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

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 $(\sqrt{19}\times \sqrt{19})R{23}^{\circ }$ 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 toward the Pt substrate atoms if near a Pt position. According to DFT, the oxide has a band gap; STM indicates that the conduction band minimum lies $\approx 2.3$ eV above $E_F$.

Figure 1

Schematic view of the Pt${}_3$Zr unit cell ($D{0}_{24}$ structure). Dotted (red) vertical lines indicate the stacking of Zr atoms in the B and C layers. Lattice constants from Ref. 19.

Figure 2

STM images of the Pt${}_3$Zr surface annealed at 900 ${}^{\circ }$C; note the different image scales. (a) Annealing after 20 min sputtering while cooling the sample from 400 to 100 ${}^{\circ }$C. (b) Annealing after 30 min sputtering of the cold sample.

Figure 3

STM image of a 0.45-nm-deep hole on the annealed Pt${}_3$Zr surface. The inset shows an overview image. The brightness of the terraces is adjusted to show both layers simultaneously, and a slight nonlinear distortion of the image due to creep of the piezoscanner was removed (assuming that the top-layer lattice should be undistorted). Two sets of parallel lines show that the lattices of bright (Zr) atoms in both layers are in registry.

Figure 4

(a) STM image of the Pt${}_3$Zr(0001) surface oxidized at 400 ${}^{\circ }$C. (b) Atomically resolved image ($-2.5$ mV, 0.45 nA) of the oxide annealed at 1000 ${}^{\circ }$C. (c) Fourier transform of (b).

Figure 5

(a), (b) STM images of the Pt${}_3$Zr(0001) surface oxidized at 400 ${}^{\circ }$C and annealed at $T\ge 850{}^{\circ }$C [(a) 2 V, 0.1 nA; (b) $-14$ mV, 0.5 nA]. The inset shows a line profile taken across the protrusion in the middle of the image.

Figure 6

(a) STM image of a surface partially covered by ZrO${}_2$ ($-2$ mV, 5.8 nA). The height difference between the two oxide levels is $\approx 0.22$ nm, corresponding to one metal layer. (b) Apparent height of an oxide island [bright region marked “oxide” in (a)] with respect to the metal (marked “Pt”). The data were taken with a tunneling current of 0.14 nA, except for the data point at $-2$ mV, 5.8 nA. The broken lines are guides to the eye.

Figure 7

Calculated models for a ZrO${}_2$(111) trilayer adsorbed on Pt/Pt${}_3$Zr(0001): (a) Top view, the rhombus shows the unit cell. (b) Side view, vertical dotted lines indicate atoms on top of each other.

Figure 8

Projected density of states for ZrO${}_2$ adsorbed on Pt/Pt${}_3$Zr(0001), averaged over the O1/Pt and Zr/Pt configurations for (a) the PBE and (b) the vdW-DF geometry. For both geometries, the DOS was evaluated with the PBE functional.