Self-assembly of aromatic thiols on Au(111)

We have studied the self-assembly of two aromatic thiols, 4-mercaptopyridine (4MPY) and 4-hydroxythiophenol (4HTP), on Au(111) from aqueous solutions with in situ scanning tunneling microscopy and ex situ X-ray photoelectron and Auger electron spectroscopies. Upon adsorption of the molecules, islands of nanometer scale appear immediately on flat Au(111) terraces, which is in sharp contrast to the formation of depressions in the self-assembly of the n-alkanethiol monolayers. The islands increase both in size and number initially, and decrease slowly after reaching maximum values in about 15 min. The formation of the islands can be attributed to the Au atoms on the surface which become highly mobile due to the strong binding to the thiols. The decrease of the islands at the late stage is due to the fact that atoms in small islands diffuse into nearby big islands and surface step edges. Both 4MPY and 4HTP molecules are randomly distributed on the surface immediately after adsorption, but slowly arrange into domains of ordered structures. For 4MPY the ordered structure has lattice constants, a = 5.1±0.3 angstroms, b = 15.8±0.3 angstroms and α = 78±3°, whereas 4HTP packs into a (√3×√3)R30° lattice.