Thermal activation field evaporation

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

As field evaporation is a thermally activated process, the field evaporation rate is given by... 

Since evaporation can be induced at high fields without thermal activation, a sample can be prepared without thermal disorder of the surface. The fields required for this are estimated most conveniently by assuming that evaporation occurs by depression of the Schottky saddle and resorting to Eq. (49). The magnitude of the field required for evaporation at T = 0°K for the more interesting metals are listed in Table V. 

In pulsed-laser stimulated field desorption, if the field is high enough, the adsorbed species can be thermally field desorbed, most probably within one to a few atomic vibrations. If the activation barrier of evaporation has been reduced by the applied field to much less than the surface diffusion barrier, then the adsorbed species will be desorbed before they have any chance of interacting with other atoms or molecules on the surface. Thus the desorbed species should represent well the... 

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