Metal overlayers on organic functional groups of self-organized molecular assemblies. V. Ion scattering spectroscopy and x-ray photoelectron spectroscopy of Ag/COOH interfaces

Abstract

In the present work, x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) were used to characterize mercaptoundecanoic acid [MUA, HS(CH2)10 COOH] and mercaptohexadecanoic acid [MHA, HS(CH2)15COOH] self-assembled monolayers (SAMs) before and after depositing up to 1.0 nm Ag at ∼10−7 Torr. Results from XPS O 1s, Ag 3d, and C 1s spectra do not indicate that a complex is formed (no chemical shifts) between Ag and the COOH of MUA or MHA at 295 K. Although no direct evidence for Ag/COOH interactions is evident, the initial value for ISS Ag peak intensities for 1.0 nm of Ag on COOH of MHA is unchanged after 1 h and is only reduced to 68% of the initial value after 15 h. For MUA, the ISS Ag peak intensity for 1.0 nm is constant for about 5 min and is gradually reduced until it is not detectable about 90 min after Ag deposition. These results and those we obtained earlier for Ag on octadecanethiol [ODT, HS(CH2)17CH3] might be explained by weak or no interactions between Ag and COOH and by different defect densities in the SAMs. The initial Ag ISS peak intensity for 1.0 nm Ag on MHA increases from 295 K as the SAM temperature is reduced to reach a maximum at about 200 K. The increased ISS intensity from Ag/COOH vs Ag/CH3 is consistent with lower defect densities in the MHA vs ODT.