Growth of One-Dimensional Pd Nanowires on the Terraces of a Reduced ${\mathrm{SnO}}_2(101)$ Surface

Palladium, vapor-deposited at room temperature on a reduced ${\mathrm{SnO}}_2(101)$ surface, forms one-dimensional islands, one atomic layer high, 5 Å wide, and up to 350 Å long. Scanning tunneling microscopy shows that neighboring islands do not merge. First-principles calculations reveal the atomistic processes that lead to this, for metal oxide substrates unusual, overlayer growth. Formation of 1D islands is mediated by a large anisotropy in surface diffusion, strong Pd-Sn interaction, and the lack of stable binding sites at the sides of the Pd islands. Nucleation is defect mediated, and the initial nucleation site determines the width of the resulting nanocluster.

Figure 1

Structure of (a) the oxidized, (b) the reduced ${\mathrm{SnO}}_2(101)$ surface, and (c), (d) the reduced surface with one and three atom-wide Pd stripes.

Figure 2

Empty-states STM images of (a) the clean reduced ${\mathrm{SnO}}_2(101)$ surface (inset $6\times 6{\mathrm{nm}}^2$) and (b), (c), (e) with increasing coverages of Pd deposited at room temperature. Displayed in (d) are line profiles of a step edge and a Pd cluster taken at the positions of the white lines in (c). (f) shows the surface in (e) after annealing.

Figure 3

Calculated potential energy surfaces for a single Pd atom on different $\mathrm{Pd}/{\mathrm{SnO}}_2$ surface structures (see text). Top Pd, first layer Sn, and second layer O atoms of the $\mathrm{Pd}/{\mathrm{SnO}}_2$ slabs are shown with shaded large gray, small gray, and large dark spheres, respectively. The surface unit cells are indicated by dashed lines. Positions of possible Pd binding sites (taken from fully relaxed calculations) are marked with diamonds (missing O rows), $+$ and $\times$. The barriers between these binding sites along the deep blue troughs are less than 0.12 eV.

Figure 4

Constant current STM images of (a) a ${\mathrm{SnO}}_2(101)$ surface slightly damaged by ${\mathrm{He}}^{+}$ sputtering and (b) after deposition of 0.35 ML of Pd.