Surface structures of ZrO$_2$ films on Rh(111): From two layers to bulk termination.

We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2-10 monolayers (ML) using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Zirconia was deposited using a UHV-compatible sputter source, resulting in layer-by-layer growth and good uniformity of the films. For thicknesses of 2-4 ML, a layer-dependent influence of the substrate on the structure of the thin films is observed. Beyond this thickness, films show a (2 $\times$ 1) or a distorted (2 $\times$ 2) surface structure with respect to cubic ZrO$_2$(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase occurs for annealing temperatures of up to 730 {\deg}C; transformation to the thermodynamically stable monoclinic phase occurs after annealing at 850 {\deg}C or above. High-temperature annealing also breaks up the films and exposes the Rh(111) substrate. We argue that the tetragonal films are stabilized by oxygen deficiency, while the monoclinic films are only weakly defective and show band bending at defects and grain boundaries. This observation is in agreement with positive charge being responsible for the grain-boundary blocking effect in zirconia-based solid electrolytes. Our work introduces the tetragonal and monoclinic 5 ML-thick ZrO$_2$ films on Rh(111) as well-suited model system for surface-science studies on ZrO$_2$ as they do not exhibit the charging problems of thicker films or the bulk material and show better homogeneity and stability than the previously-studied ZrO$_2$/Pt(111) system.