Formation and exchange processes of alkanethiol self-assembled monolayer on Au(111) studied by thermal desorption spectroscopy and scanning tunneling microscopy

The exchange phenomena from hexanethiol ( CH3(CH2)5SH: HT) to octadecanethiol ( CH3(CH2)17SH: ODT) on a Au(111) surface have been studied by thermal desorption spectroscopy (TDS) and scanning tunneling microscopy (STM). The TDS study showed the existence of (CH3(CH2)5S)2 (C6–C6), CH3(CH2)5S–S(CH2)17CH3 (C6–C18), and (CH3(CH2)17S)2 (C18–C18) dimer species in the monolayer. Desorption peaks for each dimer species strongly depend on the immersion time in the ODT solution, which revealed the details of the exchange process in the HT monolayer. Namely, originally dimerized species of C6–C6 are gradually replaced by C6–C18 dimers, followed by the replacement of C18–C18 dimers. The STM study showed that the exchange occurred not randomly but at specific domains and in specific domain boundary regions. In addition, STM of annealed samples revealed that there exist at least two types of domains corresponding to those seen in the TDS peaks. Dimerization occurs in these regions. Our results show that the specific domains and domain boundaries act as important areas for exchange phenomena and monolayer formation.