AVS 49th International Symposium
    Surface Science Tuesday Sessions
       Session SS1-TuA

Paper SS1-TuA8
O@sub2@ and NO Island Formation on Al(111)

Tuesday, November 5, 2002, 4:20 pm, Room C-108

Session: Ultrafast Phenomena & Dynamics at Surfaces
Presenter: J.Z. Sexton, University of California, San Diego
Authors: J.Z. Sexton, University of California, San Diego
A.C. Kummel, University of California, San Diego
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The oxidation of aluminium is a fundamentally important process that is not well understood. Scanning tunneling microscopy was employed to study the mechanisms for the oxidation of Al(111) with thermal O@sub2@ and NO at medium to high oxygen coverage regime (20-50% ML). The STM-UHV studies provided the following observations: 1) Oxygen islands on the Al(111) surface, prepared with thermal oxygen, are elongated and non-compact. 2) Al(111) step edges change shape upon O@sub2@ chemisorption to relieve strain from oxide islands. 3) Islands produced with thermal nitric oxide (NO) produce round, compact islands in contrast to the non-compact, elongated islands formed with thermal oxygen. 4) Above a critical oxygen coverage (30-40% ML), Al-atom protrusions appear within oxygen islands. These protrusions increase with increasing coverage and indicate the onset of the phase transition from isolated chemisorbed oxygen islands to an ionic Al@sub2@ O@sub3@ amorphous layer. 5) Pre-existing oxygen features can locally enhance the sticking coefficient. This local enhancement of oxygen adsorption on the Al(111) surface is likely due to a perturbation in the local electronic structure surrounding an oxygen feature. The enhanced O@sub2@ chemisorption results from lattice strain coupled to a work-function change in the proximity of pre-existing oxygen islands. The lattice strain at high oxygen coverages eventually yields the phase transition to amorphous Al@sub2@ O@sub3@.