Chemistry Reference
In-Depth Information
metal wires, one of them modified with a SAM of the molecule of interest, are
mounted in a crossed geometry with one of the wires perpendicular to an external
magnetic field. The junction separation is controlled by deflecting this wire with
the Lorentz force generated by a small dc current. The selected SAM was based on
linear oligophenyleneethylene with either one or two thioacetyl functional groups
on the ends of the molecule. Upon metal-SAM-metal contact, molecules with one
or two functional groups behave as molecular diodes or wires, respectively, due
to the different molecule-metal contacts. Here the importance of chemisorption in
order to improve the electrical contact becomes evident.
Another way to improve the metal-molecule contact consists of applying a force,
e.g., with a conducting AFM tip, while measuring the electrical conductance. For
the case of SWNTs connected to a gold electrode and explored with metallized
AFM tips acting as mobile electrodes, it has been shown that a minimum applied
force of
30 nN is required to obtain an optimum electrical contact and that
above 50 nN the radial deformation of the molecule induces a semiconducting-
like behaviour due to the opening of an electrical gap (Gomez-Navarro
et al.
,
2004).
In conclusion, the development of nanometre-scale techniques such as scanning
tunnelling microscopy, which permit imaging, manipulation and the performance
of electronic, vibrational and optical spectroscopy at submolecular level offers a
myriad of possibilities in the coming years to understand the phenomenology of
interactions between single molecules and conductive substrates. Spin-polarized
STMs have also emerged in recent years (Bode
et al.
, 1998), adding magnetic
characterization of single molecules at the nanometre scale.
∼
Self-assembly at submonolayer coverages
In the previous section we have explored the 0D case of individual molecules on
inorganic surfaces, which is experimentally achieved for very low coverages. If the
density of molecules at the surface is increased or
T
sub
increased then molecules
may interact laterally and form aggregates or assemblies. We thus have the interplay
of molecule-molecule in addition to the molecule-substrate interactions. Indeed
the geometry and chemical nature of the molecules are at the origin of the potential
assemblies. Here one can realize the great possibilities that chemical synthesis can
offer, since molecules can be chemically modified by e.g., adding functional groups
and thus engineering the assemblies.
The first chosen example refers to the control of surface-supported supramolec-
ular assembly by judicious distribution of substituents in porphyrin molecules
(Yokoyama
et al.
, 2001). Substituted porphyrin molecules OMBD-deposited on a
Au(111) surface formmonomers, trimers, tetramers or extendedwire-like structures
