Structure-Dependent Charge Transport and Storage in Self-Assembled Monolayers of Compounds of Interest in Molecular Electronics: Effects of Tip Material, Headgroup, and Surface Concentration

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Abstract
The electrical properties of self-assembled monolayers (SAMs) on a gold surface have been explored to address the relation between the conductance of a molecule and its electronic structure. We probe interfacial electron transfer processes, particularly those involving electroactive groups, of SAMs of thiolates on Au by using shear force-based scanning probe microscopy (SPM) combined with current−voltage (i−V) and current−distance (i−d) measurements. Peak-shaped i−V curves were obtained for the nitro- and amino-based SAMs studied here. Peak-shaped cathodic i−V curves for nitro-based SAMs were observed at negative potentials in both forward and reverse scans and were used to define the threshold tip bias, VTH, for electric conduction. For a SAM of 2‘,5‘-dinitro-4,4‘-bis(phenylethynyl)-1-benzenethiolate, VII, VTH was nearly independent of the tip material [Ir, Pt, Ir−Pt (20−80%), Pd, Ni, Au, Ag, In]. For all of the SAMs studied, the current decreased exponentially with increasing distance, d, between tip and substrate. The exponential attenuation factors (β values) were lower for the nitro-based SAMs studied here, as compared with alkylthiol-based SAMs. Both VTH and β of the nitro-based SAMs also depended strongly on the molecular headgroup on the end benzene ring addressed by the tip. Finally, we confirmed the “memory” effect observed for nitro-based SAMs. For mixed SAMs of VII and hexadecanethiol, I, the fraction of the charge collected in the negative tip bias region that can be read out at a positive tip bias on reverse scan (up to 38%) depended on the film composition and decreased with an increasing fraction of I, suggesting that lateral electron hopping among molecules of VII occurs in the vicinity of the tip.