1,6-hexanedithiol monolayers on Au(111): A multitechnique structural study

Monolayers of 1,6-hexanedithiol [HS(CH2)(6)SH] deposited on Au(lll) from th e gas phase were characterized by scanning tunneling microscopy (STM), graz ing incidence X-ray diffraction (GIXD), and low-energy atom diffraction (LE AD). Molecular resolution STM images suggest that the molecules lie prone i n a striped arrangement with an inter-row spacing of 5 A. For the films pre pared at an elevated temperature, two uniaxial incommensurate phases were f ound by GIXD. With respect to the surface substrate net, the diffraction pa tterns of both phases can be described by rectangular (p x root 3) nets, wh ere p is 4.24 +/- 0.01 and 4.30 +/- 0.01. These values of p correspond to s pacings of 12.23 +/- 0.04 and 12.40 +/- 0.02 Angstrom along the nearest-nei ghbor (NN) direction of the substrate, whereas the spacing along the next-n earest-neighbor direction is 5 if in both cases. The LEAD patterns can be d escribed by a 3 x 1 superlattice with respect to the mesh observed by GIXD. Lattice nonuniformity and angular broadening along the NN direction were o bserved by GIXD. The structure of the striped phases is consistent with the molecules being fully extended and flat on the surface with their molecula r C-C-C plane parallel to the surface. Using different growth protocols, in cluding liquid-phase deposition, the order of the striped phases was observ ed to change considerably; however, no evidence of nucleation of other orde red phases was found. Even if denser phases exist, the striped phases may a ct as effective kinetic traps preventing the transition to other denser pha ses. The results of both varied growth conditions and performed annealing e xperiments can be explained by the strong molecule/substrate interaction in the striped phases, which is a consequence of the strong, but not site-spe cific, interaction of both sulfur atoms with the gold surface.