Combined neutron and X-ray diffraction has been used to structurally characterize the crystalline monolayer structures of fluoroalkane monolayers, C6 to C16, adsorbed on graphite at sub-monolayer coverages and over the temperature range from 10 K until the layers melt. In all cases the molecules are found to have their carbon backbones parallel to the graphite surface although the experimentally determined patterns are not particularly sensitive to the orientation of the fluoroalkanes about their long axis on the surface. The layers appear to be incommensurate with the underlying substrate. A significant number of the fluoroalkanes also exhibit both a high and a low temperature phase, attributed to the onset of a rotator phase at higher temperatures. Interestingly, the temperature range of the high temperature phase is much greater than found for the alkanes. The scattering cannot distinguish the chirality of these helical molecules on the surface.
(by A. Inaba)
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Fig. 1. Experimental neutron (dots) and X-ray (crosses) diffraction patterns and the calculated patterns (solid lines) arising from a centered unit cell for 0.7 monolayer coverage on graphite of perfluorohexadecane.
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Fig. 2. Illustration of the unit cell for perfluorotetradecane on graphite.
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Fig. 3. Illustration of the unit cell for perfluoropentadecane on graphite.
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Fig. 4. Experimental neutron (dots) and X-ray (crosses) diffraction patterns and the calculated patterns (solid lines) from pgg unit cell for 0.7 monolayer coverage on graphite of perfluoroheptane.
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Fig. 5. Experimental neutron (dots) and X-ray (crosses) diffraction patterns and the calculated patterns (solid lines) from primitive unit cell for 0.7 monolayer coverage on graphite of perfluoroheptane.
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Fig. 6. Illustration of the pgg unit cell for perfluorononane on graphite.
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Fig. 7. Illustration of the primitive unit cell for perfluorononane on graphite.
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