| Abstract
| - The molecular-scale structure and phase behavior of single-component Langmuir films of 4‘-methyl-4-mercaptobiphenyl (MMB) and 4‘-perfluoromethyl-4-mercaptobiphenyl (FMMB) on mercury were studiedusing surface tensiometry, grazing incidence X-ray diffraction, and X-ray reflectivity. At low coverages, acondensed but in-plane disordered single layer of surface-parallel molecules is found for both compounds. Athigh coverages, both compounds exhibit in-plane-ordered phases of standing-up molecules. For MMB, thebiphenyl core dominates the structure, yielding a centered-rectangular unit cell with an area Ax of 21.8 Å2/molecule, with molecules tilted by ∼14° from the surface normal in the nearest-neighbor direction, and acoherence length ξ of >1000 Å for the crystalline domains. For FMMB, the perfluoromethyl group dominatesthe structure, yielding a hexagonal unit cell with untilted molecules, an area Ax of 24.2 Å2/molecule, and amuch smaller ξ of ∼110 Å. The structure is discussed in comparison with self-assembled monolayers ofMMB on crystalline Au(111) and similar-length alkanethiolate SAMs on Au(111) and on mercury. Thedifferences in the structure are discussed and traced to the differences in the substrate's surface structure, andin the molecular cross section and rigidity.
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