1 Introduction - Wiley-VCH · 2012-09-26 · would be (i) self - assembled monolayer ( SAM ) method, (ii) Langmuir – Blodgett ( LB ) technique, and (iii) layer - by - layer ( LbL

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Introduction Katsuhiko Ariga

Developments of nanotechnology and microtechnology have been tremendous and are having huge social impact. Based on these technologies, various tools and machines are signifi cantly miniaturized, leading to compact and effi cient informa-tion processing and communication, as seen in mobile computers and cellular phones. Handy and wearable devices have been developed that enhance com-munication and reduce traffi c congestion and overpopulation in certain areas, which may produce reductions in power consumption and environmentally unfriendly emissions. In order to obtain ultrasmall functional systems, advanced nanotechnology - based fabrication for highly precise small structures plays a central role. Most of them are called top - down nanofabrication methods. For example, photolithographic techniques have been widely used for miniaturization of structures especially in silicon - based technology. Unfortunately, these top - down lithographic approaches require a combination of instrumentation, clean - room environment, and materials that are accompanied by rapid cost increases. Indus-tries moving along the current direction may encounter unavoidable limitations due to economical reasons and/or technical reasons.

Therefore, alternate methodology, bottom - up approaches, will become indispen-sable (Figure 1.1 ). In the bottom - up approaches, the principles of self - assembly are central to construct nanostructures through spontaneous processes. Self - assembled processes are sometimes capable of forming highly integrated and complicated three - dimensional structures in an energyless one - step process. However, such assemblies are not often predictable and designable. Therefore, nanostructure for-mation in three - dimensional ways remains as fundamental sciences rather than well - established methodologies. If the dimensions of objects are reduced from three to two, the situation drastically changes. We already have an established strategy to make well - organized two - dimensional fi lms (ultrathin fi lms) through molecular self - assembly with the aid of external processes such as substrate dipping and solution casting. Three representative methodologies for thin - fi lm preparation would be (i) self - assembled monolayer ( SAM ) method, (ii) Langmuir – Blodgett ( LB ) technique, and (iii) layer - by - layer ( LbL ) assembly. In particular, these methods are good ways to provide organic ultrathin fi lms. Therefore, studies on organic ultrathin fi lms would be good starting points for bottom - up nanotechnology.

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Organized Organic Ultrathin Films: Fundamentals and Applications, First Edition. Edited by Katsuhiko Ariga.© 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.

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Figure 1.1 Top - down approach and bottom - up approach for fabrication of micro/nanostructures.

In this book, we describe the fundamentals and applications of organic ultrathin fi lms upon classifi cations of fabrication strategies. Here, their outlines are sum-marized. Chapter 2 explains the self - assembled monolayer (SAM) method (Figure 1.2 ). The SAM method provides a monolayer strongly immobilized on a solid support. This method utilizes the strong interaction between the heads of the amphiphiles and the surface of the solid support, as seen in covalent linkages between silanol amphiphiles and a glass or metal oxide surface and strong interac-tions between thiol amphiphiles and a gold surface. These strong interactions with the solid surface sometimes allow molecules very different from those of typical amphiphiles to form monolayer structure on the surface. The formed SAM struc-ture have great potential for a wide range of applications including sensors and various devices The formation of self - assembled monolayers is a powerful tool for surface modifi cation.

In Chapter 3 , the Langmuir – Blodgett (LB) technique is introduced (Figure 1.3 ). The LB technique is the most powerful method of achieving molecular assemblies with precisely layered structures. In this method, an insoluble monolayer of amphiphile molecules is fi rst spread on the surface of a water phase. The monol-ayer can be highly compressed through lateral pressure application. The fi nally obtained highly condensed monolayer is transferred onto a solid support in a

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layer - by - layer manner by dipping the support through the monolayer. Film thick-ness (the number of the layers) is easily tuned in nanometer level just by control-ling dipping cycles. The monolayer - forming amphiphile must have an appropriate hydrophilic – hydrophobic balance. The profi le of monolayer compression can be interesting research subject of molecular assembly in two dimensions.

The LB method requires rather expensive apparatus, and water - soluble mole-cules are not usually appropriate targets. As compensation for these disadvanta-geous features, another type of technique for layered ultrathin fi lms was developed.

Figure 1.2 Self - assembled monolayer (SAM).

Figure 1.3 Langmuir – Blodgett (LB) fi lm.

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The so - called layer - by - layer (LbL) assembly is explained in Chapter 4 (Figure 1.4 ). Unlike the LB method, the LbL assembly can also be applicable to a wide range of water - soluble substances. In addition, this assembly method can be conducted using a very simple procedure with nonexpensive apparatuses such as beakers and tweezers. A typical LbL procedure is based on electrostatic adsorption. In the case of a solid support negative surface charge, adsorption of thin layer of cationic poelectrolyte neutralized surface change and subsequent overadsorption reverts surface charges. The subsequent process changes the surface charges alternately between positive and negative. Therefore, layered assembly can be continuously conducted to provide ultrathin fi lms with desired thickness and layer sequence.

Chapter 5 describes the other types of organic ultrathin fi lms and hybrid thin fi lms some of which form assembling structures in solution (not on a solid surface) For example, formation of lipid bilayer structures in aqueous solution (Figure 1.5 ) and their transformation to thin fi lms on a solid support by casting are exemplifi ed. Lipids and related amphiphiles possess hydrophobic tails and a hydrophilic head. When they are dispersed in aqueous media, these molecules are usually assembled into bilayer fi lms by avoiding unfavorable contact between hydrophobic parts of the molecules with external water media. Such organization often results in spherical assemblies having a water pool inside and lipid bilayer shell. These are called liposomes and/or vesicles. The formation mechanisms of these objects are basically identical to those for cell membranes. Casting of these dispersions onto a solid substrate leads to thin - fi lm formation with multiple lipid layers. Upon appropriate designs of amphiphiles, their assemblies can extend to more complicated morphologies such as ribbons, sheets, and tubes.

In this book, various organic ultrathin fi lms are described according to these categories, self - assembled monolayer (SAM), Langmuir – Blodgett (LB) fi lms, layer - by - layer (LbL) assembly, and the other thin fi lms such as lipid bilayers. Although

Figure 1.4 Layer - by - layer (LbL) assembly.

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the main aim of this book is to give an introduction to organic ultrathin fi lms, some of the examples (especially in LbL assembly) are thin fi lms of inorganic components. This means that a strategy useful for organic components can be applicable for inorganic nano - objects and their hybrids with organic components. We partially include this inorganic feature in a book entitled “ Organic Ultrathin Films ” , because we want to demonstrate the wide versatility of the described methods and the availability of this typical bottom - up nanotechnology for all kinds of materials.

Figure 1.5 Formation of liposome (or vesicle).