Surface Geometry of ${\mathbf{C}}_{60}$ on Ag(111)

The geometry of adsorbed ${\mathrm{C}}_{60}$ influences its collective properties. We report the first dynamical low-energy electron diffraction study to determine the geometry of a ${\mathrm{C}}_{60}$ monolayer, $\mathrm{Ag}(111)\mathrm{\text{−}}(2\sqrt{3}\times 2\sqrt{3})30°\mathrm{\text{−}}{\mathrm{C}}_{60}$, and related density functional theory calculations. The stable monolayer has ${\mathrm{C}}_{60}$ molecules in vacancies that result from the displacement of surface atoms. ${\mathrm{C}}_{60}$ bonds with hexagons down, with their mirror planes parallel to that of the substrate. The results indicate that vacancy structures are the rule rather than the exception for ${\mathrm{C}}_{60}$ monolayers on close-packed metal surfaces.

Figure 1

LEED patterns from $\mathrm{Ag}(111)\mathrm{\text{−}}(2\sqrt{3}\times 2\sqrt{3})R30°$ for incident beam energies of (a) 101 eV, (b) 365 eV, and (c) 504 eV, for a sample $T$ of 32 K. Reciprocal unit cells for the substrate (long arrows) and overlayer (short arrows) are indicated in (a).

Figure 2

(a) Vacancy site structure, showing three layers of Ag. The mirror plane of the molecule is parallel to the mirror plane of the substrate (dashed line). There are two such parallel orientations of the molecules, one as shown, and one rotated 180°. The top hexagon is surrounded by 3 hexagons and 3 pentagons—180° rotation interchanges these (as well as some others that are not visible). (b) Side view of the vacancy structure, viewed from the bottom of (a), along the dashed line, including the atoms in the box on (a). The structural parameters are given in Table ; the buckling is magnified and the atoms are shown with a reduced size for clarity.

Figure 3

Experimental (bold line) and best-fit calculated spectra for the $\mathrm{Ag}(111)\mathrm{\text{−}}(2\sqrt{3}\times 2\sqrt{3})R30°$ structure. Individual beam $R$ factors are given. The overall $R$ factor is 0.36.