Structural investigations of ${\mathrm{Al}}_5{\mathrm{Co}}_2\left(2\overline{1}0\right)$ and (100) surfaces: Influence of bonding strength and annealing temperature on surface terminations

Structural investigations of the ($2\overline{1}0$) and (100) surfaces of ${\mathrm{Al}}_5{\mathrm{Co}}_2$ using experimental ultrahigh vacuum techniques and ab initio computational methods are presented in this work. Both surfaces are identified and show bulk terminations where only specific atoms remain. These atoms can be seen as truncated parts of chemically bonded atomic motifs which have been identified in the bulk in a previous work [M. Meier et al., Phys. Rev. B 91, 085414 (2015)]. Whereas the ($2\overline{1}0$) surface presents a single termination, the structure of the (100) topmost layers is found to be highly dependent on the preparation conditions, especially on the annealing temperature. This behavior, also observed for the (001) surface studied previously, can be partly explained when considering the bonding strength of the truncated motif parts with the subsurface.

Figure 1

(Left) Layer stacking perpendicular to the [100] direction [blue (red) spheres are Al (Co) atoms]. The strongest chemical bonds are represented by lines connecting atoms. The ($2\overline{1}0$) plane is forming a ${30}^{\circ }$ angle with $\vec{b}$. A $2\times bcos\left({30}^{\circ }\right)$ distance is needed to describe the complete structure in the $\left[120\right]$ direction. ${\mathrm{Al}}_{1,2,3}$ and ${\mathrm{Co}}_{1,2}$ refer to the five nonequivalent atomic positions. (Right) Chemically bonded motifs present in the bulk: planar motif (${\mathrm{Co}}_2$)${}_3$(${\mathrm{Al}}_2$)${}_3$ and 3D motif (${\mathrm{Co}}_1$)(${\mathrm{Al}}_3$)${}_6$ viewed from the top.

Figure 2

(Left) Calculated projected surface (S) layer DOS for the ($2\overline{1}0$) surface (${\mathrm{P}}_B$ model) for the simplified slab containing 160 atoms (plain blue line) and for the nonsimplified slab (dashed black line). Here, the spin is not considered. (Right) Calculated projected S layer DOS for the ($2\overline{1}0$) surface (${\mathrm{P}}_B$ model) with spin-up (plain blue line) and spin-down (dashed black line) contributions.

Figure 3

LEED pattern at 20 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2$($2\overline{1}0$) surface prepared at 973 K showing a ($2\times 1$) reconstruction.

Figure 4

(a) $75\times 75{\mathrm{nm}}^2$ STM image of the ${\mathrm{Al}}_5{\mathrm{Co}}_2$($2\overline{1}0$) surface prepared at 973 K ($V_b=-2$ V; $I_t=0.1$ nA) and (b) $20\times 20{\mathrm{nm}}^2$ ($V_b$ = $-1$ V; $I_t=0.2$ nA). The unit cell of the reconstruction is also shown.

Figure 5

(a) Top and side view of the complete P termination. The T and B lines are shown. Blue and green spheres are Al atoms, red spheres are Co atoms. The magnitudes of the basis vectors of the reconstructed unit cell are $c$ and $4bcos\left({30}^{\circ }\right)$. (b) ${\mathrm{P}}_A$ model generated by removing the Al pentagons every two B lines, while keeping the T lines unchanged, (c) ${\mathrm{P}}_{A-\text{4Co}}$, (d) ${\mathrm{P}}_B$ obtained by removing all atoms on every second B line, (e) ${\mathrm{P}}_{B-\text{4Co}}$, (f) ${\mathrm{P}}_C$ formed by removing the Co atoms of every second T line, (g) ${\mathrm{P}}_D$ obtained by removing all of its atoms on every second T line and (h) ${\mathrm{P}}_{D-\text{2Co}}$.

Figure 6

Calculated surface energies for all seven models (continuous lines) matching the experimental LEED and STM results and non reconstructed terminations P and F (dashed lines), plotted as a function of ${\mu }_{\mathrm{Al}}-{\mu }_{\mathrm{Al}}^{\mathrm{bulk}}$.

Figure 7

Experimental STM images, ${\mathrm{P}}_B$ and ${\mathrm{P}}_{B-4\mathrm{Co}}$ simulated STM images (at $V_b=\pm 2$ V).

Figure 8

Calculated DOS for the ${\mathrm{P}}_B$ model and for the ${\mathrm{P}}_{B-4\mathrm{Co}}$ model, projected on surface (S) and subsurface planes ($\mathrm{S}-1$) to ($\mathrm{S}-2$) (thick lines). The dotted lines show the bulk DOS of corresponding bulk planes.

Figure 9

UPS data and calculated projected DOS for surface (S) and subsurface ($\mathrm{S}-1$) layers for ${\mathrm{P}}_B$ (dashed black line) and ${\mathrm{P}}_{B-4\mathrm{Co}}$ (blue line) models.

Figure 10

(a) LEED pattern at 60 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2$($2\overline{1}0$) surface prepared at 823 K with $x$ and $y$ oblique unit mesh domains (orientated at $\pm {60}^{\circ }$ with respect to the $c$ axis) parameters. (b) LEED pattern at 20 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2$($2\overline{1}0$) surface prepared at 823 K, showing a diffuse hexagonal unit cell ($h_1$ and $h_2$). (c) $30\times 30{\mathrm{nm}}^2$ STM image ($V_b=-1$ V; $I_t=0.1$ nA) showing one domain.

Figure 11

(a) LEED pattern at 40 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2$($2\overline{1}0$) surface prepared at 1043 K, (b) $50\times 50{\mathrm{nm}}^2$ STM image ($V_b=-2$ V; $I_t=0.08$ nA, ${45}^{\circ }$ rotated).

Figure 12

LEED pattern at 55 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2$(100) surface prepared at 973 K showing a ($2\times 1$) reconstruction.

Figure 13

STM images of ${\mathrm{Al}}_5{\mathrm{Co}}_2$(100) surface prepared at 973 K: (a) $150\times 150{\mathrm{nm}}^2$ ($V_b=-2$ V; $I_t=0.2$ nA) and (b) $10\times 10{\mathrm{nm}}^2$ ($V_b=-1$ V; $I_t=0.2$ nA).

Figure 14

Side view of all non reconstructed terminations and corresponding labels.

Figure 15

Calculated surface energies for all ($2\times 1$) reconstructed models, plotted as a function of ${\mu }_{\mathrm{Al}}-{\mu }_{\mathrm{Al}}^{\mathrm{bulk}}$.

Figure 16

(a) Complete A termination with Al atoms in green and blue, Co atoms in red, (b) ${\mathrm{A}}^{-2}$, (c) ${\mathrm{A}}^{-1}$, (d) ${\mathrm{A}}^{+1}$, the circles correspond to missing atoms (compared to the complete A termination).

Figure 17

${\mathrm{A}}^{+1}, {\mathrm{A}}^{-1}$, and ${\mathrm{A}}^{-2}$ simulated STM images (at $V_b=\pm 2$ V).

Figure 18

UPS data and calculated DOS for surface (S) and subsurface ($\mathrm{S}-1$) layers (${\mathrm{A}}^{+1}$).

Figure 19

(a) LEED pattern at 25 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2$(100) surface prepared at 823 K showing diffused spots along $b_r^{*}$, corresponding to a mixture of different reconstructions, ($2\times 1$), ($3\times 1$), and ($4\times 1$), (b) $20\times 20{\mathrm{nm}}^2$ STM image of the same surface ($V_b=-2$ V; $I_t=0.1$ nA).

Figure 20

Structure of the (a) ($3\times 1$)+ and (b) ($4\times 1$)+ models, + referring to models where additional atoms (in ovals) remain in the line in-between space.

Figure 21

(a) Simulated STM images ($V_b=-2$ V) for the ${\mathrm{A}}^{+1}$ ($2\times 1$) model (left) and ($3\times 1$)+, ($4\times 1$)+ models (right) corresponding to the experimentally observed surface prepared at lower temperature (823 K), (b) simulated STM image for the A model corresponding to the experimentally observed surface at higher preparation temperature (1043 K).

Figure 22

(a) LEED pattern at 40 eV of the ${\mathrm{Al}}_5{\mathrm{Co}}_2$(100) surface prepared at 1043 K and (b) $30\times 30{\mathrm{nm}}^2$ STM image ($V_b=-2$ V; $I_t=0.08$ nA).

Figure 23

Calculated surface energies for the different models matching the (100) surface prepared at different temperatures, plotted as a function of ${\mu }_{\mathrm{Al}}-{\mu }_{\mathrm{Al}}^{\mathrm{bulk}}$.

Figure 24

Side view of terminations for the different surfaces studied. Terminations intersect covalently bonded motifs, leaving groups of atoms at their surfaces [a group of one atom ${\mathcal{C}}_1$ for the ${\mathrm{A}}^{+1}$ model (100), a group of three atoms ${\mathcal{C}}_3$ for the ${\mathrm{P}}_{6\text{Al}\text{miss}}^{(\sqrt{3}\times \sqrt{3})R{30}^{\circ }}$ model (001), and a group of eight atoms ${\mathcal{C}}_8$ for the ${\mathrm{P}}_B$ model ($2\overline{1}0$)]. Inset is showing a top view of the (001) surface, highlighting the triangular shaped ensembles forming a $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ reconstruction.

Figure 25

Calculated surface energies for the best models for all different surfaces studied, plotted as a function of ${\mu }_{\mathrm{Al}}-{\mu }_{\mathrm{Al}}^{\mathrm{bulk}}$.