Charge Transport of Phosphonate Self-assembled Monolayers across Large-area Molecular Junctions

Using indium tin oxide (ITO) substrates as bottom and eutectic gallium and indium (EGaIn) as top electrodes, we successfully formed alkyl phosphate self-assembled monolayers (SAMs) on ITO and molecular junctions in the form of ITO-PO3(CH2)nH//GaOx/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) characterization is 0.90 n-1 with a log-injection current density (log10|J0|) of 1.87 A/cm2 at an applied bias of 0.5 V, which is confirmed by the impedance spectroscopy results from the resistance of SAM (RSAM), indicating the evolution of RSAM with molecular length is the main origin of decay of charge transport rate in the phosphate junctions. The dielectric constant (εr) values of PO3(CH2)nH SAMs measured in junctions are 2.28-2.60, indicating the semiconductor of 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 possible optical properties of a hybrid electronic system of organic molecules with phosphonic acid anchoring group, semiconductor and metallic electrodes.