Oxidation of a Tb₂O₃(111) Thin Film on Pt(111) by Gas-Phase Oxygen Atoms

Abstract

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.

Growth, Structure, and Stability of the High-Index TbOx(112) Surface on Cu(111)

“…The lattice parameter of 3.66 Å estimatedfor TbO x (111) films on Pt(111) is closer to the value for bulk-terminated TbO 2 (111),22 possibly because the lattice constant of Pt(111) is larger than that of Cu(111). Both of the TbO x (111) films are characterized by smaller lattice parameters than CeO 2 (111) films grown on these metal substrates 12,18,44.…”

“…deficient fluorite structure wherein the Tb cations arrange into a hexagonal structure in registry with Pt(111) while oxygen vacancies are randomly distributed 22. The lattice parameter of 3.66 Å estimatedfor TbO x (111) films on Pt(111) is closer to the value for bulk-terminated TbO 2 (111),22 possibly because the lattice constant of Pt(111) is larger than that of Cu(111).…”

“…[7][8][9][10][11][12][13][14][15][16][17][18] Researchers have also investigated ceria deposition onto Si(111) surfaces. 19,20 Recent studies show that Sm 2 O 3 and Tb 2 O 3 films grown on Pt(111) also adopt the (111) orientation of the cubic fluorite structure, 21,22 thus enabling investigations of the chemistry of well-defined REO surfaces other than ceria. Investigations with model thin films have indeed provided fundamental insights for understanding the surface chemical 4 properties of CeO x (111), particularly the reactivity toward organic oxygenates.…”

Reducible Oxides as Ultrathin Epitaxial Films

“…The reversibility of the structural and morphological modifications is observed also in this case [105]. Terbium oxide films were prepared with similar methods, i. e. using reactive deposition and post growth annealing in O 2 [106]. The LEED pattern up to 4 ML thickness shows the (1.32 × 1.32) structure, compatible with the terbium oxide lattice parameter of the Tb 2 O 3 phase in a fluorite structure with disordered oxygen vacancies in close analogy with the case of samarium oxide [107].…”

“…The films could not be oxidized by using thermal treatments in O 2 at P ~ 7×10 -7 mbar and temperatures from 95 to 1000 K, while plasma-generated atomic oxygen was shown to be effective in oxidizing a film at 300 K to TbO 2 and in generating weakly bound surface O species. The reduced Tb 2 O 3 phase could be re-obtained by annealing TbO 2 in vacuum at 1000 K [106].…”