Organized Organic Ultrathin Films E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 326

Veröffentlichungsjahr: 2012

Beschreibung

This handy reference is the first comprehensive book covering both fundamentals and recent developments in the field with an emphasis on nanotechnology.

Written by a highly regarded author in the field, the book details state-of-the-art preparation, characterization and applications of thin films of organic molecules and biomaterials fabricated by wet processes and also highlights applications in nanotechnology

The categories of films covered include monomolecular films (monolayers) both on a water surface and on a solid plate, Langmuir-Blodgett films (transferred multilayer films on a solid plate from a water surface), layer-by-layer films (adsorbed multilayer films on a solid support), and spontaneously assembled films in solution.

Leseprobe

Table of Contents

Cover

Related Titles

Title page

Copyright page

Preface

List of Contributors

1 Introduction

2 Self-Assembled Monolayer (SAM)

2.1 Introduction

2.2 Preparation and Characterization

2.3 Functions and Applications

2.4 Future Perspective

3 Langmuir–Blodgett (LB) Film

3.1 Concept and Mechanism

3.2 Preparation and Characterization

3.3 Functions and Applications

4 Layer-by-Layer (LbL) Assembly

4.1 Concept and Mechanism

4.2 Preparation and Characterization

4.3 Functions and Applications

4.4 Brief Summary and Perspectives

5 Other Thin Films

5.1 Bilayer Vesicle and Cast Film

5.2 Self-Assembled Fibers, Tubes, and Ribbons

5.3 Polymer Brush Layer

5.4 Organic–Inorganic Hybrids

5.5 Colloidal Layers

5.6 Newly Appearing Techniques

Index

Related Titles

Decher, G., Schlenoff, J. B. (eds.)

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Sequential Assembly of Nanocomposite Materials

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Kumar, C. S. S. R. (ed.)

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ISBN: 978-0-470-63927-6

The Editor

Dr. Katsuhiko Ariga

National Institute for Materials

Science (NIMS)

International Center for Materials

Nanoarchitectonics (MANA)

1-1 Namiki

Tsukuba-Shi

Ibaraki 305-0044

Japan

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Preface

Most important structures in the world are organized organic ultrathin films. They are much more important than the other well-known structures such as nanotubes, nanoparticles and graphene. Why do I (you) think so? Answers may be found within our body. Organic ultrathin films are universal structural units of life. Our body and inside mechanisms are made through assembly of functional units where various interfacial environments of thin films such as cell membranes provide medium for highly efficient molecular conversion, energy conversion, information conversion, and the other important life activities. These functions are surprisingly precise, specific and efficient as well as highly flexible, dynamic, and soft. Not limited to structures found inside of our bodies, organic ultrathin films are seen everywhere around us from soap bubbles to technologically important surface coatings and drug carriers. In addition, technique and science for organized organic ultrathin films can be regarded as one of the most practically advanced nanotechnologies where nanometer-level control of film structures and molecular functions are successfully realized. Organized organic ultrathin films are most important targets in our studies in many research fields including advanced biotechnology and nanotechnology.

This book describes fundamentals and frontiers of four major topics of organized organic ultrathin films: (i) self-assembled monolayer (SAM); (ii) Langmuir–Blodgett (LB) film; (iii) layer-by-layer (LbL) assembly; (iv) other thin films (bilayer vesicle and cast film). All the authors of these chapters are highly experienced in both basic education and advanced research, thus this book satisfies readers with research purposes and educational aims. Organized organic ultrathin films are most important structures in the world. Therefore, this book is a most valuable science guide for you.

Katsuhiko ArigaAugust 2012

List of Contributors

Katsuhiko Ariga

National Institute for Materials Science (NIMS)

International Center for Materials Nanoarchitectonics (MANA)

1-1 Namiki

Tsukuba-Shi

Ibaraki 305-0044

Japan

Mineo Hashizume

Tokyo University of Science

Department of Industrial Chemistry

12-1 Ichigayafunagawara-machi

Shinjuku-ku

Tokyo 162-0826

Japan

Ken-ichi Iimura

Utsunomiya University

Graduate School of Engineering

Department of Advanced Interdisciplinary Science

7-1-2 Yoto

Utsunomiya

Tochigi 321-8585

Japan

Teiji Kato

Utsunomiya University

Graduate School of Engineering

Department of Advanced Interdisciplinary Science

7-1-2 Yoto

Utsunomiya

Tochigi 321-8585

Japan

Toshihiro Kondo

Ochanomizu University

Graduate School of Humanities and Sciences

Division of Chemistry

2-1-1 Ohtsuka

Bunkyo-ku

Tokyo 112-8610

Japan

Takeshi Serizawa

Tokyo Institute of Technology

Department of Organic and Polymeric Materials

2-12-1-H121 Ookayama

Meguro-ku

Tokyo 152-8550

Japan

Kohei Uosaki

National Institute for Materials Science (NIMS)

International Center for Materials Nanoarchitectonics (MANA)

1-1 Namiki

Tsukuba-Shi

Ibaraki 305-0044

Japan

Norihiro Yamada

Chiba University

Faculty of Education

1-33 Yayoi-cho

Inage-ku

Chiba 263-8522

Japan

Ryo Yamada

Osaka University

Graduate School of Engineering Science

Division of Materials Physics

1-3 Machikaneyama

Toyonaka

Osaka 560-8531

Japan

1

Introduction

Katsuhiko Ariga

bottom-up approach, top-down approach, Langmuir–Blodgett films, layer-by-layer process, self-assembled monolayer, liposomes/vesicles

Developments of nanotechnology and microtechnology have been tremendous and are having huge social impact. Based on these technologies, various tools and machines are significantly miniaturized, leading to compact and efficient information processing and communication, as seen in mobile computers and cellular phones. Handy and wearable devices have been developed that enhance com­munication and reduce traffic 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. Industries 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 indispensable (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 for­mation 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 films (ultrathin films) through molecular self-assembly with the aid of external processes such as substrate dipping and solution casting. Three representative methodologies for thin-film 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 films. Therefore, studies on organic ultrathin films would be good starting points for bottom-up nanotechnology.

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 films upon classifications of fabrication strategies. Here, their outlines are summarized. 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 interactions 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 structure 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 modification.

Figure 1.2 Self-assembled monolayer (SAM).

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 first spread on the surface of a water phase. The monolayer can be highly compressed through lateral pressure application. The finally obtained highly condensed monolayer is transferred onto a solid support in a layer-by-layer manner by dipping the support through the monolayer. Film thickness (the number of the layers) is easily tuned in nanometer level just by controlling dipping cycles. The monolayer-forming amphiphile must have an appropriate hydrophilic–hydrophobic balance. The profile of monolayer compression can be interesting research subject of molecular assembly in two dimensions.

Figure 1.3 Langmuir–Blodgett (LB) film.

The LB method requires rather expensive apparatus, and water-soluble molecules are not usually appropriate targets. As compensation for these disadvantageous features, another type of technique for layered ultrathin films was developed. 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 films with desired thickness and layer sequence.

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

Chapter 5 describes the other types of organic ultrathin films and hybrid thin films 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 films on a solid support by casting are exemplified. 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 films 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-film formation with multiple lipid layers. Upon appropriate designs of amphiphiles, their assemblies can extend to more complicated morphologies such as ribbons, sheets, and tubes.

Figure 1.5 Formation of liposome (or vesicle).

In this book, various organic ultrathin films are described according to these categories, self-assembled monolayer (SAM), Langmuir–Blodgett (LB) films, layer-by-layer (LbL) assembly, and the other thin films such as lipid bilayers. Although the main aim of this book is to give an introduction to organic ultrathin films, some of the examples (especially in LbL assembly) are thin films 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.

2

Self-Assembled Monolayer (SAM)

Toshihiro Kondo, Ryo Yamada, and Kohei Uosaki

self-assembled monolayers, organothiols, organosilanes, functions, applications

2.1 Introduction

Construction of a molecular device with the desired functionality to fix and arrange molecules in order on a solid surface is one of the chemist’s dreams. The attempts of fixing a molecular layer with various functionalities on a solid substrate and of controlling the surface properties have been carried out since Langmuir and Blodgett investigated Langmuir–Blodgett (LB) films, which are monolayers and multilayers transferred from the air/water interface onto a solid surface, in the early part of the last century [1–3]. Gaines [4] first summarized the details of LB films that were recently updated by Roberts [5]. Because the molecules are physisorbed onto the solid surface in LB films, their structures easily change and soon become random. In 1980, on the other hand, Sagiv found that molecules with a long alkyl chain can be fixed to the solid surface through the covalent linkage between the trimethoxysilyl (-Si(OCH3)3) or trichlorosilyl (-SiCl3) group of the molecule and the surface hydroxyl (-OH) group of the solid substrate, and that an advanced orientation can be achieved by the interaction between the alkyl chains, and pointed out the similarity to the LB film [6]. Since the molecular layer can be spontaneously formed with a high orientation, this process is called self-assembly (SA) and the formed molecular film a self-assembled monolayer (SAM). Later, Nuzzo and Allara found in 1983 that an alkanethiol can react with a gold substrate to form Au–S bonds and that a highly oriented SAM with the hydrophobic interaction between the alkyl chains is formed on the gold surface [7], thus construction of a highly oriented molecular layer can be achieved on the electroconductive substrate, and the alkylthiol SAM on gold has been rapidly extended to both basic science and applications along with active research [8–13]. Recently, the modification of silicon surfaces of organic molecules through a covalent Si–C bonding has been realized by a wet reaction [14–18] and this research that makes the best use of the semiconductor property of the substrate has been actively carried out.

The molecule, which formed the SAM, consists of three parts as shown in Figure 2.1.

Figure 2.1 A schematic illustration of the SAM on a solid substrate.

The first part is the surface binding group (dark gray circle in Figure 2.1), which spontaneously binds to the substrate surface by a covalent linkage, so that this group depends on the substrate materials. In the case of using the oxide surface, which has a surface hydroxyl group, as the substrate, the trimethoxysilyl (-Si(OCH3)3) or trichlorosilyl (-SiCl3) group is used as the surface binding group. When the thiol (-SH) group is used as a surface binding group, the semiconductor, such as GaAs [19], CdSe [20], and In2O3 [21], in addition to the metal, such as Pt, Ag, Cu, etc., besides Au, are used as a substrate. On these substrates, disulfide (-S–S-), selenol (-SeH), and isocyanide (-NC) can also be bonded [10]. When Si, as mentioned above, Ge, or diamond [22] are used as the substrate, the terminal olefin group can be used as a surface binding group [14–18]. The second part is the alkyl chain (rectangle in Figure 2.1), and the energy associated with its interchain van der Waals interaction is of the order of a few kcal/mol, which means that this interaction is exothermic [8]. The last part is the terminal group (light gray circle in Figure 2.1), which is generally a methyl group.

In this chapter, we introduce the fundamental science of the SAMs, such as molecular layer structures and formation processes of the SAMs, and applications of the SAMs to construct the surface with various functionalities.

2.2 Preparation and Characterization

2.2.1 Organothiols on Au

Organothiols adsorb on metal surfaces via metal–S bond formation, as shown in Figure 2.2. SAMs of alkanethiols can be prepared in solutions and vapors of molecules [7, 8, 10]. Self-assembly in solutions are commonly used because it is easy to prepare, control, and molecules that have small vapor pressure can be used. Typically, a several mM solution of alkanethiols in organic solvents, such as ethanol and hexane, is used to form SAMs.

Figure 2.2 Schematic presentation of the SAMs of alkanethiols on metal substrates.

A (111)-oriented gold thin film is the most widely used substrate since it is stable under ambient conditions and can be easily formed on various substrates, such as a glass slide, mica and silicon, by thermal vacuum evaporation [23]. A polished surface of a gold polycrystal is also used as a substrate [24]. Grains having atomically flat (111) terraces are grown when the substrate is heated around 300 °C during the deposition. Flame annealing is also employed to obtain flat and wide (111) terraces [23].

Most of the commercially available alkanethiols are usually used without further purification, though the residual impurity of sulfur is known to disturb the formation of an SAM of pyridinethiol on a Au(111) surface [25]. To form a monolayer, a clean substrate is immersed in the solution for 1–24 h. Sometimes, a much longer period, such as several days, is required for completion of the monolayer formation [26]. Both temperature and solvent strongly affect the density of the defects and domain size [27, 28]. These effects are discussed later.

The kinetics of the SA process in solution was monitored by a quartz crystal microbalance (QCM) [29], which can measure mass changes on surfaces. The SA process was found to be divided in an initial fast adsorption and subsequent slow steps [29–32]. The coverage reaches 80% during the initial first adsorption process (∼10 min). The slow adsorption process continues for up to several hours.

Figure 2.3 shows the infrared spectrum of ferrocenyl-undecanethiol (FcC11SH) monolayers formed on the Au(111) surface as a function of immersion time. The peak position assigned to CH vibration of the CH2 group was found to shift from the frequency attributed to liquid-like phases of poly-methylene to that attributed to solid-like phases [32]. This result indicates that the alkyl chains in the monolayers become more solid-like during the slow adsorption process, thus, the slow adsorption process is interpreted as a defect-healing process.

Figure 2.3 (a) IR absorption spectrum of a FcC11SH monolayer on the Au (111) electrode. Modification time was 10 min. (b) Evolution of peak positions of the methylene asymmetric (upper panel) and symmetric (lower panel) stretching modes.

Reprinted from [32].

Figure 2.4 shows schematic drawings of the top and side views of the monolayer. The alkyl chain is tilted from the surface normal about 30 ° with the all-trans conformation. This tilt angle comes from the conditions for closed packing of the alkyl chains. Close inspection of the IR data revealed that the plane defined by an all-trans carbon molecular skeleton alternatively changes its direction [33].

Figure 2.4 (a) Model of molecular arrangement with respect to Au(111) surface. Shadow circles and small open circles represent positions of molecules and gold atoms, respectively. A diagonal slash indicates the azimuthal orientation of the plane defined by the C–C–C backbone of an all trans-hydrocarbon chain. (b) Side view of the monolayer. Circles represent sulfur atoms.

A high-resolution scanning tunneling microscopic (STM) image using large tunneling impedance revealed the molecular arrangement and local structures of the SAMs [34]. Figure 2.5 shows a molecularly resolved STM image. The basic molecular arrangement is (√3 × √3)R30 ° with respect to the Au(111) surface. Close inspection of the structure revealed small differences in the height among the molecules [35–37]. The structure considering these modulations is called c(4 × 2) of (√3 × √3)R30 °. The c(4 × 2) structure is attributed to the different orientations of the alkyl termination due to the different twist angles among the alkyl chains or small deviations of the sulfur atom positions from the hexagonal symmetry, indicating the existence of two kinds of sulfur positions [30, 38]. The different sulfur adsorption sites can result in a variable electronic structure and height in the monolayer. In fact, the position of the sulfur atom on a gold surface is still under debate.

Figure 2.5 An STM image of a Au(111) surface covered with an octanethiol monolayer. The rectangular cell represents the unit cell for c(4 × 2) of (√3 × √3)R30 °. Lines represent nearest neighbor (B) and two next nearest neighbor directions of the Au atoms of the substrate.

Reprinted from [36].

An STM image revealed various defect structures as shown in Figure 2.6a. One significant feature is the pit-like structure. The holes are not pinholes in the monolayer but depressions of the Au surface created during the monolayer formation, as shown in Figure 2.6b. These depressions of the Au surface are called vacancy islands (VIs) of the gold surface. The VIs are known to be formed during the very initial stage of the SA and grow via an Ostwald ripening process [39, 40].

Figure 2.6 (a) An STM image of a Au(111) surface covered with decanethiol (C10SH) SAM. A schematic model of VIs (b), domain boundary due to tilt angles (c) and stacking misfits (d).

The other defect structure is the domain boundary. Typically, a domain boundary consists of void lines with a space of single or several molecules. These defects originate from the misfits in the tilt angles, stacking geometry and rotational direction of the c(4 × 2) geometry. Figures 2.6c and d show models of the typical domain boundaries caused by rotational and stacking misfits, respectively.

The defect density can be reduced by annealing after the SAM formation [41, 42], increasing the temperature of the solution during the SAM formation [27] and changing the solvent [28]. Figure 2.7 shows STM images of the Au(111) surface modified with decanethiol in ethanol, dimethylformamide (DMF) and toluene at room temperature [28]. The density of the VIs and size of the grain have changed.

Figure 2.7 STM images of a Au(111) surface covered with a decanethiol monolayer modified in 1 mM solution in (a) ethanol (b) DMF and (c) toluene for 1 h.

Reprinted from [28].

Alkanethiols are known to change their orientations as a function of coverage [34, 43–46]. Figure 2.8 schematically shows the relationship between the coverage and orientation of the molecules. When the coverage is low, alkanethiol molecules do not form ordered structures (Figure 2.8a). As the coverage increased, the so-called pin-stripe patterns are formed (Figure 2.8b). In this structure, molecules are oriented parallel to the surface plane and arranged in a head-to-head configuration, that is, thiols are pointing to each other. After the pin-stripe phase, interdigit structures, in which the alkyl chains are stacked with those in the next rows, appeared (Figure 2.8c and d). As the coverage increased, the molecules stand up and form an island that consisted of a √3 × √3 molecular arrangement (Figure 2.8e).

Figure 2.8 Schematic representation of evolution of structures of decanethiol on Au(111) as a function of coverage (a–e). See text for details.

Reprinted from [30].

The SAMs of organothiols are known to be reductively desorbed by applying negative potential in alkaline aqueous solutions as shown in Eq. (2.1) [47].

(2.1) 

As a reverse process, thiol SAMs are expected to be oxidatively formed by applying a positive potential in solutions, containing thiolate molecules.

The desorption and readsorption processes of alkanethiol SAMs in electrochemical environment were investigated by various electrochemical techniques, scanning tunneling microscopy (STM), and surface X-ray diffraction (SXRD) [48–52]. Figure 2.9 shows cyclic voltammograms of a Au(111) electrode in 20 mM KOH ethanol solutions containing 100 µM and 1 mM hexane thiol (C6SH) [52]. When the potential was swept in the negative direction, the cathodic peak (negative current peak) due to the reductive desorption of the monolayer was observed around −800 mV. The anodic peak (positive current peak) due to the oxidative adsorption of the molecules was observed at −700 mV during the positive potential scan.

Figure 2.9 Cyclic voltammograms of a Au(111) electrode measured in 20 mM KOH ethanol solution containing 100 µM (dashed line) and 1 mM C6SH (solid line).

Reprinted from [52].

Figure 2.10 shows sequential STM images during oxidative adsorption and reductive desorption of C6SH taken in 20 mM KOH ethanol solution containing 60 µM of C6SH [52]. The white arrow in the images is the marker indicating the same location of the surface. Initially, the potential of the Au electrode was −950 mV, at which C6SH was desorbed. Double-stripe patterns attributed to herringbone structure of Au(111) surface were observed, as indicated by the black arrows in Figure 2.10a. This result indicates that the Au(111) surface is reconstructed and no molecules are chemisorbed on the surface, as expected.

Figure 2.10 In situ STM images of a Au(111) electrode in 20 mM KOH ethanol solution containing 60 µM hexanethiol. Images (a–h) were sequentially taken. Arrows and numbers beside the image indicate the frame direction of STM image and electrochemical potential of the substrate during the scanning. A hexanethiol monolayer is known to desorb around −800 mV. Oxidative adsorption of desorbed molecules took place around −700 mV.

Reprinted from [52].

When the potential was swept positively and reached the potential at which the anodic peak was observed, −700 mV, herringbone structure disappeared and the step structure of the gold changed as indicated by the pointing finger. No significant features assigned to the thiol molecules were clearly observed. This result indicates that the gold atoms became highly mobile. The thiol molecules were supposed to be adsorbed on the surface and make gold atoms mobile [53]. The VIs of the gold surface is observed in Figure 2.10c. When the potential of the electrode was more positive than −690 mV, typical features of the gold surface covered with SAMs of alkanethiols were observed. The desorption of the thiol molecules and formation of the double-stripe patterns due to the reconstruction of the Au(111) surface were again observed when the potential was swept negatively (Figures 2.10e–h).

The SAMs of organothiols on Au are becoming popular and conventional. This technique is used to functionalize not only single-crystalline surfaces but also nanoparticles and nanorods [54, 55]. Spontaneous adsorption of organodithiols to metal electrodes is used to fabricate single-molecular devices [56].

2.2.2 Organosilanes on SiOx Surfaces

Organosilane monolayers are formed via hydrolysis of the anchor group of molecules and hydroxyl group on the surfaces as shown in Figure 2.11. Whereas the most widely studied system is the alkyl-tri-chlorosilanes on SiO2 including native oxide, glass and mica surfaces, the SAMs of organosilanes can be formed on various kinds of oxide surfaces [8]. Since the oxide surfaces play important roles in many electronic devices, such as field effect transistors, organosilane monolayers are used to control the interfacial properties of the devices [57].

Figure 2.11 Schematic presentation of the SAMs of alkyl-tri-chlorosilanes and alkyl-tri-methoxysilanes on SiO2 surfaces.

One of the important characteristics of the SAMs of alkyl-tri-chlorosilanes is the lateral crosslinking of molecules. Due to this crosslinking, the SAMs become robust, but a long-range, two-dimensional order is not realized. The structural order, growth rate and growth mechanism are influenced by many environmental parameters, such as humidity, water content of the solution, wettability of the surface, pH and temperature [8], because the hydrolysis reaction is influenced by these factors. It was shown that most of OH group on the surface does not form chemical bonds with molecules and physical adsorption through the water layer plays an important role, as shown in Figure 2.12 [58].

Figure 2.12 Schematic presentation of mechanism and structure of alkyl-try-chlorosilanes on SiO2 surface.

Reprinted from [58].

The structural evolution during the SAM formation was studied by in situ atomic force microscopy [59]. The solution containing 0.5 mM octadecyltrichlorosilane in toluene was injected into the atomic force microscope (AFM) cell to initiate the SAM formation. Figure 2.13 shows a series of AFM images taken after the injection of the solution. Initially, fractally shaped islands were observed to grow and cover the surface. This result indicates that the growth mechanism is a diffusion-limited aggregation. It is concluded that the polysiloxane oligomers formed in the solution adsorb onto the active centers of the surface.

Figure 2.13 AFM images of the mica surface during octadecylsiloxane (ODS) monolayer growth in toluene solution. The concentrations of ODS and water were 0.5 mM and 4.5 mM, respectively. Image size: 1 µm × 1 µm.

Reprinted from [59].

2.2.3 SAMs on Si Surface via Si–C Bonding

The modification of the silicon surfaces by covalent attachment of organic molecules is realized by the chemical reaction between the olefin and hydrogen-terminated Si surfaces as shown in Figure 2.14. The H-terminated Si surface is prepared by chemical etching of the Si surface in NH4F solution [60]. Si–H bonds on the surface are cleaved to generate radicals by heat [14], radical initiators [61], ultraviolet light [62], electrochemical [63, 64], and sonochemical [65] methods.

Figure 2.14 Schematic presentation of the SAMs of alkanes directly connected to Si surface via a Si–C bond.

Figure 2.15 shows the proposed formation mechanism of the SAMs on Si [15–18, 66]. The initial reaction is removal of hydrogen on the H-terminated Si surface and formation of dangling bonds (Si radical) by UV light or thermal activation. The dangling bond reacts with the CC bond, and this reaction results in the formation of the carbon radical. The carbon radical abstracts a hydrogen atom from the H-terminated surface and a new reactive dangling bond is formed. This chain reaction results in the formation of monolayers.

Figure 2.15 Proposed reaction scheme of the SAM formation.

Reprinted from [66].

Figure 2.16 shows the H-terminated Si(111) surface before and after the monolayer formation, respectively [67]. The H-terminated Si(111) surface was prepared by chemical etching in 40% NH4F. The monolayer was formed by immersing the Si substrate in a solution containing CH2CH–(CH2)8–CH3 and CH2CH–(CH2)8–SH (20 : 1) under UV irradiation for several hours. As shown in Figure 2.16, atomically flat terraces were observed before and after the SAM formation. Although the two-dimensional molecular order has not been confirmed, the SAM is uniformly formed on the surface.

Figure 2.16 AFM images of H–Si(111) surface before (a) and after (b) modification of CH2CH–(CH2)8–CH3 and CH2CH–(CH2)8–SH.

Reprinted from [67].

The growth rate and conformational order were investigated by attenuated total reflection infrared spectroscopy and sum frequency generation (SFG) spectroscopy [68–71]. Figure 2.17