Delicate surface reaction of dialkyl sulfide self-assembled monolayers on Au(111)

Self-assembly of n-dioctadecyl sulfide (ODS) on Au(111) has been closely in vestigated by using X-ray photoelectron spectroscopy (XPS), in which the bi nding condition of sulfur on Au(111) was determined by the S(2p) XPS peak p osition, and the surface density and chain conformation of adsorbed molecul es were determined by the relative XPS peak intensity, C(1s)/S(Pp). The sur face reaction of ODS on Au(111) was unstable unlike ODT SAM, and it was cha nged drastically by small variation of adsorption condition. When adsorptio n was carried out in 1 mM CH2Cl2 solution at room temperature, ODS molecule s mostly formed fully adsorbed SAMs, intact without C-S cleavage. This was evaluated by the C(1s)/S(2p) intensity, which was twice as strong as ODT SA M, and by the S(2p) peak which appeared as a doublet at the position of "un bound" sulfur [S(2p(3/2)) at similar to 163 eV], suggesting "physisorption" of ODS on Au(111). On the other hand, when a different condition for SAM f ormation was used (e.g., high temperature, long time immersion, or CHCl3 as a solvent), the C(1s)/S(2p) intensity decreased to a value smaller than OD T SAM, and the S(2p) peak was shifted to lower binding energies, the "bound " (162 eV) and "free" (161 eV) sulfur positions. In these SAMs, different s urface reactions including carbon-sulfur (C-S) bond cleavage seem to occur rather than nondestructive adsorption. High-resolution atomic force microsc ope images revealed that ODS SAM, prepared by 24-h immersion in 1 mM CH2Cl2 solution at room temperature, formed a hexagonal lattice with the lattice constant, d = 0.46 nm, which is nearly equal to the close-packed distance b etween alkyl chains and totally incommensurate against gold adlattice. Our data suggest a unique self-assembling process of ODS SAM, in which the chai n-chain interaction is expected to be more predominant rather than the mole cule-substrate interaction unlike ODT SAM.