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Abstract
The focus of this PhD thesis was the technological realization of area contacts to molecular
monolayers. In particular, the assembly of Metal-Molecule-Metal junctions by means of
soft lithography was investigated. The major challenge is the assembly of the top contact
without destroying the molecular layer. Here two approaches were pursued to evaluate new
routes of sandwiching biomolecular layers between two metal electrodes.
For the fundamental investigation of electron transport through a self-assembled monolayer
(SAM) a liquid metal was used as the top contact. This technique was combined
with planar microelectrodes of well dened electrode size. An array of 64 micro electordes
(Multy Electrode Array, MEA) realized on a silicon chip served as the bottom electrode.
Due to its low toxicity eutectic gallium-indium (EGaIn) was selected as the liquid top
contact. By coating the electrodes with a SAM prior to the EGaIn deposition, Metal-
SAM-Metal junctions were generated. Initially SAMs of alkanethiols were used as model
system for the evaluation of these junctions. It was possible to determine direct tunneling
as the dominant charge transport mechanism for these molecules. Additionally a transition
to Fowler-Nordheim tunneling was observed at high bias voltage. Subsequently, SAMs of
oligopeptides were investigated as a biological model system. The self assembly properties
were found to be much weaker for oligopeptides than for alkanethiols, resulting in higher
defect rates of peptide junctions. However, direct tunneling was found to be the dominant
charge transport mechanism for these molecules as well.
In the second approach a printing technique for metal lms was combined with Nanoimprint
Lithography (NIL) for the fabrication of printed crossbar structures. This required
the development of a proper electrode layout and the fabrication of NIL stamps by electron
beam lithography (EBL). Thus, a new process was developed that combines NIL with
a universal printing procedure. Utilizing NIL allowed the patterning of metal layers with
structures of parallel wires containing feature sizes as small as 50 nm. With these electrodes
crossbar arrays were noninvasively assembled by printing an array of top electrodes perpendicular
to an array of bottom electrodes. All cross points of the devices are electrically
addressable. Furthermore, a layer of conducting polymer was incorporated into the junctions
to further evaluate the crossbar functionality. From current-voltage measurements the
conductivity of the incorporated conducting polymer could be determined. Additionally
the ndings were compared to simulation, conrming the results.
In comparison the drop electrode approach allows the fundamental investigation of the
charge transport in SAMs. But on account of the liquid top contact this approach lacks the
possibility of a commercial application. The printed crossbar approach on the other hand
is superior in this point. The combination of the next generation lithography technique
NIL with a universal printing procedure makes this approach feasible for applications in
the future. However, the direct contacting of SAMs remains challenging.
VII