CHAIN-LENGTH-DEPENDENT STRUCTURE OF ALKANETHIOLS FORMING DIMERS ON AU(111)

It was commonly accepted the alkanethiols (C2H2n+1SH) self assemble on Au(111) into the (root 3 x root 3)R30 degrees structure by chemisorpt ion of the sulfur atoms to the three-fold hollow sites. However, the o bservation of (p x root 3) structures and evidence for dimerized sulfu rs on the substrate have questioned this model. We have imaged alkanet hiols on Au(111) for 4 less than or equal to n less than or equal to 8 by scanning tunneling microscopy under ambient conditions. We found ( p x root 3) structures for n less than or equal to 6 and the (root 3 x root 3)R30 degrees structure for n greater than or equal to 7 both fo r high densities of (22 +/- 2)Angstrom(2) per molecule indicating a de pendence of structure on chain-length. We can describe our observation s by two distinct crystalline phases of dimerized thiol molecules. (C) 1998 Elsevier Science B.V.