SURFACE-STRUCTURE AND THERMAL MOTION OF NORMAL-ALKANE THIOLS SELF-ASSEMBLED ON AU(111) STUDIED BY LOW-ENERGY HELIUM DIFFRACTION

Low energy helium diffraction has been used to study the packing and thermal motion of the terminal CH3 groups of monolayers of n-alkane thiols self-assembled on Au(111)/mica films and a Au(111) single crystal surface. At low temperatures (< 100 K), the terminal CH3 groups are arranged in domains containing a hexagonal lattice with a lattice constant of 5.01 angstrom. As the length of the carbon chain is shortened, an abrupt decrease in the diffraction peak intensities is observed for CH3 (CH2)9SH/Au(111)/mica, and no diffraction is observed for CH3 (CH2)5SH/Au(111)/mica. This is indicative of a sudden decrease in surface order at around ten carbon atoms per chain. A semi-quantitative estimation of the average domain size of each monolayer surface shows a maximum of 46 angstrom at intermediate chain length [CH3 (CH2)13SH/Au(111)/mica], decreasing to 26 angstrom at longer [CH3 (CH2)21SH/Au(111)/mica] and 41 angstrom at shorter [CH3 (CH2)9SH/Au(111)/mica] chain lengths. No phase transitions could be detected at the surfaces of these monolayers from 35 K to 100 K, but as expected for a soft material, the thermal motion of the n-alkane thiol molecules increases with increasing surface temperature and reduces the diffraction intensities to zero at around 100 K. The relative mean square displacements of the surface CH3 groups along the directions perpendicular and parallel to the surface have been calculated from the temperature dependence of the diffraction peak intensities using the standard Debye-Waller formalism. The measured values are in good agreement with the results from a recent molecular dynamics simulation.