We used X-ray photoelectron spectroscopy and temperature-programmed desorption (TPD) to investigate the oxidation of Tb 2 O 3 (111) films on Pt(111) by gaseous oxygen atoms. We find that plasma-generated O atom beams are highly effective at completely oxidizing the Tb 2 O 3 films to TbO 2 at 300 K, for film thicknesses up to at least seven layers. Heating to ∼1000 K in ultrahigh vacuum restores the films to the Tb 2 O 3 stoichiometry and produces two distinct O 2 TPD features centered at ∼385 and 660 K, which we attribute to the release of lattice oxygen from the surface vs bulk trilayers, respectively. We also find that the adsorption of plasma-activated oxygen at 90 K produces a weakly bound state of oxygen on the TbO x films which desorbs between ∼100 and 270 K during TPD. This oxygen state is consistent with a form of chemisorbed oxygen, possibly an atomic and/or molecular species that bonds on-top of Tb atoms at the surface. TPD experiments of the oxidation of Tb 2 18 O 3 films by 16 O atom beams demonstrate that oxygen desorption below about 500 K originates almost entirely from the oxygen that is "added" to the Tb 2 O 3 film and that all isotopic combinations of O 2 desorb from the bulk above 500 K, though the relative amount of 18 O to 16 O which desorbs above 500 K is lower than that determined from the isotopic composition of the oxidized TbO x films. These results support the idea that oxygen desorption below 500 K originates from oxygen species that are localized at the surface and further suggest that the oxide structure only partially accommodates oxygen atoms that incorporate into lattice sites at 300 K.

We investigated the growth and partial reduction of Sm 2 O 3 (111) thin films on Pt(111) using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Sm 2 O 3 (111) films are high quality and grow in a defective fluorite structure wherein the Sm cations adopt a hexagonal (1.37 × 1.37) lattice in registry with the Pt(111) surface, while oxygen vacancies are randomly distributed within the film. STM measurements show that Sm 2 O 3 (111) film growth on Pt(111) occurs by the Stranski-Krastanov mechanism, in which a single O−Sm−O trilayer initially forms, followed by the growth of well-defined, multilayer islands. The Sm 2 O 3 (111) films undergo partial reduction during annealing at 1000 K in ultrahigh vacuum. LEED and STM provide evidence that a fraction of the Sm 2 O 3 in the first layer, closest to the Pt(111) substrate, decomposes to produce well-ordered domains of rocksalt SmO(100) during reduction, and that Sm 2 O 3 from the third and higher layers concurrently spreads onto the first layer to form a more contiguous second layer of Sm 2 O 3 (111). We show that the SmO(100) and Sm 2 O 3 (111) lattices can form a coincidence structure with minimal strain to the Sm-atom sublattices, and that satellite features observed in the LEED patterns are consistent with the coexistence of SmO(100) and Sm 2 O 3 (111) domains as well as the proposed Sm 2 O 3 (111)/SmO(100) coincidence structure. Lastly, we find that reoxidation of the partially reduced films restores the original Sm 2 O 3 (111) crystal structure, and significantly improves the film quality, as reflected by a flatter film morphology and better connectivity among oxide domains. An implication from this study is that the formation of (100)-oriented monoxide structures is a general characteristic of the reduction of rare-earth oxide thin films on hexagonally close-packed metal surfaces.

Low energy electron microscopy and micro-illumination low energy electron diffraction (µLEED) were used to investigate the structural properties of TbO x films grown on Cu(111) in ultrahigh vacuum.Our results reveal that the morphology and structure of the terbia films depend sensitively on the surface temperature during growth. Deposition at room temperature produces terbia films that uniformly cover the surface but lack long-range order that is detectable with LEED. Annealing in O 2 induces the formation of three-dimensional TbO x islands that grow as dendrites and cause the disordered terbia to dewet from the Cu(111) substrate. LEED measurements show that the terbia dendrites are crystalline with a hexagonal (1.4×1.4) structure with respect to the Cu(111) surface, which is consistent with cubic fluorite-like TbO x (111). In contrast to room temperature growth, deposition at 500°C produces mainly rectangular TbO x islands that coexist with smaller quantities of irregularly-shaped islands. Utilizing µLEED, we identify the rectangular domains as the cubic fluorite-

We used temperature-programmed reaction spectroscopy (TPRS) and synchrotron-radiation-based photoelectron spectroscopy (PES) to investigate the adsorption and oxidation of methanol (CH 3 OH) on Tb 2 O 3 (111) and TbO 2 (111) thin films grown on Pt(111). We find that methanol mainly desorbs from the Tb 2 O 3 surface through both molecular and recombinative processes and that a relatively small amount of adsorbed methanol (<20%) dehydrogenates to CH 2 O and H 2 O with these species desorbing between about 160 and 300 K. Oxidation of the terbia film enhances the surface reactivity as ∼50% of the adsorbed methanol on TbO 2 oxidizes to mainly CH 2 O and water as well as CO 2 that desorbs near 600 K during TPRS. Quantification of the product yields suggests that all of the excess surface O atoms, resulting from oxidation of Tb 2 O 3 to TbO 2 , are removed during TPRS by reaction with adsorbed CH 3 OH. We did not detect CO or H 2 production under any conditions. PES measurements show that several adsorbed intermediates form on the TbO x surfaces at temperatures as low as 140 K, including mainly methoxy (CH 3 O−) as well as smaller quantities of a more oxidized species, thought to be either CH 2 O 2 or CHO 2 . XPS spectra collected as a function of the surface temperature provide evidence that the adsorbed CH 3 O− groups serve as the main intermediate for both CH 2 O and CH 3 OH formation at temperatures below 400 K, while the more oxidized species is a spectator to CH 2 O formation but undergoes complete oxidation on TbO 2 at temperatures above 450 K. The high reactivity of the TbO 2 surface correlates with the presence of labile oxygen atoms that are generated during oxidation of the Tb 2 O 3 film.