Structural investigation of monolayers prepared by deposition of (CH3S)(2) on the (111) face of single-crystal gold

Danışman, Mehmet Fatih
Casalis, L
Bracco, G
Scoles, G
Although self-assembled monolayers made of long chains of n-alkanethiols [CH3(CH2)(n-1)SH] on Au(111) have been extensively studied in the past, the driving forces behind the appearance of the (3 x 2root3) superlattice observed at full coverage are still not completely understood. To focus on the role played by the sulfur headgroup minimizing the interactions between chains and to prevent a possible X-ray-induced damage, we have carried out a He atom diffraction study of the adsorption of the shortest (n = 1) thiol radical, obtained by dissociative adsorption of (CH3S)(2), on the Au(111) surface. Contrary to current literature reports, domains of the hexagonal (root3 x root3)R30degrees structure are observed to convert into the well-ordered (3 x 2root3) superstructure after annealing times of a few hours. The observation of peaks forbidden by the (3 x 2root3) model proposed by Fenter et al. (Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216) suggests a distortion of the symmetry, of the unit cell. The observed chemisorption and physisorption energies were in good agreement with the literature.