Growth and Structure of Tb₂O₃(111) Films on Pt(111)

Abstract

We investigated the formation and structure of Tb 2 O 3 (111) thin films grown on Pt(111) using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find that the Tb 2 O 3 (111) films adopt an oxygen-deficient cubic fluorite structure, wherein the Tb cations form an approximately (1.32 × 1.32) lattice in registry with the Pt(111) substrate. STM shows that terbia film growth follows the Stranski−Krastanov mechanism, in which Tb 2 O 3 (111) initially forms a well-connected wetting layer up to a Tb 2 O 3 coverage of ∼2 ML (monolayer), whereas multilayer Tb 2 O 3 islands develop as the coverage increases thereafter. The Tb 2 O 3 (111) wetting layer yields a sharp (3 × 3) LEED pattern relative to the primary Tb spots that we attribute to diffraction from a 3/4 coincidence lattice formed at the Tb 2 O 3 (111)−Pt(111) interface. STM further shows that oxygen vacancies are randomly distributed within the lattice of the Tb 2 O 3 (111) wetting layer. The (3 × 3) LEED pattern diminishes during the transition to island growth but re-emerges as the islands ripen at Tb 2 O 3 coverages beyond about 5 ML. We attribute the re-emergent LEED pattern to a (3 × 3) superstructure of oxygen vacancies within the Tb 2 O 3 (111) islands, the formation of which is likely mediated by the (3 × 3) Tb 2 O 3 −Pt(111) coincidence lattice. The (3 × 3) structure persists to Tb 2 O 3 coverages of at least 10 ML and remains stable after annealing to temperatures up to 1100 K. An implication of this study is that Tb 2 O 3 (111) surfaces with well-defined, ordered oxygen vacancies can be stabilized to better study the role of structure and oxygen vacancies on the chemical reactivity of TbO x surfaces.