Formation and structure of alkylthiocyanate self-assembled monolayers on Au(111)

Abstract

Alkylthiocyanates (ATC) self-assembled monolayers (SAMs) were formed by the spontaneous adsorption on Au(111) using both solution and ambient-vapor deposition methods. Deposition method, temperature effect, and chain length dependence for ATC SAM formation were examined. The surface structures and adsorption characteristics of ATC SAMs on Au(111) were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). STM observation showed that octylthiocyanate (OTC) SAMs formed in solution at room temperature have unique surface structures including the formation of ordered and disordered domains, vacancy islands, and structural defects. Moreover, we revealed for the first time that the adsorption of OTC on Au(111) in solution at 50°C leads to the formation of SAMs containing small ordered domains, whereas the SAMs formed by vapor deposition at 50°C have long-range ordered domains, which can be described as (√3 × √19)R5° structures. In addition, dodecanethiocyanate (DTC) SAMs fabricated by vapor deposition at 50°C have better quality than solution deposition. They have various surface structures including c(4 × 2) structure. XPS measurements of the peaks in the S 2p and N 1s regions for both OTC and DTC SAMs showed that vapor deposition is the more effective method compared to solution deposition for obtaining high-quality SAMs by adsorption of alkylthiocyanates on gold. The results obtained will be very useful in understanding the SAM formation of organic thiocyanates on gold surfaces.