The local removal of material by a probe is possible with inhomogeneous materials, especially in the case of a thin layer with weak interactions to the support. A moderate pressure of the tool induces a shifting or removal of parts of the layer. For manipulations in the nanometer range, scanning probe techniques are preferred due to the existence of nanometer tools (the scanning tips) and a positioning system with sub-nanometer precision (cf. Section 4.4).
For additive surface transport processes, the writing probe can be used as reservoir for the material to be deposited. The so-called "dip-pen nanolithography" holds surface-active molecules on the tip ofa scanning force microscope. Contact ofthe tip with a substrate which has a high affinity for the adsorbed molecules results in a transfer of these molecules via a water meniscus to the surface, and a nanostructure is formed according to the x-y movement of the tip relative to the substrate (Fig. 55). A prerequisite is sufficient mobility ofthe molecules on the tool surface and a high affinity for the substrate surface. The latter is given for compounds forming self-assembled monolayers (SAM) on surfaces. Examples are alkylthiols such as octadecylthiol on
a gold substrate, yielding a monolayer of nanometer thickness. Spots of about 15 nm diameter and structures with widths of 50-70 nm were realized with 16-mercaptohex-anoic acid or octadecanethiol on gold .
The principle of transport on surfaces can be scaled down to the atomic range. Instead of mechanical probes, fields can be applied for force transfer. Atoms with a tendency towards high electrical polarization, such as neutral alkaline metal atoms, are moved by electrical fields on monocrystalline surfaces. In this way the manipulation of Cs atoms on GaAs surfaces has been demonstrated . Electrically conductive micro-and nanoparticles show an even better surface mobility in the electrical field. The speed of electromigration increases with substrate temperature and particle size .
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