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12.3.2. Filling Macrocycle Arrays
Not only linear ligands but also macrocylic ligands as porphyrines and phthalo-
cyanines can be used in the surface-assisted self-assembly approach. Thus, meso-
tetrapyridyl porphyrines form on Ag(111) surfaces' densely packed monolayers
containing two orientations of the macrocycles (due to packing effects).
Co-depositing of iron atoms onto this monolayer leads to selective incorporation
of the metal atoms into the porphyrin macrocycles whereby the template structure
is strictly preserved (Fig. 12.12) [32]. The immobilization of the molecular
reactants allows the identification of single metalation events in a novel reaction
scheme.
This ''filling'' approach opens up appealing opportunities, especially because
it seems to be easily applied to a large variety of porphyrin or related macrocycle
species organized on surfaces (which can be metalated by iron and other metal
centers). Specifically, the ''filling approach'' allows the formation of low-dimen-
sional metallo-porphyrin architectures by using preorganized immobilized macro-
cycle template arrangements that are subsequently functionalized by metallation.
Moreover, novel porphyrin compounds can be created because procedures can be
conceived where the addition of a metal center enters as a final step. Current work
addresses the elucidation of the coordination characteristics of the involved metal
centers, which will also set the base for the investigation of the physical properties
(e.g., the magnetism) of single metal centers in the formed extended metal ion
networks.
12.4. SELF-ASSEMBLY OF CARBON NANOTUBES (CNTs)
Remarkable electronic properties make carbon nanotubes (CNTs) promising
building blocks for molecular or nanoscale devices. In comparison with
molecules, CNTs represent an ideal link between the nanoscopic molecular
world and their macroscopic implementation. CNTs can fulfill this role because
of their typical dimensions involving both nanometer diameter and micrometer
lengths. To use them in this interlinking role, CNTs need to be assembled with
nanometer precision into hierarchical arrays over large scales of areas; at the
same time, they have to be connected to partially macroscopic device compo-
nents (e.g., electrodes) [33]. At the nanoscale, different techniques are pursued to
achieve highly ordered structures. Thus, most of the available methods rely on
fabrication of CNTs on prepatterned substrates or catalysts [34]. In a different
approach, self-assembly techniques take use of the capillary forces leading to
three-dimensional micro-patterns of aligned CNT films [35]. Thereby, a water-
spreading method on prepatterned substrates is used to direct the growth of
highly ordered CNT films. Although this process is still restricted to the
micrometer length scale, improvement of the feature resolutions seems possible
(Fig. 12.13).
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