Inelastic Electron Tunneling Spectra of Neutral and Basic Zinc Dithiophosphates on Native Aluminum Oxide Surfaces

Abstract

Inelastic electron tunneling (IET) spectra of pure neutral and basic isopropyl, isobutyl, and p-t-octyl phenyl ZnDTPs are reported. These pure compounds were specially synthesized in order to examine the IET spectra of the neutral and basic ZnDTP salts separately. This is of interest since the commercial ZnDTPs, whose spectra were reported previously, consist of mixtures of neutral and basic ZnDTP salts for all types of primary, secondary, and aryl ZnDTPs used today. Thus, an examination of each representative pair was made, i.e. isobutyl (primary), isopropyl (secondary), and p-t-octyl phenyl (aryl), in order to investigate these structural modifications on the IET spectra and relate the data to the adsorption characteristics. Generally, the conclusions drawn are in agreement with those from previous work on commercial ZnDTPs. Aryl ZnDPTs dissociate on adsorption, and do not retain the P= S group. Alkyl ZnDTPs retain the P=S group on adsorption. Differences among the neutral and basic forms of the pure alkyl ZnDTPs investigated in this work were found. An unusual OH absorption frequency was observed in the IET spectrum of the basic isobutyl ZnDTP, corresponding to a strongly hydrogen-bonded OH group, possibly due to a surface zinc hydroxide phase. The lowest frequency for the P= S group, adsorbed on the surface, was found for the neutral isopropyl ZnDTP, suggesting this species was more strongly bonded to the surface than the other ZnDTPs examined in this work.