Molecular Simulations on the Elongation Dynamics of Gold Nanowires in Vacuum and in Solvents

Abstract

We report the mechanical elongation of a gold nanowire in vacuum and in solvents via molecular simulation. First, by combining molecular dynamics (MD) simulations with density-functional-theory (DFT) calculations, we demonstrate that the second-moment approximation of the tight-binding (TB-SMA) potential is most suitable to describe the interactions between Au atoms for finite Au clusters including nanowires. Second, we investigate the impact of the crystallography orientation, length, elongation rate and temperature on the ductile elongation properties of nanowires in a vacuum. Two typical break-junction structures, the monatomic chain and the zigzag helical long chain structure, were found; however, they are found under different length, rate and temperature combinations. Third, we investigate the effect of an inert solvent. The gold nanowires are elongated in a simple Lennard-Jones solvent (propane). Extensive MD runs demonstrate the effect of non-polar inert solvent on the elongation properties of Au nanowires is minimal below the melting point of gold nanowires. Fourth, we study the elongation of gold nanowires in a benezenedithiol (BDT) solution, in which the BDT molecules can bond to the Au atoms of the nanowire. Applying the grand canonical Monte Carlo (GCMC) technique, we show that the packing density of the bonded BDTs on the surface of Au nanowire is larger than that on a flat Au (111) surface. Combined GCMC and MD simulation results show that the presences of BDT molecules have the effect of increasing the average ductile elongation substantially compared to that in vacuum, even at room temperature. The findings from this work are helpful in understanding the underlying mechanism of the formation of Au-BDT-Au molecular break-junctions used in molecular conductance measurements.