α,ω-Alkanedibromides\nForm Low Conductance\nChemisorbed Junctions with Silver Electrodes

Chemical groups capable of connecting molecules physically\nand\nelectrically between electrodes are of critical importance in molecular-scale\nelectronics, influencing junction conductance, variability, and function.\nWhile the development of such linkage chemistries has focused on interactions\nat gold, the distinct reactivity and electronic structure of other\nelectrode metals provides underexplored opportunities to characterize\nand exploit new binding motifs. In this work we show that α,ω-alkanedibromides\nspontaneously form well-defined junctions using silver, but not gold,\nelectrodes through application of the glovebox-based scanning tunneling\nmicroscope-based break junction method. We systematically evaluate,\nthrough a series of additional studies, whether these molecular components\nform physisorbed or chemisorbed contact geometries, and if they undergo\nsecondary chemical reactions at the silver surface. Critically, we\nfind that the same junctions form when using different halide, or\ntrimethylstannyl, terminal groups, suggestive of an electronically\ntransparent silver–carbon(sp<sup>3</sup>) contact chemistry.\nHowever, the experimental conductance of the junctions we measure\nwith silver electrodes is ∼30× lower than that observed\nfor such junctions comprising gold–carbon(sp<sup>3</sup>) contacts,\nwhich does not align with predictions based on first-principles calculations.\nWe further exclude the possibility that the proposed silver alkyl\nspecies undergo α- or β-hydride elimination reactions\nthat result in a distinct contact chemistry through conductance measurements\nof control molecules that cannot undergo such processes. Applying\ninsights provided from prior temperature-programmed desorption studies\nand a robust series of atomistic simulations, we ultimately propose\nthat in these experiments we measure alkoxide-terminated junctions\nformed from the reaction of the chemisorbed alkyl with oxygen that\nis coadsorbed on the silver surface. This work, in demonstrating that\nhigh conductance contact chemistries established using model gold\nelectrodes may not be readily transferred to other metals, underscores\nthe need to directly characterize the interfacial electronic properties\nand reactivity of electrode metals of wider technological relevance.