Charge transport of phosphonate self-assembled monolayers across large-area molecular junctions

Abstract

Using indium tin oxide (ITO) substrates as the bottom electrode and eutectic gallium and indium (EGaIn) as the top electrode, we successfully formed alkyl phosphate self-assembled monolayers (SAMs) on ITO and molecular junctions in the form of ITO–PO₃(CH₂)_nH//GaO_x/EGaIn (n = 8, 10, 12, 14, or 16). The tunneling decay coefficient (β, decay of charge transport rate vs. n) of junctions obtained from direct current (DC) methods is 0.90 n⁻¹ with a log-injection current density (log₁₀|J₀|) of 1.87 A cm⁻² at an applied bias of 0.5 V, which is confirmed by the impedance spectroscopy results from the resistance of the SAM (R_SAM), indicating that the evolution of R_SAM with molecular length is the main origin of decay of charge transport rate in the phosphate junctions. The dielectric constant (ε_r) values of PO₃(CH₂)_nH SAMs measured in junctions range from 2.28 to 2.60, indicating that the semiconductor ITO substrates do not affect the dielectric and electrical responses of junctions. This work provides a compatible platform for studying the interfacial charge transport, dielectric responses, and even potential optical properties of a hybrid electronic system of organic molecules with phosphonic acid anchoring groups, semiconducting substrates and metallic electrodes.