Pt$_3$Zr(0001): A substrate for growing well-ordered ultrathin zirconia films by oxidation

TL;DR

This study investigates the surface structure and oxidation behavior of Pt$_3$Zr(0001), demonstrating its suitability as a substrate for growing well-ordered ultrathin zirconia films with specific structural and electronic properties.

Contribution

The paper provides detailed atomic-scale insights into the surface structure, oxidation process, and bonding characteristics of Pt$_3$Zr(0001), introducing it as a promising substrate for ultrathin zirconia film growth.

Findings

Successful formation of a ZrO$_2$ trilayer with a specific superstructure

Weak binding of oxide trilayer to substrate with Zr atoms bonding mainly to Pt

Oxide exhibits a bandgap with conduction band minimum about 2.3 eV above Fermi level

Abstract

We have studied the surface of pure and oxidized Pt$_3$Zr(0001) by scanning tunneling microscopy (STM), Auger electron microscopy, and density functional theory (DFT). The well-annealed alloy surface shows perfect long-range chemical order. Occasional domain boundaries are probably caused by nonstoichiometry. Pt$_3$Zr exhibits ABAC stacking along [0001]; only the A-terminated surfaces are seen by STM, in agreement with DFT results showing a lower surface energy for the A termination. DFT further predicts a stronger inward relaxation of the surface Zr than for Pt, in spite of the larger atomic size of Zr. A closed ZrO$_2$ film is obtained by oxidation in $10^{-7}$ mbar O$_2$ at 400 $^\circ$C and post-annealing at $\approx 800 ^\circ$C. The oxide consists of an O-Zr-O trilayer, equivalent to a (111) trilayer of the fluorite structure of cubic ZrO$_2$, but contracted laterally. The oxide forms a $(\sqrt{19} \times \sqrt{19})$R23$^\circ$ superstructure. The first monolayer of the substrate consists of Pt and contracts, similar to the metastable reconstruction of pure Pt(111). DFT calculations show that the oxide trilayer binds rather weakly to the substrate. In spite of the O-terminated oxide, bonding to the substrate mainly occurs via the Zr atoms in the oxide, which strongly buckle down towards the Pt substrate atoms, if near a Pt position. According to DFT, the oxide has a bandgap; STM indicates that the conduction band minimum lies $\approx 2.3$ eV above $E_\mathrm{F}$.