α,ω-Alkanedibromides Form Low Conductance Chemisorbed Junctions with Silver Electrodes.

Abstract

Chemical groups capable of connecting molecules physically and electrically between electrodes are of critical importance in molecular-scale electronics, influencing junction conductance, variability, and function. While the development of such linkage chemistries has focused on interactions at gold, the distinct reactivity and electronic structure of other electrode metals provides underexplored opportunities to characterize and exploit new binding motifs. In this work we show that α,ω-alkanedibromides spontaneously form well-defined junctions using silver, but not gold, electrodes through application of the glovebox-based scanning tunneling microscope-based break junction method. We systematically evaluate, through a series of additional studies, whether these molecular components form physisorbed or chemisorbed contact geometries, and if they undergo secondary chemical reactions at the silver surface. Critically, we find that the same junctions form when using different halide, or trimethylstannyl, terminal groups, suggestive of an electronically transparent silver-carbon(sp³) contact chemistry. However, the experimental conductance of the junctions we measure with silver electrodes is ∼30× lower than that observed for such junctions comprising gold-carbon(sp³) contacts, which does not align with predictions based on first-principles calculations. We further exclude the possibility that the proposed silver alkyl species undergo α- or β-hydride elimination reactions that result in a distinct contact chemistry through conductance measurements of control molecules that cannot undergo such processes. Applying insights provided from prior temperature-programmed desorption studies and a robust series of atomistic simulations, we ultimately propose that in these experiments we measure alkoxide-terminated junctions formed from the reaction of the chemisorbed alkyl with oxygen that is coadsorbed on the silver surface. This work, in demonstrating that high conductance contact chemistries established using model gold electrodes may not be readily transferred to other metals, underscores the need to directly characterize the interfacial electronic properties and reactivity of electrode metals of wider technological relevance.