Interfering electron paths in single-molecule junctions

Abstract

The central theme of this dissertation is the experimental investigation of electrical transport through single-molecule junctions using the mechanically controlled break-junction (MCBJ) technique at room temperature. Of particular interest is the phenomenon of quantum interference, wherein the wave-like nature of the electron plays a crucial role in the electrical conductance of various molecules. Chemical design and the influence of external stimuli such as mechanical manipulation or applying strong electric fields can impact the interference, of which the latter is also used to investigate conductance switching due to conformational changes.

Chapter 1 serves as an introduction to the field of molecular electronics, discussing the origin of the field and landmarks, and provides theoretical considerations concerning charge transport in metal-molecule-metal junctions. Finally, a general introduction on quantuminterference is given.

Chapter 2 broadly discusses the MCBJ experimental method, where the mechanical and electrical equipment for two set-ups is discussed alongside the modus operandi for doing fast-breaking experiments. Subsequently, data analysis using machine learning methods will be discussed. Ultimately, reference measurements on bare gold electrodes and an oligo(phenylene-ethynylene) (OPE) molecule were performed as a benchmark for roomtemperature high-bias and current-voltage characterization on other molecules. For high-bias experiments we note that measurements can be performed reliably and shows in the case of the OPE molecule that its conductance increases as a function of applied bias voltage in agreement with the single-level model.

Chapter 3 revolves around the study of the electrical properties of molecules containing a cyclophane core. For molecules containing a paracyclophane core we observe that ortho-connections suppress the conductance more so than a para-connection as compared to a meta-connection. Additionally, we find para-connections to cyclophane units to be the common denominator in showing mechanosensitive behaviour, i.e., in which themolecule changes its conductance strongly due to mechanical deformation.

In Chapter 4 we have developed a method to reconstruct the observed destructive quantum interference dip in a molecule with a naphthalenophane core, opting for establishing a closer link between theory and experiment. Two complementary techniques at room temperature were used for this study: (i) the MCBJ technique, which allows for large statistical sampling fortifying the robustness of the dip reconstructionmethod; (ii) the alternating-current scanning tunneling microscopy break-junction technique (ACSTM- BJ) allowing for the continuous simultaneous measurements of the conductance and the corresponding thermopower, providing additional information on the destructive quantum interference dip. We find a sinusoidal response of the thermopower across the conductance dip without a sign change. Theoretical calculations on conductance and thermopower including electrode distance and energy alignment variations emphasize the crucial role of thermal fluctuations at roomtemperature.

For Chapter 5 we change pace and shift towards molecular switches. Three differently anchored norbordaniene molecules were investigated under high-bias circumstances. For all compounds, we find two conductance states. We find no full switching between two conductance states, as the two states are present across a wide-range of applied bias voltages and no clear population differences between the states are found. Alternatively to the explanation of the switching within the molecule itself, one can argue that either we observe two different configurations of the molecular junction or that interactions of the short linkers of the molecules, by interactions with the gold surface, quench the switching between the states unlike previous published results using a molecule with the same backbone but with longer linkers.

Chapter 6 investigates the effect of chemical design on the conductance of macrocyclic structures, studying them with different substituents. We observe a clear difference in conductance between para- and meta-connections in the core using thiophene and benzene substituents, consecutively. Here, the created para- connected path shows a higher conductance than its meta counterpart. Different connections, para and meta, in molecules with the same backbone show less of an effect on the conductance of the molecular junction. Additionally, preliminary results of one of the compounds using room temperature current-voltage characteristics shows a negative differential conductance and hysteretic behaviour.

Lastly, in Chapter 7 we conclude the obtained results from this dissertation and place them in a broader perspective.