Structure and local reactivity of the Au(111) surface reconstruction

The close-packed (111) surface of gold is well known to show a $22\times \sqrt{3}$ reconstruction on single nm lengths with a long-range herringbone pattern on scales of a few hundred nm. Here we investigate the local reconstruction using density functional theory and compare the results to scanning tunneling microscopy experiments. Moreover, we use hydrogen and fluorine as probe atoms to investigate changes in the ability of the Au(111) surface to catalyze the reactions involved in the formation of molecular nanostructures. We find a small variation of the reactivity across different surface sites and link those results to the local coordination environment of the face-centered-cubic (fcc), hexagonal-close-packed (hcp), and ridge regions. Finally, we scrutinize a commonly used approximation in density functional studies, namely that Au(111) is atomically flat and a perfect termination of the fcc lattice.

Figure 1

(a) Schematic representation of the Au(111)/$22\times \sqrt{3}$ reconstruction, showing how 23 surface atoms fit into 22 lattice sites by compressing the top layer of the surface with the additional atoms colored dark red. The positions corresponding to lined-up fcc and hcp sites are indicated by the vertical lines. (b) STM image of the Au(111) herringbone reconstruction ($V_{\mathrm{tip}}=0.9$ eV, $I_{\mathrm{set}}=1$ nA; Ref. 29).

Figure 2

Calculated structure of the reconstructed Au(111) surface, using the vdWDF/PBE density functional. (a) Site character $c$: distance from the ideal fcc and hcp site for each atom in the top layer, as discussed in the text. (b) Structure of the top layer. The color coding in the top half denotes the atom type, with yellow atoms being of fcc and green atoms being of hcp type. On the bottom half, the color indicates atomic height, with yellow atoms being the closest to the ideal fcc lattice continuation from below. (c) Calculated top-layer height with respect to the ideal fcc lattice and an experimental line scan (Ref. 29).

Figure 3

Top: Nearest neighbor (NN) distribution for the top layer of the reconstructed Au surface. Shown are the number of neighboring atoms located within the distances shown in the legend, where $d$ is the bulk NN spacing. (middle) In the ideally terminated fcc surface, the nearest-neighbor count shows a uniform surface coordination. Bottom: The top-layer height profile for our six-layer calculation as in Fig. 2 is provided for reference.

Figure 4

Energies of H (a) and F (b) atoms adsorbed on the frozen Au(111) surface. The two types of hollow site are clearly distinguishable in both fcc and hcp region for H adsorption, while only a small difference in the hcp site type is observed for F adsorption. (c) The surface atom height profile is repeated from Fig. 2c.

Figure 5

The symbols show the position of the integrated $d$-band centers with respect to the Fermi level for all atoms in the six-layer Au $22\times \sqrt{3}$ unit cell according to Eq. (2). The bottom two layers have been kept fixed as indicated by the parentheses in the legend. Each layer has a correspondingly colored dashed line which indicates the equivalent position of the $d$-band center for an unreconstructed unit cell. The thick solid line shows the $d$-band center for bulk gold.