Paper SS1-FrM7
Reactions of Atomic Hydrogen with Self Assembled Monolayers

Friday, October 19, 2007, 10:00 am, Room 608

The interaction of atomic hydrogen with hydrocarbon and semi-fluorinated self assembled monolayers (SAM) has been studied in-situ using x-ray photoelectron spectroscopy (XPS) and ex-situ contact angle measurements. Results indicate that atomic hydrogen (AH) reactions with alkanethiolate SAMs are strongly dependent upon the hydrocarbon chain length. For short chained SAMs (ie. C₉ and C₁₂), AH permeates rapidly through the hydrocarbon film and reacts primarily with the native thiolate bond resulting in chain desorption, as evidenced by a simultaneous loss in both carbon and sulfur. Conversely, sulfur was removed preferentially from longer chained alkanethiolates (ie. C₁₆ and C₁₈), followed by the AH mediated erosion of the hydrocarbon film. The different reaction pathways for the long and short chain SAMs are due to effect of chain length on the residence time of AH in the hydrocarbon overlayer. In contrast to the short chain SAMs, AH reactions within the hydrocarbon film become important due to the increased length of the alkyl chain. Reactions of AH with semi-fluorinated SAMs (e.g. Si-(CH₂)₂(CF₂)₅(CF₃)) were consistent with a kinetically controlled process initiated by H atom abstraction from C-H bonds within the SAM. Secondary reactions of the carbon-centered radicals formed in the organic film with AH leads to the desorption of fluorocarbon radicals (e.g. CF₃(CF₂)₅). As a result of this reaction mechanism, the rate of fluorocarbon loss from the film is proportional to the adsorbate coverage and the flux of AH. The decrease in contact angle that occurs as a result of AH exposure is directly proportional to the coverage of CF₃ and CF₂ groups, providing a convenient route to control the surface wettability and other interfacial properties. In comparison to low energy electron modification, reactions of AH with semi-fluorinated SAMs are found to be more effective in etching the organic film.