Thin film multilayer systems technique

From optical point of view, an OLED structure can be considered as a multilayer thin-film system composed of absorbing and nonabsorbing materials, as shown in Figure 6.27. Therefore, the optical properties and optimal structure of such a multilayer device can be investigated by applying thin-film optical analysis techniques. Based on the theory of optical admittance analysis for analyzing the optical properties of a thin-film system [92], the optical properties of an OLED thin-film system can be simulated to reduce the ambient reflection. 

Fabrication of organic thin films based on sponfaneous molecular assembly has been considered as one of fhe powerful approaches to create novel supramolecular systems. In this context, multilayer films were fabricated by layer-by-layer electrostatic deposition techniques based on the electrostatic interaction between dsDNA and the positively charged polymer poly(diallyldimethylammonium chloride) (PDDA) on GC surfaces. A uniform assembly of PDDA/DNA multilayer films was achieved, based on the adsorption of the negatively charged DNA molecules on the positively charged substrate [55]. 

Polvimide-Metal Interfaces. Several technological applications including semiconductor packaging and metallization demand a reliable and durable adhesion properties of the metal films. In the development of multilayer devices consist of alternating layers of metal and polyimides several reliable techniques are needed to study both thin films and their interfaces. The usefulness of the nuclear scattering techniques to study the metallization and the associated interfacial elemental diffusion processes under the effects of various temperature and humidity treatments on the metal-polyimide systems, such as Al, Cu, N, and Au on Du Pont Kapton type H have already been reported (21., 22.). Only a couple of examples are presented here to illustrate the ERD application. 

The same methods to introduce grain boundaries, i.e. bi-crystal substrates, step-edges and bi-epitaxial techniques, also apply to the other superconductors [14.52, 14.107-14.109], The details of the interfacial interactions between the substrates and the thin films need to be investigated in order to find the optimum conditions and geometries for each individual system. The chemical interactions must also be considered when integrating the superconductors into multilayer structures. Intermediate non-superconducting layers that are inert to the YBCO may react chemically with the other superconductors. It is also necessary to carefully consider the ex situ processes of the Tl- and Hg-based superconductors producing multilayer structures. 

Exfoliation of the perovskite related layer structures is more difficult than for the clays and acid phosphates discussed earlier but can be achieved by intercalation of large bulky amines. Treacy etal. reported that the layered perovskite HCa2Nb30io could be made to form unilamellar sheets by first intercalation of an amine polyether. Spontaneous exfoliation of the layers occurs on subsequent exposure of the intercalated phase to a suitable solvent. Exfoliation techniques have been extended to other systems using tetra(n-butyl)-ammonium hydroxide (TBAOH) by Mallouk and others. A number of examples of the protonated layered perovskite phases that intercalate bases have been exfoliated. The Dion-Jacobson phases typically exfoliate to form plates but others including Ruddlesden-Popper tantalates curl to form tubular scrolls . Part of the interest in these single layer dispersions arises from their use as building blocks in the layer-by-layer self-assembly of thin films. Single layers derived from exfoliated perovskites can be attached to or alternately stacked with polycationic layers to produce thin films. Tiled monolayer structures and multilayer perovskite heterostructures result from the self-assembly. 

In Physical vapour deposition (PVD) method, the solid material to be deposited is evaporated in a vacuum system through physical techniques, followed by condensation and deposition as a thin film on a cooler substrate. PVD is a very versatile method for manufacturing of pure metal films, alloys or compounds of thickness up to 50 pm [67]. At relatively high temperature, a thermal treatment is generally necessary to homogenize the composition of a multilayer deposit [68]. 

Total reflection (or reflectance) XRF (TXRF) is a technique that looks at thin film samples or thin-layer samples with the ability to detect elements from Na to U. A schematic TXRF spectrometer and a commercial system can be seen in Figure 8.17a and b. The Bruker S2 PICOFOX consists of an air-cooled Mo anode metal-ceramic X-ray tube, a multilayer monochromator, and an XFlash SDD and can be equipped with a 25 position autosampler. 

As already mentioned, a wide variety of photochemical and photophysical properties of clay-dye systems have been reported. Motivated by the progress in controlling photoprocesses by organizing species in the interlayer space of layCTed materials has led researchers to fabricate intercalation compounds as thin films. For such purposes, films with precisely controlled thickness and the multilayered structures with alternate heteroaggregates are required. Accordingly, the layer-by-layer deposition technique and the LB method have been conducted. 

Formation of ultrathin non-conducting polymer films has also been reported by Hammond et al. using ionic attraction on the surface of the carboxylate terminated SAM allowing layer-by-layer deposition of ionic multilayers [33,34,139-141]. These systems have been combined with patterning techniques and have been examined with respect to their optically responsive properties. Viologen-based polyelectrolytes have also been shown to adsorb onto a carboxylate SAM surface, forming an electroactive thin film [146]. 

The discussion of stress concentration near a film edge in the next section is followed by a brief review of linear elastic fracture mechanics concepts, a prelude to a discussion of delamination and cracking due to film residual stress. A survey of these topics set in the context of fracture mechanics has been presented by Hutchinson and Suo (1992). The chapter also includes descriptions of various experimental techniques for evaluating the fracture resistance of interfaces between films and substrates. In addition, representative experimental results on the interface fracture resistance, as a function of interface chemistry and environment, are presented for a variety of thin film and multilayer systems of scientific and technological interest. 

The focus in the thin film research impact area is to develop a fundamental understanding of how morphology can be controlled in (1) organic thin film composites prepared by Langmuir Blodgett (LB) monolayer and multilayer techniques and (2) the molecular design of membrane systems using ionomers and selected supported liquids. Controlled structures of this nature will find immediate apphcation in several aspects of smart materials development, particularly in microsensors. 

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