Кафедра фізико-математичних наук

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Browsing Кафедра фізико-математичних наук by Author "Korsunska, N."

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    Effects of thermal treatment on the complex structure of luminescence emission of Li-doped ZnO screen-printed films
    (2023) Chukova, O.; Borkovska, L.; Khomenkova, L.; Korsunska, N.; Ponomaryov, S.; Androulidaki, M.; Stratakis, E.
    The ZnO–Li films were synthesized and investigated in an attempt to explore and develop RE-free phosphor materials capable of emitting intense visible light in a wide spectral range. The effects of both heterovalent doping with lithium and high-temperature annealing on the optical properties of ZnO films were studied. The films were deposited on the Al2O3 substrate using the screen-printing method and annealed at 800–1,000°C in air for 0.5–3 h. Both doping and annealing result in the transformation of the shape of reflectance spectra in the range of 300–400 nm and the shift of absorption edge to the long-wavelength region. At the same time, the bandgap value estimated taking into account the exciton peak position and its binding energy is independent of Li-doping. The feature at 300–400 nm and the shift of absorption edge are ascribed to the appearance of the absorption band that excited the yellow photoluminescence band. The photoluminescence spectra of undoped and Li-doped films show the emission bands in the ultraviolet and visible spectral ranges. The ultraviolet emission is due to ZnO exciton recombination. The visible emission band comprises several components peaked at 430, 482, 540, 575, and 640 nm. Their relative intensities depend on Li-doping, annealing temperature, and annealing duration. The 430- and 482-nm luminescence bands were observed in Lidoped films only. Their excitation spectra show the peak located at 330–340 nm, indicating that the energy significantly exceeds the ZnO bandgap energy. Consequently, the 430- and 482-nm luminescence bands are attributed to an additional crystal phase formed under annealing. Other components of visible emission bands are ascribed to the defect-related emission of ZnO. The possible nature of these bands is further discussed. Li-doping and annealing at intermediate temperatures result in blue emission and an enhancement of other visible bands, which makes ZnO–Li films a perspective material in photonic applications.
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    Influence of compacting pressure on the electrical properties of ZnO and ZnO:Mn ceramics
    (2024) Korsunska, N.; Markevich, Iryna; Stara, T.; Polishchuk, Yu.; Ponomaryov, S.; Kozoriz, K.; Tsybrii, Zinoviia; Melnichuk, O.; Melnichuk, L.; Venger, E.; Khomenkova, Larysa
    Undoped and Mn-doped ZnO ceramics were prepared from the powders compacted at different pressures and sintered in air at high temperature. Their structural, optical, light emitting and electrical characteristics as well as the distribution of chemical elements were studied. It was found that an increase in compacting pressure stimulates an increase in direct current conductivity in both undoped and doped samples. In the case of doped samples, this effect was accompanied by a decrease in the height of potential barriers at the grain boundaries. It is found that electron concentration in ceramic grains, estimated from the modelling of infrared reflection spectra, remained relatively constant. The analysis of luminescence spectra and spatial zinc distribution revealed that the increase in compacting pressure results in the accumulation of interstitial zinc at the grain boundaries forming channels with enhanced conductivity. These findings provide an explanation for the evolution of electrical properties of ceramic samples with compacting pressure.
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    Optical properties of Zn0.75Mg0.25O:Mn ceramics
    (2023) Markevich, I.; Korsunska, N.; Stara, T.; Polishchuk, Yu.; Vorona, I.; Kozoriz, K.; Ponomaryov, S.; Melnichuk, O.; Melnichuk, L.; Cremades, A.; Khomenkova, Larysa
    Mn-doped ZnO, MgO and Zn0.75Mg0.25O samples ([Mn] = 0.1 at.%) were produced by conventional solid-state technique and investigated by means of XRD, EPR, absorption, photocurrent, photo- and cathodoluminescence methods. It was shown that Zn0.75Mg0.25O solid solution with hexagonal structure has the bandgap of Eg ~3.65 eV. The quenching of host defect-related luminescence in ZnO:Mn and in hexagonal Zn0.75Mg0.25O:Mn was observed, while the Mn-related emission being absent. The energy level of Mn2Z+n center in hexagonal Zn0.75Mg0.25O:Mn was found to be at 2.16 eV below conduction band (c-band) bottom and all excited states of Mn2Z+n ions, including the lowest one, reside in c-band, as it takes place in ZnO:Mn. It is concluded that the necessary condition to obtain Mn-related light emission in Mn-doped alloys is to make deeper the lowest excited level of Mn2Z+n ions. One of the solutions is to produce Zn1-xMgxO:Mn solid solution with the bandgap energy larger than 4.0 eV using nonequilibrium fabrication approaches.
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    Peculiarities of photoluminescence excitation in ZnO ceramics doped with group-I elements
    (2022) Korsunska, N.; Markevich, I.; Stara, T.; Kozoriz, K.; Melnichuk, L.; Melnichuk, O.; Khomenkova, Larysa
    Extrinsic luminescence, excitation, and absorption spectra of ZnO ceramics doped with acceptors (lithium, copper, or silver), as well as undoped ZnO ceramics sintered in various atmospheres, have been studied. It is shown that the acceptor doping leads to the appearance of luminescence bands in the visible spectral interval, and their intensity significantly exceeds the intensity of the corresponding emission from undoped specimens. A selective maximum at 390–400 nm, which is usually absent in the excitation spectra of self-activated luminescence bands in undoped ZnO specimens, is found to dominate in the excitation spectra of those bands. It is supposed to be caused by the interaction between the emitting centers and defects arising near the impurities, with the Auger process being the most probable mechanism of energy transfer from these defects to the emitting centers. By sintering ZnO ceramics in the presence of carbon, it is shown that the appearance of the selective maximum in the excitation spectra occurs due to the extraction of oxygen from ZnO ceramics. An assumption has been done concerning the nature of the centers responsible for the excitation of extrinsic luminescence.