Rudyk Y. Optical-spectral, electric and heat conductivity properties of the nanostructured materials based on zinc oxide

The work is devoted to the purposeful modification of the optical-spectral, electric, heat conductivity and absorption properties of zinc oxide by means of lowering of system dimensionality and changes of the technological processes in order to obtain the effective multifunctional materials for the optoelectronic and sensor devices. In result of investigations of the heat conductivity properties in the composites based on zinc oxide it has been found the quantum confinement effect which appears as increasing of the heat conductivity of the thermal paste at transition from the micropowder to the nanopowder. It was shown that in the nanocomposite material based on zinc oxide the excitonic component of the heat conductivity is also considerable since the free excitons in zinc oxide are stable even at room temperature due to their large binding energy (60 meV) which is much more higher than in GaN (24 meV). The effectiveness of the mathematical modeling methods employment for the optimization of the technological process was demonstrated since on the basis of zinc oxide thin films there was obtained the ultraviolet filter with an optimized spectral characteristic. The photoluminescence spectra of ZnO nanorods with 2 mkm length were investigated. The correspondence between the radiation bands and mechanisms explaining their appearance was proposed. Beside the exciton band in the UV spectral region there was detected the band with a maximum at 577 nm connected with the transition between the donor-acceptor pairs: of hydroxide (OH–) and interstitial oxygen Оі with the energy of 2,15 eV. The second more intensive band with a maximum at 701 nm arises due to the donor-acceptor transition OH–→OZn0/- (ion of hydroxide → oxygen vacancy) with the energy of 1,77 eV. It has been shown that vacuumization considerably affects the photoluminescence parameters and decay time of the photoresponse of ZnO nanowires and nanorods. In the case of ultraviolet radiation at the wavelength of 360 nm detecting the recovery time for the samples with the nanostructures grown by the hydrothermal methods was found to be much higher in comparison with those for the sample with the nanostructures grown from the vapor phase. It has been found that UV detector based on ZnO nanorods shows a much higher rise time of photocurrent value upon illumination in comparison with the detector with ZnO nanowires. The high rise time indicates that the traps in the ZnO nanorods are involved into the photocurrent generation process. The light emitting diodes with optimized characteristics have been created. The performed investigations confirmed presence of the homojunction exhibiting the considerable breakthrough in creation of ZnO with p-type conductivity. Such an achievement would be considered as a basis for the considerable increase of the light emitting diodes effectiveness without increasing of the working voltage. The observed electroluminescence spectra of the created light emitting diodes were found to possess the broad multi-color emission bands that would be perceived by eye as a white light. This confirms possibility to apply such a device asautonomouslight source without additional applications of the luminophores. The laser generation was realized in the massifs of ZnO microprisms and nanorods in vicinity of 388 nm. The coherent feedback in ZnO microprismscan be provided by two basic mechanisms. In the first case the coherent feedback is provided by multiple reflections from the end facets of the microprisms serving as a Fabry–Perot resonator. In the second case, the coherent feedback is provided by multiple scattering events (random lasing from ZnO). Investigations of the wettability processes on the surface of different types of microstructuresbased on zinc oxide testifies that reducing of thewater contact angleis slower in the case of UV irradiation than under the plasma treatment, probably due to differences in the energy level delivered during treatment and the subsequent surface alteration. The dependence of the wettability for the three types of the microstructures on their surface morphology at ion etching and controlled ultraviolet irradiation was investigated. The samples with a larger surface roughness and surface-to-volume ratio were found to possess a considerably higher water contact angle and a time of transition from the superhydrophobic to the superhydrophilic state. On the basis of existing prototype the effective resistive sensor of ammonia, based on application of nanostructured ZnO was created. Such an approach provided the developed working surface leading to increase of the sensor sensitivity and widening of its working temperature region.