Na Adsorption on the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ Surface: From Two-Dimensional Gas to Nanocluster Array

We have systematically investigated Na adsorption on the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ surface at room temperature using scanning tunneling microscopy (STM). Below the critical coverage of 0.08 monolayer, we find intriguing contrast modulation instead of localized Na adsorbates, coupled with streaky noise in the STM images, which is accompanied by monotonic work function drop. Above the critical coverage, Na clusters emerge and form a self-assembled array. Combined with first-principles theoretical simulations, we conclude that the Na atoms on the ($7\times 7$) surface are, while strongly bound ($\sim 2.2\mathrm{e}\mathrm{V}$) to the surface, highly mobile in “basins” around the Si rest atoms, forming a two-dimensional gas phase at the initial coverage, and that the cluster at the higher coverage consists of six Na atoms together with three Si adatoms.

Figure 1

Filled state STM images (at $-1.3\mathrm{V}$ sample bias, 20 pA) of the $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}(7\times 7)$ surface with Na coverage at (a) 0; (b) 0.02; (c) 0.04; (d) 0.06; (e) 0.08; and (f) 0.10 ML. A ($7\times 7$) unit cell is marked by two green triangles in (a), where F is the FHUC and U is the UHUC. At coverage (f) of 0.10 ML, Na clusters start to form (two blue arrows). (g)–(l) show the contract modulation upon Na adsorption for images (a)–(f) by subtracting the image of the clean ($7\times 7$) surface.

Figure 2

(a) The surface work function change ($\Delta \varphi$) as a function of the Na coverage ($\theta$). (b) The corresponding number of Na clusters per unit cell. The inset shows the entire work function change up to the Na saturation coverage ($\Delta \varphi =-2.5\mathrm{e}\mathrm{V}$). Two inflection points are clearly evident at 0.08 and 0.22 ML, dividing into the three distinctly different adsorption stages.

Figure 3

Filled and empty state images of the Na clusters with bias voltage of $-2.0\mathrm{V}$ for (a) and $+0.8\mathrm{V}$ for (b). (c) The structural model of the Na cluster with the highest $E_b$ ($2.13\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}$). One of the Na adatoms near the Si dimer is pushed outwards (red arrow), the structure being triply degenerated. (d),(e) Simulated filled and empty state STM images, at $-2.0\mathrm{V}$ and $+0.8\mathrm{V}$, respectively. The top part of (c) shows the adsorption scheme for a single Na atom. The green hexagon indicates an “attraction basin.” The three open circles (black arrow marked) correspond to the lowest energy sites ($E_b=2.26\mathrm{e}\mathrm{V}$ for the FHUC and 2.20 eV for the UHUC).