Charge transport through oligoarylene self-assembled monolayers: interplay of molecular organization, metal-molecule interactions, and electronic structure

The electrical properties of two molecular wires-a novel aryl moiety, 6-(5-pyridin-2-ylpyrazin-2-yl)pyridine-3-thiol (PPPT), and the well studied 1,1';4',1."-terphenyl-4-thiol (TPT)-organized in self-assembled monolayers (SAMs) are measured using metal-molecule-metal (MMM) mercury-drop junctions. Current measured at the same bias voltage through PPPT is found to be more than one order of magnitude lower than through TPT. To interpret and understand these results, characterization of the structure, organization of the SAMs, and theoretical analyses of the molecular systems are discussed. X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) indicate that although PPPT forms high-quality SAMs on both Au and Ag substrates, it exhibits a lower packing density (by 20 %) and less orientational order than TPT. In addition, electronic structure calculations with density functional theory (DFT) reveal that the electron-withdrawing nitrogen atoms in the PPPT aryl backbone stabilize the valence molecular electronic structure and pull negative charge from the thiol sulfur. This behavior can influence both charge-injection barriers and metal-molecule binding interactions in the MMM junctions. The current-voltage data are interpreted on the basis of a hole-tunneling, through-bond mechanism. Conductance analysis through a model for off-resonant tunneling transport suggests that a comparatively small difference in the charge-injection barrier can explain the factor of ten difference in observed conduction.