Langmuir-Blodgett - films

Thin films of materials play an important role in modern electronics. Langmuir-Blodgett films (Mort, 1980) are prepared from surface-active molecules that adsorb at the surface of an aqueous solution. The surface is gently compressed to produce a close-packed monolayer, which can be coated on appropriate substrates by dipping at constant surface pressure. The layers so produced are of a precise thickness, and repeated dipping gives multilayers. For example, diacetylenes and other polymerizable 

Among the various LCDs, the three most widely used are (i) dynamic scattering display, (ii) twisted nematic display and (iii) guest-host display (Shanks, 1982). 

Dynamic scattering displays, which are the first generation LCDs, make use of a nematic LC with a negative dielectric anisotropy (As = — 0) doped with an 

During the remainder of the 1930s, Langmuir and Blodgett carried out a brilliant series of studies on multilayer films of a variety of chemicals, supplemented by studies in Britain, especially at the ill-fated Department of Colloid Science in Cambridge (Section 2.1.4). Then the War came, and momentum was lost for a couple of decades. After that, L-B films came back as a major topic of research and have been so ever since (Mort 1980). It is current practice to refer to molecular films, made by various techniques (Swalen 1991), but the L-B approach remains central. 

Molecular films are of intense current concern in electronics. For instance, diacetylenes and other polymerisable monomer molecules have been incorporated into L-B films and then illuminated through a mask in such a way that the illuminated areas become polymerised, while the rest of the molecules can be dissolved away. This is one way of making a resistance for microcircuitry. L-B films have also found a major role in the making of gas-sensors (Section 11.3.3). 

A review of what has come to be called molecular electronics (Mirkin and Ratner 1992) includes many striking discoveries, such as a device based on azoben .ene (Liu et al. 1990) that undergoes a stereochemical transition, trans-to-cis, when irradiated with ultraviolet light, but reverts to trans when irradiated with visible light. The investigators in Japan found that L-B films of their molecules can be used for a 

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. 

The LB technique is an alternative for the SAM systems. LB allows obtaining ultrathin films with controlled thickness and a well-defined molecular orientation. In this method, amphiphilic molecules, which are spread on a liquid surface (so-called Langmuir film), are deposited on a solid surface by dipping it in the solution. The molecules are transferred from the air-water interface to the solid substrate. 

Changes in the collapse pressure of monolayers prepared from mixtures of I and 5 as a function of will, therefore, be ideal. In the presence of interactions, rrc m will deviate, of course, from ideal behavior. Expanding Eq. (1) (or Eq. 5) and neglecting the higher order terms led to 

Monolayers are best formed from water-insoluble molecules. This is expressed well by the title of Gaines s classic book Insoluble Monolayers at Liquid-Gas Interfaces [104]. Carboxylic acids (7-13 in Table 1, for example), sulfates, quaternary ammonium salts, alcohols, amides, and nitriles with carbon chains of 12 or longer meet this requirement well. Similarly, well-behaved monolayers have been formed from naturally occurring phospholipids (14-17 in Table 1, for example), as well as from their synthetic analogs (18,19 in Table 1, for example). More recently, polymerizable surfactants (1-4, 20, 21 in Table 1, for example) [55, 68, 72, 121], preformed polymers [68, 70, 72,122-127], liquid crystalline polymers [128], buckyballs [129, 130], gramicidin [131], and even silica beads [132] have been demonstrated to undergo monolayer formation on aqueous solutions. 

Composition of the subphase is of paramount importance for monolayer formation. Electrolytes, by virtue of their ability to salt-out organic molecules, often render surfactants insoluble and, thus, permit the formation of a well-behaved monolayer. A case in point is the previously cited example of optically active (jV-(a-methylbenzyl)stearamides [114]. Stable monolayers could not be formed with these molecules on water. Stable monolayers readily formed, however, on strong aqueous acid solutions [114], 

Scrupulous cleanliness is an absolute must in working with monolayers [104] all materials used should be of the highest purity solvents and solutions should be dust-free glassware should be cleaned with a special cleaning solution and kept separate from other equipment the film balance should be kept in a clean area on a vibration-free table and surgical gloves should be worn at all times. These necessary precautions have been elaborated in frightening detail in a recent publication [114] and a review [107]. 

Monolayers and multilayers on solid substrates have come a long way since their initial preparation by Katherine Blodgett during the 1930s [133, 134], Deservedly termed as Langmuir-Blodgett (LB) films , they have flourished 

Various deposition modes can be obtained using a parameter 0 given by Hoing and coworkers [57]  

Recently, ultrathin films of poly(o-anisidine) and poly(ethoxyaniline) have been fabricated for application in nanotechnology [64-66]. Matsura and co-workers [67] have fabricated monolayers of 3-carotene using a Langmuir-Blodgett film technique together with the flow-orientation method. They have utilised XRD, UV-visible and FTIR techniques to elucidate the film-structure of P-carotene indicating that P-carotene orients perpendicular to the air-water interface. It was found that the films are, however, well-ordered both in the stacking direction and the in-plane direction. 

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A belief that solid interfaces are easier to understand than liquid ones shifted emphasis to the former but the subjects are not really separable, and the advances in the one are giving impetus to the other. There is increasing interest in films of biological and of liquid crystalline materials because of the importance of thin films in microcircuitry (computer chips ), there has been in recent years a surge of activity in the study of deposited mono- and multilayers. These Langmuir-Blodgett films are discussed in Section XV-7. 

Interferometry is based on the fact that light reflected from the front and back interfaces of a film travels different distances, producing interference effects. The method has been applied to Langmuir-Blodgett films (Section XV-7) and to soap films (Section XrV-8) [147-149]. 

Transmission electron microscopy (TEM) can resolve features down to about 1 nm and allows the use of electron diffraction to characterize the structure. Since electrons must pass through the sample however, the technique is limited to thin films. One cryoelectron microscopic study of fatty-acid Langmuir films on vitrified water [13] showed faceted crystals. The application of TEM to Langmuir-Blodgett films is discussed in Chapter XV. 

Films spread at liquid-liquid interfaces or on liquids other than water are discussed followed by the important effects of charged monolayers on water. Finally, the most technologically important application of Langmuir films, the Langmuir-Blodgett film deposited on a solid substrate, is reviewed. 

Deposited Langmuir-Blodgett films take on many of the same stmctures as the Langmuir monolayers discussed in Section IV-4C, and they are often compared to the self-assembling monolayers described in Section XI-IB. The area... 

G. G. Roberts, Langmuir-Blodgett Films, Plenum, New York, 1990. 

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Bourdieu L, Ronsin O and Chatenay D 1993 Molecular positional order in Langmuir-Blodgett films by atomic force microscopy Science 259 798... 

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Nakamura T, Tachibana H, Yumara M, Matsumoto M and Tagaki W 1993 Structure and physical properties of Langmuir-Blodgett films of Cgg with amphiphilic matrix molecules Synth. Met. 55 3131-6... 

Goldenberg L M, Williams G, Bryce M R, Monkman A P, Petty M C, Flirsch A and Sol A 1993 Electrochemical studies on Langmuir-Blodgett films of 1-fert-butyl-1,9-dihydrofullerene-60 J. Chem. Soc., Chem. Commun. 1310-12... 

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Nakamura E 1995 Langmuir-Blodgett film of amphiphilic Cgg carboxylic acid Langmuir 660-5... 

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C2.4 Organics films (Langmuir-Blodgett films and self-assembled monolayers)... 

Daniel M F, Lettington O C and Small M 1983 Investigation into the Langmuir-Blodgett film formation ability of amphiphiles with oyano head groups Thin Solid Films 99 61-9... 

Tredgold R FI, Jones S D, Evans S D and Williams P I 1986 Aluminium oxide as a substrate for the deposition of Langmuir-Blodgett films J. Mol. Electron. 2 147-9... 

Sohwartz D K 1997 Langmuir-Blodgett film struoture Surf. Sc/. Rep. 27 245-334... 

Peterson I R 1984 Optioal observation of monomer Langmuir-Blodgett film struoture Thin Solid Films 116 357-66... 

Peterson I R 1987 Langmuir-Blodgett films structure and application J. Mol. Electron. 3 103-11... 

Rothberg L, Higashi G S, Allara D L and Garoff S 1987 Thermal disordering of Langmuir-Blodgett-films of cadmium stearate on sapphire Chem. Phys. Lett. 133 67-72... 

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Tredgold R H, Allen R A and Hodge P 1987 X-ray-diffraction and optical studies of Langmuir-Blodgett films formed from azobenzene derivatives Thin Solid Films 155 343-52... 

Taohibana FI, Nakamura T, Matsumoto M, Komizu FI, Manda E, Niino FI, Yabe A and Kawabata Y 1989 Photoohemioal switohing in oonduotive Langmuir-Blodgett films J. Am. Chem. See. 111 3080-1... 

Barnik M I, Kozenkov V M, Shtykov N N, Palto S P and Yudin S G 1989 Photoinduoed optioal anisotropy in Langmuir-Blodgett films J. Mol. Electron. 5 53-6... 

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Jones R, Tredgold R FI, Floorfar A and Flodge P 1984 Eleetrioal-eonduetivity in Langmuir-Blodgett films of porphyrins—inplane and through-the-film studies Thin Solid Films 113 115-28... 

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