Inelastic Electron Tunneling Spectroscopy of Silane Coupling Agents Adsorbed on Alumina

摘要

Inelastic electron tunneling spectroscopy (IETS) has been used to obtain vibrational spectra of alkoxysilanes adsorbed on aluminum oxide. The spectra reveal that: (1) monoalkoxysilanes are not adsorbed whereas trialkoxysilanes are readily adsorbed, (2) the reaction mechanism of triethoxysilanes and trimethoxysilanes is the same, and (3) the reaction mechanism under anhydrous conditions utilizes surface OH o OD groups present on alumina. All of these results suggest the following hypotheses: Trialkoxysilanes can condense directly with OH groups on an alumina surface to form siloxane interfacial bonds. Such bonds are probably stabilized subsequently by condensation between chemisorbed silane molecules to form siloxane oligomers. On the other hand, monoalkoxysilanes appear to be unable to bond to an alumina surface, either because their reactivity is intrinsically lower or because they are not able to condense further to form a non-volatile polysiloxane layer in situ. Their intrinsic reactivity will, of course, be reduced because they have fewer reactive sites per molecule. Nevertheless, they clearly act as catalysts for further oxidation of the aluminum surface, as evidenced by the rapid increase in junction resistance on doping the substrate with them.