Surface Structure of an Ultrathin Alumina Film on ${\mathrm{Ni}}_3\mathrm{Al}(111)$: A Dynamic Scanning Force Microscopy Study

The surface structure of an ultrathin alumina film on a ${\mathrm{Ni}}_3\mathrm{Al}(111)$ substrate has been studied by dynamic scanning force microscopy. The alumina film exhibits a hexagonal superstructure with a lattice parameter of 4.14 nm and a $(1/\sqrt{3}\times 1/\sqrt{3})R30°$ substructure. Two domains rotated by 24° are present. The film is terminated by a hexagonal lattice of oxygen ions with a lattice parameter of 0.293 nm, which is rotated by 30° with respect to the substrate lattice. The nodes of the 4.14 nm superstructure and the 2.39 nm substructure are pinned on points of the substrate lattice, where the surface atomic lattice is almost commensurable. The oxygen lattice is perfectly hexagonal close to these nodes and disordered in the surrounding regions.

Figure 1

Damping image of the alumina film formed after oxidation at 1000 K of a clean ${\mathrm{Ni}}_3\mathrm{Al}(111)$ surface. A highly ordered hexagonal nanostructure consisting of dark single and hexagonal arranged objects is imaged. Two domain structures with an angular difference in orientation of about 24 degrees can be distinguished. Dark contrast corresponds to large damping values and vice versa. ($46.2\times 46.2{\mathrm{nm}}^2$, $\Delta f=-67.0\mathrm{Hz}$).

Figure 2

Damping image of a single domain of the nanostructured thin alumina film. Three hexagonal structures with lattice constants of 0.81 nm, 2.39 nm, and 4.14 nm can be distinguished. ($11.7\times 11.7{\mathrm{nm}}^2$, $\Delta f=-76.1\mathrm{Hz}$).

Figure 3

(a) Detuning image with atomic resolution of the thin alumina film on ${\mathrm{Ni}}_3\mathrm{Al}(111)$ ($12.3\times 12.3{\mathrm{nm}}^2$, $\Delta f=-13.4\mathrm{Hz}$). The self-correlation (b) shows hexagonal structures with lattices of 4.14 nm, 2.39 nm, and 0.293 nm. The Fourier transformations of images (a) and (b) appear in the inset. The spots from the two large superstructures appear in the center of the Fourier transform. Close to the six spots from the atomic lattice, several satellites are present which are in registry with the 2.39 nm superstructures (e.g. in the rectangle).

Figure 4

(a) Model explaining the origin of the superstructures. A hexagonal lattice of 0.293 nm (smallest unit cell/blue) is superimposed to the ${\mathrm{Ni}}_3\mathrm{Al}(111)$ surface lattice (small black unit cell with a parameter of 5.03 nm) after a rotation of 30° (see text). The nodes of the $(\sqrt{67}\times \sqrt{67})R47.78°$ large unit cell (dark grey/red) with a parameter of 4.12 nm almost coincide with nodes of the substrate lattice. The light grey/green unit cell with a 2.38 nm lattice parameter is the $(1/\sqrt{3}\times 1/\sqrt{3})\mathrm{\text{−}}R30°$ substructure of the 4.12 nm superstructure. The same situation is reproduced by a rotation of 24° of the large unit cell. (b) Fourier filtered image of Fig. 3a after a selection of the 6 spots corresponding to the atomic lattice and the satellites which are responsible for the modulation of the atomic lattice.