Unusual Cluster Shapes and Directional Bonding of an fcc Metal: $\mathrm{Pt}/\mathrm{Pt}(111)$

Small clusters of Pt adatoms grown on Pt(111) exhibit a preference for the formation of linear chains, which cannot be explained by simple diffusion-limited aggregation. Density functional theory calculations show that short chains are energetically favorable to more compact configurations due to strong directional bonding by $d_{z^2}$-like orbitals, explaining the stability of the chains. The formation of the chains is governed by substrate distortions, leading to funneling towards the chain ends.

Figure 1

STM images of 0.015 ML $\mathrm{Pt}/\mathrm{Pt}(111)$ deposited by (a) PLD at $T=180\mathrm{K}$ and (b) TD at 145 K. The inset in (a) shows a part of the image with atomic resolution (note the “wrap-around” color map used for better contrast of both substrate and adatom chain). The line scan in (b) shows that the chain between the tick marks has a length of 8 Pt atoms (6 more than the dimer).

Figure 2

Fraction of straight chains for different adatom cluster sizes $n$. Experimental data for $n=3$ are from TD only (0.01 ML, $T\approx 140\mathrm{K}$); $n>3$ data include both TD ($T\approx 140\mathrm{K}$) and PLD ($T\approx 180\mathrm{K}$) experiments with 0.015 ML Pt [15]. Error bars ($2\sigma$) include counting statistics only. For the funneling simulation, the inset shows the sites leading to attachment at the sides of a chain (light gray regions) and at its ends (rhombs).

Figure 3

(a) DOS of the Pt adatoms in triangular trimers and in the center of a 3-atom chain. (b) Charge-density difference in a plane parallel to the surface through a linear 3-atom chain with respect to a superposition of isolated adatoms (energy range $-5.4$ to $-4.9\mathrm{eV}$). Positive (negative) values are shown with white (gray) background, and crosses mark the positions of the Pt atoms.

Figure 4

Calculated selected diffusion barriers (a),(b) and (c) substrate bond lengths surrounding a 3-atom chain, indicated by the thickness of the bonds [24]. Adatoms can easily traverse long (thin) bonds, not short (thick) ones. Arrows in the left indicate a few possible diffusion paths.