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, VT (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, Jo, although in conformational flexible molecules field-induced effects are expected.