No abstract

We present a study using several techniques of the growth of decanethiol monolayers deposited on singlecrystal gold surfaces. Through independent measurements of coverage, energetics, and structure as a function of the growth rate and temperature, we provide a quantitative, in-depth description of the molecular processes by which these aliphatic molecules ''self-assemble'' into highly ordered structures in the absence of a solvent. We find that the multiple-energy scales present in these systems produce distinct adsorption mechanisms, structures, and growth regimes, indicating a complexity that is likely to be a general characteristic of this broad class of self-assembling systems. ͓S0163-1829͑98͒03019-7͔

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.

Monolayers of 1,6-hexanedithiol [HS(CH2)6SH] deposited on Au(111) from the gas phase were characterized by scanning tunneling microscopy (STM), grazing incidence X-ray diffraction (GIXD), and low-energy atom diffraction (LEAD). Molecular resolution STM images suggest that the molecules lie prone in a striped arrangement with an inter-row spacing of 5 Å. For the films prepared at an elevated temperature, two uniaxial incommensurate phases were found by GIXD. With respect to the surface substrate net, the diffraction patterns of both phases can be described by rectangular (p × 3) nets, where p is 4.24 ( 0.01 and 4.30 ( 0.01. These values of p correspond to spacings of 12.23 ( 0.04 and 12.40 ( 0.02 Å along the nearest-neighbor (NN) direction of the substrate, whereas the spacing along the next-nearest-neighbor direction is 5 Å in both cases. The LEAD patterns can be described by a 3 × 1 superlattice with respect to the mesh observed by GIXD. Lattice nonuniformity and angular broadening along the NN direction were observed by GIXD. The structure of the striped phases is consistent with the molecules being fully extended and flat on the surface with their molecular C-C-C plane parallel to the surface. Using different growth protocols, including liquid-phase deposition, the order of the striped phases was observed to change considerably; however, no evidence of nucleation of other ordered phases was found. Even if denser phases exist, the striped phases may act as effective kinetic traps preventing the transition to other denser phases. The results of both varied growth conditions and performed annealing experiments can be explained by the strong molecule/substrate interaction in the striped phases, which is a consequence of the strong, but not site-specific, interaction of both sulfur atoms with the gold surface.