Lateral Manipulation of Single Adsorbates and Substrate Atoms With the Scanning Tunneling Microscope
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van der Waals and short-range chemical forces can be used to move atoms or molecules along the surface. No bias voltage or tunneling current is necessary. Apart from this technique, additional advances using the effects caused by the electric field generated by the bias voltage between tip and sample and by the current flowing through the gap region can be used for atomic or molecular modification.1 A few experimental prerequisites must be taken into account if one wants to perform manipulation on single atoms or small molecules. Especially when working on metal surfaces, it is important to operate the STM at low temperatures since single metal atoms and weakly bonded adsorbates are mobile at ambient temperatures. To freeze this motion, one usually must cool even below liquidnitrogen temperature. Furthermore the manipulation of single atoms/molecules means working at extremely low coverage—that is, using only a few atoms (for example 100) for a long time (several days). This requires both high cleanliness and the ability to deposit small amounts of adsorbates in a controlled manner. One can achieve these results surrounding the whole STM by a liquidhelium radiation shield and depositing small amounts of adsorbates through a small hole in the shield. Finally a flexible and precise tip-positioning procedure
and—when creating and investigating nanostructures—a high stability of the whole system are necessary. Figure la shows a schematic STM setup for manipulation. If the apex of the tunneling tip is far away from the adsorbate, only long-range forces like van der Waals forces are exerted on the adsorbate. When the tip comes close enough to the adsorbate, one enters the regime of short-range chemical forces. Since both distance and orientation of this chemical bond between tip apex and adsorbate can be controlled by adjusting the tip position, this situation is termed a "tunable chemical bond."2 The procedure for lateral manipulation is remarkably simple and appears in Figure lb: In the STM imaging process, the tip is scanned at distances of a few atomic diameters above the surface (a) and—in the constant current mode—follows contours of constant local electronic density of states.5 For manipulation the tip is brought close to the adspecies (b) by reducing the tunneling resistance. Then the tip is moved parallel to the surface (c) to a predetermined place. The particle is thereby pulled (or pushed) to the desired location (d). The tip is then drawn back to the scanning distance (e), and a new STM picture is scanned to check the result of the manipulation. The magnitude of the force acting on an atom/molecule is not directly accessible to the STM. The force is however a function of the distance between tip and adsorbate. Controlling the distance thus gives a measure of the exerted force. Moreover since the tunneling current at low voltages is proportional to the bias voltage and depends exponentially on the distance,5 the tunneling resistance can be used as a rough measure to control the force acting on the adsorba
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