Formation of long-range-ordered self-assembled monolayers of dodecyl thiocyanates on Au(111) via ambient-pressure vapor deposition

Abstract

The surface structure, binding condition, and reductive desorption behavior of self-assembled monolayers (SAMs) of dodecyl thiocyanate (DDTC, C12–SCN) on Au(111) formed via solution and ambient-pressure vapor depositions at 50 °C for 24 h were examined by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). STM imaging clearly revealed that adsorption of DDTC molecules on Au(111) in a 1 mM ethanol solution led to the formation of short-range-ordered dodecanethiolate (C12S) SAMs with a domain size ranging from several nanometers to 20 nm, whereas the SAMs formed via vapor deposition had long-range-ordered C12S SAMs of size exceeding 60 nm, which were characterized as having a (4 × √3)rect packing structure. XPS measurements showed that the DDTC SAMs formed via vapor deposition on Au(111) contained approximately one-fourth the amount of unbound sulfurs and CN species compared to solution-deposited SAMs. CV measurements also showed that vapor-deposited SAMs had a sharp reductive desorption peak at −1.028 V, whereas solution-deposited SAMs had two broad desorption peaks at −0.671 and −0.946 V, implying that vapor-deposited SAMs are more uniform and electrochemically stable. The present results clearly demonstrate that DDTC SAMs on Au(111) with a high degree of structural order, homogeneous interface, and high electrochemical stability can be prepared by ambient-pressure vapor deposition.