The structure and stability of alkanethiol self-assembled monolayers on gold

Self-Assembled Monolayers (SAMs) have attracted a great deal of interest in recent years due to their potential applications in molecular assembly as model surfaces. In the literature, most SAMs studies are conducted on evaporated gold (Au) on silicon wafer, with Chromium (Cr) or Titanium (Ti) as an interlayer to improve adhesion. However, both Cr and Ti are diffusable inside the Au layer where the former diffuses faster than the latter. Once Cr or Ti diffuses to the Au surface, SAM will desorb easily after assembly. In addition, the high surface free energy of Au is reduced by adsorption of organic molecules from the environment; hence, complete wetting of Au cannot be achieved under ambient conditions. Typically, organic contaminants on Au are removed via exposure to a strong oxidant, including "piranha" solution and oxygen plasma generated by UV (ultraviolet) light. However, these treatments leave a surface oxide which can affect the properties of SAMs deleteriously. Another surface cleaning method---flame annealing---has frequently been used to clean the Au substrate. This process not only removes the organic contaminants, but also shaves the "rolling hills" on the evaporated Au surface. The structure and stability of SAMs formed on evaporated, piranha-etched and annealed Au/Cr/Si and Au/Ti/Si substrates are compared in detail. We found that only SAMs on annealed Au/Ti/Si substrate are crystalline, densely-packed and stable under sonication. To our knowledge, this is the first study to define the relationship between SAMs pretreatment, structure and stability. Ex situ Ellipsometry, Fourier Transform Infrared (FTIR) were used to characterize SAMs' structure.