Deposition of Metal Overlayers at End-Group-Functionalized Thiolate Monolayers Adsorbed at Au. 1. Surface and Interfacial Chemical Characterization of Deposited Cu Overlayers at Carboxylic Acid-Terminated Structures

Abstract

This paper describes the preparation and characterization of interfacial structures formed by the deposition of thin (~0.2 and 5 nm) overlayer Alms of Cu at carboxylic acid-terminated thiolate monolayers adsorbed at Au. Infrared reflection spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) have been used to probe both compositional and structural details of the interfaces. The monolayers are formed by the chemisorption of carboxylic acid-terminated alkanethiols (HO2C-(CH2)nSH, n = 10 and 15) from dilute ethanolic solutions at the surface of evaporated Au films. Prior to Cu deposition, the IRS data indicate that the polymethylene chains form a densely packed surface phase for n = 15 and a more disordered surface phase for n = 10. For n = 10 and n = 15, the terminal carboxylic acids exist in both free and hydrogen-bonded forms. Deposition of a Cu overlayer by resistive evaporation and subsequent exposure to the laboratory ambient leads to the formation of a carboxylate salt as revealed by the detection of both the va(COO-) and vs(COO-) modes by IRS. The IRS data also show that Cu deposition induces disorder in the underlying polymethylene chain structure relative to that of the uncoated monolayer. Both the XPS and IRS data reveal that Cu is bound in its +2 oxidation state to the carboxylate. Images from AFM indicate that the crystallite size (~20 nm) of the Cu overlayer is markedly less than that (~200–300 nm) of anchoring metal substrate. The potential application of this adsorbate-substrate system to fundamental studies of the adhesion of metal overlayers at surface functionalized polymeric materials is briefly discussed. © 1992, American Chemical Society. All rights reserved.