Formation and Two-dimensional Ordering of Organic Thiocyanate Self-Assembled Monolayers on Au(111)

Abstract

Self-assembled monolayers (SAMs) formed by organic thiols on gold have been extensively studied due to a fundamental interest in their surface science and their many potential applications in corrosion inhibition, biosensors, nanopatterning, and molecular electronics [1-3]. The surface structures and adsorption characteristics of the alkylthiocyanate (ATC) SAMs on Au(111) prepared by solution and ambient-pressure vapor deposition methods were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). 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 × 2√19)R5° structures (Fig. 1A). Dodecylthiocyanates (DTC) can also form well-ordered SAMs having a c(4 2) superlattice, as in the case of alkanethiol SAMs (Figs. 1B and C). Our STM and XPS measurements showed that vapor deposition is the more effective method compared to solution deposition for obtaining high-quality SAMs by adsorption of OTC thiocyanates on gold. The results obtained will be very useful in understanding the SAM formation of organic thiocyanates on gold surfaces.