Mechanical, electrical and chemical properties of nanostructured materials

In recent years, metallic nano-structures have attracted considerable attention as they exhibit exceptional size/shape dependent properties and greatly enhanced performances in many industrially and technologically important phenomena such as catalysis, chemical reactivity, selectivity, and stability. Evidently, to utilise such minuscule particles with specific morphology and architecture requires precise control of the growth and structure. Thanks to the outstanding works of several groups, tuning of growth conditions, with a sufficient degree, to decipher the rules governing the evolution of the growth front, and thus to tailor nano-crystals with certain structural characteristics is now possible. One such example is the high resolution transmission electron microscopy experiment that revealed an unprecedented structural transformation of a silver nanowire into a square nanotube with the smallest possible cross-sectional area through application of an axial strain. Another example is seed-mediated growth experiment in which surface diffusion of atoms at the facets of a growing seed is found to play a critical role in determining the growth front of a seed and thus shape or morphology taken by the final product in the solution phase synthesis of metal nano-crystals. While previous studies have highlighted the macroscopic description of processes, there is less understanding as to whether individual atomic-scale processes poses any significant role in controlling the structural characteristics of nano-products. Clearly, the development of a microscopic modelling of cluster growth and obtaining stable structures requires a complete understanding of the atomistic processes underlying micro-structural evolution of these materials. To this end, atomistic simulations are real alternative to examine the structure and dynamics of growing particles at the atomic scale and thereby specify the important individual atomistic processes taking place in the course. In the Thesis, extensive calculations on energetics, dynamics, and electronics of nano-materials, ranging from a nanowire to nanoclusters with various shapes and morphology, were performed using molecular statics (MS), molecular dynamics (MD) and electronic calculations based on semi-empirical, many-body potentials extracted from the embedded atom method (EAM) and density functional theory (DFT). In the simulations, all model systems for examining the Ag nano-materials are carefully chosen to simulate more realistic representations of experimentally investigated metal nano-structures and were initially arranged in their perfect lattice positions. The standard conjugate gradient method is utilized to fully minimize the total energy of the system. The MS and MD simulations were carried out after eliminating the non-zero initial stress in the model systems. In MD simulations, the atoms were coupled to a Nose-Hoover thermostat to keep system in the equilibrium at higher temperatures and the equations of motion are integrated with steps of 1 fs using the Verlet algorithm. The activation energy barriers for the diffusion processes were calculated using an accurate and efficient technique of the nudged elastic band (NEB). In the first part of the Thesis, the atomic nature of spontaneous transformation of a silver nanowire into a nanotube under tensile strain was explored through MS, MD and DFT calculations. It was found that due to the substantially increased compressive strain in the core of the wire, induced by the applied tensile strain along the length direction, the fcc wire undergoes a structural phase transition into a tubular wire, in a perfect agreement with the experimental observation. MS calculations predicted that this particular type of structural phase transformation is controlled by the nature of the applied strain, the length of the wire and the initial cross-sectional shape: More specifically, for such a perfect structural transformation, the <100> axially oriented fcc nanowire needs (1) to be formed by stacking A and B layers of an fcc crystal, both possessing the geometry of two interpenetrating one-lattice-parameter-wide squares, containing four atoms each, (2) to have an optimum length of eight layers, and (3) to be exposed to a combination of low and high stress along the length direction. DFT calculations, further, indicated that bond between Ag atoms in the tubular structures are much stronger compared to the bond between Ag atoms in the nanowire. In contrast to the experimental findings, MD simulations resulted in temperature dependent-structural transformation of the nanowire. In the second part of the Thesis, in order to understand the shape evolution of Ag nanoclusters of various shape and morphology at the atomistic scale, the energy barriers and reaction rates of different pathways were determined using NEB technique and MD simulations based on the EAM potentials. Activation energy barrier calculations revealed that the adatom deposited on {111} facet would preferentially migrate to the edges to find energetically more favorable sites on the {111} facet and then would diffuse to the neighboring {110} facets through an exchange mechanism, in particular. MD simulations for a total of 300 repeated events at different temperatures also confirmed the energetic predictions: regardless of temperature, the leading diffusion mechanism in the mobility of a single atom from {111} facet to the other facets is by far the exchange of the adatom with an edge atom. Notable number of hopping attempts occurs only when temperature is high enough. In order to determine the atomistic nature of the specific growth modes proposed in the experimental work, further MD runs were conducted to simulate growth on various silver nano-seeds at 500K. The deposition rate were identified on the basis suggested by the statistics of repeated 300 single adatom diffusion on the cubic Ag clusters. The simulations on nano-cubes confirm that metal nano-seeds enclosed by {100} facets can be directed to grow into octopod, concave, truncated cube, and cuboctahedron when the relative surface diffusion and deposition rates are finely tuned as suggested in the experiment. Atomic level processes also play a significant role in controllably fine tuning of the two competing rates of surface diffusion and depositon. Regardless of temperature and initial shape of the nano-seeds, the exchange of the deposited atom with an edge atom of the seed is by far the governing diffusion mechanism between the neighboring facets, and thus is the leading atomistic process determining the conditions for fine tuning of macroscopic processes. In conclusion, the presented molecular static and dynamic simulations are the first simulations to understand the underlying atomistic mechanisms of the experimentally determined growth modes of metal nano-clusters.The results further offer insights into the atomistic nature of specific structural transformation into a nanotube with the smallest possible cross-section. Therefore, Thesis not only presents a clear understanding of the underlying physics behind the growth and stability of metal nano-crystals, but also provides new insight into their surface dynamics and structure at the atomic scale and hence contributes to the development of nanotechnology.