Headgroup Effects on the Formation and Structure of Self-Assembled Monolayers on Au(111)

Abstract

To examine the effects of headgroups on the formation and structure of self-assembled monolayers (SAMs) on Au(111), the SAMs derived from dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS), cyclohexanethiol (CHT) and cyclohexyl thiocyanate (CHTC), benzenethiol (BT) and phenyl thiocyanate (PTC), and benzyl mercaptan (BM) and benzyl thiocyanate (BTC) were characterized by the scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), contact angle (CA), and cyclic voltammetry (CV). It was found that DMTS with trisulfides remarkably affects the formation of vacancy islands (VIs) due to higher content of sulfur per molecule compared to DMDS. DMTS SAMs formed at RT for 1 h in a 0.01 mM ethanol solution had short-range ordered phase, whereas those formed in a 1 mM ethanol solution showed disordered phase. DMTS SAMs contained 20 – 30% of VIs, which are 3 – 10 times higher than those of DMDS SAMs. STM observations revealed that DMDS SAMs had ordered phase, whereas DMTS SAMs were mainly composed of disordered phase with a partial ordered phase. XPS measurement showed that S 2p peaks for DMDS or DMTS SAMs were appeared at 161.1 – 161.2 and 162 eV, indicating that both SAMs chemisorb at gold surface. S 2p peak with a strong intensity for DMTS SAMs at 161.1 eV implies the presence of atomic sulfur layer emerged from the adsorption of trisulfide on Au(111). Multiple reductive desorption peaks for DMTS SAMs were observed around -910 ~ -990 mV, implying the existence of atomic sulfur layer, which is supported by the XPS results. Moreover, to understand the self-assembly phenomena and structure of organic thiocyanates on Au(111), we also examined the formation and surface structure of SAMs formed by alicyclic and aromatic thiocyanates. CHT SAMs prepared at RT for 1 h in a 1mM ethanol solution had ordered phase on an entire Au(111) surface, while CHTC SAMs had a mixed phase containing ordered and disordered phases. CHTC SAMs with ordered domains showed (5 × 2√3)R35° lattice structure, similar to CHT SAMs. BT SAMs prepared via vapor phase deposition at 50 °C for 1 h showed row structure with a distance of 1.26 ± 0.08 nm between the rows. PTC SAMs also showed similar structure with a spacing of 1.23 ± 0.14 nm between the rows. On the other hand, BM SAMs showed highly ordered (√3 × √3)R30° lattice structure regardless of deposition methods, whereas BTC SAMs showed different structure depending on the deposition methods. BTC SAMs prepared via vapor phase deposition at 50 °C showed ordered phase with increasing deposition time. Ordered domains for BTC SAMs prepared after 24 h deposition can be assigned to (3 × 5√3)rect unit cell with a molecular density of 27.1 Å2/molecule. On the contrary, BTC SAMs prepared via solution phase deposition were composed of disordered phase. The structural differences between thiol and thiocyanate SAMs are attributed to the different growth kinetics of thiocyanate SAMs are slower than those of thiol SAMs, as demonstrated in a previous work. From this study, we clearly revealed that the chemical structure of headgroup significantly affects the formation and surface structure of SAMs on Au(111).