*2.1. Preparation of SAMs*

A great merit of SAM-forming thiols is that they form spontaneously on electronically soft metal substrates such as gold surfaces from liquid or vapor media under mild conditions [5,59]. Well-defined SAMs are formed by immersing the clean metal surface into a thiol solution, commonly ethanol or aqueous solution, for a certain period of time (hours to days), followed by washing with the same solvent. The quality of the SAMs is determined by the variety and concentration of thiol solution, temperature, soaking time, and the metal surface structure. Vapor deposition is also used to prepare SAMs, but the SAM morphology is here hard to control. Single-crystal, atomically planar gold electrodes are the best substrates to investigate the SAM properties, which can then be characterized using several in situ techniques including electrochemical scanning tunneling microscopy (in situ STM). Nanomaterials with porous structure and high surface area are also used as substrates [40,60]. It is noteworthy that a potential-assisted method to accelerate the SAM formation process has been developed [9,61,62]. The potential range is then a key parameter that strongly affects the quality of the SAMs, which can be evaluated simply by the surface coverage.
