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Langmuir 2000, 16, 1172-1179
Coordinate Covalent Cobalt-Diisocyanide Multilayer Thin Films Grown One Molecular Layer at a Time Michael A. Ansell, Elizabeth B. Cogan, and Catherine J. Page* Department of Chemistry, University of Oregon, Eugene, Oregon 97403 Received February 18, 1999. In Final Form: September 16, 1999 Multilayer thin films consisting of alternating layers of cobalt and 1,4-diisocyano-benzene (DiCNB) synthesized layer by layer on an amine functionalized silicon substrate were first reported in 1996. This paper describes further work in characterizing these films using X-ray photoelectron spectroscopy (XPS), ellipsometry, attenuated total reflectance-infrared spectroscopy (ATR-IR), and reflection-absorption infrared spectroscopy (RAIRS). Ellipsometry shows the films grow uniformly, suggesting sequential binding of metal and diisocyanide components. Control experiments confirm that both components are necessary for film growth and that 1-isocyano-2-phenylethane, a mono-functional isocyanide, caps the film and terminates further growth. Although the ellipsometry data suggests uniform film growth consistent with DiCNB molecules oriented perpendicular to the plane of the substrate, in accord with a simple model of these films, a more complex picture of the resulting film constituents is seen with further analysis. FTIR and XPS results indicate the presence of significant amounts of oligomeric isocyanide species in the films. We also report the growth of uniform multilayers of cobalt and 1,6-diisocyanohexane.
Introduction We first reported the preparation of cobalt-diisocyanide multilayer films in 1996,1 along with preliminary ellipsometry and ATR-FTIR spectra that indicated regular growth and incorporation of isocyanide moieties in the film, respectively. Since that time, our attention has been focused on further understanding and modifying the chemistry and structure of the cobalt-diisocyanide multilayer thin films. These films are examples of layered organic/inorganic materials that provide a structural template for incorporating properties of technological interest, including nonlinear optical properties,2,3 twodimensional magnetic properties, catalytic activity,4 selective chemical responses for sensor applications,5-7 or modified electrical properties for electrode surfaces.8 Developing and understanding the methods required to build such films one molecular layer at a time is necessary to enable exquisite control of structure and function. In our original communication,1 we showed evidence of the layer-by-layer synthesis of films grown on an amine functionalized surface by repeated sequential immersion in cobalt (II) chloride and 1,4-diisocyanobenzene (DiCNB) solutions to build a multilayered structure. The layering scheme is shown in Figure 1. A later study9 reported very preliminary work on CoII-1,6-diisocyanohexane (DiCNH) multilayers. This paper presents results of a more extensive study on the growth and structure of cobalt* To whom correspondence should be addressed. Phone: 541346-4693. Fax: 541-346-4643. E-mail:
[email protected]. (1) Ansell, M. A.; Zeppenfeld, A. C.; Yoshimoto, K.; Cogan, E. B.; Page, C. J. Chem. Mater. 1996, 8, 591. (2) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (3) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (4) Gao, H.; Angelici, R. J. J. Am. Chem. Soc. 1997, 119, 6937. (5) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (6) Rong, D.; Kim, Y. I.; Hong, H.-G.; Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. Coord. Chem. Rev. 1990, 97, 237. (7) Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883. (8) Lin, S.; McCarley, R. L. Langmuir 1999, 15, 151. (9) Ansell, M. A.; Cogan, E. B.; Neff, G. A.; Page, C. J. Supramol. Sci. 1997, 4, 21.
Figure 1. Layering scheme for deposition of Co-DiCNB multilayers.
1,4-diisocyanobenzene (Co-DiCNB) multilayers using ellipsometry, XPS, ATR-FTIR, and RAIRS, as well as some further preliminary work on the Co-DiCNH system. Several other types of multilayer thin films with alternating inorganic and organic layers have been recently reported. Perhaps the most well-studied are zirconium bis(phosphonate) thin films.10-14 Other chemistries that have been explored for the production of multilayers include the use of silanes,2,15-18 pyrazines with (10) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (11) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (12) Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S.-L.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855. (13) Zeppenfeld, A. C.; Fiddler, S. L.; Ham, W. K.; Klopfenstein, B. J.; Page, C. J. J. Am. Chem. Soc. 1994, 116, 9158. (14) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567.
10.1021/la990180u CCC: $19.00 © 2000 American Chemical Society Published on Web 11/27/1999
Coordinate Covalent Cobalt-Diisocyanide Multilayer Thin Films
Ru,19 dithiols with Cu,20 and diamines with Ru,21 NiPt(CN)4,22 or Co-Pt(CN)4.23 Each system has been characterized with various combinations of ellipsometry, XPS, IR, AFM, and grazing angle X-ray diffraction and have been shown to display regular layer growth. Although the films are often represented as well-ordered materials, the evidence to support this picture bears closer examination. X-ray diffraction studies at low angles (
