TiO2 and SiO2 surfaces were chemically modified via covalent attachment of a variety of organosilane molecules, and electron stimulated desorption from these surfaces was examined. The study focussed on, but was not limited to, bound trimethylsilyl and tri(methyl-d3)silyl groups. Relative parent and fragment secondary ion yields are strongly dependent upon the substrate material. The significantly lower ion yields found in the case of modified TiO2 are explained by the relative positions of the TiO2 valence band and the ionization potentials of (CH3)3Si. and CH3. radicals, such that resonance neutralization of near-surface (CH3)3Si+ ions is energetically favorable. Such neutralization processes are energetically unfavorable at the SiO2 surface. Total dissociation cross sections (1.0 keV electron bombardment) were measured for trimethylsilyl groups bound to these surfaces. The cross section was found to be a factor of 3.7 lower for modified TiO2 than for modified SiO2. This difference does not appear to be explicable in terms of differences between the secondary electron yields of the two substrates, but may instead be an indication that "recapture" (neutralization processes involving bond reformation) occurs with significant probability at the tiO2 surface.
Bibliographical noteFunding Information:
The bulk of this work was performed at the University of Minnesota Regional Instrumentation Facility for Surface Analysis, which is funded by the National Science Foundation under grant CHE: 7916206. We would like to thank Dr. M. Knotek, Sandia Laboratories, Albuquerque. NM, for performing preliminary ESD threshold energy measurements and for helpful discussions. T.R.H. is grateful to the Dow Chemical Company and the Procter and Gamble Company for financial support during this work. The Physical Electronics Division of Perkin-Elmer Corporation provided both financial support for T.R.H. and aided in the design and production of the reactor for surface chemical modification.