DIRECT OBSERVATION OF 4-MERCAPTOPYRIDINE AND BIS(4-PYRIDYL) DISULFIDEMONOLAYERS ON AU(111) IN PERCHLORIC-ACID SOLUTION USING IN-SITU SCANNING-TUNNELING-MICROSCOPY

Organic monolayers of 4-mercaptopyridine (PySH) and bis(4-pyridyl) dis ulfide (PySSPy) self-assembled on an Au(lll) electrode in perchloric a cid solution were studied using electrochemical scanning tunneling mic roscopy (STM). Both monolayers formed from PySH and PySSPy exhibited t he same surface feature on Au(111), appearing as a ladderlike structur e. An ordered structure with intermolecular distances of 0.49 and 1.42 nm was consistently observed for the PySH and PySSPy monolayers descr ibable as Au(111)-SPy, which was explained by a rectangular unit cell of p(5 x root 3R-30 degrees). High-resolution STM images revealed the internal structures and micro-orientation of chemisorbed pyridinethiol ate molecules. They were oriented with the molecular plane of the pyri dine rings mostly perpendicular to the surface, but the molecular axis considerably tilted with respect to the surface normal. It was also f ound that intermolecular interactions through the sulfur portions were thought to exist between two pyridinethiolate molecules, as is expect ed to be found in dimer-state adsorption. Thus, we report, for the fir st time, the detailed investigation of pyridinethiolate monolayers on Au(111) in solution at the molecular level.