Role of Molecular Dipoles in Charge Transport across Large Area Molecular Junctions Delineated Using Isomorphic Self-Assembled Monolayers

Delineating the role of dipoles in large area junctions that are based on self-assembled monolayers (SAMs) is challenging due to molecular tilt, surface defects, and interchain coupling among other features. To mitigate SAM-based effects in study of dipoles, we investigated tunneling rates across carboranesisostructural molecules that orient along the surface normal on Au (but bear different dipole moments) without changing the thickness, packing density, or morphology of the SAM. Using the Au-SAM//Ga₂O₃-EGaIn junction (where “//” = physisorption, “–” = chemisorption, and EGaIn is eutectic gallium–indium), we observe that molecules with dipole moments oriented along the surface normal (with dipole moment, p = 4.1D for both M9 and 1O2) gave lower currents than when the dipole is orthogonal (p = 1.1 D, M1) at ±0.5 V applied bias. Similarly, from transition voltage spectroscopy, the transition voltages, V_T (volt), are significantly different. (0.5, 0.43, and 0.4 V for M1, M9, and 1O2, respectively). We infer that the magnitude and direction of a dipole moments significantly affect the rate of charge transport across large area junctions with Δ log|J| ≅ 0.4 per Debye. This difference is largely due to effect of the dipole moment on the molecule-electrode coupling strength, Γ, hence effect of dipoles is likely to manifest in the contact resistance, J_o, although in conformational flexible molecules field-induced effects are expected.