Understanding the formation of metal-molecule junction at the atomistic limit via conductance measurement

Single molecular junction offers an atomic scale laboratory to control and manipulate the quantum electronic transport. However, formation of metal-molecule junction is not straightforward and requires significant attention. Employing two isomers of bipyridine, 4, 4′ bipyridine and 2, 2′ bipyridine between gold electrodes, here, we investigate the formation of a metal-molecule bond by studying charge transport through single molecular junctions using a mechanically controlled break junction technique at ambient condition. While both molecules form molecular junctions during the breaking process, closing traces show the formation of molecular junctions unambiguously for 4, 4′ bipyridine via a  conductance jump from the tunneling regime, referred as ‘jump to molecular contact’, being absent for 2, 2′ bipyridine. Through statistical analysis of the experimental data, along with molecular dynamics and first-principles calculations, we provide a microscopic origin behind the formation and evolution of molecular junction. Our findings reveal that contact formation is greatly influenced by both the structure of the molecular backbone and the evolution of the junction during breaking or making process, providing an important insights for using a single molecule in an electronic device.