Binding structure, breaking forces and conductance of Au-Octanedithiol-Au molecular junction under stretching processes: a DFT-NEGF study

Au-n-octanedithiol-Au molecular junction (Au-SC8S-Au) has been investigated using density functional theory combined with the nonequilibrium Green's function approach. Theoretically calculated results are used to build the relationship between the interface binding structures and single-molecule quantum conductance of n-octanedithiol (SC8S) embodied in a gold nanogap with or without stretching forces. To understand the electron transport mechanism in the single molecular nanojunction, we designed three types of Au-SC8S-Au nanogaps, including flat electrode through an Au atom connecting (Model I), top-pyramidal or flat electrodes with the molecule adsorbing directly (Model II), and top-pyramidal Au electrodes with Au atomic chains (Model III). We first determined the optimized structures of different Au-SC8S-Au nanogaps, and then predicted the distance-dependent stretching force and conductance in each case. Our calculated results show that in the Model I with an Au atom bridging the flat Au (111) gold electrodes and the SC8S molecule, the conductance decreases exponentially before the fracture of Au-Au bond, in a good agreement with the experimental conductance in the literature. For the top-pyramidal electrode Models II and III, the magnitudes of molecular conductance are larger than that in Model I. Our theoretical calculations also show that the Au-Au bond fracture takes place in Models I and III, while the Au-S bond fracture appears in Model II. This is explained due to the total strength of three synergetic Au-Au bonds stronger than an Au-S bond in Model II. This is supported from the broken force about 2 nN for the Au-Au bond and 3 nN for the Au-S bond.

Automated summary

This study investigates the Au-n-octanedithiol-Au molecular junction (Au-SC8S-Au) using density functional theory combined with the nonequilibrium Green's function approach. The relationship between the binding structures at the interface and the single-molecule quantum conductance of n-octanedithiol (SC8S) in a gold nanogap is established through theoretical calculations. Different models of Au-SC8S-Au nanogaps are designed to understand the electron transport mechanism in single molecular nanojunctions. The calculated results show variations in conductance and bond fractures depending on the specific model and bond strength.