Deposited monolayer films

The TiO2(110) surface can be modified to alter the surface chemistry of adsorbed methoxides. Recently Vohs et al. [73] have reported temperature programmed desorption studies of methanol on TiO2(110)-supported V2O5. Deposited monolayer films of vanadia converted some of the adsorbed methanol to formaldehyde and water, while multilayer films of vanadia on the TiO2(110) surface were found to be inactive for methanol oxidation. Furthermore, adsorption studies of formaldehyde indicated that formaldehyde production from... 

The adsorption of deposited monolayer films can be very tenacious, especially when the solid substrate can react with the deposited material. Carbox-yhc acid monolayers deposited on metal or metal oxide surfaces, for example, almost certainly form metal soaps that are extremely difficult to remove by any means other than direct chemical etching. For example, such strongly adsorbed films are very useful in friction and wear studies of boundary lubrication (see Chapter 18). 

Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

In order to study the structure of Langmuir films of polymers spheres, most researchers deposited the films on solid substrates using the LB technique [158-162] and analyzed the structure using a microscope. A modified version of the LB method allowing the transfer of particle monolayers is outlined in Figure 8a. 

The quantity and quality of the deposited monolayer on a solid support is measured by a so-called transfer ratio, tr. This is defined as the ratio between the decrease in monolayer area during a deposition stroke, Al, and the area of the substrate, As. For ideal transfer, the magnitude of tr is equal to 1. Depending on the behavior of the molecule, the solid substrate can be dipped through the film until the desired thickness of the film is achieved. Different kinds of LB multilayers can be produced and/or obtained by successive deposition of monolayers on the same substrate (see Figure 4.11). The most common one is the Y-type multilayer, which is produced when the monolayer deposits on the solid substrate in both up and down directions. When the monolayer deposits only in the up or down direction, the multilayer structure is called either Z-type or X-type. Intermediate structures are sometimes observed for some LB multilayers, and they are often referred to be XY-type multilayers. 

No direct method exists by which monolayer film structures on water can be studied. Therefore, the LB method has been used to study molecular structures in past decades. The most useful method for investigating the detailed LB-deposited film structure is the well-known electron diffraction technique (or the scanning probe microscope [Birdi, 2002a]). The molecular arrangements of deposited mono-and multilayer films of fatty acids and their salts, using this technique, have been reported. The analyses showed that the molecules were almost perpendicular to the solid surface in the first monolayer. It was also reported that Ba-stearate molecules have a more precise normal alignment compared to stearic-acid monolayers. In some investigations, the thermal stability of these films has been found to be remarkably stable up to 90°C. 

A process similar to that described earlier for TiOi was used to deposit Z1O2 films [61]. Self-assembled monolayers with terminal sulphonate or methyl groups on Si were nsed as snbstrates no film growth occurred on bare Si. The deposition solution was Zr(S04)2 dissolved in an aqueous HCl solution at 70°C. 

Figure 15. (a) A schematic representation of organic amphiphilic molecules at air-water interface, (b) Schematic representation of the various deposition modes for monolayer films and the resulting L-B assemblies. (Reproduced with permission from H. Kuhn, D. Mobius, and H. Bucher, Physical Methods of Chemistry, Vol. I, Part IIIB, A. Weissberger and B. W. Rossiter, Eds., Wiley, New York, 1972, p. 577.)... 

Ethene adsorbed on Cu/ZnO at 300 K. (52), Fig. 15E, gives closely similar wavenumbers and relative intensities to the spectrum on Cu(100) with absorptions at 1550, 1290, and 920 cm-1—so much so that the spectrum may imply the dominance of (100) facets on the Cu particles. Ethene adsorbed on cold-deposited Cu films studied by SERS gives bands at 1544, 1278, and 896 cm 1 (227). A 3-monolayer covering of Cu on Ru(0001) gives a similar spectrum with somewhat shifted band positions at 1504, 1248, and 880 cm-1 (218). 

Monolayer Films of Phthalocyanine Derivatives. A series of organic derivatives of phthalocyanines were prepared that have two important characteristics of materials to be deposited by the Langmuir-Blodgett technique (1) they are soluble in volatile organic solvents, and (2) they form monomolecular films on the surface of water. Further study of deposited films of these phthalocyanine derivatives will be necessary in order to determine the exact orientations on the surface, but regardless of their orientations, they offer interesting possibilities for construction of thin films of ordered arrays of molecules on the surface of gas sensors. 

In an effort to carry the success of 29 forward from homogeneous solution [77] to films, the team at Miami has turned to 31 [78], While 31 forms excellent monolayer films at the air-water interface, decent anthracenic fluorescence signatures are not produced until the films are diluted substantially with stearic acid and deposited onto hydrophobic glass. As may be anticipated, neat films of 31 only show broad excimeric emissions. Light collection problems dog the experiments on the films on water. No sensitivity of the emission towards pH is found, so experiments with saxitoxin are not reported at this stage. We note the closely related PET system 32 which shows good sensing of membrane-bounded protons in micellar media [79],... 

---