Stability, structure, and barrier properties of self-assembled films on metal supports

Abstract

The first part of this thesis details the use of self-assembled monolayers (SAMs) derived from the adsorption of n-alkanethiols [CH 3(CH2).-ISH] onto copper to protect the underlying metal from corrosion. Due to their dense packing and crystalline structure, these films provide a barrier that impedes the transport of oxygen and water to the copper surface. As measured by electrochemical impedance spectroscopy, the resistance provided by these films increases by 4.2 MQ*cm 2 for each methylene unit in the adsorbate that forms the SAM for chain lengths n 16. Efforts to form thicker, more protective SAMs on copper utilized the assembly of long-chain [omega]-alkoxy-nalkanethiols [CH 3(CH2)p- 10(CH2)mSH; m = 11, 19, 22; p = 18, 22] that contain an internal ethereal unit. The barrier properties of these ether-containing SAMs depend on the chain length of the adsorbate and the position of the ethereal unit along the hydrocarbon chain. For all SAMs studied, the crystalline, densely packed structure of the film dramatically affects its resistance against the transport of corrosive agents. The eventual loss in protection of these films is attributed to oxidation and subsequent roughening of the underlying copper surface which perturbs the crystalline hydrocarbon lattice of the SAM. Upon prolonged exposure to 1 atm of 02 at 100% relative humidity (RH), the SAMs that exhibited the most stable crystalline structures were more effective in maintaining their barrier properties at superior levels. The results indicate that the design of barrier coatings requires a selection of adsorbates that can achieve dense packing and high crystallinity and are able to maintain their structural properties. The second part of this thesis discusses the use of underpotential deposition (upd) of silver and copper on gold to affect the structure and stability of an adsorbed n-alkanethiolate SAM. Thiols adsorb onto gold surfaces modified by submonolayer quantities of silver or copper and form SAMs with macroscopic properties similar to those of SAMs on gold, as evidenced by wetting and ellipsometric thickness measurements. Nevertheless, the molecular-level features of these films are distinct from those of SAMs on the native metals (gold, silver, or copper). First, the presence of the upd metal alters the binding and molecular structure of the adsorbed thiol, resulting in a more dense packing and a different orientation for the terminal methyl (-CH3) group than on gold. In addition, the presence of a silver upd adlayer improves the thermal stability of the adsorbed monolayer while the presence of either a silver or copper upd layer improves the stability of the SAM against exchange with competing adsorbates at room temperature. The improved stability of the SAMs on upd-modified gold is attributed to a stronger ligation between the adsorbed sulfur and the upd metal. These results demonstrate that a single atomic layer of silver or copper is sufficient to achieve the adhesion of evaporated films of silver or copper films while alleviating the problems associated with oxidation of these substrates.