Atomically resolved force microscopy images of complex surface unit cells: Ultrathin alumina film on NiAl(110)

In this Brief Report we present atomically resolved images of the ultrathin alumina film on NiAl(110). For the first time detailed images of the complex microstructure for both reflection domains have been obtained by frequency modulation dynamic force microscopy using a very stable, custom built, dual mode scanning force and scanning tunneling microscope. Measurements have been performed under ultrahigh vacuum conditions at 5 K with a quartz tuning fork as a force sensor. The high spatial resolution allows to derive 28 atomic positions in real space in the surface unit cell by simple graphical analysis. This has been successful without application of filtering or correlation methods, emphasizing the potential of this force microscopy method on complex oxide surfaces. With respect to topographic height even quantitative agreement with theory could be achieved, here shown for selected structural elements within the unit cell. Furthermore, deeper insight into a wavelike morphological feature could be obtained. Consistency with a published density-functional theory model and connections to other data from the literature are discussed.

Figure 1

High-resolution FM-DFM images of the ultrathin alumina film on NiAl(110). (a) Domain $B$; (b) domain $A$. The geometric relation between the oxide and the small NiAl(110) surface unit cell is illustrated. Atomic positions are marked with crosses. Scan area $3\times 3{\text{nm}}^2$ each. (a) $\Delta f=-6.2\text{Hz}$, $A_{\text{OSC}}=1.4\text{Å}$; (b) $\Delta f=-6.7\text{Hz}$, $A_{\text{OSC}}=1.7\text{Å}$.

Figure 2

(a) Stack of ten superimposed overlays with crosses of the surface unit cell taken from Fig. 1a of domain $B$. The spread in each set of crosses indicates the uncertainty due to imaging noise, weak contrast, and reading errors. (b) Average overlay as determined from (a) giving the mean positions of the groups of crosses. (c) Geometry and dimensions of the two oxide overlayer domains $A$ and $B$, their orientation relative to each other and to the NiAl(110) substrate ($b_1=10.55\text{Å}$, $b_2=17.88\text{Å}$, $\alpha =88.7°$). (d) Average positions of domain $A$ (crosses) reflected onto result for domain $B$ [circles from (b)] for comparison. (e) Positions from the averaged overlays of four independent images (domain $B$). The spread here allows to check reproducibility. (f) Average of positions from (e).

Figure 3

Comparison of the atomic positions of the averaged surface unit cells for both domains (crosses) with those of the topmost oxygen (gray dots) and aluminum (black dots) atoms in the DFT model (Ref. 11). The dashed and solid gray lines indicate the surface unit cell position and rectangular blocks of eight oxygen atoms from Ref. 11, respectively. (a) Domain $B$; (b) domain $A$.

Figure 4

FM-DFM image of a $B$ domain (scan range $4.1\times 4.8{\text{nm}}^2$, $\Delta f=-5.5\text{Hz}$, $A=1.4\text{Å}$). The wavelike topographic feature of the oxide film exhibits two inequivalent but equidistant crests. They differ in the arrangement, contrast, and number of their brightest protrusions. The length of a complete period of the feature is $18\text{Å}$. The right part shows an average of 322 line profiles (gray curve) lined up along the short end (indicated by the dashed double arrow line) of the three unit cells broad rectangle marked in the image. The black curve has been smoothed. Every second crest is less pronounced. The dashed and solid line segments in the line profile are set to spacings of a quarter of a unit cell length.

Figure 5

(a) Topography image of rectangular blocks of eight protrusions labeled with 1–8 and A–H, respectively; domain $A$. Each two parallel rows of four positions (dashed lines) are inclined in opposite directions along the long side of each rectangular block. Scan range $2.5\times 1.6{\text{nm}}^2$, $\Delta f=-6.7\text{Hz}$, $A_{\text{OSC}}=1.7\text{Å}$. (b)–(c) Line profiles along dashed lines from (a) in comparison to calculated absolute heights for corresponding oxygen positions from the DFT model (boxed crosses). Please note that this figure shows individual line profiles giving rise to a larger scale range in $z$ direction as compared to the average profile given in Fig. 4.