Growth, solvent effects, and thermal desorption behavior of octylthiocyanate self-assembled monolayers on Au(111)

Abstract

The growth process, solvent effects, and thermal desorption behavior of octylthiocyanate [OTC, CH₃(CH₂)₇S–CN] self-assembled monolayers (SAMs) on Au(111) were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). To investigate their growth processes, octanethiol [OT, CH₃(CH₂)₇S–H] and OTC SAMs were prepared in 0.5 μM ethanol solution at room temperature as a function of immersion time: 10 min, 1 h, 2 h, and 24 h. STM imaging revealed that OT SAMs underwent a phase transition from the liquid phase containing striped-phase domains to the closely packed c(4 × 2) phase. OTC SAMs underwent a different phase transition from the liquid phase containing aggregated molecules to the disordered phase containing striped-phase domains. The adsorption amounts of OTC SAMs formed after immersion for 10 min and 24 h were measured to be 16% and 30% smaller than those of OT SAMs under the same conditions. STM and XPS results show that the growth kinetics of OTC SAMs on Au(111) are much slower than those of OT SAMs. Hexane resulted in OTC SAMs of higher structural quality than ethanol, DMF, or toluene. TDS measurements revealed that the relative desorption intensities of octanethiolate (C8S⁺, monomer) and dioctyl disulfide (C8SSC8⁺, dimer) to octanethiol (C8SH⁺) fragments for OTC SAMs were much weaker than those of OT SAMs. This is because desorption of monomers and dimers is strongly suppressed by low surface coverage of OTC SAMs, as revealed by STM observations.