. The thickness of such monolayers is of the order of a few nanometers, depending on the materials being used. With this method, multilayer structures can also be produced, either by repeated deposition of the same layer or by the deposition of alternate layers. In this manner, multilayers containing several hundred individual components can be obtained. In general, the thermal and mechanical stabilities of such layers are, however, limited. 

We conclude that the microscopic etch mechanism must be the same for single crystals and sputter deposited, polycrystalline ZnO Al. For the latter, the tendency for crater formation is masked by inhomogeneous chemical or physical properties like porosity, composition or, in case of dynamic deposition, multilayered ZnO Al films. This multilayer structure results from the fact that structural properties of ZnO Al deposited by a sputter process varies depending on the position of the film relative to the race track of the sputter target [131,132]. This dependence is important for the etch rate of in-line sputter deposited films [133]. 

In the former case, the ions migrate among the interstitial defects, which may be relevant only to small ions such as Li+. This leads to a transference number close to 1 for the cation migration. In the other case, the lattice contains both anionic and cationic holes, and the ions migrate from hole to hole [39], The dominant type of defects in a lattice depends, of course, on its chemical structure as well as its formation pattern [40-43], In any event, it is possible that both types of holes exist simultaneously and contribute to conductance. It should be emphasized that this description is relevant to single crystals. Surface films formed on active metals are much more complicated and may be of a mosaic and multilayer structure. Hence, ion transport along the grain boundaries between different phases in the surface films may also contribute to conductance in these systems. 

Fig. 9.26 shows data for multilayer structures of different layer spacing measured at a temperature of 100 K. Several features in the spectra are observed whose energy depends on the quantum well thickness and are identified as the subband thresholds. The energies of the subband transitions are shown in Fig. 9.26(b) as a function of the layer spacing. Three quantum levels are detected and the solid lines show the theoretical values based on Eq. (9.26). The effective masses used to fit the data are 0.3/ o for electrons and for holes. 

Finally, we will consider briefly the formation of multilayer thin films by layer-by-layer deposition of hydrogen-bonded polymer pairs [51,52]. In this way a multilayer structure is obtained from potentially miscible polymer pairs. The stability of these films very much depends on the presence of hydrogen bonds, and pH may be used as an external trigger to erase the layered structure [53,54] and selectively dissolve one of the components [25-27]. This procedure allows for the preparation of microporous films not unlike the nanoporous films obtained by dissolution of the hydrogen-bonded side groups from self-assembled block copolymer-based comb-shaped supramo-lecules [15,17,18]. 

The multilayered structure and electroluminescent mechanism of OLEDs is illustrated in Figure 4.45. Depending on whether small organic molecules or long repeating-unit polymers are used (Figure 4.46), the diodes are referred to as OLEDs or PLEDs, respectively. Under positive current, electrons and holes are injected into the emissive layer from opposite directions - from the cathode and anode, respectively. The metal... 

Apart from the formation of ultrathin surface-attached PEL-PEL complexes it is very interesting whether the PEL brushes can be also used for the formation of PEL multilayer assemblies. The so-called layer-by-layer (LBL) technique is a simple and powerful method to form well-defined multilayered structures [80]. For the formation of such multilayer assemblies the brushes are dipped alternately into polyelectrolyte solutions, one consisting of a positively charged polyelectrolyte, the other of a negatively charged polyelectrolyte. It is usually assumed that in this LBL deposition process the driving force for each monolayer formation is charge overcompensation [81, 82]. The stability of the multilayered system formed by LBL process in different environments is one of the limitations of this process. Since the attachment of the first layer depends solely on the interaction of... 

Abstract Two types of membrane are presented free-standing films which are formed from aqueous polyelectrolyte solutions and membranes prepared by alternating electrostatic layer-by-layer assembly of cationic and anionic polyelectrolytes on porous supports. Layer-by-layer assemblies represent versatile membranes suitable for dehydration of organic solvents and ion separation in aqueous solution. The results show that the structuring of the polyelectrolytes in the liquid films and the permeability of the multilayer membranes depends on different internal and environmental parameters, for example molecular weight, polymer charge density, ionic strength, and temperature. 

Murray, C.A., andAllara, D.L. (1982) Measurement of the molecule-silver separation dependence of surface enhanced Raman scattering in multilayered structures. Journal of Chemical Physics, 76, 1290-1303. 

As already mentioned, the reflection modulation technique is relatively simple but for thicker or absorbing thin films it becomes much more complicated due to the multiple reflection effects in the used multilayer structure (Fig. 1) which may lead to erroneous results, if not correctly taken into account [10,20]. In that case, the measurement of the incidence angle dependence of the modulation intensity is required. Through a correct analysis of experimental data one can get both real and imaginary parts of r, as well as its anisotropy and ri3 tensor components). 

The stracture formed by donor acceptor BHJ blends is dependent on the solvent used. Studies of polyfluorene PCBM blends show that choice of solvent controls the segregation of materials within the blend film so that a vertical multilayer structure forms spontaneously in some cases (Bjorstrom and Moons, 2007). Similarly, the choice of solvent in a polymer-polymer blend controls the vertical arrangement of materials and the device performance (Arias et al, 2002). Exploitation of these effects may open new avenues for materials-processing procedures in the organic photovoltaic field. 

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