Large area tunneling junctions have received great attentions recently for study of charge transport across molecules, for its simple fabrication, high yield of working device, and reproducibility of results. Controversy, however, still remains, such as: i) whether the molecular dipole in tunneling junctions is affecting the charge transport, and, ii) junctions with same molecule and same electrode materials have different behaviors?<br/>To study the role of dipoles, we used thiolcarborane molecules to form junctions that have similar structure but different dipoles (both magnitude and directions). We found that the embedded dipoles will affect the tunneling rate across the junctions. We also statistically study the charge transport across alkyl-amide thiols that has different dipole structure (through a study of kurtosis and skewness). It has been discovered that the molecular dipoles align to the electric field leading to a conformational change. Hence a perturbation in charge transport was observed revealing the role of dipole in molecular junctions.<br/>One possible cause to the variation in charge transport from similar test-beds is the difference at the contact between the molecules and the two leads. The contact interfaces (electrode/SAM) and the electronic properties, are highly dependent on the substrate morphology and top electrode structure. We, however, successfully fabricated metal surface with significantly lower roughness and polished the top eutectic gallium indium electrode with acid etching. We demonstrate that using ultra-smooth substrates can lead to more ordered SAMs with better defined open interface. As a result of high-quality SAMs, when polished EGaIn tip contacts the SAMs interface forming a junction, finer molecular information can thus be derived from charge transport results, which was over shadowed in the highly defective and poorly defined large area junctions.

Revisiting the structure-properties relation in charge transport across large area molecular tunneling junctions

253rd National Meeting (2017)

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