(571c) Molecular Dynamics and Density Functional Theory Studies on the Formation of Ultrathin Gold Nanowires in Vacuum and in Organic Solvents

The molecular break-junction technique involves pulling a gold nanowire apart in a solution containing the organic molecules (such as 1,4-Benzenediothiolate (BDT) molecules) whose properties are to be measured. We have first studied the structure and energy evolutions of a finite gold nanowire during elongation through a combination of classical molecular dynamics (MD) simulations and ab initio density functional theory (DFT) calculations in vacuum environment. We employed three classical force fields, the glue model, the embedded atom method (EAM), and the tight-binding second moment potential (TB-SMA) to independently investigate the nanomechanical stretching behavior of Au nanowire for small system involving hundreds of gold atoms. The comparisons of energy relaxations obtained by using classical force fields and DFT calculations indicate that the TB-SMA potential is the most accurate classical potential in describing the structural properties of gold nanowires. The study of large system which involves thousands of gold atoms shows that TB-SMA potential can accurately describe the mechanical response under tensile force in the sense that it gives the single Au-Au bond strength comparable with DFT result. Moreover, MD simulations were performed for three different crystallographic orientations Au (001), Au (110) and Au (111) with the elongation rate of 1 m/s at two temperatures 0.01 K and 298 K. The classical force fields lead to different rupture configurations. For the TB_SMA potential, monatomic chains ranging from one to five atoms in length are obtained under certain crystallographic orientation and temperature. For the glue model, the nanowires show very high ductility at elevated temperature without occurrence of monatomic chains. We then put Au nanowires in organic solvents to study the adsorption structure of molecules on Au wires under tension. A particular relevant system is BDT molecules in a tetrahydrofuran (THF) solvent, since the first molecular break-junction experiment was performed on this system, leading to the measurement of the I-V characteristics of a single BDT molecule. Grand canonical Monte Carlo (GCMC), canonical Monte Carlo (MC) and MD simulation are applied to study the adsorption of BDT/THF mixtures on gold nanowires and the elongation and breakage of gold nanowires in organic solvents. These results will help us to get a better understanding in the original discrepancy between experimental and theoretical study in I-V characteristics of Au/BDT/Au devices.