Research assistance, University of Rochester
Structurally, nanoporous (NP) system can be regarded as a network of interconnected nanowires as its constituting ligaments. My research is investigating the mechanical behaviors of single-crystalline Cu nanowires aligned in the <001>, <110> and <111> crystallographic directions under tensile loading and their triple junction network using molecular dynamics (MD) simulations. Shear strain tensor analysis is used to differentiate deformation mechanisms accommodating strain among nanowires and capture the necking point during stretching. In addition, a computational analysis method is used to quantify plastic and elastic deformation. The nanowires in different crystallographic orientations behave differently in elongation. Nanowire in the <110> orientation shows enhanced ductility compared to the <001> and <111> directions because of the extended twining activity while dislocation glides lead to necking in the other two nanowires. In general, dislocation activity accommodate 10% to 20% plastic deformation after yielding while most of the plastic strain generates within bulk and surface atoms. We also investigated the deformation behaviors of triple junctions. We found out that yield strength of triple junction structure is quite close to the strength of NP structure. This means that deformation behavior of the NP system can be represented as a function of junctions rather than only single ligaments.
Abstract: When fabricating nanowires in a doubly-clamped beam configuration it is possible for a residual axial stress to be generated. Here, we show that material characterisation of metal and semiconductor nanowires subjected to residual axial stress can be problematic. Benchmark measurements of the Young's modulus of nanowires are performed by sectioning a doubly-clamped nanowire into two cantilevered wires, eliminating residual axial stress. Use of models for doubly-clamped beams that incorporate the effects of residual stress are found to lead to ambiguity in the extracted Young's modulus as a function of displacement fit range, even for nanowires with no residual stress. This is due to coupling of bending and axial stress effects at small displacements, and the limited displacement range of force curves prior to fracture or plastic deformation. This study highlights the importance of fabricating metal and semiconductor nanowires that exhibit little or no residual axial stress for materials characterisation.
Pub.: 29 Jun '17, Pinned: 29 Jun '17
Abstract: When reducing the size of metallic glass samples down the nanoscale regime, experimental studies on the plasticity under uniaxial tension show a wide range of failure modes ranging from brittle to ductile ones. Simulations on the deformation behavior of nanoscaled metallic glasses report an unusual extended strain softening and are not able to reproduce the brittle-like fracture deformation as found in experiments. Using large-scale molecular dynamics simulations we provide an atomistic understanding of the deformation mechanisms of metallic glass nanowires and differentiate the extrinsic size effects and aspect ratio contribution to plasticity. A model for predicting the critical nanowire aspect ratio for the ductile-to-brittle transition is developed. Furthermore, the structure of brittle nanowires can be tuned to a softer phase characterized by a defective short-range order and an excess free volume upon systematic structural rejuvenation, leading to enhanced tensile ductility. The presented results shed light on the fundamental deformation mechanisms of nanoscaled metallic glasses and demarcate ductile and catastrophic failure.
Pub.: 02 Jun '16, Pinned: 29 Jun '17
Abstract: Plasmonic metal nanostructures have shown great potential in sensing applications. Among various materials and structures, monolithic nanoporous gold disks (NPGD) have several unique features such as three-dimensional (3D) porous network, large surface area, tunable plasmonic resonance, high-density hot-spots, and excellent architectural integrity and environmental stability. They exhibit a great potential in surface-enhanced spectroscopy, photothermal conversion, and plasmonic sensing. In this work, interactions between smaller colloidal gold nanoparticles (AuNP) and individual NPGDs are studied. Specifically, colloidal gold nanoparticles with different sizes are loaded onto NPGD substrates to form NPG hybrid nanocomposites with tunable plasmonic resonance peaks in the near-infrared spectral range. Newly formed plasmonic hot-spots due to the coupling between individual nanoparticles and NPG disk have been identified in the nanocomposites, which have been experimentally studied using extinction and surface-enhanced Raman scattering. Numerical modeling and simulations have been employed to further unravel various coupling scenarios between AuNP and NPGDs.
Pub.: 29 Jun '17, Pinned: 29 Jun '17