A Grazing Incidence X-Ray Diffraction Study of Self-Assembled Monolayers
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A GRAZING INCIDENCE X-RAY DIFFRACTION STUDY OF SELF-ASSEMBLED MONOLAYERS 3 3 P. Fenterl, 3 , Jun Li 2,3 , P. Eisenberger 1 , T. A. Ramanarayanan , K. S. Liang 1) Dept. of Physics, Princeton Univ., Princeton N.J. 2) Dept. of Chemistry, Princeton Univ., Princeton N.J. 3) Exxon Research and Engineering, Annandale N.J.
ABSTRACT We describe the two dimensional structure of n-Alkyl Thiols, CH3(CH2)17SH (C18), self-assembled on the surfaces of Au(111) and Ag(111). By using Grazing Incidence X-ray Diffraction (GIXD), we show that C18 forms ordered and dense monolayers. Although the properties of Au and Ag are very similar (for instance, the lattice constants of Au an Ag are nearly identical), the structure of C18 on these two substrates is very different. Introduction There has been a great deal of interest in the properties of organic monolayers adsorbed on solid surfaces [1,2,31. Self-assembled (SA) monolayers [4,5,6,7] form by chemisorption of molecules to a substrate from the liquid phase. Because of the many competing interactions present (such as the inter-molecular, head group/substrate and head group/head group interactions...) it is often difficult to isolate the effect of changing only one of these interactions. In this regard, the comparison of the monolayer structure on Au(111) and Ag(111) is very valuable. Because the properties of Au and Ag are very similar (and in particular their lattice constants are nearly identical), the differences in the monolayer structure in these two cases will reflect only a change in the head group/substrate interaction. Although there have been many experiments on the properties and structure of CH 3 (CH 2 )17 SH (C 18 ) adsorbed on Au(111) [4,6,7,8], relatively less work has been performed on the C18 /Ag(111) system[9,10]. From these experiments, it has been inferred that the structures of the monolayer in these two cases is different. In particular, IR absorption studies have observed a striking difference in the molecular tilt. In this paper, we compare the intermolecular structure of C 18 adsorbed on Au(111) and Ag(111) using grazing incidence x-ray diffraction (GIXD) [11,12] to characterize the film structure.
Experimental The preparation details have been published previously [13]. Briefly, we have chosen to use single crystal substrates, which were prepared by mechanical polishing and chemical etching to produce a (111) surface orientation (within ±0.30). This was followed by a series of sputtering and annealing cycles in a standard ultra-high vacuum system to produce a clean and well ordered surface.
Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
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The samples were then transferred to a dilute solution of C18 in ethanol (-lmM), in which the monolayers spontaneously adsorbed over the course of -24 hours. In the case of Ag substrates, because of the reactivity of Ag to atmospheric gases, the substrate was transfered to the C1 8 solution through a load/lock, and was exposed only to a dry N 2 environment. The GIXD measurements were performed at
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