Decoding Forming Processes of Different Contact Configurations in Au- and Ag-Electrode Single-Molecule Junctions

Revealing the details of the configuration evolution process at the atom-sized level is fundamentally significant for understanding the interface configuration and further the electron transport property of molecular junctions. However, it is still prohibitively difficult to directly detect the geometric structure of the molecular junction with atom-sized precision in experiment presently. Here, ab initio-based adiabatic and nonadiabatic geometry evolution simulation methods are used to reveal the forming processes of single-molecule junctions. By applying this method, we systematically investigate the forming processes of 1,4-diethynylbenzene molecular junctions with gold and silver electrodes. The numerical results demonstrate that due to different hardnesses of gold and silver, when fabricating the 1,4-diethynylbenzene molecular junction with Ag electrodes by stretching, the ethynyl end is very easy to be adsorbed on the hollow position of the Ag electrode by spontaneously pushing the tip Ag atom aside. However, for the Au-electrode system, the Au atom on the electrode tip is very difficult to be pushed aside, so the ethynyl end is generally adsorbed on the tip atom of the Au electrode. These specific yet significant differences in the forming processes of ethynyl-ended molecular junctions thereby result in the higher conductance of Ag-electrode systems compared with that of Au-electrode ones in experimental measurements.