Voids, thin films

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-film composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has little effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich solid phase that forms the membrane and a polymer-poor liquid phase that forms the membrane pores or void spaces. 

It is a characteristic of the ion-by-ion mechanism for CdSe deposition from NTA solutions that thin films are highly scattering, and this scattering decreases as film thickness increases. It is likely that this scattering is due to voids in the thin films, which are a result of the low density of initial nuclei. 

In common with CdSe deposition from selenosulphate baths, cluster deposition of PbSe normally resulted in specular films, while ion-by-ion films were initially highly scattering as thin films but eventually (usually) became specularly reflecting with increase in thickness. As for CdSe, the development of specularity with thickness of ion-by-ion films could be explained by filling in of voids between the large crystals. 

Thin film technology is becoming one of the important technologies today. While there are infinite varieties of thin film fabrication methods, most amorphous thin films seem to exhibit fractal-like atomic structures. Depending on the fabrication conditions, a thin film grows on the substrate into columnar structures with many voids interdispersed in the thin film.80 These structures can be seen in the field ion microscope, and compositional variation can be analyzed with the atom-probe. In addition, formation of atomic clusters inside the thin film can be substantiated with the observation of a large fraction of cluster ions in field evaporation by the atom-probe. 

Quantitative simulation of spectra as outlined above is complicated for particle films. The material within the volume probed by the evanescent field is heterogeneous, composed of solvent entrapped in the void space, support material, and active catalyst, for example a metal. If the particles involved are considerably smaller than the penetration depth of the IR radiation, the radiation probes an effective medium. Still, in such a situation the formalism outlined above can be applied. The challenge is associated with the determination of the effective optical constants of the composite layer. Effective medium theories have been developed, such as Maxwell-Garnett 61, Bruggeman 62, and other effective medium theories 63, which predict the optical constants of a composite layer. Such theories were applied to metal-particle thin films on IREs to predict enhanced IR absorption within such films. The results were in qualitative agreement with experiment 30. However, quantitative results of these theories depend not only on the bulk optical constants of the materials (which in most cases are known precisely), but also critically on the size and shape (aspect ratio) of the metal particles and the distance between them. Accurate information of this kind is seldom available for powder catalysts. 

Sometimes interdiffusion between two metals is uneven and as such may lead to a situation of creating vacancies or voids. This type of imbalance is the result of possible unequal mobilities between a metal couple. These voids occur individually near the common interface. The voids, like bubbles, coalesce, resulting in porosity and loss of strength. Many thin-film couples exhibit this phenomenon, which is referred to as Kirkendall void creation. Al—Au, Cu—Pt, Cu-Au are just a few examples. To be specific, it has been found (7), for instance, that in the case of Au-Ni about five times more Ni atoms diffuse into Au than Au atoms diffuse into Ni. 

With large-surface-area membrane filter media, the interpretation of the true bubble point can be further complicated because of the diffusion of the test gas through the media. Because the filter media are more than 70% void space, a liquid-wetted membrane is virtually a thin film of liquid across which a test gas will diffuse, governed by Fick s law. 

Fig. 11 Craze in commercial polystyrene showing the characteristic steps nucleation through void formation in a pre-craze zone, growth of the fibrillar structure of the widening craze by drawing-in of new matrix material in the process zone, and final breakdown of the fibrillar matter transforming a craze into a crack (the crack front is more advanced in the center of the specimen, shielded by a curtain of unbroken fibrils marked by the arrow). The fibril thickness depends—of course—on the molecular variables, the strain rate-stress-temperature regime of the crazing sample and on its treatment (preparation, annealing) and geometry (solid, thin film) for PS typical values of between 2.5 and 30 nm are found [1,60,61]...

Polysaccharides may exercise a protective action in an emulsion and foam as a thin film at liquid-liquid (emulsion) and liquid-air (foam) interfaces. The hydrophile-lipophile balance in the macromolecules as well as <(>, determines whether or not the emulsion is an oil-in-water or water-in-oil dispersion (Void and Void, 1983 Dickinson, 1992). 

NLO effects in the thin films. Hyperbranched polymers should be ideal matrix materials as they offer three-dimensional spatial separation of the NLO chromophores in the spherical architecture, and their void-rich topological structure should help minimize optical loss in the NLO process. 

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