New Insights in the c(4×2) Reconstruction of Hexadecanethiol on Au(111) Revealed by Grazing Incidence X-ray Diffraction

Abstract

The c(4 x 2) structure of C16H33SH alkanethiol monolayers self-assembled on Au(111) has been studied by grazing incidence X-ray diffraction. This structure coexists on the surface with the (radical3x radical3)R30 degrees phase. The structural refinement of the c(4 x 2) phase has been accomplished by omitting the fractional order reflections common to both structures. The surface unit cell consists of four symmetry-independent molecules with atomic displacements related by couples, such that only two nonequivalent chains are present in the surface cell. The stability between neighbor chains is due to van der Waals interactions. The substrate plays an important and non-negligible role in the c(4 x 2) reconstruction. The lateral and normal substrate relaxations to the surface plane are small, and gold atom displacements are lower than 0.25 angstroms but contribute very strongly to the fractional order intensities. The molecular chains form a close packed structure tilted by 37 degrees from the surface normal with no indications of dimer formation between closest S atoms.

The interface structure of n-alkylthiolate self-assembled monolayers on coinage metal surfaces

“…There are two relatively recent SXRD studies, one of the (√3x√3)R30° phase of dodecylthiolate [86], the other of the (2√3x3)rect. phase of hexadecylthiolate [46]. As remarked earlier, the long chain thiolates at high coverage invariably lead to a coexistence of the (√3x√3)R30°…”

“…A more recent SXRD investigation of the (2√3x3)rect. phase of hexadecylthiolate (n=16) [46], however, based on a very much larger data set than that which appears to have been used in the original 1994 study, concluded that the two inequivalent S headgroup adsorption sites were the fcc and hcp hollow sites. This investigation sought to minimise the problem of the coexistence of the (√3x√3)R30° and (2√3x3)rect.…”

“…As mentioned earlier, the investigation of the (2√3x3)rect. phase of hexadecylthiolate [46] led to a best-fit model involving co-occupation (within the larger unit mesh) of molecules in fcc and hcp hollow sites. It is important to note, however, that both of these studies predate the more recently published evidence for the role of Au adatoms in the surface, so these structural models were not tested.…”

Surface Dipoles and Work Functions of Alkylthiolates and Fluorinated Alkylthiolates on Au(111)

“…Evidence against the dimer model was presented by scanning tunneling microscopy [42] and by electron spectroscopy experiments [43]; in the latter, thiolate dimers were found only at temperatures above 375K. In recent He and X-ray diffraction experiments it was concluded that in the (4 2) c × superstructure alkyl-thiolate molecules adsorb as monomers on the Au(111) surface [44,45].…”

Of interfaces and spin transport across nanoscale layers

“…A persisting controversy surrounds the precise adsorption site of the sulfur headgroup on the Au(111) surface. Time of flight scattering and GXRD studies emphasize adsorption at (fcc and hcp) hollow sites [111,112], where the sulfur head-group binds to three Au surface atoms, while photoelectron diffraction (PED) and normal incidence x-ray standing wave (NIXSW) studies prefer on-top adsorption, where sulfur binds to one Au atom [113,114] . Computational studies offer a third possibility.…”