MSE PhD Final Defense: Zelin Miao

MSE PhD Final Defense: Zelin Miao

TITLE: Chemical Principles for Engineering Properties of Single-Molecule Circuits

ADVISOR: Masha Kamenetska (Chemistry, Physics, MSE)

CHAIR: Karl Ludwig (Physics, MSE)

COMMITTEE: Sahar Sharifzadeh (ECE, MSE); Sahar Sharifzadeh (ECE, MSE); Jörg Werner (ME, MSE, Chemistry)

ABSTRACT: Since the pioneering proposal by Aviram and Ratner to use donor-bridge-acceptor molecular systems as active circuit elements, known as “molecular rectifiers”, the field of molecular electronics has advanced rapidly. Over the past few decades, numerous intriguing electronic properties associated with single molecules have been demonstrated. Much of this progress is driven by experimental techniques such as break-junction (BJ) methods, which enable precise characterization and manipulation of charge transport at the nanoscale. In these approaches, metal-molecule junctions are repeatedly formed by allowing molecules to self-assemble between two metallic electrodes, and the conductance is analyzed in a statistically meaningful manner under ambient or other conditions. Integrating inorganic materials, such as coordination complexes with tunable charge states, into nanoscale electronic junctions enables access to a broader spectrum of transport phenomena by introducing additional spin degrees of freedom. These emergent properties are central to the development of advanced spin-based technologies, including single-molecule qubits, spin valves, and molecular memory devices. However, significant challenges remain, as conventionally ex situ synthesized coordination compounds often disassemble upon contact with electrode surfaces, hindering their direct electrical characterization. To overcome this limitation, an in situ assembly approach has been developed to construct quasi-one-dimensional (1D) coordination wires within junctions using organic ligands, where metal atoms are incorporated from the external environment during junction elongation. Here, I show that the electron density distribution over the ligands is a decisive factor governing robust coordination wire assembly, and demonstrate that the identity of the metal centers can be extended to other species through electrochemical control, thereby yielding junctions with distinct conductance properties. Single-molecule conductance measurements of a series of diradical wires featuring uniquely constrained anchor groups are also presented. Our results provide experimental evidence that radical-electrode coupling can significantly affect conductance trends with increasing molecular length. In particular, strong electronic coupling can attenuate the previously reported anti-ohmic behavior in radical systems, where conductance increases with molecular length. Overall, this thesis establishes chemical principles that govern robust junction assembly and electron transport behavior, highlighting how linker and molecular design and interfacial electronic coupling can be leveraged to tailor the functionalities of molecular circuits.