Theoretical study of the formation, evolution, and breaking of gold nanowires

Real time imaging experiments with metal nanowires (NWs), in particular gold under stress, that show their formation, evolution, and breaking, were obtained with high resolution electron microscopy. In order to understand these results, we use density functional theory (DFT) based methods to simulate the evolution of Au NWs. First we use a tight-binding molecular dynamics (TBMD) method to understand the mechanisms of formation of very thin gold NWs. We present realistic simulations for the breaking of these NWs, whose main features are very similar to the experimental results. We show how defects lead to the formation of one-atom constrictions in the Au NW, which evolves into a one-atom-thick necklace chain. Similarly to the experimental results, we obtain that these necklaces can get as long as five-atoms from apex to apex. Before breaking, we obtain relatively large Au-Au bond distances, of the order of 3.0-3.1 Angstrom. A further pull of the wire causes a sudden increase of one of the bond distances, indicating the breaking of the NW. To get some more insight into the electronic structure aspects of this problem, we considered several of our tight-binding structures before breaking and studied them in detail using an ab initio method based on the DFT. By pulling the wire quasi-statically in this case, we also observed the breaking of the wire at similar distances as in the TBMD. This result was independent of the exchange-correlation potential used-either the local density approximation (LDA) or the generalized gradient approximation (GGA). The pulling force before rupture was obtained as 2.4 nN for the LDA, and 1.9 nN for the GGA. Finally, we also present a detailed analysis of the electronic structure properties for the Au neck atoms, such as the density of states and charge densities, for some configurations before the rupture.