Surface reaction mechanisms as a function of surface composition, adsorbate coverage, and adsorbate acidity: I. Trimethylgallium on metal-treated silicon. II. Hexafluoropentanedione, methanol, and acetone on copper
Cadwell, Linda Ann
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Permalink
https://hdl.handle.net/2142/19513
Description
Title
Surface reaction mechanisms as a function of surface composition, adsorbate coverage, and adsorbate acidity: I. Trimethylgallium on metal-treated silicon. II. Hexafluoropentanedione, methanol, and acetone on copper
Author(s)
Cadwell, Linda Ann
Issue Date
1994
Department of Study
Chemical and Biomolecular Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Engineering, Materials Science
Language
eng
Abstract
The heterogeneous reaction of trimethylgallium was determined as a function of adsorbate coverage, surface composition, and surface temperature. As the average of trimethylgallium on Si(100) is increased, the reaction mechanism evolves from intramolecular decomposition, forming methane at very low coverage, to intermolecular interactions at half monolayer coverage, which form ethylene and facilitate methyl-gallium bond scission, to the desorption of gallium-alkyl species and methyl species at multilayer coverage. Desorption is favored at higher coverage, since reaction sites on Si(100) are blocked. There is no significant difference between the reaction mechanism of a saturation exposure of trimethylgallium on a sample held at 100 K and one held at 280 K, besides the increased adsorption of trimethylgallium into the multilayer adsorption state on the former. Surfaces with 0.5 ML of aluminum or gallium metal on Si(100) form the same surface reconstructions, but exhibit dramatically different reactivities towards trimethylgallium. On the gallium-treated surface desorption is favored, whereas on the aluminum-treated surface trimethylgallium decomposes by 250 K. Methyl groups from the trimethylgallium are transferred to the surface aluminum atoms and trimethylaluminum desorbs. Gallium atoms are left on the surface. The reactivities of the group III organometallics on group III metal-rich surfaces depend on the electronegativity differences between the coordinated metal atom in the adsorbate and the metal on the surface. Reactions favor the formation or maintenance of the strongest alkyl-metal bonds. The reactivities of acetone and hexafluoropentanedione on Cu(110), and the reactivity of methanol on Cu(210), were determined with and without oxygen coadsorption. Acetone, a weakly acidic molecule, was unreactive on Cu(110). Hexafluoropentanedione, a strongly acidic molecule, decomposed on Cu(110). H$\sb2$O, CO$\sb2$, CO, HF and CF$\sb3$FCO desorbed from the surface. The coadsorption of oxygen with hexafluoropentanedione on Cu(110) at 100 K resulted in the etching of copper through the production of Cu(II)bishexafluoroacetylacetonate at 440 K. Methanol, a moderately acidic molecule, desorbs intact from Cu(110), however, on the more corrugated Cu(210) surface methanol undergoes partial dehydrogenation to produce formaldehyde. Formaldehyde and hydrogen desorbed at 400 K. The yield of formaldehyde is increased in the presence of adsorbed oxygen.
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