AN ATOMIC-FORCE MICROSCOPY STUDY OF A SELF-ASSEMBLED OCTADECYL MERCAPTAN MONOLAYER ADSORBED ON GOLD(111) UNDER POTENTIAL CONTROL

We have used an atomic force microscope (AFM) to study a self-assembled octadecyl mercaptan monolayer on Au(111) under 100 mM NaClO4 solution. We controlled Au surface potential by electrochemical methods, so that the monolayer could be imaged in situ under different surface charge conditions. The monolayer is quite stable over a wide range of potentials but is destroyed when surface potential is higher than 1590 mV or lower than a value between -370 and -755 mV vs Ag/AgCl. The failure of the film at negative potentials depends on how much O2 is dissolved in the solution. The disintegration of the film is found to be irreversible. We measured the film thickness, and, by comparing the images of film with that of Au after the monolayer was stripped off, the proposed (square-root 3 X square-root 3)R30-degrees overlayer structure was confirmed. We have also done electrochemical voltammetry measurements which agree with the AFM results.