Molecular ordering of organosulfur compounds on Au(111) and Au(100): Adsorption from solution and in ultrahigh vacuum

Low-energy electron diffraction and reflection-absorption infrared spectroscopy were used to study the monolayers formed by the adsorption of n-alkane thiols [HS(CH₂)_m CH3] on both (111) and (100) single-crystal gold substrates. Samples were prepared by dosing either from solution (m=15, 17, 18, and 21) or in ultrahigh vacuum (m=0-9). On Au(111), ordered surface structures are obtained which can be indexed as (n√3×√3) R30°, where n varies from 1 to 6. On Au(100), the adsorption of short chain thiols leads to the formation of a c(2×2) overlayer while the longer chain homologs show additional diffraction spot splittings. It is also found that chain length influences both the character of the diffraction seen and perturbs the reactive sticking probability of molecules dosed in UHV. Infrared studies, reveal that the polymethylene chains of the monolayers formed on Au(100) are comprised of nearly all-trans conformations and are less canted than the comparable structures formed on Au(111). A simple model is proposed to account for the differences seen on the two surfaces and a mechanism for the adsorption and assembly of organic thiols into ordered monolayer phases on gold is presented.