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Langmuir 1993,9, 1927-1929
Notes Multilayer Sol-Gel Films Using the Langmuir-Blodgett Deposition Method
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Huesein Samha and M. Keith DeArmond'
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Department of Chemistry and Biochemistry, New Mexico State University, La8 Cruces, New Mexico 88003-0001
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Received February 1,1993. In Final Form: April 6,1993
The preparation sf organized film assemblies of functional molecules has as an ultimate goal a variety of useful applications includingsensors, insulating and conducting coatings, optical coatings, and, ultimately, molecular electronic devices. The two distinct approaches to these goals have been that of the self-assembled (SA) film and the Langmuir-Blodgett (LB) film. The SA technique has the advantage that the films produced by covalent binding to a substrate are durable, thermallystable, and chemically resistant to a variety of reagents. Key disadvantages are the general limitations in chemical function possible for SA systems, the difficulty in controlling and varying the concentration of the functional species, and the problems in obtaining multilayer films. For example,the formation of covalent sulfur bonds to gold or silver surfaces,l-' while producing stable, well-ordered films, has not produced multilayer films. The other common SA system, polysiloxane, originally developed by Sagiv and co-workersa10 does also produce stable chemically inert films, but functional and multilayer films are again not easily prepared. Sagiv has produced bifunctional monolayer assemblies'O that, in early attemptsll did not produce quality multilayer films, most recently have produced better quality multilayer films. The Kodak group of Ulman has also developed a chemical procedure12 for preparation of multilayer siloxane f i from <richl+ rosiloxanes that have terminal functional groups. In contrast, LB films having multilayer8 are readily produced and functional amphiphilic molecules are often used.13J4 The strategy of mixed LB films using a fatty acid with a functionalorganic or metal complexcan provide some versatility with this film meth0d.l"" However, the ~~
(1) Porter, M. D.;Bright, T. B.; Allara, D.L.; Chideey, C. D.J. Am. Chem. Soc. 1987,109,3669. (2) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1988,110,6660. (3) Chidaey, C. D.; Loiacono, D. N. hngmuir 1990,6,682. (4) Nuzzo, R. G.; Dubois, L. H.; Allara, D.L. J. Am. Chem. SOC.1990, 112,668. (5) Widrig, C. A.; Alvee, C. A.; Porter, M. D. J. Am. Chem. SOC.1991, 113, 2806. (6) Kim, Y. T.; Bard, A. J. Langmir 1992,8,1096. (7) Folkere, J. P.; Laibmis, P. E.; Whitesidea, G. M. Langmuir 1992, 28,1330. (8) N e a r , L.; Sagiv, J. J. Am. Chem. SOC. 1986,105,674. (9) Gun,J.; Sagiv, J. J. Colloid Interface 1983,113,457. (10) Maoz, R.; Netzer, L.; Gun,J.; Sagiv, J. J. Chem. Phye. 1988,86 (11112). (11)Pomeranz, M.; Segmuller, A.; Netzer, L.; Sagiv,J. Thin Solid Films 1986,132,153. (12) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989,6,101. (13) Miller, C. J.; McCord, P.; Bard, A. J. Langmuir 1991, 7, 2781. (14) Fujihira, M.; Niahiyama, K.; Yamada, H. Thin Solid Film 1981, 132, 77. (16)Des Enfanta,R.; Martinez.,T.;DeArmond, M. K. ACS Symp. Ser. 1992, No. 499,46. (16) Samha,H.;DeArmond, M. K. Coord. Chem. Rev. 1991,111,73. (17) Samha,H.; Martinez., T.; DeArmond, M. K. Langmuir 1992,8, 2001.
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Figure 1. P A isotherm of OTS on the surface of water at 20 OC.
fragility of the film and the low thermal stability are significant disadvantages of this film technique. Obviously, a technique combining the advantages of the SA method and the versatility and simplicity of the LB film technique would be valuable. Recently Bilewicz and Majdals have produced mixed monolayer films with a thiol and alcohol by the LB film method. The gold substrate used gave an SA film that provided stability and the long chain alcohol gave an LB film that permits variation of the concentration of ubiquinone added in the film. No multilayer filmsof this type were reported. Lately two r e p ~ r t s lof~ the * ~ preparation of siloxane films using the LB method have been published; however no attempt to produce a multilayer film was apparent. We wish to report production of multilayer siloxane films, with functional molecules incorporated,using a simpleLB film method combined with a sol-gel procedure. Octadecyltrichlorosilane,OTS, and dodecyltrichlorosilane, DTS, were Aldrich products and were used to produce LB films of siloxane. Ru(bpy)z(nbpy)(C104)2,1, and Ru(tm-bpy)s(ClO4)2,2 (bpy is 2,2'-bipyridme, tm-bpy is 4,4',6,6'-tetramethyL2,2'-bipyridine, and nbpy is 4,4'dinonadecyl-2,2'-bipyridine), were incorporated as functional species in the LB films. These compounds were available in our laboratory from previous study and were synthesized using standard procedures. When the freshly prepared benzene solution of siloxane(2.68 X 103 M OTS) is fdtered on the surface of a water subphase, on an alternatedouble sided NIMA trough, by passing it through a pipette blocked with glasswool, a monolayer of OTS is formed on the subphase surface. The layer spreads for about 16 min before it is compressed to give a rigid close packed LB film of OTS (Figure 1). In the preparation of the mixed LB films of OTS/Ru2+ complexes (1 and 2) (Figure 2A),the Ru2+complex benzene solution is mixed with the freshly prepared OTS benzene solution of 1:lO molar ratio and then the solution mixture is filtered on the surface of the water subphase and the formed monolayer is compressed. Using the same procedure, the sandwich structure multilayer films (Figure 2B) are (18) Bilewicz, R.; Majda, M. Longmuir 1991, 7,2794. (19) Richardeon, T.;Roberts, G. G.; Holder, S.; Lacy, D. Thin Solid Film 1992,2101211,299. (20) Yaae, K.; Schwiegk, S.; Lieaer, G.; Wagner, G. Thin Solid Film 1992,130, 213.
0743-1463/93/2409-1921$04.00/00 1993 American Chemical Society
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Figure 3. P A isothermsof pure DTS monolayer spread on the surface of water subphaee and compressed after (a) 30, (b) 80, and (c) 180 min and after being compressed once to a surface pressure of 25 mN/m and then relaxed at surface preaeure
