We report direct evidence of a unit mesh containing more than one hydrocarbon chain at the surface of a self-assembled monolayer of long-chain n-alkanethiols. Our helium diffraction measurements for a monolayer of n-octadecanethiol on Au(111) are consistent with a rectangular primitive unit mesh of dimensions 8.68×10.02 Å containing four crystallographically distinct hydrocarbon chains. This packing arrangement can also be described as a c(4×2) superlattice with respect to the fundamental simple hexagonal [(√3×√3)R30°] array of lattice parameter 5.01 Å previously observed for monolayers of other n-alkanethiols on gold. No temperature-dependent phase behavior is observed in the temperature range where surface diffraction is measurable (30–100 K) and cycling up to temperatures as high as 50 °C caused no observable change in the diffraction. It is proposed that this larger unit mesh is the result of a patterned arrangement of rotations of the hydrocarbon chains about their molecular axes. This patterned arrangement must be different than the herringbone structure expected by simple analogy to bulk n-alkanes.

We report the observation by scanning tunneling microscopy (STM) and low energy atom diffraction, of new, striped, structures at the surface of monolayers of n-alkane thiols [CH3 (CH2)n−1 SH with n=8,10,12] self-assembled on the (111) face of single crystal gold. These structures can be prepared by slow (room temperature) or thermally accelerated treatment of the well known c(4√3×2√3)R 30° phase formed by self-assembly in solution, or can be accessed directly by molecular beam deposition. With respect to the unit mesh of the gold substrate, the new striped structures can be described as p×√3 overlayers where 7.5≤p≤13. The discovery of these phases has implications for the understanding of the growth mechanisms and the pursuit of applications of this widely studied class of materials.

Low-energy helium atom diffraction measurements of the surface structure of n-alkanethiol films deposited from a molecular beam on to the (111) face of gold single crystals (at an impingement rate on the order of 10 11(1 molecules cm -2 s -1 ) show that the thiols form "striped" overlayers. These structures are similar to those previously seen by Dubois et al. in a recent low energy electron diffraction (LEED) study of vapor-deposited overlayers (J. Chem. Phys. 1993, 98, 678), by Poirier et al. in a scanning tunneling microscope (STM) study of low coverage solution-grown short-chain thiols monolayers (Langmuir 1994, 10, 3383), and more recently by us, Poirier, and Tarlov by thermal treatment of the full-coverage c(4 3×2 3)R30°phase formed in the standard way by self-assembly from solution (J. Chem. Phys. 1994, 101, 11031). The surface periodicity of the monolayer structures observed in the present study can be described (with respect to the Au(111) surface lattice) in terms of a rectangular p× 3 unit mesh where p, the periodicity of the stripes, scales linearly with the length of the adsorbed thiol. The absolute value of the stripes' period is, with a maximum deviation of 3%, 1.9 times the length of the corresponding fully stretched thiolate fragment which coincides with the length of the corresponding fully stretched dialkyl disulfide. The present results, analyzed in the context of the others, confirm the presence of coverage-dependent and chain-lengthdependent phase behavior in these systems and suggest that, at the lowest "full" coverages, the molecules may assume a near-flat configuration on the gold substrate.

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 Å. 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 Å at intermediate chain length [CH3(CH2)13SH/Au(111)/mica], decreasing to 26 Å at longer [CH3(CH2)21SH/Au(111)/mica] and 41 Å 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. [J. Hautman and M. Klein, J. Chem. Phys. 93, 7483 (1990).]

We describe the first experimental study of the molecular structure of the self-assembled monolayer of CH&H*)&H formed on the Ag(ll1) surface. Using both low-energy He diffraction and grazing incidence X-ray diffraction, we show that the monolayer formed on Ag is both incommensurate and rotated with respect to the Ag lattice, with a lattice spacing which is very similar to n-alkane chains in bulk hydrocarbon crystals. Lastly, by comparing the results of the X-ray and He diffraction experiments on the same sample, we observe that the outermost surface of the monolayer is less ordered than the interior.