2025 Theses Doctoral

Tuning Conductance in Single-Molecule Electronics

Lee, Woojung

This thesis explores approaches to controlling electron transport, which is essential for enabling single-molecule electronics as functional electronic components. The dissertation is structured into five chapters, each exploring a different method for modulating the conductance of single-molecule electronics.

Chapter 1 introduces the fundamentals of single-molecule electronics, including key concepts and theoretical principles for understanding their electronic behavior. It also presents the scanning tunneling microscope-based break-junction (STM-BJ) technique as the experimental platform used in this study.

Chapter 2 investigates temperature-dependent conductance in oligo[n]phenylene molecular wires, where electron tunneling occurs coherently. Conventionally, coherent tunneling is considered temperature-independent process; however, this chapter explains how thermal energy affects dynamic molecular conformation, resulting in conductance variations. Specifically, experimental findings, supported by density functional theory (DFT) calculations, reveal that thermal energy induces enhanced dihedral planarization, leading to an increase in π-orbital coupling and electron tunneling probability.

Chapter 3 explores conductance tunability in redox-active and light-switchable single-molecule electronics using ferrocene-based molecular junctions. STM-BJ measurements show that the oxidized state of ferrocene forms a direct Fe-Au bond with Au electrode, leading to significantly higher conductance compared to junctions with additional chemical linkers such as thioethers.

Chapter 4 further validates the bonding mechanism between the metal center atom (M) of Group VIII metallocenes (M = Fe, Ru, and Os) and the Au electrode. It explores the formation of ferrocene-, ruthenocene- and osmocene-based single-molecule electronics using STM-BJ measurements. Experimental and theoretical results demonstrate that Group VIII metallocenes can directly bind to gold electrodes without chemical linkers, highlighting their potential for fabricating redox-active and high-conducting single-molecule devices.

Chapter 5 introduces an approach to enhancing conductance through enhancing antiaromaticity in thiophene-based one-dimensional topological insulators. Unlike typical molecules with exponential conductance decay, this thiophene-based diradical system exhibits length-enhanced conductance. Furthermore, modulating oxidation states through photochemical and electrochemical methods increases antiaromaticity, significantly enhancing conductance of single-molecule electronics.

This thesis presents diverse methods for modulating conductance in single-molecule electronics by introducing unique electron transport mechanisms, utilizing thermal, optical, chemical, and electrochemical approaches.