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Langmuir 1996, 12, 436-440
Formation of Two-Dimensional Mixed Crystals on Graphite Observed by Scanning Tunneling Microscopy F. Stevens, D. J. Dyer, and D. M. Walba* Department of Chemistry and Biochemistry and Optoelectronic Computing Systems Center, Campus Box 215, University of Colorado, Boulder, Colorado, 80309-0215 Received July 3, 1995. In Final Form: September 18, 1995X Scanning tunneling microscopy (STM) is a powerful tool for observing organic molecules in real space. In this study, five similar ferroelectric liquid crystal (FLC) materials (four unsaturated isomers and the saturated parent) adsorbed on graphite were examined by scanning tunneling microscopy as neat compounds and as mixtures. The unit cells of adsorbed monolayers formed by the mixtures showed distinct differences from the unit cells of the pure compounds, suggesting that mixed monolayers were formed. The saturated compound formed two structures having different unit cells, and it appeared that the difference in energies was very small. Changing the location of the double bond had no observable effect on the adsorbed monolayers, and the isolated double bonds were not observed.
Introduction In scanning tunneling microscopy (STM) an atomically sharp metal tip is scanned over a conducting surface while a voltage is applied between the tip and the surface. In typical experiments, the tip height is adjusted during scanning to maintain the current at a constant value. In this way, an atomic-scale map of surface conductivity and topography can be obtained. Furthermore, it has been found that if a smooth, flat, conducting surface is covered with an organic compound, the molecules are sometimes found to adsorb to the surface, forming a monolayer of 2-D crystal. This adsorbed monolayer typically modulates the tunneling current and images of individual adsorbed organic molecules at up to atomic resolution can be obtained.1 By allowance for observation of adsorbates at molecular resolution and in real space, STM provides a unique opportunity to study surface structure. We are particularly interested in studying the interactions of liquid crystals (LCs) with solid surfaces, as these interactions are important for LC device fabrication. STM provides a method for characterizing the solid surface contacting liquid crystal films in certain circumstances. Specifically, it is known that when a LC sample is placed on highly oriented pyrolytic graphite (HOPG), 2-D crystals of LC molecules often grow on the HOPG.2 These crystal monolayers can then be imaged by STM under the LC film in situ, providing the desired molecular-level characterization of the solid surface (2-D crystal) contacting the LC film. In such crystal monolayers it is widely accepted that aromatic moieties image brighter (that is, exhibit more efficient tunneling) by STM than aliphatic regions. However, little data on compounds possessing isolated functional groups has been reported, and a general theory of STM imaging of organic molecules is still lacking. In connection with another project, unsaturated isomers 2a-d of the saturated chiral smectic liquid crystal 13,4 were recently synthesized in these laboratories [Figure 1].5 These compounds were attractive for STM study as many had LC phases at room temperature, and it was * To whom correspondence should be addressed: phone, (303) 492-6247; FAX, (303) 492-5894. X Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298-1328. (2) Smith, D. P. E.; Ho¨rber, H.; Gerber, Ch.; Binnig, G. Science 1989, 245, 43-45. (3) Walba, D. M.; Ros, M. B.; Sierra, T.; Rego, J. A.; Clark, N. A.; Shao, R.; Wand, M. D.; Vohra, R. T.; Arnett, K. E.; Velsco, S. P. Ferroelectrics 1991, 121, 247-257.
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Figure 1. Structures of the five liquid crystals observed.
hoped that the isolated double bond would be observed. STM images were indeed successfully obtained for all five compounds. Mixtures of each of the unsaturated compounds 2a-d with the saturated parent 1 were also studied in the hope that the double bond would act as a marker to allow identification and analysis of mixed crystals. The results of this study are presented below. Experimental Section Each STM sample was prepared by placing a small amount (
