Stretching-induced conductance variations as fingerprints of contact configurations in single-molecule junctions

摘要

Molecule-electrode contact atomic structures are a critical factor thatcharacterizes molecular devices, but their precise understanding and controlstill remain elusive. Based on combined first-principles calculations andsingle-molecule break junction experiments, we herein establish that theconductance of alkanedithiolate junctions can both increase and decrease withmechanical stretching and the specific trend is determined by the S-Au linkagecoordination number (CN) or the molecule-electrode contact atomic structure.Specifically, we find that the mechanical pulling results in the conductanceincrease for the junctions based on S-Au CN two and CN three contacts, whilethe conductance is minimally affected by stretching for junctions with the CNone contact and decreases upon the formation of Au monoatomic chains. Detailedanalysis unravels the mechanisms involving the competition between thestretching-induced upshift of the highest occupied molecular orbital-relatedstates toward the Fermi level of electrodes and the deterioration ofmolecule-electrode electronic couplings in different contact CN cases.Moreover, we experimentally find a higher chance to observe the conductanceenhancement mode under a faster elongation speed, which is explained by abinitio molecular dynamics simulations that reveal an important role of thermalfluctuations in aiding deformations of contacts into low-coordinationconfigurations that include monoatomic Au chains. Pointing out theinsufficiency in previous notions of associating peak values in conductancehistograms with specific contact atomic structures, this work resolves thecontroversy on the origins of ubiquitous multiple conductance peaks inS-Au-based single-molecule junctions.