Studies On Novel Structures And Mechanical Properties Of Metal Nanowires And Graphene Nanomaterials

Low-dimensional nanomaterials have attracted intensive interest owing to theirfascinating properties and potential applications in the new generation ofhigh-performance nano electromechanical devices. In particular, some outstandingproperties induced by novel microstructures arise, which are not remarkable in their bulkcounterparts. Exploration of the structural and mechanical behavior of nanoscalematerials has been one of the essentially fundamental areas in nanotechnology. In thisdissertation, molecular dynamics (MD) simulations, as well as density functional theory(DFT) based first-principles calculations have been used to study the structural andmechanical properties of metal nanowires and graphene nanomaterials. The followingresults and conclusions are obtained:1. Elastic and dynamic properties of multi-shell nickel nanowires dependent onthe structure and size. Helical structures of nickel nanowires and their crystallinecounterparts with0.64-1.52nm in effective diameters are obtained by using simulatedannealing method, similar to those produced in experiments. Their size and structuraldependence of Young’s modulus is clarified by a quantum Sutton-Chen many bodypotential based MD simulation. It is demonstrated that the interaction energy between thesurface and the core of nanowires has little effect on their stiffness, in spite of the factthat the energy is notable. In addition, longitudinal oscillation is characterized byParrinello-Rahman constant-pressure algorithm. The dynamic response shows dramaticaldifference between helical and crystalline structures. Furthermore, the observedlongitudinal oscillation to the external loading is qualitatively explained in theframework of a continuum-medium mechanical model. These results may provide asupplemental method to distinguish the two types of nanowires in experiments bychecking their Young’s modulus or longitudinal modes.2. Uniaxial force induced structural phase transitions and pseudoelasticity inmetal nanowires. Firstly, the solid-solid phase transition from regular crystallinestructures to their novel helical counterparts in metal nanowires is observed inexperiments. However, the possibility of inverse transition remains unexplored. Thecoupled effects of size, uniaxial force, and relaxing temperature on the structural phase transitions in copper nanowires are systematically studied by using the embedded atommethod (EAM) potential based MD simulations and density functional theory (DFT)calculations. Both the reported phase transition from the crystalline structure to thehelical one and the unexpected inverse transition are observed. In other words, helicalstructures are dominant for effective diameters less than0.8nm, while the uniaxial forcemay lead to helical-crystalline phase transition for thicker ones. We also observe that thetransition from helical12-7-1nanowire to crystalline [110] nanowire is much easier thanthe inverse transition. Considering that the Young’s modulus and the conductance of ananowire sensitively depend on its atomic geometry, the reversible solid-solid phasetransition should be valuable in the mechanically controllable breaking junction (MCBJ)experimental analysis and practical applications in the future nanoscale interconnectors,sensors and actuators.Secondly, iron nanowires exhibit tensile-strain induced martensitic phase transitionand <110>â†'<001> reorientation. However, the spontaneous reorientation andpseudoelasticity that happens in face-centered cubic metal nanowires has never beenrevealed in body-centered cubic nanowires. We demonstrate that, in terms of EAMpotential based MD simulations on iron nanowires with cross-sections between10.8×10.42and59.3×56.72, the <110>â†'<001> reorientation and pseudoelasticitycan be induced by the axial strain. The reorientation mechanism shows an intricatedependence on the deformation temperature. Specifically, the original <110> nanowiresand the reoriented <001> counterparts exhibit pseudoelastic behavior with recoverystrain on the order of30%. We further show that the unique stress-strain response in ironnanowires leads to an interesting phenomenon, i.e. the spontaneous <110>â†'<001>reorientation can be initiated by the external loading with a small compressive strain onthe order of4%. The temperature and stress dependent mechanical response in ironnanowires makes it an ideal candidate for intelligent nanomaterials such as smart sensors.3. The integrated effects of stress and temperature on the mechanical behaviorand defects evolution in graphene nanomaterials. Firstly, as a building block fornanoelectronic devices at the molecular scale, carbon linear atomic chains (LACs) showparticular promise for logical molecular switches. However, the key issue limiting theirapplication is the lack of effective technique to produce LACs. Our AIREBO potentialbased MD simulations further studies the structural evolution and breaking mechanismsof graphene nanoribbons (GNRs) under stress and wide range of temperatures, which areyet to be fully explored by experiments. Non-hexagons and long LACs are produced at high temperatures, whose main features are similar to the observation from electronbeam irradiation experiments. We show how defects lead to the formation of definitelystable LACs. In contrast, tensile deformation modes become brittle at low temperaturesdue to localized defects which are attributed to insufficient thermal energy. In thesimulation of zigzag GNRs at high temperatures, we unexpectedly obtained an amazingfracture of the armchair-zigzag bridge that is connected by non-hexgons. These resultsprovide an efficient way of making long carbon LACs as well as GNRs withhybrid-edges through stress-temperature-controlled procedure, which is expected byincorporating MCBJ technique and Joule heating procedure employed in experiments.Secondly, graphene may be damaged unexpectedly by external force or constantstrain in practical applications, resulting in the lifespan and performance degradation ingraphene-based high-performance electrical or mechanical devices. In our simulations,nano-damage in suspended graphene monolayer is created via a rigid C60molecule. Weshow that the self-healing procedure happens under adequate heat treatment. It suggestsan efficient two-stage self-healing mechanism in damaged graphene: local curvatureintroduced by defects around the damage and curved surface smoothed via defectsreconstruction which leads to damage shrinking. In addition, our simulations indicate thatthermal fluctuation and the size of damage affect the self-healing capability of graphene.These results offer additional insights for realizing self-healing nano-devices composedof graphene, as a supplement of current healing methods including ion irradiation andchemical treatment.