Tereshchuk V. Dynamics of formation of ordered quasi-one-dimensional solid-state nanosystems

The formation of quasi-one-dimensional structures is based on the use of nanowires as basic elements. This dissertation is devoted to the study of two variants of the evolution of their morphology. In the first one, the cross-section of the nanowire becomes periodically modulated as a result of spontaneous transformation of the surface in the pre-melting mode (the temperature of the nanowire is much lower than the melting point of the material from which it was synthesized). In the final stage, such modulations lead to the break-up of the nanowire into a chain of isomeric nanodroplets. Provided proper control over the kinetics of the process, the effect of thermal instability of nanorods can be used to create waveguides of subwavelength optics. In the second case, the base nanowire is used as a quasi-one-dimensional substrate for further synthesis of an ordered system of nanoclusters - i.e. the creation of the core-shell nanowires. Homogeneous in diameter nanowires are significantly inferior to core-shell systems in the ability to "capture" light per unit mass. Therefore, the establishment of methods for controlling the geometric parameters of core-shell nanowires will dramatically reduce the mass of the "mirror" - detector in the optical resonator with the subsequent ability to detect mechanical motions of the studied objects up to quantum effects. The main part of the dissertation consists of four sections, which are devoted to: (i) the establishment of physical mechanisms responsible for spontaneous periodic modulation of the surface and the subsequent break-up of the nanorod; (ii) determining the patterns of formation of nanoclusters on the nanowire in the synthesis mode; (iii) development of methods for controlling the parameters of both the disintegration and the synthesis processes for the formation of systems with predetermined morphology and physical properties. The first section of this dissertation is devoted to a detailed analysis of the break-up processes of cylindrical nanowires with a face-centered cubic crystal lattice depending on the system temperature. The same section presents a description of the Monte Carlo model on the basis of which the research was conducted. Based on numerical experiments, it was found that the manifestation of the anisotropy of surface energy density can significantly affect the kinetics of the break-up process and in some cases cause large deviations of the cross-sectional modulation wavelengths with maximum growth increment from the predictions of existing models. In this regard, an analytical model was built, based on which an expression was obtained which takes into account the dependence of the wavelength of modulations of the nanowire radius on the anisotropy of surface energy density, which allowed to predict with high accuracy the wavelength of surface modulations for nanowires of different orientations. In the second section, the Monte Carlo kinetic method was used to study the physical mechanisms responsible for the disintegration of nanowires with a diamond-like cubic crystal structure into chains of nanoparticles. Depending on the temperature and orientation of the nanowire relative to its internal crystal structure, the wavelengths of modulations of the cross-section exceed its initial radius by 4-18 times. It is shown that the reason for the formation of surface modulations of nanowires with ultra-short wavelengths 4.5<λ<2π is the effect of roughening transition, which depending on the system temperature may appear on some or all faces that form the lateral surface of the nanostructure. The third section investigates the kinetics of nanowires with a cubic body-centered crystal lattice at different stages of their break-up and analyzes the dependence of the wavelength of surface perturbations of nanorods with FCC and BCC crystal lattices on external influences, which lead to surface diffusion intensification. It was determined that for certain orientations of nanowires with BCC lattice the roughening transition effect plays a major role in the disintegration process and causes the formation of terraces on their lateral surface and the development of short-wave perturbations of the cross-section. The fourth section analyzes the physical mechanisms responsible for the formation of an ordered sequence of nanoclusters synthesized on nanowires with a diamond-like crystal lattice. It was established that the main factors determining the self-ordering of the nanocluster system are the surface diffusion of atoms and the shadow effect. Based on the obtained results, it was determined that changing the rate of supply of atoms to the system is an effective method of ensuring a high level of periodicity in the location of the nanoclusters along the base nanowire and allows to synthesize nanostructures with desired geometric parameters.