Xe adsorption on a ${\mathrm{C}}_{60}$ monolayer on Ag(111)

Low-energy electron diffraction (LEED) experiments and grand canonical Monte Carlo simulations were carried out to study the adsorption of Xe on a substrate composed of a monolayer of ${\mathrm{C}}_{60}$ molecules on a Ag(111) surface. LEED adsorption isobars indicated that the adsorption occurs in steps, with the Xe initially adopting a structure having the same unit cell as the ${\mathrm{C}}_{60}$. Isosteric heats corresponding to the first two steps were measured to be $234\pm 8$ and $204\pm 14\mathrm{meV}$, respectively. For the simulations, the interaction potential of Xe with the composite substrate was modeled as the sum of two parts: the Xe-Ag part was computed using an ab initio van der Waals potential that varies as an inverse-distance cubed and the $\mathrm{Xe}\text{−}{\mathrm{C}}_{60}$ part was computed using a spherically averaged ${\mathrm{C}}_{60}$ potential [E. S. Hernandez et al., J. Low Temp. Phys. 134, 309 (2004)]. The resulting adsorption potential is highly corrugated, with the most attractive sites located in the threefold hollows between the ${\mathrm{C}}_{60}$ molecules, forming a honeycomb array. The simulations (at temperatures ranging from $55\text{to}90\mathrm{K}$) show that these attractive sites are filled first, followed by adsorption in two types of secondary sites, where a competition exists due to steric hindrance. The thermodynamic properties of film growth obtained in the simulation are in good agreement with the experiment.

Figure 1

(Color online) (a) LEED pattern from the ${\mathrm{C}}_{60}$ monolayer on Ag(111) at $T=32\mathrm{K}$ for an incident beam energy of $300\mathrm{eV}$. The unit cells (in reciprocal space) for Ag(111) (large rhombus) and the $(2\surd 3\times 2\surd 3)R30°$ structure (small rhombus) are indicated. (b) Schematic structure model for the ${\mathrm{C}}_{60}$ monolayer (Ref. 19.)

Figure 2

LEED intensity isobars for Xe (a) adsorption and (b) desorption at $1.7\times {10}^{-7}\mathrm{mbar}$ on the ${\mathrm{C}}_{60}$ monolayer on Ag(111). These intensities are the averaged integrated intensities of the three bright first-order Ag(111) diffraction spots (indicated by circles on the inset LEED pattern).

Figure 3

(Color online) Family of adsorption isobars showing the intensity of the substrate diffraction spot (shown in Fig. 2) as a function of $T$ for (from right to left) $5.2\times {10}^{-7}$, $1.7\times {10}^{-7}$, $8.5\times {10}^{-8}$, and $1.4\times {10}^{-8}\mathrm{mbar}$.

Figure 4

(Color online) Locations in $P$ and $T$ of the first and second isobar steps, respectively. The $y$ axis is on a natural logarithmic scale ($P$ in $\mathrm{Pa}\approx {10}^{-2}\mathrm{mbar}$). The isosteric heats derived from the slopes of the lines are indicated.

Figure 5

(Color online) Diagram of the simulation cell, not to scale. The red dots represent the positions of the centers of the ${\mathrm{C}}_{60}$ molecules.

Figure 6

(Color online) Schematic view of the positions of sites $H$ (black triangles), $B$ (red squares), and $C$ (blue circles) projected on the simulation cell. The empty circles represent the centers of the ${\mathrm{C}}_{60}$ molecules. Each set of sites is contained on three planes parallel to the base of the cell at different heights.

Figure 7

(Color online) Adsorption potential as a function of distance above the sites $H$ (solid black line), $B$ (dashed red line), and $C$ (long-dashed blue line).

Figure 8

(Color online) Adsorption isotherms for $T=60$, 65, 77, and $90\mathrm{K}$ (from left to right). The dotted lines are guides to the eyes.

Figure 9

(Color online) Contour plots of the 2D density $n_2(x,y)$ for phases (a) $H$ and (b) $BC$. Contours correspond to $N=0.5{\mathrm{\AA }}^{-2}$ (black, outer), $1{\mathrm{\AA }}^{-2}$ (red, middle), and $2{\mathrm{\AA }}^{-2}$ (blue, inner).

Figure 10

(Color online) Effective potentials due to both the substrate with Xe atoms at the $H$ sites at the locations of (a) the $C$ sites and (b) the $B$ sites (solid-blue line). The bare substrate (unfilled $H$ sites) potential is also shown (dashed line) for comparison.

Figure 11

Relative coverage as a function of $z$ for $T=60\mathrm{K}$ and $P=\left(\mathrm{a}\right)$ $1.4\times {10}^{-6}\mathrm{mbar}$, (b) $2.4\times {10}^{-5}\mathrm{mbar}$, and (c) $5.8\times {10}^{-5}\mathrm{mbar}$.

Figure 12

Relative coverage as a function of $z$ for $T=60\mathrm{K}$ and $7.2\times {10}^{-4}\mathrm{mbar}$.

Figure 13

(Color online) ln $P$ vs $1∕T$ for (a) $N=0.01{\mathrm{\AA }}^{-2}$ and (b) $N=0.05{\mathrm{\AA }}^{-2}$. The lines are the linear fits and the isosteric heats derived from the slopes are given on the graphs.

Figure 14

(Color online) Experimental (red, left axis) and simulated (blue, right axis) adsorption isobars at $P=1.7\times {10}^{-7}\mathrm{mbar}$.