Self-assembly of aromatic thiols on Au(111)

We have studied the self-assembly of two aromatic thiols, 4-mercaptopyridin e (4MPY) and 4-hydroxythiophenol (4HTP), on Au(lll) from aqueous solutions with in situ scanning tunneling microscopy and ex situ X-ray photoelectron and Auger electron spectroscopies. Upon adsorption of the molecules, island s of nanometer scale appear immediately on flat Au(lll) terraces, which is in sharp contrast to the formation of depressions in the self-assembly of t he n-alkanethiol monolayers. The islands increase both in size and number i nitially, 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 sur face which become highly mobile due to the strong binding to the thiols. Th e decrease of the islands at the late stage is due to the fact that atoms i n small islands diffuse into nearby big islands and surface step edges. Bot h 4MPY and 4HTP molecules are randomly distributed on the surface immediate ly after adsorption, but slowly arrange into domains of ordered structures. For 4MPY the ordered structure has lattice constants, a = 5.1 +/- 0.3 Angs trom, b = 15.8 +/- 0.3 Angstrom and alpha = 78 +/- 3 degrees, whereas 4HTP packs into a (root 3 x root 3)R30 degrees lattice. (C) 1999 Elsevier Scienc e B.V. All rights reserved.