Interplay between bulk atomic clusters and surface structure in complex intermetallic compounds: The case study of the ${\mathrm{Al}}_5{\mathrm{Co}}_2\left(001\right)$ surface

The ${\mathrm{Al}}_5{\mathrm{Co}}_2$ crystal is a complex intermetallic compound, whose structure can be described by a stacking of chemically bonded atomic motifs. It is a potentially new catalytic material for heterogeneous hydrogenation. A single crystal of this phase has been grown by the Czochralski technique in order to study the influence of the three-dimensional bulk substructure on the two-dimensional surface using both experimental ultrahigh vacuum surface techniques and ab initio methods based on the density functional theory. Some bulk properties are first presented, focusing on chemical bond strengths, the determination of the Al and Co chemical potentials in ${\mathrm{Al}}_5{\mathrm{Co}}_2$, the vibrational properties, and the specific heat. Then, the combination of experimental and computational approaches allows the identification of the surface structure, which was found to depend on the surface preparation conditions. In all cases, the surface terminates at specific bulk layers (Al-rich puckered layers) where various fractions of specific sets of Al atoms are missing, identified as ${\mathrm{Al}}_3$ atoms left at the surface resulting from cluster truncation. Finally, electron density of states calculations and spectroscopic measurements were compared and indicate a strong $sp\text{−}d$ hybridization of the topmost pure Al layer with subsurface Co atoms. This could influence the surface reactivity and the catalytic performances of this material.

Figure 1

Atomic structure of ${\mathrm{Al}}_5{\mathrm{Co}}_2$. Top: layer stacking along the [001] direction. The layers P1 and P2, or F1 and F2, are related to each other by a rotation of ${180}^{\circ }$. Bottom: in-plane structure of the flat (F) and puckered (P) layers. Co atoms are displayed in red, Al atoms at the mean plane position (in the P layer) and in the F layer are in blue, Al atoms above the mean plane position in the P layer are in dark blue, and Al atoms below are in light blue.

Figure 2

(a) Defect concentrations at 300 K as a function of the chemical composition, (b) defect concentrations at 1000 K as a function of the chemical composition. The color code in (b) is the same as in (a).

Figure 3

Defect formation energies in ${\mathrm{Al}}_5{\mathrm{Co}}_2$ (in eV) for (a) Al-rich side and Co-rich side, (b) for stoichiometric ${\mathrm{Al}}_5{\mathrm{Co}}_2$ and chemical potentials at 300 K as a function of the chemical composition in ${\mathrm{Al}}_5{\mathrm{Co}}_2$.

Figure 4

Convoluted DOS in bulk ${\mathrm{Al}}_5{\mathrm{Co}}_2$. (a) Contribution of the P and F layers, (b) contributions of Co and Al atoms (partial DOS).

Figure 5

Isodensity surface ($0.015 e^{-}/{\AA }^{-3}$) of the difference between the total charge density and the superposition of atomic charge densities. The strongest bonds within the two types of atomic motifs are displayed. Only ${\mathrm{Al}}_1$-type atoms and atoms involved in the considered motifs are shown.

Figure 6

(a) Calculated vibrational density of states for ${\mathrm{Al}}_5{\mathrm{Co}}_2$ and contributions of Al and Co to the VDOS. Full line is for Co atoms and dashed line is for Al atoms, (b) phonon dispersion relations along high-symmetry directions $\left[K\right(\frac{2}{3},\frac{2}{3},0),H(\frac{2}{3},\frac{2}{3},\frac{1}{2}),A(0,0,\frac{1}{2}),\Gamma (0,0,0),M(\frac{1}{2},0,0),L(\frac{1}{2},0,\frac{1}{2})]$.

Figure 7

Specific-heat measurement for the ${\mathrm{Al}}_5{\mathrm{Co}}_2$ compound (solid blue curve) compared with the calculated one (solid black curve). The contribution of phonons is represented by the dots. The inset shows a fit of the low-temperature data in a $C_p/T$ versus $T^2$ plot. The equation corresponds to $C_p/T=\alpha T^2+\gamma$, with $\alpha$ and $\gamma$ the coefficients related the lattice specific heat and the electronic specific heat determined by the fit.

Figure 8

(a) Variation of the elemental composition at the (001) surface of ${\mathrm{Al}}_5{\mathrm{Co}}_2$ as a function of the photoelectron takeoff angle. The measurements at ${60}^{\circ }$ are more surface sensitive than those performed at $0^{\circ }$. The dotted lines are only guides for the eyes. (b), (c) XPS Al $2p$ and Co $2p_{3/2}$ core-level lines, respectively, measured at $0^{\circ }$ takeoff angle. Dots represent the experimental spectra and thin lines are the deconvoluted spectra. The arrow in (c) outlines the position of the satellite peak in the Co $2p_{3/2}$ spectrum.

Figure 9

LEED pattern at 20 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2\left(001\right)$ surface prepared at high temperature (973 K) showing a $\left(\sqrt{3}\times \sqrt{3}\right){R30}^{\circ }$ reconstruction.

Figure 10

STM images ($-2$ V, 0.1 nA) of the ${\mathrm{Al}}_5{\mathrm{Co}}_2\left(001\right)$ surface prepared at 973 K. (a) $70\times 70{\text{nm}}^2$, (b) $20\times 20{\text{nm}}^2$.

Figure 11

Topmost layer for (a) complete P layer and the three surface models considered in this study: (b) ${\mathrm{P}}_{6\text{Al}\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$, (c) ${\mathrm{P}}_{15\text{Al}\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$, (d) ${\mathrm{F}}_{15\text{Al}\&\mathrm{Co}\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$ with the atoms which are slightly protruding above the mean surface plane position in dark blue, protruding underneath the mean surface plane position in light blue, and the basic and reconstructed unit cells represented with black lines. Co atoms for the F termination are presented in red.

Figure 12

Calculated surface energies for the different models (continuous lines) as well as for nonreconstructed terminations P and F (dashed lines), plotted as a function of ${\mu }_{\text{Al}}-{\mu }_{\text{Al}}^{\mathrm{bulk}}$.

Figure 13

Simulated STM images of the ${\mathrm{P}}_{6\text{Al}\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$ model (left), experimental STM images (middle), and simulated STM images of the ${\mathrm{P}}_{15\text{Al}\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$ model (right) for different bias voltages [+2 V (top), +1 V, +0.5 V, $-0.5$ V, $-1$ V, $-2$ V (bottom)]. The size of images is $5\times 5{\text{nm}}^2$.

Figure 14

Calculated DOS for the ${\mathrm{P}}_{6\text{Al}\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$ model projected on surface (S) and subsurface planes (S-1) to (S-3) (thick lines). The dotted lines show the bulk DOS of equivalent atoms (Eq-S for the bulk DOS of the equivalent atoms present in the surface layer and Eq-S-1 to Eq-S-3, the bulk DOS of the equivalent atoms present in the corresponding subsurface layers).

Figure 15

(a) Experimental and (b) simulated STM images $\left(5\times 5{\text{nm}}^2\right)$ of the surface. The two arrows indicate the two different distances $d_1$ and $d_2$ (see text). (c) The ${\mathrm{P}}_{6\text{Al}\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$ model where vectors indicate atomic displacements occurring during relaxation.

Figure 16

(a) UPS and STS (200 points from $-2$ V to +2 V, 0.18 nA, 0 to 3.3 nA range, grid $10\times 10$, on a $50\times 50 {\mathrm{nm}}^2$ surface) data, (b) total bulk convoluted DOS and S + S-1 layers convoluted DOS.

Figure 17

(a) LEED pattern at 19 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2\left(001\right)$ surface prepared at low temperature (823 K), showing the $\left(2\times 2\right)$ reconstruction. (b) STM image of the ${\mathrm{Al}}_5{\mathrm{Co}}_2\left(001\right)$ surface after annealing at 823 K [$20\times 20{\text{nm}}^2,-2$ V bias voltage, 0.12 nA, with $\left(2\times 2\right)$ reconstructed unit cell showed with black lines].

Figure 18

(a) Model ${\mathrm{P}}_{9\text{Al}\mathrm{miss}}^{(2\times 2)}$ and (b) model ${\mathrm{P}}_{10\text{Al}\mathrm{miss}}^{(2\times 2)}$.

Figure 19

Simulated STM images of ${\mathrm{P}}_{10\text{Al}\mathrm{miss}}^{(2\times 2)}$ (left), experimental STM images (middle), simulated STM images of ${\mathrm{P}}_{9\text{Al}\mathrm{miss}}^{(2\times 2)}$ (right), for different bias voltages (+1 V and $-1$ V). The size of images is $5\times 5{\text{nm}}^2$.

Figure 20

LEED patterns of ${\mathrm{Al}}_5{\mathrm{Co}}_2\left(001\right)$ after annealing ($T=1180$ K for 1 h), recorded at $T=134$ K, for three different incident beam energies. The reciprocal unit-cell vectors are shown in the 87 eV pattern.

Figure 21

Side view of the relaxations of the surface layers. The blue spheres are Al and the red spheres are Co. $d$ is the interlayer spacing, $\Delta$ is the average pucker in the surface layers, and the percent deviation from the bulk spacings is also given. The bulk interlayer spacing is 1.88 Å and the average pucker of the bulk P layers is 0.46 Å.

Figure 22

Comparison of experimental and theoretical LEED I(E) curves for 14 independent beams. The individual Pendry factors (Rp) are shown. The total $R$ factor is 0.32.

Figure 23

(a) Domains highlighted on a $20\times 20{\text{nm}}^2$ experimental STM image obtained for the surface prepared at 973 K, (b) schematic illustrating the three possibilities to form the reconstruction (973 K), (c) domains highlighted on a $20\times 20{\text{nm}}^2$ experimental STM image obtained for the surface prepared at 823 K, (d) schematic illustrating the four possibilities to obtain the reconstruction (823 K).

Figure 24

Calculated surface energies for the different models described in Tab. 4 plotted as a function of ${\mu }_{\mathrm{Al}}-{\mu }_{\mathrm{Al}}^{\mathrm{bulk}}$ (surface energy for the ${\mathrm{P}}_{\mathrm{random}}$ model has not been calculated, because of its size (>600 atoms) needed to simulate the disorder). ${\mathrm{P}}_{3\mathrm{Al}\mathrm{miss}}^{(1\times 1)}$ is in red(observed experimentally at high temperature 1180 K), ${\mathrm{P}}_{10\mathrm{Al}\mathrm{miss}}^{(2\times 2)}$ is in blue (observed experimentally at low temperature 823 K), ${\mathrm{P}}_{6Al\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$ is in orange (seen for the surface prepared at 973 K) and ${\mathrm{P}}_{\mathrm{complete}},{\mathrm{P}}_{3\mathrm{Al}\mathrm{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$ for comparison purposes are in green (not observed experimentally).