Electrical conductance of single conjugated oligomers

Using individual molecules as building blocks for electronic devices opens
a new route for further miniaturization of future electronics. The
realization relies on better understanding and control of charge transport
at the level of single molecules. In this thesis we use a mechanically controllable break junction technique to
measure the electronic transport properties of single conjugated oligomers.
This technique allows precise control of the distance between two atomic sized
contacts, which is able to match the size of a single molecule. Via a liquid cell we are able
to investigate molecules in a controlled liquid environment. Using oligo-phenylene
ethynylene (OPE) molecules as our model system, we start with
an OPE-dithiol molecule to understand the properties of a metal-molecule-metal
junction. To overcome variations in individual conductance traces,
we introduce a robust statistical analysis of repeatedly formed molecular
junctions. We then move on and study the role of contacts in molecular
conductance. Surprisingly, we find out that for OPE-monothiols, clear well-defined molecular
signals due to aromatic coupling can be observed. Finally, we
show that the conductance of redox molecular junctions can be controlled by an
electrochemical gate. This thesis is structured as follows: Chapter 1 is an introduction to single-molecule electronics.
Chapter 2 introduces theoretical models of the conductance of single molecules. Chapter 3 describes
the basic principle of mechanically controllable break junctions, the samples and the setup.
Chapter 4 compares the breaking process in passive pure solvents and to which
anchoring molecules are added. On this basis a robust statistical
analysis without any data selection is developed. In chapter 5 we discuss
the effects of contacts and side groups on molecular conductance. Chapter 6 shows
a comparison study of OPE-dithiols and OPEmonothiols. In Chapter 7 we demonstrate
electrochemical gating of single redox molecules.