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Download fileRole of Molecular Dipoles in Charge Transport across Large Area Molecular Junctions Delineated Using Isomorphic Self-Assembled Monolayers
journal contribution
posted on 13.10.2017, 00:00 authored by Jiahao Chen, Symon Gathiaka, Zhengjia Wang, Martin ThuoDelineating
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 carboranesisostructural 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.