No abstract

Atomistic simulations are employed to probe the deformation behavior of experimentally observed top‐down and bottom‐up face‐centered cubic silver nanowires. Stable, <110> oriented nanowires with a rhombic and truncated‐rhombic cross section are considered, representative of top‐down geometries, as well as the multiply twinned pentagonal nanowire that is commonly fabricated in a bottom‐up approach. The tensile deformation of a stable, experimentally observed structure is simulated to failure for each nanowire structure. A detailed, mechanistic explanation of the initial defect nucleation is provided for each nanowire. The three geometries are shown to exhibit different levels of strength and to deform by a range of mechanisms depending on the nanowire structure. In particular, the deformation behavior of top‐down and bottom‐up nanowires is shown to be fundamentally different. The yield strength of nanowires ranging from 1 to 25 nm in diameter is provided and reveals that in addition to cross‐sectional diameter, the strength of the nanowires is strongly tied to the structure. This study demonstrates that nanowire structure and size may be tailored for specific mechanical requirements in nanometer‐scale devices.

Previous atomistic simulations and experiments have attributed size effects in the elastic modulus of Ag nanowires to surface energy effects inherent to metallic surfaces. However, differences in experimental and computational trends analyzed here imply that other factors are controlling experimentally observed modulus changes. This study utilizes atomistic simulations to determine how strongly nanowire geometry and surface structure influence nanowire elastic modulus. The results demonstrate that although these factors do influence the elastic modulus of Ag nanowires to some extent, they alone are insufficient to explain current experimental trends in nanowire modulus with decreasing dimensional scale. Future work needs to be done to determine whether other factors, such as surface contaminants or oxide layers, contribute to the experimentally observed elastic modulus increase.

Using molecular statics simulations and the embedded atom method, a technique for bending silver nanowires and calculating Young's modulus via continuum mechanics has been developed. The measured Young's modulus values extracted from bending simulations were compared with modulus values calculated from uniaxial tension simulations for a range of nanowire sizes, orientations and geometries. Depending on axial orientation, the nanowires exhibit stiffening or softening under tension and bending as size decreases. Bending simulations typically result in a greater variation of Young's modulus values with nanowire size compared with tensile deformation, which indicates a loading-method-dependent size effect on elastic properties at sub-5 nm wire diameters. Since the axial stress is maximized at the lateral surfaces in bending, the loading-method-dependent size effect is postulated to be primarily a result of differences in nanowire surface and core elastic modulus. The divergence of Young's modulus from the bulk modulus in these simulations occurs at sizes below the range in which experiments have demonstrated a size scale effect on elastic properties of metallic nanowires. This difference indicates that other factors beyond native metallic surface properties play a role in experimentally observed nanowire elastic modulus size effects.

The plastic deformation of a pentagonal silver nanowire is studied by nanoindentation using an atomic force microscope ͑AFM͒. AFM images of the residual indent reveal the formation of a neck and surface atomic steps. To study the microscopic deformation mechanism, the indentation force-depth curve is converted to an indentation stress-strain curve and compared to the tensile stress-strain curves predicted by the atomistic simulations of pentagonal silver nanowires. The indentation stress-strain curve exhibits a series of yielding events, attributed to the nucleation and movement of dislocations. The maximum stress measured during nanoindentation ͑2 GPa͒ is comparable to the tensile yield strength predicted by atomistic simulations.