The interaction of alkali metals with oxide surfaces

Thesis

The thesis is an investigation of the atomic-level interactions of Li and Na with SrTiO₃ surfaces from the perspectives of structure (STM), chemistry (XPS), and quantum mechanics (DFT). Li and Na were deposited onto a variety of SrTiO₃ reconstructions on the (001) and (111) terminations in UHV using an in-house-built UHV evaporator. Coverage and annealing temperature were varied to systematically study the evolution of surface structure by STM, which was the primary technique of the thesis. It is shown that the underlying termination and the reconstruction of the substrate prior to deposition significantly influence the interaction strengths. The polar bulk termination of SrTiO₃(111) seems to be stabilised by a Li monolayer, indicated by the presence of areas with a moiré superlattice over a bulk-like (1×1) periodicity and atomic flatness. Multiple reduction-induced reconstruction transformation were observed, four of which resulted in previously undiscovered SrTiO₃ reconstructions. DFT indicates that reconstructions differ in their alkali-metal interaction strengths due to differences in the surface density of O 2p-derived dangling bonds, and that Li and Na induce different surface structures due to differences in the degree of covalency between Li–O and Na–O bonds. A Li monolayer is, furthermore, calculated to quench metallic surface states on Ti-terminated SrTiO₃(111)-(1×1). XPS shows that surface structure transformations are driven by alkali-metal reduction filling Ti 3d orbitals, which happens in two stages: an initial stage upon annealing below 600 °C, where only the surface is affected, and a later stage, where the entire surface region is affected. The thesis is concluded with a remark on takeaways for researchers active in battery interfaces.

Authors: Forssberg, A

• DOI: 10.5287/ora-aokoabmey
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: battery interfaces; nanoscience; 2D materials; surface science; polar surfaces; reconstruction transformation; oxide physics
• Subjects: Two-dimensional materials; Surface chemistry; Nanoscience; Solid state physics