Кафедра фізико-математичних наук

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Browsing Кафедра фізико-математичних наук by Author "Isaieva, Oksana"

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    Effect of dopant loading and calcination conditions on structural and optical properties of ZrO2 nanopowders doped with copper and yttrium
    (2024) Khomenkova, Larysa; Marchylo, Oleg; Polishchuk, Yulia; Ponomaryov, Semyon; Isaieva, Oksana; Vorona, Igor; Melnichuk, Liudmyla; Portier, Xavier; Melnichuk, Olexandr; Korsunska, Nadiia
    Undoped, Cu and/or Y doped ZrO2 nanopowders were synthesized with Zr, Y, and Cu nitrates using a co-precipitation approach. Their structural and optical properties were examined regarding dopant content (0.1–8.0 mol.% of CuO and 3–15 mol.% of Y2O3) and calcination conditions (400 °C– 1000 °Cand, 1,2 or 5 h) through Raman scattering, XRD, TEM, EDS, AES, EPR,UV–vis and FTIR diffused reflectance methods. The results showed that both Cu and Y dopants promoted the appearance of additional oxygen vacancies in ZrO2 host, while the formation of tetragonal and cubic ZrO2 phases was primarily influenced by the Y content, regardless of Cu loading. The bandgap of most of the powders was observed within the 5.45–5.65 eV spectral range, while for those with high Y content it exceeded 5.8 eV. The (Cu,Y)-ZrO2 powders with 0.2 mol.% CuO and 3 mol.% Y2O3 calcined at 600 °Cfor 2 h demonstrated nanoscaled tetragonal grains (8–12 nm) and a significant surface area covered with dispersed CuxOspecies. For higher calcination temperatures, the formation of CuZr 2+ EPR centers, accompanied by tetragonal-to-monoclinic phase transformation, was found. For fitting of experimental FTIR reflection spectra, theoretical models with one, five, and seven oscillators were constructed for cubic, tetragonal, and monoclinic ZrO2 phases, respectively. Comparing experimental and theoretical spectra, the parameters of various phonons were determined. It was found that the distinct position of the high-frequency FTIR reflection minimum is a unique feature for each crystalline phase. It was centered at 700–720 cm−1, 790–800 cm−1, and 820–840 cm−1 for cubic, tetragonal, and monoclinic phases, respectively, showing minimal dependence on phonon damping coefficients. Based on the complementary nature of results obtained from structural and optical methods, an approach for monitoring powder properties and predicting catalytic activity can be proposed for ZrO2–based nanopowders.
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    Photoluminescence quantum yield of carbon dots: emission due to multiple centers versus excitonic emission
    (2024) Dimitriev, Oleg; Kysil, Dmytro; Zaderko, Alexander; Isaieva, Oksana; Vasin, Andrii; Piryatinski, Yuri; Fahlman, Mats; Nazarov, Alexei
    Carbon dots (CDs) are recognized as promising fluorescent nanomaterials with bright emission and large variations of photoluminescence quantum yield (PLQY). However, there is still no unique approach for explanation of mechanisms and recipes for synthetic procedures/chemical composition of CDs responsible for the enhancement of PLQY. Here, we compare photophysical behavior and PLQY of two types of CDs synthesized by different routes, leading to the different extent of oxidation and composition. The first type of CDs represents a conjugated carbon system oxidized by F, N and O heteroatoms, whereas the second type represents a non-conjugated carbon system oxidized by oxygen. Photophysical data, photoemission spectroscopy and microscopy data yield the suggestion that in the first case, a structure with a distinct carbon core and highly oxidized electron-accepting shell is formed. This leads to the excitonic type non-tunable emission with single-exponent decay and high PLQY with a strong dependence on the solvent polarity, being as high as 93% in dioxane and as low as 30% in aqueous medium, but which is vulnerable to photobleaching. In the second case, the oxidized CDs do not indicate a clear core–shell structure and show poor solvatochromism, negligible photobleaching, low PLQY varying in the range of 0.7–2.3% depending on the solvent used, and tunable emission with multi-exponent decay, which can be described by the model of multiple emission centers acting through a clustering-triggered emission mechanism. The obtained results lead to a strategy that allows one to design carbon nanomaterials with principally different PLQYs that differ by orders of magnitude.