Formation of Pt-induced Ge atomic nanowires on Pt/Ge(001): A density functional theory study

Pt deposited onto a Ge(001) surface gives rise to the spontaneous formation of atomic nanowires on a mixed Pt-Ge surface after high-temperature annealing. We study possible structures of the mixed surface and the nanowires by total energy (density functional theory) calculations. Experimental scanning-tunneling microscopy images are compared to the calculated local densities of states. On the basis of this comparison and the stability of the structures, we conclude that the formation of nanowires is driven by an increased concentration of Pt atoms in the Ge surface layers. Surprisingly, the atomic nanowires consist of Ge instead of Pt atoms.

Figure 1

Schematic representation of Pt-Ge(001) surfaces. (a) $\beta$ surfaces have $\frac{1}{4}$ ML of Pt atoms in the top layer with possible positions given by the indices. (b) Adsorption sites for ad-dimers on the ${\beta }_6$ surface (Pt atoms at positions 0 and 6). (c) and (d) Adsorption of a NW (E) on the $\gamma$ surface (Pt atoms at positions 1, 2, 5, and 6) and the ${\gamma }^{\ast }$ surface (additional Pt atoms in the third layer under the NW), respectively.

Figure 2

(Color online) Left: simulated filled-state STM image of the ${\beta }_6$ surface (Pt atoms at positions 0 and 6) at bias $V=-0.70\text{V}$ (Ref. 16). Contours are added to guide the eye. Red (dark gray) and green (light gray) disks show positions of Pt and Ge atoms, respectively. Right: experimental STM image of the $\beta$ surface at $V=-0.3\text{V}$ (Ref. 18).

Figure 3

(Color online) (a)–(d) Simulated STM images at bias $V=-1.50\text{V}$ (Ref. 16). The positions of NW adatoms are represented by yellow (white) disks. (a) The structure after optimizing a Pt NW on the ${\beta }_6$ surface; (b) Pt NW on Ge(001); (c) (sunken) Pt NW on the $\gamma$ surface; (d) Ge NW on the ${\gamma }^{\ast }$ surface, see Fig. 1. (e) Experimental STM image of a wide trough; $V=-0.50\text{V}$ and $I=0.50\text{nA}$ (Ref. 13). (f) Experimental STM image of a NW; $V=-1.35\text{V}$ and $I=0.50\text{nA}$ (Ref. 18)