Mechanically controllable break junction in liquid environment : a tool to measure electronic transport through single molecules

This thesis discusses electronic transport measurements of single molecules
in a liquid environment with the ultimate goal of demonstrating
a molecular transistor addressed by a liquid gate. A mechanical controllable
break junction with an integrated liquid cell is used as the
measurement tool. We first studied the influence of different solvents
on the conductance of gold junctions in the regime of tunneling and
true metallic contacts. These measurements served as control experiment
for the electrical characterization of single molecules. We then
measured the conductance traces of thiolated C60 molecules while varying
the electrode separation. The shape of the curves was found to be
strongly influenced by the solvent. By using a resonant tunneling model,
the electronic tunneling rates could be extracted. Finally, the effect of a
liquid gate was tested on several tetrathiafulvalene (TTF) compounds.
This thesis is organized in the following way.
• The first chapter presents two different approaches for producing
nanogaps in order to contact single molecules. One is based on
electrochemical wires grown in pores, whereas the other method
concerns the electrochemical narrowing of lithographically defined
gaps.
• The second chapter introduces the break junction and gives a short
overview over its research field. Discussed are the geometry and
the mechanical properties of the break junction and finally the
conductance of atomic contacts.
• Chapter 3 is dedicated to the fabrication of break junction samples
and the measurement setup.
• Electric conductance measurements of atomic contacts in liquid
environments are presented in the forth chapter. This work was
done with the perspective of measuring single molecules in the
break junction. Also discussed is tunneling through alkanethiols.
• Chapter 5 presents electric transport measurements through thiolated
C60 molecules in a liquid environment. When varying the
electrode separation a peak in the conductance traces has been
observed. By applying a resonant tunneling model, we were able
to extract electronic tunneling rates.
• In Chapter six transport measurements through tetrathiafulvalene
(TTF)-based molecules with an electrochemical gate are shown.
A possible gate effect is discussed based on the I-V characteristics.