Bukivskyi A. Nature and kinetics of photoluminescence of PbCdI2 heterogeneous solid solutions

The thesis is devoted to the investigation of the mechanisms of formation of excited states of various nature, their dynamics, and the kinetics of radiative recombination in Pb1-XCdXI2 heterogeneous semiconductor solid solutions under excitation by visible and X-ray photons. Investigations of the surface morphology and characteristics of the crystal structure of Pb1-XCdXI2 heterogeneous solid solutions were carried out. The presence of PbI2 phases of 4H-polytype and CdI2 phases of 4H-polytype and also PbI2 and CdI2 of 2H-polytype has been shown. The formation of PbI2 nanoclusters (NCLs) of various sizes that naturally occurs in the crystalline matrix of CdI2 has been discovered and studied. These NCLs are quantum disks with an average diameter of 2.6 nm and a height of 1.4 nm, and are located within the atomic layers. 20 It was found that Pb1-XCdXI2 heterogeneous semiconductor solid solutions exhibit intense PL and also XL at room temperature. The nature of the optical transitions that form individual PL bands of these materials was established and an energy diagram of the excited levels of Pb1-XCdXI2 heterogeneous solid solutions was constructed. It has been shown that at room temperature, the luminescence of such nanostructured materials, predominantly, is determined by the presence of deep intrinsic luminescent states on the surface of PbI2 NCLs, including self-trapped (STE) excitons. Using the model of excitons self-trapping on deformed surface chemical bonds in nanoparticles, the mechanism of self-trapping of excitons that is associated with the deformation of ion-covalent bonds of Pb-I on the PbI2 NCL surface has been proposed. It was found that the transition from the delocalized to the STE state occurs mainly as a result of thermal activation or quantum mechanical tunneling at temperatures of 300 and 4.2 K, respectively. It was shown that these processes at room temperature are fast and occur in the sub-nanosecond and nanosecond time intervals, and their rate depends on the NCL size. The nature of XL of the investigated crystals was established and it was shown that it is analogous to the mechanisms of PL with certain differences due to a significant difference in the energy of the X-ray and visible photons. Based on the results of the individual PL bands kinetics studies and their mathematical analysis, the lifetimes and the rate of recombination of the corresponding excited states were established. It was shown that the optical response of these materials is fast, and the materials, respectively, are high-speed and, accordingly, promising for the production of highly efficient, fast-acting, uncooled scintillation detectors for X- and γ-rays based on them. The establishment of the nature of PL of these materials and the corresponding radiative recombination processes made it possible to determine the ways of improving the scintillation properties, such as the luminescence intensity, the radiation resistance, and the speed of these crystals.