Research Topic

Metal Oxides

We study how oxide surfaces change with phase, thickness, oxygen chemical potential, and the material beneath them.

Bulk oxides are often too insulating or structurally complex for an unambiguous atomic-scale experiment. A well-defined oxide film on a metal offers a controlled alternative. The substrate provides electrical conductivity, while the film retains the coordination, defects, adsorption sites, and phase behavior that make an oxide chemically interesting.

Zirconia is our established model system. It allows film growth, atomic-resolution microscopy, spectroscopy, and density-functional calculations to be compared on the same structure. We use that foundation to move toward polar and reducible perovskite oxides, where termination and defect chemistry are even more strongly coupled.

Thickness selects the zirconia surface structure.

ZrO₂ films on Rh(111) do not approach the bulk structure in a single step. Two- to four-layer films retain substrate-dependent reconstructions; at approximately five layers a tetragonal row structure appears, while stronger annealing produces monoclinic domains. The sequence makes film thickness and thermal history explicit experimental variables rather than hidden sample details.

STM image of the tetragonal row structure of a zirconia film on Rh(111) — Tetragonal row structure of a five-layer ZrO₂ film on Rh(111), imaged by STM. Adapted from Lackner et al., Surface Science (2019).

Supported metals probe the chemistry of the film.

Metal atoms do not nucleate in the same way on every zirconia surface. Cluster density and preferred adsorption sites vary with both the deposited metal and the oxide structure. Au, for example, forms islands whose position and charge state reflect the local zirconia environment. These model interfaces provide a direct route from atomic structure to metal–oxide interaction.

STM image of gold clusters on an ultrathin zirconia film — Au clusters on ultrathin zirconia, imaged by STM. Adapted from Choi et al., J. Phys. Chem. C (2016).

The support becomes part of the reaction.

Under reducing conditions, a substoichiometric zirconia layer spreads over supported ZrO₂ particles on Rh, Pt, and Ru. Oxidation reverses the process. The film remains formally Zr⁴⁺ while excess electrons are transferred to the metal substrate. This reversible encapsulation is a structurally resolved example of strong metal–support interaction, and it shows why the electronic reservoir beneath an oxide cannot be treated as passive.

STM image of an ultrathin zirconia film surrounding a thicker zirconia island — Ultrathin zirconia formed around thicker ZrO₂ islands after reduction. Adapted from Lackner et al., J. Mater. Chem. A (2019).

Polar perovskites add a new boundary condition.

Setvin and co-workers showed that cleaved KTaO₃(001) can compensate its polarity through several distinct surface structures. Annealing produces alternating KO and TaO₂ stripes only four to five atoms wide; exposure to water creates a hydroxylated overlayer instead. This work is not presented as an S² Lab result. It defines the type of atomically controlled polar-oxide problem that we intend to pursue on KTaO₃ and SrTiO₃.

AFM image of the atomic-scale KO and TaO2 labyrinth on KTaO3(001) — Atomic-scale KO/TaO₂ labyrinth on annealed KTaO₃(001), imaged by AFM. Adapted from Setvin et al., Science (2018).

Current work

Toward controlled perovskite-oxide surfaces.

S² Lab is developing this line of work toward KTaO₃ and SrTiO₃ under controlled gases, water, electrical bias, and temperature. The near-term goal is to prepare reproducible terminations and then connect their atomic structure to local electronic and mechanical response.

Related publications