Thin films orientation effects

In very thin films, new effects may take place, because the finite thickness is responsible for further modifications of the Madelung potential. This shows up, for example, when one considers unsupported MgO films, with thicknesses n ranging from 1 to 6 atomic planes and several orientations ((100), (110) and (211)). As a first gross approximation, the atoms may be assumed to remain at their bulk positions. The application of the self-consistent tight-binding method yields the gap width A and the ionic charges Qs borne by the surface atoms, as a function of their coordination number Zg. The results are as follows ... 

This characteristic of RAIR can be observed experimentally. Fig. 8 shows the transmission spectrum of polydimethylsiloxane (PDMS) while Fig. 9 shows the RAIR spectrum of a thin film of PDMS spin-coated onto a chromium substrate. It can be observed that the bands near 1024 and 1095 cm have similar intensities in the transmission spectra but the band at higher frequencies is clearly much more intense in the RAIR spectrum. This change in relative intensity when PDMS is deposited onto a reflecting substrate is related to optical effects and is not related to orientation effects. 

The particular colors that are observed at different angles will depend critically on the thickness of the thin film coating. Precision instrumentation is required to carefully control film thickness during production. The magnitude of the optical effect depends on the density of flakes in the ink, while the quality of the optical effect depends on the precise orientation or alignment of these flakes with respect to the paper surface. 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

Processes which employ combinations of these strategies have proved to be much more effective at yielding uniform long-range ordered patterns than single strategies. Table 1 describes the methods used to control the microdomain orientation of a variety of thin film block copolymers [41,42,66-108],... 

A number of researchers have used surface energy libraries to examine the self-assembly of block copolymer species in thin films. It is well known that substrate-block interactions can govern the orientation, wetting symmetry and even the pattern motif of self-assembled domains in block copolymer films [29]. A simple illustration of these effects in diblock copolymer films is shown schematically in Fig. 6. However, for most block copolymer systems the exact surface energy conditions needed to control these effects are unknown, and for many applications of self-assembly (e.g., nanolithography) such control is essential. 

Fig. 6 Illustration of surface energy effects on the self-assembly of thin films of volume symmetric diblock copolymer (a). Sections b and c show surface-parallel block domains orientation that occur when one block preferentially wets the substrate. Symmetric wetting (b) occurs when the substrate and free surface favor interactions with one block B, which is more hydrophobic. Asymmetric wetting (c) occurs when blocks A and B are favored by the substrate and free surface, respectively. For some systems, a neutral substrate surface energy, which favors neither block, results in a self-assembled domains oriented perpendicular to the film plane (d). Lo is the equilibrium length-scale of pattern formation in the diblock system...

Fig. 7 2D thickness-surface energy gradient library for mapping the effects of these parameters on the self-assembly of PS-b-PMMA block copolymer thin films. See text for a fuU description. Lq is the equilibrium self-assembly period and h is the film thickness. Dashed white lines delineate the neutral surface energy region, which exhibits nanostructures oriented perpendicular to the substrate plane. (Derived from [18] with permission)... 

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