Structural analysis of the intermetallic surface compound ${\mathrm{CePt}}_5/\mathrm{Pt}\left(111\right)$

We report on a detailed low-energy electron diffraction (LEED) and low-temperature scanning tunneling microscopy (STM) study of the intermetallic surface compound ${\mathrm{CePt}}_5$ on Pt(111). Depending on the thickness we observe various diffraction patterns and superstructures. In the low-thickness regime a slightly compressed $(2\times 2)$ superstructure is aligned along the $\langle 1\overline{1}0\rangle$ direction of the Pt(111) substrate. STM reveals another, much larger superstructure with a periodicity of ($9.02\pm 0.45$) nm presumably responsible for the strongly broadened LEED spots. At about 3 unit cells (u.c.) the surface is dominated by a $(3\sqrt{3}\times 3\sqrt{3})R{30}^{\circ }$ pattern as revealed by LEED satellites and Fourier-transformed high-resolution STM images. It is interpreted as a moiré pattern between the film and the substrate. We precisely determine the superstructure of the intermetallic film to $(\frac{10}{9}\sqrt{3}\times \frac{10}{9}\sqrt{3})R{30}^{\circ }$ with respect to the Pt(111) substrate. Above 3 u.c. the satellites progressively disappear. A model is developed that consistently describes this thickness-dependent transition. For ${\mathrm{CePt}}_5$ films with a thickness between 6 and 11 u.c. the lattice of the compressed $(2\times 2)$ superstructure rotates back into the substrate's $\langle 1\overline{1}0\rangle$ directions.

Figure 1

Pristine Pt(111). Arrows mark the crystallographic directions. (a) LEED image obtained at a primary electron energy $E_{\mathrm{p}}=63$ eV. The missing spot in the lower left is covered by the electron gun. (b) STM topography showing three atomically flat terraces separated by two monoatomic step edges (scan parameters: $U=+1.0$ V, $I=300$ pA). (c) Atomic resolution image as obtained on a defect-free surface area ($U=+1.0$ mV, $I=90$ nA). The corresponding FT is shown in the inset.

Figure 2

2 u.c. ${\mathrm{CePt}}_5$ on Pt(111). (a) LEED pattern ($E_{\mathrm{p}}=63$ eV) and (b) large-scale STM topography showing structures $B$, $C$, the nonperiodic (np) structure, and a minority surface reconstruction $m$ (scan parameters: $U=+1.0$ V, $I=300$ pA). (c) High-resolution differential conductance map and (d) the FT of structure $B$. The superstructure pattern and the lattice of ${\mathrm{CePt}}_5$/Pt(111) are marked in blue and green, respectively.

Figure 3

3 u.c. ${\mathrm{CePt}}_5$ on Pt(111). (a) LEED image ($E_{\mathrm{p}}=63$ eV) and (b) STM image of a surface dominated by structure $C$. Minor surface areas display structure $B$ and the np structure (black). The inset shows the surface of an atomically flat terrace with domain walls (some of which are highlighted by white lines) oriented along the $\langle 1\overline{1}0\rangle$ directions of the Pt(111) substrate (scan parameters: $U=+1.0$ V, $I=300$ pA). (c) High-resolution image and (d) its FT with blue and green marks illustrating the superstructure and the atomic lattice of ${\mathrm{CePt}}_5$/Pt(111), respectively. Note that the atomic lattice is rotated by ${30}^{\circ }$ relative to Pt(111). The moiré pattern forms a $(3\sqrt{3}\times 3\sqrt{3})R{30}^{\circ }$ superstructure of the atomic lattice as highlighted by the (1,1) and (2,2) spots (marked by black arrows). The (3,3) spot (white arrow) coincides with the (1,0) spot of the intermetallic film.

Figure 4

(a) LEED image ($E_{\mathrm{p}}=63$ eV) and (b) STM topography of a 3 u.c ${\mathrm{CePt}}_5$ intermetallic film on Pt(111). The surface is almost completely covered with structure $C$ which is highlighted by a bright tone in (b). The inset shows a rendered three-dimensional representation of the surface area within the box. (c) At 3.75 u.c. ${\mathrm{CePt}}_5$ the surface fraction covered by structure $C$ has decreased significantly. This is compensated by a larger fraction of the surface being covered by structure $C^{\prime }$ (dark tone). (d) and (e) show an STM image and the LEED pattern of 4.5 u.c. ${\mathrm{CePt}}_5$/Pt(111) which are dominated by structure $C^{\prime }$.

Figure 5

Structural model of structures $C$ and $C^{\prime }$ observed for ${\mathrm{CePt}}_5$/Pt(111). In correspondence with the STM images presented in Figs. 4 and 4 bright (dark) surface areas illustrate structure $C$ ($C^{\prime }$). White, yellow, brown, and green spheres represent the Pt atoms and the red (between Pt atoms) and blue (top of Pt atom) the Ce atoms.

Figure 6

LEED patterns of ${\mathrm{CePt}}_5$ films grown on Pt(111) with thickness (a) $t=7$ u.c., (b) 8 u.c., and (c) 10 u.c ($E_{\mathrm{p}}=50$ eV). Note the rotation by ${30}^{\circ }$. (d) and (e) show the simultaneously recorded topographic STM image and differential conductance map, respectively, of a 10 u.c. ${\mathrm{CePt}}_5$ film (scan parameters: $U=+1.0$ V, $I=300$ pA, $U_{\mathrm{mod}}=10$ mV). Structures $C^{\prime }$ and $D$ are separated by domain boundaries. The black arrow marks a minority structure of the surface and the white arrow points to a dislocation line. (f) and (g) High-resolution (left panel) and corresponding FT images (right) acquired on structures $C^{\prime }$ and $D$, respectively ($U=+1.0$ V, $I=300$ pA). In either case blue and green marks indicate the moiré pattern and the ${\mathrm{CePt}}_5$ unit cell, respectively.