Interaction of oxygen with Al(111) studied by scanning tunneling microscopy

Abstract

The interaction of oxygen with Al(111) was studied by scanning tunneling microscopy (STM). Chemisorbed oxygen and surface oxides can be distinguished in STM images, where for moderate tunnel currents and independent of the bias voltage the former are imaged as depressions, while the latter appear as protrusions. An absolute coverage scale was established by counting O adatoms. The initial sticking coefficient is determined to s_o=0.005. Upon chemisorption at 300 K the O adlayer is characterized by randomly distributed, immobile, individual O adatoms and, for higher coverages, by small (1×1) O islands which consist of few adatoms only. From the random distribution of the thermalized O adatoms at low coverages a mobile atomic precursor species is concluded to exist, which results from an internal energy transfer during dissociative adsorption. These ‘‘hot adatoms’’ ‘‘fly apart’’ by at least 80 Å, before their excess energy is dissipated. A model is derived which explains the unusual island nucleation scheme by trapping of the hot adatoms at already thermalized oxygen atoms. Oxidation starts long before saturation of the (1×1) O adlayer, at coverages around Θ_O≂0.2. For a wide coverage range bare and O_ad covered surfaces coexist with the surface oxide phase. Upon further oxygen uptake both chemisorbed and oxide phase grow in coverage. Oxide nucleation takes place at the interface of O_ad islands and bare surface, with a slight preference for nucleation at upper terrace step edges. Further oxide formation progresses by nucleation of additional oxide grains rather than by growth of existing ones, until the surface is filled up with a layer of small oxide particles of about 20 Å in diameter. At very large exposures up to 5×10⁵ L they cover the entire surface as a relatively smooth, amorphous layer of aluminum oxide. The difference in Al atom density between Al metal and surface oxide is accommodated by short range processes, with no indication for any long range Al mass transport. Based on our data we discuss a simpler two step model for the interaction of oxygen with Al(111), without making use of an additional subsurface oxygen species. The complex spectroscopic data for the O/Al(111) system are rationalized by the wide coexistence range of bare and O_ad covered surface with surface oxide and by differences in the electronic and vibronic properties of the surface atoms depending on the number of neighboring O adatoms in the small O_ad islands.