Defect study of self-assembled monolayers by chemical etching

Self-assembled monolayers (SAMs) of 4-methyl-4′-mercaptoethyl-biphenyl (HS-CH2-CH2-C6H4-C6H4-CH3) and hexadecane thiol (HS-(CH2)15-CH3) adsorbed on Au(111)/mica were investigated by cyanide etching in order to identify defects in the monolayer. The etch pits formed around a defect were examined ex situ by scanning tunneling microscopy. For both thiols removal of gold atoms begins in the vicinity of terrace edges and leads to the formation of triangular pits on the gold terraces. The defect densities of both thiols are comparable and the etch rate is slightly higher for the alkane thiol compared to the biphenyl thiol. This feature combined with a charge permeability orders of magnitude higher than for alkane thiols makes biphenyl based thiols a promising material for modifying electrochemical properties of electrodes.