Structure-property relationships in molecular tunnel junctions

Abstract

Understanding how molecular structure impacts charge transport is one of the central questions in molecular electronics. By modifying the structure of molecules, electronic properties could vary significantly. However, many aspects remain unclear – especially the interactions between metal and molecules at the interface. To address this issue, this thesis investigates the characterization of molecular junctions and relationships between molecular structure and junction properties. Based on self-assembled monolayers (SAMs), my research focused primarily on characterization of oligophenylene dithiol (OPD) series in tunneling regime. A variety of molecules with different functional groups were successfully synthesized, and their SAMs were characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, Kelvin probe, reflection absorption infrared spectroscopy (RAIRS), and conducting probe atomic force microscopy (CP-AFM). Additionally, we applied off-resonance single level model (orSLM) to experimental current-voltage (I-V) characteristics, which provides a quantitative analysis for I-V curves. By extracting key electronic parameters – HOMO to Fermi level energy offset εh and metal-orbital coupling Γ– we were able to quantify the effects of molecular change in molecular junctions. In Chapter 3, we studied the mixed molecular junctions that contain two different molecules and showed that conductance is dependent on molecular ratio exponentially, not linearly. Our results also showed that exponential relationship holds for Γ and the work function difference ΔΦ. In Chapter 4, we studied the relationship between nuclear magnetic resonance (NMR) in the solution state and the conductance in SAM-based molecular junctions. A strong exponential correlation was observed between proton chemical shift and conductance, suggesting that NMR could serve as a predictive tool in molecular junctions. The cause of this correlation was likely to be the difference in HOMO density extending to the molecular backbone. Lastly, Chapter 5 focused on the effect of protonation on transport properties. Protonation in bipyridine-based molecular junctions decreased εh by shifting HOMO closer to the Fermi level. However, it also significantly reduced the Γ, leading to overall decrease in conductance. Overall, this thesis provides a quantitative framework for understanding how molecular features impact charge transport in molecular tunnel junctions. These findings offer interesting insights into the structure-property relationships in molecular electronics.