Self-Assembly of Thin Films

Surfactant templating that was initially used for the preparation of mesostructured bulk materials was soon extended to thin films. Indeed, thin films have potential applications as membranes, catalysts, and sensors or as components of 

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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...

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. 

Figure 2.26 (a) Scheme for an electrostatically self-assembled multilayer (PAHOs)4(Apo-COx)3(COx) electrode, (b) Catalytic current response to p-d-glucose concentration for self-assembled nanostructured thin films of PAH-Os/COx/(PAH-Os/(ApoGOx)3 and (PAH-Os/ApoCOx)3/PAH-Os/COx, where ApoCOx is FAD-free glucose oxidase. Taken from Ref [219]. 

Figure 12.5 (a) Layer-by-layer deposition of glucose oxidase and the polyallylamine Os3 +n + -polypyridine polyelectrolyte on the electrode, (b) Typical catalytic current responses for different glucose concentrations obtained by self-assembled nanostructured thin films based on different architectures (i) PAH/Os/GOx, (ii) cysteamine/GOx/PAH-Os, (iii) PAH/GOx/ -Os, and (iv) (PAH-Os)2/(GOx)i. All measurements were performed in 0.1 M tris buffer at pH 7.5. Part (b) Reproduced with permission from Ref. 34a. Copyright Wiley-VCH Verlag GmbH Co. KGaA. 

Huang, N., Xiao, Z., Huang, D. and Yuan,C. (1998) Photochemical disinfection of Escherichia coli with a Ti02 colloid solution and a self-assembled Ti02 thin film. Supramol. Sci. 5, 559-564. 

Figure 29 Self-assembled polyelectrolyte thin film via the technique [341], With permission of Elsevier Science...

Figure 9.19 Simultaneous 2/imx 2 pm image of film topography (left) and surface potential (right) of a seven-bilayer, 30 nm MFeCIs, Ppy/SPS bilayer films. The Z-scale ranges are 50 nm for the topography and 50 mV for the surface potential. (Reprinted with permission from Sensors and Actuators A Physical, Layer-by-layer self-assembled conductive thin films for MEMS applications byB. Bush, G. Xu, C. Carraroand R. Maboudian, 126, 1, 194—200. Copyright (2006) Elsevier Ltd)...

Investigations on the self-assembly in thin films of an amorphous P2VP-PS-P2VP triblock copolymer, forming cylindrical microdomains in bulk, showed that the orientation of microdomains due to the P2VP/substrate interactions persisted in the entire film in contrast to the diblock case [14], This was viewed as a result of the formation of an interconnected structure in the triblock coming from the formation of loops within the microdomains. More recently, AFM and SAKS measurements on a PEO-PBO-PEO amorphous-semicrystalline triblock thin film revealed the presence of a semicrystaUine PEO monolayer at the substrate, comprised of unfolded chains, and PBO blocks at the air/polymer siuface in a looped conformation (15]. 

Li etal. looked at the SHG properties of self-assembled monolayer thin films of tetra-para-pyridylporphyrin [H2(TPyP)] and some derivatives on fused quartz and silicon surfaces. By covalently bonding the TPyP molecules to an alkyl- or phenyl-siloxy group, the centrosymmetric... 

Similarly, Wang and coworkers [271] reported that hyperbranched dendritic structures possessing poly(3-alkylthiophene) arms self-assemble into thin films with morphological features, as well as electrical and optical properties that reveal a surprising degree of structural order. Typical conductivities varied between 42 and 65 sec/cm. 

LefSvre N et al. Self-assembly in thin films of mixtures of block copolymers and homopolymers interacting by hydrogen bonds. Macromolecules 2010 43(18) 7734—7743. 

Block copolymers are polymers that consist of two or more polymer chains attached at their ends. They can self-assemble into thin films having segregated blocks with domain sizes from 10 to 200 nm. This domain size and shape can be controlled by the molecular weight, segment size, and block interactions. By selective etching of one of the blocks, an ordered thin film that can be used as a template is produced with the ranaining block. This type of template synthesis is a fairly new and emerging method with a variety of copolymers as well as applications. ... 

Hodak, J., et al.. Layer-by-layer self-assembly of glucose oxidase with a poly(aUylamine)ferrocene redox mediator. Langmuir, 1997.13(10) p. 2708-2716. Yoo, D., et al.. New electro-active self-assembled multilayer thin films based on alternately adsorbed layers of polyelectrolytes and functional dye molecules. Synthetic Metals, 1997. 85(1-3) p. 1425-1426. 

Liu, Y.J. and Claus, R.O., Strong enhancement of optical absorbance from ionic self-assembled multilayer thin films of nanocluster Pt and polymer dye. J Appl Phys, 1999. 85(1) p. 419-124. 

Many templating applications require patterning of block copolymer thin films. Block copolymer self-assembly in thin films varies significantly from the bulk due to interfacial interactions at the polymer-substrate and polymer-air interfaces. The important points are summarized below for a more in-depth review of these materials, see Reference 21. In this section, we focus on film thicknesses (h) equal to or larger than the bulk spacing do-... 

The Bag of Tricks Controlling Self-Assembly in Thin Films... 

It will be clear that L-B films are intrinsically linked to self-assembly of molecules, and this has been recognised in the title of a recent overview book (Ulman 1991), An Introduction to Ultra thin Organic Films from Langnmir-Blodgett to Self-Assembly An Overview. 

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