Karbovnyk I. Mechanisms of nanophases formation and electronic processes in layered crystalline and hybrid functional materials

The dissertation deals with an important problem of modern physics of semiconductors and dielectrics which is related to the understanding of mechanisms of formation of crystalline, composite and hybrid nanostructures and peculiarities of electronic processes in such structures. The relation between electronic processes and optical and electrical properties of crystalline and hybrid nanostructures under the influence of physical fields is thoroughly studied. Mechanisms of nanophases formation on the surface and in the bulk of CdI2 crystals are described. The growth kinetics of cone-shaped nanoclusters is described by a diffusion model based on the interdiffusion approximation for the individual components. The growth rate of nanoclusters is shown to depend on the time evolution of the concentration gradient of Cd2+ ions in the near-reaction zone. The influence of non-stoichiometry, impurities and the role of anion/cation sublattices on the processes that determine the properties of these phases are evaluated. The structure of local centers formed by non-stoichiometric Cd atoms is explained and the model describing the parameters of such centers is proposed. The formation of microtubes with rectangular cross section in the volume of CdI2 layered crystals doped with Bi3+ trivalent impurity ions has been observed. The formation of microtubes is related to the rolling of I–Cd–I triple layers containing impurity ions, which takes place as a result of the presence of uncompensated charges at the edges of these layers. A model explaining the formation of a bimolecular clusterby impurity atoms and a cadmium vacancy is presented. Novel thin organic films were fabricated by thermal vacuum deposition of luminescent liquid crystalline organic molecules on different substrates. It is demonstrated that combining specific substrate material with various organic molecules allows to observe multicolor emission, ultimately opening the way to designing a white light emitting structure. The possibility of tuning the multicolor light emission by changing the excitation energy is shown. The influence of external electric field and polarized laser radiation from outside the absorption range on the ordering of luminescent molecules in the process of deposition has been studied. Clear signs of specific molecular aggregation for the films deposited with electric field applied perpendicularly to the substrate were observed by atomic force microscopy.It was shown that irradiation of molecules by non-resonant polarized laser beam during deposition strongly decreases the formation of aggregated molecules. Nanostructured films obtained under influence of electric field and external irradiation show linearly polarized luminescence.The corresponding degrees of polarization were determined. Linearly polarized luminescence was achieved in nanostructured films formed by deposition of dipolar molecules on a polymer pre-modified substrate.Emission properties of nanostructured films with linearly polarized luminescence, including the distribution of luminescence intensities, were proven to remain stable over time. The framework for computational studies of nano¬tube-insulator model composites has been developed. With¬in this framework, simulations of electrical conductivity considering a large number of interconnected nanotubes across the boundary edges of the representative volume element can be performed.The framework accounts for the intrinsic conductivity of nanoelements as well as for electron tunneling between adjacent tubes. The algorithm of total conductivity calculations using element-wise conductivity matrices was implemented. Performed calculations indicated dependence of the conductivity percolation threshold in the nanotubes network on the concentration of nanotubes in the simulated volume and on the nanotube aspect ratio. The increase of the nanotube-based composite electrical conductivity with the tunneling cut-off distance was demonstrated. The nanocomposite structures based on PEDOT:PSS polymer matrix were synthesized. Two types of nanofillers were used to reinforce base matrix: single-walled carbon nanotubes and multiwalled carbon nanotubes. Systematic evaluation of the influence of radiation on the electrical response of such hybrid nanocomposites is presented. Variations of resistance and conductivity of nanocomposites depending on the volume fraction of nanotubes in the matrix, ionizing radiation dosage and temperature are analyzed.