of Fig. 4a, the sulfur atom sits on the top of the gold atom. The lone pair in the sulfur is attracted to this charge depletion region, and 0.3e is transferred from the sulfur lone pair or- bital to the gold charge depletion area. Thus when the sulfur atom in the thioglycolic acid is adsorbed on the Au(111) (Fig. 4a), the electrostatic interaction responsible for the bonding comes from the monopole term and the dipole moments in the adsorbate and substrate $^{31}$. Around the sulfur atom, there is a “ring” of accumulation of electron charge. The electro- static interaction is dominated by the attractive ionic term modified by a repulsive dipolar term $^{31}$. The sulfur atom stays on the top of the gold atom, that is, the S atom only forms a bond with one gold atom. If the mercaptan-hydrogen atom is detached from the sulfur (Fig. 4b), the S-Au bond is largely covalent with some ionic character $^{32}$. The sulfur in the SCH$_{2}$COOH forms bonds with two gold atoms of the Au(111) surface. The interaction between the HSCH$_{2}$COO − and the gold surface is the similar to the thioglycolic acid case (Fig. 4c), except that the O1 atom form an additional bond with the gold atom. There is big depletion of electrons around the gold atom. The p$_{x}$ , p$_{y}$ orbits in the O1 atom gain extra electrons, but the p$_{z}$ orbit loses some electrons. Thus, this O1-Au bonding is a convolution between the ionic bond and the covalent bond. Fig. 4d suggests that the S-Au bonds for the SCH$_{2}$COO − is a covalent bond with some ionic character. From the above discussion, we propose the following picture. When the HSCH$_{2}$COOH molecule is adsorbed on the gold substrate, it lies down on the surface 8 (Fig. 1a). When the 2,2’-dithiodiacetic acid is put on the gold surface, the SCH$_{2}$COOH molecules form the SAM on the substrate $^{9}$. If the thioglycolic acid is deposited on the gold surface with defects, the mercaptan hydrogen atom can be dissociated from the S atom and the HSCH$_{2}$COOH molecule becomes SCH$_{2}$COOH $^{11,12}$. Some SCH$_{2}$ COOH molecules lie down on the substrate (see Fig. 1b), but others rise up to a certain level. Thus, in the case of SCH$_{2}$COOH, different configurations may admix and the adsorption appears to be randomly oriented $^{9}$. If the carboxylic acid group in the HSCH$_{2}$COOH molecule is deprotonated by increasing the pH value, the resulting HSCH$_{2}$COO − lies on the surface (Fig. 1c). However, when the mercaptan hydrogen atom in the HSCH$_{2}$COO − molecule is ruptured from the sulfur, the resulting SCH$_{2}$COO − molecule rises up to a certain level 8 (Fig. 1d). F. The adsorption mechanism 12