Ordering and growth of Xe films on the 10-fold quasicrystalline approximant Al${}_{13}$Co${}_4$(100) surface

Xe adsorption on the (100) surface of the complex alloy Al${}_{13}$Co${}_4$ has been carried out using low-energy electron diffraction (LEED) and grand canonical Monte Carlo (GCMC) simulations. This surface is an approximant to the 10-fold surface of decagonal Al-Ni-Co, on which Xe adsorption has been studied previously. The adsorption behavior on the periodic surface is largely similar to that on the quasicrystal (layer-by-layer growth, hexagonal ordering near the onset of the second-layer adsorption), but it also has some differences, such as the complete lack of registry of the Xe layer with the substrate structure in the hexagonal phase, and a high sensitivity of the Xe epitaxial direction to trace impurities. In the simulations, an ordering transition was observed between the low-density and high-density monolayer regimes that involves a uniaxial compression of the monolayer film.

Figure 1

(a), (b) LEED patterns at primary beam energies 188 eV and 400 eV, respectively, at a sample temperature of 80 K. (c) Diagram of the top layer atoms of Al${}_{13}$Co${}_4$(100) unit cell. Blue (red) corresponds to atoms above (below) the center of mass of the layer; the color shading indicates the magnitude of the displacement from the center of mass of the layer. All atoms are Al, and the large ones are the “glue” atoms between the bipentagons.

Figure 2

(a) LEED pattern from the clean Al${}_{13}$Co${}_4$ surface at 65 eV. The (2,3) and equivalent diffraction spots are indicated by circles. (b)–(d) LEED patterns at the same energy after the ordering of Xe on the surface. (b) Two differently rotated domains of hexagonal Xe (red circles and blue squares) (intensity inverted for clarity). (c) One of these domains (red circles in (b)—commensurate) has diffraction spots that are coincident with the substrate (2,3) spots. (d) The range of the intensity profile shown in (e). (e) LEED intensity profile showing that the lattice angles subtended by the two different types of hexagonal Xe are ±14.32° and ±22.02°.

Figure 3

LEED patterns from an ordered Xe film, taken consecutively during the course of adsorption experiments that lasted about 1 hour. In the early experiments, the Xe is mostly in the “incommensurate” phase (red circles). This phase was gradually replaced by the “commensurate” phase (blue squares).

Figure 4

(a) LEED intensity isobars for Xe adsorption at a constant pressure of 7 × 10${}^{-6}$ mbar. (b) LEED intensity isobars for Xe adsorption at a Xe pressure of 5 × 10${}^{-6}$ mbar, enlarged to show the details at the second-layer adsorption step (near 60 K).

Figure 5

(a) Calculated adsorption potential for Xe on Al${}_{13}$Co${}_4$. (b) Calculated adsorption isotherms for Xe on A${}_{13}$Co${}_4$. The simulated temperatures are 60 K to 110 K with 10 K increment. The inset depicts the density profile along the $z$-axis at 80 K and shows the five adsorbed layers of Xe at a pressure corresponding to point e.

Figure 6

Density plots showing the probabilistic location of the adsorbed Xe atoms at 80 K (top panel) and their FTs (bottom panel). The left, middle, and right plots correspond to the submonolayer, the onset of the monolayer, and the full monolayer coverages, respectively. The apparent high resolution (fine structure) of the FTs, especially in (d) and (e), is a consequence of the periodic boundary conditions and reflects the size of the simulation cell.

Figure 7

Xe adsorption at 80 K: (top panel) adsorption isotherm, (middle panel) nearest neighbor distance by assuming a hexagonal structure, and (bottom panel) total enthalpy of the system as a function of the normalized chemical potential as defined in the text. The arrow in the middle panel indicates the location of a possible first-order transition, and the associated enthalpy is indicated in the bottom panel.

Figure 8

Pressure and temperature loci of the onset of the first-layer (square), second-layer (circle), and third-layer (triangle) formation of Xe on the Al${}_{13}$Co${}_4$(100) surface. The corresponding isosteric heats of adsorption are 241, 145, and 135 meV, respectively.

Figure 9

(a) FT from the high-density structure showing the primary symmetry directions. The solid red line establishes a primary symmetry direction that persists in all of the observations. (b) LEED pattern from the ordered structure, showing that the primary symmetry direction from (a) coincides with diffraction spots from one domain of the ordered (clean) structure. (c) Density plot for the high-density structure, showing the rows of Xe (solid red lines) that correspond to the primary symmetry direction, and the other close-packed directions (dashed lines). (d) Superimposition of the density plot (black represents Xe) onto the potential energy map, showing that Xe atoms are primarily located between surface Al atoms (red and blue dots). (e) FT from the low-density structure, indicating that the primary symmetry direction is maintained. (f) Density plot for the low-density structure with the solid red lines from (c), showing that the ordering transition observed in the simulations involves a uniaxial compression along these lines.