SELF-ASSEMBLY OF SUPRAMOLECULAR NANOSTRUCTURES: ORDERED ARRAYS OF METAL IONS AND CARBON NANOTUBES - Bio-Inspired and Nanoscale Integrated Computing - page 339

self-assembly process under 2D confinement automatically results in a highly
ordered ''grid-of-grids'' superstructure. Consequently, monolayers of [n n]
MIAs exhibit a two-fold supramolecular matrix structure, (i) internally by the
ligand-directed coordinative positioning of the metal ions and (ii) externally by
the van der Waals directed formation of the ''grid-of-grids'' superstructure. The
addressing of single metal ion arrays at the single array level was achieved by
removing one molecule from the monolayer with help of the scanning tunneling
microscopy (STM) tip. Single metal ion addressing inside of isolated metal ion
arrays could be achieved electronically through the use of scanning tunneling
spectroscopy technique (CITS) [26].
12.3. METAL ION NETWORKS (MINs)
12.3.1. Surface-Confined Assembly of Metal Ion Networks (MINs)
Method B in Fig. 12.3 introduces a shortened conceptual alternative to the above
described multistep approach in achieving extended highly ordered MIAs on
surfaces. Instead to the three-tiered self-assembly approach of method A, metal
ions and organic ligands are now ordered in only one self-assembly step under
immediate 2D confinement of the surfaces [22]. Experimentally, the organic ligand
molecules are deposited by organic molecular beam epitaxy on a metallic surface
under ultra high vacuum conditions. A more or less complete organic monolayer
is formed, on which the respective metal ions are subsequently co-deposited by
electron beam evaporation. A short annealing period (typically several minutes)
supplies the necessary mobility of the molecular components, guaranteeing the
required kinetics for accomplishing the surface-assisted self-assembly process [23].
Depending on the applied molecular ligand/metal couples, regular network
structures of different geometries, with domain sizes up to several hundred
nanometers, are formed. Within these extended, polymeric 2D network structures,
the metal ions are positioned at the crossing points in very regular distances of a
few nanometers. Scanning tunneling microscopy (STM) has been proven to be the
ideal tool to obtain structural information of the formed metal ion networks
(MINs) structures (Fig. 12.9).
One example of such a metal ion network is the self-assembly of linear
dicarboxylic acid ligands with Fe(0) metal ions on a Cu(100) surface [28]. At a
Fe/ligand ratio of 0.5/1, rectangular molecular assemblies could be obtained
consisting of dimeric iron nods that are interconnected by an organic backbone
of orthogonally arranged ligand linkers (Fig. 12.10). Within the resolution limit
of the STM, the length of the Fe-O bond of the metal ion to the coordinating
carboxylates was determined with d(Fe-O)=1.9-2.3 A ˚ (an expected range for a
Fe(II) species). The Fe-Fe distance within the dimeric nods was shown to be
between 4.5 and 5.0 A ˚ , a value that is considerably larger than in comparable
bulk structures [28]. The coordination geometry around each iron ion can be
interpreted as square-planar, which is quite unusual for Iron(II) ions. However,