Selective adsorption of toluene-3,4-dithiol on Si(553)-Au surfaces

The adsorption of small organic molecules onto vicinal Au-stabilized Si(111) surfaces is shown to be a versatile route towards controlled growth of ordered organic-metal hybrid one-dimensional nanostructures. Density functional theory is used to investigate the site-specific adsorption of toluene-3,4-dithiol (TDT) molecules onto the clean Si(553)-Au surface and onto a co-doped surface whose steps are passivated by hydrogen. We find that the most reactive sites involve bonding to silicon at the step edge or on the terraces, while gold sites are relatively unfavored. H passivation and TDT adsorption both induce a controlled charge redistribution within the surface layer, causing the surface metallicity, electronic structure, and chemical reactivity of individual adsorption sites to be substantially altered.

Figure 1

(a) Schematic structure of the clean Si(553)-Au surface. Only the topmost layers are shown. The Au (Si) atoms are represented by yellow (green) circles. The dashed lines indicate a $(1\times 4)$ surface unit cell. (b) The $(1\times 4)$ surface Brillouin zone is shown by red lines. High-symmetry points $\mathrm{\Gamma }, K$, and $M$ are shown. $K$ corresponds to the zone boundary of the $(1\times 2)$ surface. (c) Toluene-3,4-dithiol (TDT). The C, S, and H atoms are represented by dark gray, red-orange, and light gray circles, respectively.

Figure 2

Potential-energy surface for methanethiolate adsorption on (a) clean and (b) step-edge-passivated Si(553)-Au. The color scale indicates energies in eV relative to the global minimum. The Au, bulk Si, honeycomb Si, and H atoms of the surface unit cell are indicated in gold, light gray, green, and blue, respectively. Crosses label specific adsorption sites after full relaxation (see text).

Figure 3

Adsorption geometries of TDT on Si(553)-Au (top and side views). Possible reactions of the H atoms in the $-\mathrm{SH}$ groups during the adsorption process are illustrated. The dashed red lines indicate the $(1\times 4)$ surface unit cell used in the simulations.

Figure 4

Adsorption energies and geometries for dithiolate (${\mathrm{TDT}}^{\prime }$) on the clean Si(553)-Au surface (top and side views).

Figure 5

Adsorption energies and geometries of dithiolate (${\mathrm{TDT}}^{\prime }$) on Si(553)-Au with step edges passivated by hydrogen (top and side views). Color scheme is as in Figs. 3 and 4.

Figure 6

Band structures of Si(553)-Au with various adsorbates. (a) Clean surface. (b) Following step edge passivation by hydrogen. (c) Clean surface with adsorbed thiolate (${\mathrm{TDT}}^{\prime }$), following channel (ii), i.e., with step edge also passivated by two H atoms. (d) Step-edge-passivated surface with ${\mathrm{TDT}}^{\prime }$ adsorbed on the Si honeycomb, following channel (iv). In the band structures, yellow circles indicate Au states, red squares indicate states of Si atoms at the step edge, and green triangles are from doubly bonded Si atoms in the honeycomb stripe.