Molecular dynamics simulation studies of surface-stress effects in metallic nanostructures

Using molecular dynamics (MD) simulations we investigate the surface-stress-induced structural transformations and pseudoelastic behavior in palladium (Pd) crystalline nanowires. For a \u3c100\u3e initial crystal orientation our studies indicate that the surface stress can cause Pd nanowires to spontaneously undergo structural changes with characteristics that are determined by the wire cross-sectional area. Specifically, when the cross-sectional area is below 2.18nm x 2.18nm the wire changes spontaneously its crystal structure from the initial fcc structure to a body-centered-tetragonal (bct) structure. In wires of larger cross-sectional area (i.e., 2.57nm x 2.57nm) the structural transformation is achieved via a spontaneous lattice reorientation leading to an fcc wire with \u3c110\u3e orientation and {111} side surfaces. In both cases, under tensile loading and unloading Pd nanowires transform reversibly between the corresponding transformed structures and the original \u3c100\u3e structure exhibiting pseudoelastic behaviors characterized by comparable, fully recoverable, strains of up to 50%. Moreover, the temperature-dependence of the two pseudoelastic behaviors enables the shape memory effects in Pd nanowires in both cases. In the nanofilm case, our MD simulation results show that if the film is only a few nanometers thick the spontaneous reorientation of the (001) top layer leads to rolling-up of the initially planar free standing, (001)/(111) bilayer into a tubular structure. Our detailed analysis of the reorientation process indicate that the bilayer self- rolling is determined by both energetic and kinetic processes characterizing the spontaneous structural reorientation of the top (001) textured layer to the (111) orientation of the substrate layer. Specifically, the analysis of the simulation results indicate that reorientation of the (001) top layer proceeds via a mechanism characterized by nucleation from multiple sites, propagation and growth of the new (111) oriented domains embedded in the original (001) oriented layer. While individually the newly formed (111) domains grow free of defects a region containing a surface dislocation like linear defect forms at the boundary where two such domains meet. The equilibrium of the newly formed bilayer structure is attained by multiple localized bendings of the bilayer structure. The spacing (density) of the nucleation is a function of temperature and influence the radius of curvature of the resulting structure