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    Interplay between iridium doping and oxygen vacancies in ceria: Experimental and theoretical investigation of optical and electronic properties
    (Elsevier Sci Ltd, 2025) Aktas, Seda; Yildirim, Yucel; Polat, Ozgur; Sobola, Dinara; Sen, Cengiz; Demiroglu, Arsen; Caglar, Mujdat
    Cerium oxide (CeO2) films doped with iridium (Ir) were synthesized using a sol-gel spin-coating technique and systematically characterized by X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) to investigate changes in their optical and electronic properties. Complementary first-principles calculations based on the DFT + U + V approach were employed to elucidate the role of Ir doping and oxygen vacancies in CeO2's structural and electronic behavior. Theoretical and experimental findings show that Ir atoms preserve localized electronic states without significantly disturbing the surrounding charge distribution, even in the presence of oxygen vacancies. In contrast, oxygen vacancies strongly affect nearby Ce atoms, promoting their reduction to Ce3+ states while increasing the formation of additional vacancies. The combined effect of Ir doping and oxygen vacancies results in a reduced crystallite size, lattice expansion, and substantial band gap narrowing, thereby enhancing visible-light absorption and refractive index modulation. Photocatalytic activity in the UV region is diminished when Ir or oxygen vacancies are introduced individually; however, their combined presence improves performance at longer wavelengths within the visible spectrum. This study highlights how the interaction between Ir doping and oxygen vacancies can be used to tailor the redox and optical behavior of ceriabased systems, while also demonstrating the importance of accurate beyond-DFT modeling in describing correlated oxides.
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    Structural and optical evolution in CeO2 films induced by aluminum doping: A comprehensive study
    (Elsevier Sci Ltd, 2025) Bagcivan, Aybike; Aktas, Seda; Yildirim, Yucel; Polat, Ozgur; Sobola, Dinara; Sen, Cengiz; Caglar, Mujdat
    This study investigates the impact of aluminum (Al) doping and oxygen vacancies on the structural, electronic, and optical properties of Cerium Oxide (CeO2) films. The films were fabricated on glass substrates using the solgel spin coating method for both undoped and Al doped CeO2. Theoretical insights from the DFT + U + V method further explore how Al doping and oxygen vacancies influence the electronic structure and optical behavior of ceria. Experimentally, Al doping was found to increase the lattice parameters and reduce the crystallite size without forming secondary phases. Raman spectroscopy revealed a shift to lower wavenumbers in Al-doped samples, while XPS analysis confirmed the presence of both Ce3+ and Ce4+ oxidation states, along with Al3+ and O2- ions. Optical measurements showed a decrease in the optical band gap from 3.14 eV to 2.93 eV as the Al concentration increased to 5 %. Additionally, the optical dielectric constant slightly decreased, whereas optical conductivity improved with the incorporation of aluminum. Theoretical calculations show that oxygen vacancies were shown to reduce the band gap by introducing localized Ce3+ mid-gap states, while Al doping led to a gradual narrowing of the band gap without creating new states within it. The combination of Al doping and oxygen vacancies resulted in both further band gap narrowing and the appearance of mid-gap states. Optical property calculations revealed that both oxygen vacancies and Al doping reduced the intensity of the dielectric functions at 310 nm. Moreover, these two factors had opposing effects on the electron energy loss spectrum (EELS) at low energies: Al doping induced high-intensity peaks, while oxygen vacancies diminished these peaks.