${\mathrm{C}}_{60}$ superstructure and carbide formation on the Al-terminated ${\mathrm{Al}}_9{\mathrm{Co}}_2\left(001\right)$ surface

We report the formation of an ordered ${\mathrm{C}}_{60}$ monolayer on the ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio calculations. Dosing fullerenes at 300 K results in a disordered overlayer. However, the adsorption of ${\mathrm{C}}_{60}$ with the sample held between 573–673 K leads to a [4, $-2\mid 1,3$] phase. The growth of ${\mathrm{C}}_{60}$ proceeds with the formation of two domains which are mirror symmetric with respect to the [100] direction. Within each domain, the superstructure unit cell contains six molecules and this implies an area per fullerene equal to 91 Å${}^2$. The molecules exhibit two types of contrast (bright and dim) which are bias dependent. The adsorption energies and preferred molecular configuration at several possible adsorption sites have been determined theoretically. These calculations lead to a possible scheme describing the configuration of each ${\mathrm{C}}_{60}$ in the observed superstructure. Several defects (vacancies, protrusions,…) and domain boundaries observed in the film are also discussed. If the sample temperature is higher than 693 K when dosing, impinging ${\mathrm{C}}_{60}$ molecules dissociate at the surface, hence leading to the formation of a carbide film as observed by STM and LEED measurements. The formation of ${\mathrm{Al}}_4{\mathrm{C}}_3$ domains and the molecular dissociation are confirmed by XPS/UPS measurements acquired at different stages of the experiment. The cluster substructure present at the ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface dictates the carbide domain orientations.

Figure 1

LEED pattern recorded at 26 eV after dosing 1.3 MLE ${\mathrm{C}}_{60}$ with the sample held at 613 K. The reciprocal lattice vectors as* and bs* are represented by the small arrows (dashed line). Inset: LEED pattern (26 eV) recorded on the clean ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface after annealing to 953 K.

Figure 2

(a) 200 nm $\times$ 200 nm high-resolution STM image indicating that growth proceeds via step edge decoration with 0.15 MLE ${\mathrm{C}}_{60}$ $(V_b$ = +2.0 V; $I_t$ = 0.57 nA). (b) 46 $\mathrm{nm}\times 46$ nm STM image for 0.65 MLE ${\mathrm{C}}_{60}$ coverage $({\mathrm{V}}_b$ = +1.5 V; $I_t$ = 0.10 nA). (c) 32 $\mathrm{nm}\times 32$ nm high-resolution STM image for 1.3 MLE ${\mathrm{C}}_{60}$ coverage $(V_b$ = +2.0 V; $I_t$ = 0.09 nA). (d) 12 $\mathrm{nm}\times 12$ nm high-resolution STM of the dosed surface where the line indicates a change in the bias polarity $(I_t$ = 0.09 nA). (e) 6 $\mathrm{nm}\times$ 6 nm high-resolution STM image highlighting one unit cell. (f), (g) 10 $\mathrm{nm}\times 5$ nm STM image of the same region recorded several minutes apart. The bright defect is used as a reference point to directly compare the two images and one unit cell has been highlighted $(V_b=-2.0$ V; $I_t=0.2$ nA).

Figure 3

Model representing the ${\mathrm{C}}_{60}$ molecules and the underneath substrate. The circles correspond to the cage diameter (full line) and van der Waals radius (dashed line) of the fullerenes. The dark and light blue/gray spheres are, respectively, the Co and Al atoms of the substrate. The full $\left(R\right)$ and dashed $\left(S\right)$ lines are examples of equivalent ways to place the superstructure unit cell above the clean surface while keeping their vertices decorated by the $S$-1 Co atoms.

Figure 4

(a) Spectra of the Al 2p core level for the clean surface (bottom) and after 0.5-ML deposition with the sample held at 693 K (middle) and 805 K (top). (b) Spectra of the C 1s core level after 0.5-ML deposition at room temperature (bottom) and with the sample held at 693 K (middle), and at 805 K for 1 MLE deposition (top).

Figure 5

(a) UPS spectra after deposition of 0.5 MLE ${\mathrm{C}}_{60}$ with the sample held at 300 K (bottom) and 693 K (top). (b) UPS spectra for different takeoff angle obtained after deposition of 0.5 MLE ${\mathrm{C}}_{60}$ with the sample held at 805 K.

Figure 6

High-resolution STM images of the dosed surface with (a) 16 $\mathrm{nm}\times 6$ nm $(V_b=-2.5$ V; $I_t=0.17$ nA) and (b) 19 $\mathrm{nm}\times 8$ nm $(V_b=+2.5$ V; $I_t=0.07$ nA). The arrows indicate the domain boundaries.

Figure 7

(a), (b) LEED pattern obtained at 50 eV for 0.5 MLE deposition at 300 K followed by annealing to 713 K. Inset: LEED pattern (40 eV) recorded after annealing 1.3 MLE to 966 K. (c) 85 nm $\times$ 85 nm high-resolution STM image for 0.5 MLE with the sample held at 805 K. (d) 27 nm $\times$ 27 nm high-resolution STM image showing both the clean ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface $\left(G\right)$ and the new phase $\left(H\right)$. Inset: 6 nm $\times$ 6 nm STM image (FFT filtered) of a region from image (d) revealing the hexagonal structure.

Figure 8

Adsorption geometries (top view) for a ${\mathrm{C}}_{60}$ molecule adsorbed on ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001). The topmost surface layer (pure Al plane), the subsurface Co atoms $(S$-1), and the C atoms at the bottom of the molecule are represented by medium (light blue), large (dark blue), and small (yellow) spheres, respectively.

Figure 9

Histogram of C-$G$ distances between carbon atoms (C) and the center of mass of the ${\mathrm{C}}_{60}$ molecule $\left(G\right)$ in Å. The vertical straight line (gray) at 3.55 Å indicates the corresponding distance in the free ${\mathrm{C}}_{60}$ molecule.

Figure 10

Isosurface of the charge differences $\rho \left[{\mathrm{C}}_{60}\text{on}{\mathrm{Al}}_9{\mathrm{Co}}_2\text{(001)}\right]-\left\{\rho \right(\text{free}{\mathrm{C}}_{60})+\rho [{\mathrm{Al}}_9{\mathrm{Co}}_2\text{(001)}\left]\right\}$ for the T1 and H1 sites (top and side views). The atomic radius on the top view is reduced to better identify the charge transfer.

Figure 11

Isosurface of the charge differences $\rho \left[{\mathrm{C}}_{60}\text{on}{\mathrm{Al}}_9{\mathrm{Co}}_2\text{(001)}\right]-\left\{\rho \right(\text{free}{\mathrm{C}}_{60})+\rho [{\mathrm{Al}}_9{\mathrm{Co}}_2\text{(001)}\left]\right\}$ for few bridge sites (top and side views). The atomic radius on the top view is reduced to better identify the charge transfer.

Figure 12

(a) The observed (white line) and hypothetical (black line) orientations of the superstructure unit cell are superimposed over one of the two possible ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface terminations. (b) Three-dimensional representation (side view) of the two pure Al puckered planes appearing alternatively as topmost terminations on terraces. Note the arrangement and the buckling of these two layers along the b axis.

Figure 13

Schematic representation of the carbide-substrate orientation relationship. A single (001) bulk layer (arbitrary selection) of ${\mathrm{Al}}_4{\mathrm{C}}_3$ has been superimposed with two orientations on top of the Al-terminated layer of the ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface. The small spheres correspond to C atoms while the others to Al atoms [blue/gray = ${\mathrm{Al}}_9{\mathrm{Co}}_2$(001) surface; black = ${\mathrm{Al}}_4{\mathrm{C}}_3$].