Strategies for molecular junctions with expanded degrees of freedom
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Abstract
Increasing the degrees of freedom in molecule-metal junctions presents new opportunities for expanded functionality in molecular circuits and could pave the way to next generation electronics integrating single molecules as active components. For example, molecular junctions with spin degrees of freedom have been explored as a materials platform for Quantum Information Science (QIS). In this thesis, I present experimental and computational investigations of how incorporating transition metal atoms (Chapter 1), new oxygen-based linker groups (Chapter 2) and radical backbone elements (Chapter 3) into molecular junctions can expand the range of observable phenomena in molecular circuits. In Chapter 1, I present a collaborative investigation of the nature of group 8 metallocenes, which can be prepared with a range of transition metal atoms at the center of the molecule, in molecular junctions formed at both cryogenic and room temperature. We characterize the direct gold-π bonds between the molecule and the metal electrodes and their effect on molecular conductance during molecular junction evolution. Analysis of the junction persistence as well as conductance fluctuations reveals these metal containing “barrel-shaped” molecules preferentially bind to dull atomic electrode tips through van der Waals interactions. We support our measurements by modeling different binding motifs within density functional theory (DFT). Interestingly, our study finds that for closed-shell metallocenes, the electrode-molecule interface and environmental conditions have by far larger influence on electron transport than the identity of the metal atom in the molecular backbone.
In Chapter 2, I report single molecule conductance measurements with a new phenol-based linker group involving the formation of a direct, single O-Au bond to anchor the molecule to the electrode. We find that deprotonation of the phenol is necessary for molecule-metal binding, enabling pH control of junction formation. The activation of metal-phenol binding through deprotonation is supported with DFT calculations that model the binding interaction before and after deprotonation. We determine that transport through phenol systems is mediated by the molecular HOMO orbital. Accurate quantitative predictions of the electron transport properties of phenol-based linkers require higher levels of corrections to DFT which underestimates the HOMO-LUMO gap, unless supplemented by DFT+Σ methodology. Crucially, our study establishes principles for achieving pH control of the metal-molecule interface for the molecular electronics community.
Last, I demonstrate experimentally and from first principles, the unique transport properties of a family of quinoidal cyclic aromatic hydrocarbons with intermediate diradical character. These molecules exhibit “anti-ohmic” conductance, with the longer molecules having greater single molecule conductance than shorter ones. This behavior is atypical of quantum tunneling commonly observed in molecular junctions. Unlike prior examples of diradicals in molecular junctions, these compounds do not require oxidation or complex environments. We accurately model the electronic and conductance properties of these molecules by developing a modification to the 1D Su-Schrieffer-Heeger (SSH) model for cyclic quinoidal molecules with accurate experiment-based parameterization. The 1D SSH model suggests that the anti-ohmic behavior is typical of intermediate diradical molecules and predicts anti-ohmic trends across a broad range of molecular length regimes. Our work suggests the search for long range high conducting molecular wires should focus on molecules with neutral intermediate diradical character that have the increased benefit of stability, diversity, and experimental accessibility.
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2024