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Fig. 87 Scanning probe-based manipulation of single molecules on solid substrates: transfer of CO onto a Cu substrate [127]

ential treatment of one or the other interaction is achieved by adjustment of the STM potential in combination with the distance between the tip and surface. Thus single Cu atoms were moved or CO molecules were picked up and transferred by the STM tip on a crystalline Cu surface (Fig. 87) [127]). In an analogous experiment, Si atoms were moved on an Si(001) surface and arranged into lines, resulting in trench structures of 3 nm width and 2-3 nm depth [128].

Experiments at 4 K showed the feasibility of manipulating individual atoms and arranging them into structures. On crystalline Ni, adsorbed Xe atoms were assembled by the STM tip into an IBM logo in an area of 5 nm x 16 nm [129]. The information density of such structures is about 1 bit nm-2, which corresponds to 100 Tbit cm-2. However, the deposition and manipulation of individual atoms and small molecules, and therefore also the generation of structures with atomic resolution is still limited to isolated demonstrations in basic research. Therefore, the patterns created by individual noble gas atoms on crystalline surfaces at low temperatures represent milestones in the exploitation of the ultimate limits of nanostructure technology, but they do not exhibit direct practical relevance for the fabrication of nanotechnical devices.

STM allows manipulation and characterization at the molecular scale at the same time. In order to study metal-molecule contacts, the interaction of a gold atom with a pentacene molecule, both adsorbed on a thin NaCl film grown on a metal substrate, was investigated using STM [130]. An STM tip was used to bring the Au atom into close contact with the molecule. Resonant inelastic electron tunneling (IET) through the lowest unoccupied orbital of pentacene led to bond formation, and the resulting changes in bond hybridization could be imaged. The bond could be broken by IET through the molecular complex. The resulting changes were rationalized by comparison with density functional calculations.

The tip of the STM can thus be used to pick up atoms and move them on surfaces. However, it may also be used to induce motion through electronic excitations produced by the tunneling electrons. Short chains of Cu atoms terminated by a Co atom were assembled on a Cu(111) surface, and the hopping induced by tunneling electrons of the Co atom between different sites at the end of the chain (which manifested itself as low-frequency "telegraph" noise) was analyzed [131]. Density functional calculations were used to rationalize the facts that the tip location that maximized this hopping was not directly over the Co atom and that the barrier to motion increased with increasing Cu chain length.

Using a low-temperature scanning tunneling microscope (LT-STM), the controlled lateral manipulation in constant height mode of a Lander molecule (a polyaromatic molecular board supported by four legs) adsorbed on a Cu(2 1 1) surface has been demonstrated [132].

Controlled manipulation of a single molecular wire along a copper atomic nano-structure by means of low-temperature scanning tunneling microscopy (STM) has been demonstrated using a Lander molecule [133]. Irrespective of its position along the copper nanostructure, the central molecular wire maintained electronic contact with the atomic wire underneath. This effect was manifested in the STM images depending on the orientation of the legs. By STM manipulation, the molecular wire could be precisely positioned in an electronic contact conformation at the end of the atomic wire.

A novel STM manipulation scheme for controlled molecular transport of weakly adsorbed molecules has been demonstrated [134]. Single sexiphenyl molecules adsorbed on an Ag(111) surface at 6 K were shot towards single silver atoms by excitation with the tip. To achieve atomically straight trajectories, an electron resonator consisting of linear standing-wave fronts was constructed.

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