Counterion-Dependent Charge Transport in Ultrathin-Film Molecular Junctions

Counterions are ubiquitous in charged molecular films, yet their influence on charge transport in solid-state molecular junctions (MJs) remains insufficiently understood. Here, we investigate how embedded counterions regulate conductance in MJs based on ultrathin molecular films (2-9 nm) of 4,4'-Diamino-2,2'-stilbenedisulfonic acid (DAS). These DAS films were electrochemically grafted onto Au electrodes via in situ generated diazonium precursors. During film growth, cations from the supporting electrolyte are incorporated into the molecular network through electrostatic interactions with the sulfonate groups. The dense structure of the electro-grafted films enables direct deposition of Ti/Au top contacts, yielding robust large-area junctions with well-defined thicknesses. By varying the incorporated counterions (H⁺, Li⁺, Na⁺, or K⁺), we show that junction conductance is strongly counterion dependent, whereas the transport attenuation factor remains nearly unchanged. This behavior indicates that counterions primarily affect the prefactor J ₀, consistent with a counterion-dependent change in interfacial coupling. Na⁺- and K⁺-containing films favor a large population of strongly coupled sulfonate-metal pathways, leading to markedly higher conductance than H⁺- and Li⁺-containing MJs. These results identify embedded ionic species as active chemical regulators of molecule-electrode coupling and provide a practical strategy for tuning charge transport in molecular electronic devices.