Penetration of Deposited Ag and Cu Overlayers Through Alkanethiol Self-Assembled Monolayers on Gold

Abstract

The purpose of our research is to study the reactions, interactions, or penetration between vacuum deposited metals (M) and the organic functional end groups (OFGs) of self-assembled monolayers (SAMs) on Au films under controlled conditions. Metal/SAM/Au systems are models for understanding bonding at M/organic interfaces and the concomitant adhesion between the different materials. In broad terms, the M/OFGs form interacting interfaces (e.g., Cr/COOH or Cu/COOH) in which the deposit resides on top of the OFGs or weakly interacting interfaces through which the overlayer penetrates and resides at the SAM/gold interface. We present a review of XPS results from weakly interacting systems (Cu/CH₂OH, CU/CN, Ag/CH₃, Ag/COOH) and discuss in more depth the time-temperature dependence of the disappearance of the metal from the M/SAM interface following deposition using ISS on the Ag/CH₃ and Ag/COOH systems. XPS and ISS were used to characterize five alkanethiols terminated with CH₃, COOH(C—11 and C—16 chain lengths), CN and CH₂OH before and after depositing up to 1.0 nm Ag or Cu at ca. 10⁻⁷ torr. XPS spectra indicate that no strong interaction occurs between the deposited Ag and the COOH organic functional group, although a stronger interaction is evident on a C—16 COOH, and a unidentate is formed for Cu on the CH₂OH. An interaction between Cu and CN is evident, and penetration into the SAM occurs to a greater extent than for Cu on CH₂OH. The Ag interaction with CH₃ is weak. ISS compositional depth profiles for Ag on COOH and CH₃, taken from 113 to 293 K, indicate that Ag remains on the surface of the C—11 COOH for up to 1 h after deposition, whereas Ag penetrates CH₃ in less than 5 min at 295 K. The time for Ag to penetrate into a C-16 COOH is several times longer than for the C—11 COOH and depends on the SAM temperature.