Molecular dimer junctions forming: Role of disulfide bonds and electrode-compression-time

Thanks to their excellent bond strength, phenyl-based molecules with thiol anchoring groups are extensively employed to form stable single-molecule junctions. However, two critical questions are still not answered which seriously hinder high-yield establishing reliable molecular functional devices: (1) Whether molecular dimer junctions will be formed, and if this is the case, whether the dimerization is caused by intermolecular disulfide bonds or pi-pi stacking of phenyl rings; (2) Upon a mechanical-compression force, is it possible that both anchoring groups of the molecule bond to the same electrode instead of bridging two opposite electrodes, which would drastically reduce the yield of the molecular junctions. Here, combining UV-Vis/Raman spectroscopy of bulk molecules and conductance/flicker-noise measurements of single molecules, we give compelling evidence that molecular dimers naturally form under ambient conditions, primarily via disulfide bonds rather than by pi-pi stacking. We further proposed a technique, named electrode-compression-hold-on (ECHO), and reveal that the two thiol groups of phenyl-backboned molecules will bond to the same electrode upon a compression force with a prolongated ECHO time. In contrast, the compression-time-dependent phenomenon is not observed for alkyl-backboned molecules. The underlying mechanism for these unprecedented observations is elucidated, shedding light on the yield of molecular junctions.