Huseynova S. Features of the formation of long-wavelength luminescence centers in zinc oxide nanoparticles for luminescent sensors.

The work is devoted to the obtaining and study of the photoluminescent properties of undoped and doped zinc oxide nanocrystals obtained by the method of chemical deposition for optoelectronics, sensors and biomedical applications. Special attention was paid to the study of emission transitions activation processes during the interaction of nanoparticles with heavy metal ions, which are dangerous for the environment and human health. The average crystallite sizes were calculated from the shift of the fundamental absorption edge compared to bulk crystals, which for experimental samples lie in the range of 3.5–7 nm. The obtained sizes agree well with the results of studies obtained using a scanning electron microscope (SEM), where the formation of nanocrystallites up to 10 nm in size was recorded. It was shown that the size of nanocrystallites is affected by the concentration of precursors (ZnSO4 and KOH) and the presence of a stabilizer. The “red shift” of the fundamental absorption edge when doping ZnO nanocrystals with heavy metal impurities is due to an increase in the size of nanocrystallites due to the introduction of additional nucleation energy during the diffusion of heavy metal ions into the zinc sublattice. The possibility of doping ZnO nanocrystals during the growth process by chemical coprecipitation by adding aqueous solutions of metal salts - elements of groups II-III and transition elements to the ZnSO4 solution is shown. Doping with transition element ions is accompanied by a change in the nanocrystals color, which is confirmed by the presence of optical absorption lines in the visible region of the spectrum. The example of ZnO:Fe nanocrystals shows a good agreement of the energies of intracenter transitions within the Fe2+ ion, obtained from the optical absorption and photoluminescence spectra, with theoretical calculations of the energy states of the Fe2+ ion, obtained in the weak crystal field approximation (insignificant crystal environment). A study of the photoluminescence of ZnO nanocrystals obtained by the chemical coprecipitation method revealed the presence of effective radiation in the ultraviolet and visible regions of the spectrum. The position of the ultraviolet emission lines correlates with the position of the fundamental absorption edge and is due to emission transitions involving excitons bound to donors, acceptors, and recombination radiation through shallow donors. The visible emission of ZnO nanocrystals obtained by chemical co-deposition is characterized by broad non-elementary emission bands in the blue, green and orange-red regions. The decomposition of these bands into elementary Gaussian components allowed us to isolate a series of elementary emission lines localized at 2.82, 2.72, 2.62, 2.5, 2.37, 2.3, 2.17, 2.06 and 1.88 eV. It was found that neither the technological conditions of synthesis (the size of nanocrystallites) nor the temperature of the nanocrystals affect the spectral location of these lines. The absence of optical absorption lines in the visible region of the spectrum indicates the intrinsic defect nature of these emission lines.