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    Screened Coulomb hybrid density functional investigation of oxygen point defects on ZnO nanowires
    (Elsevier, 2018) Celik, Veysel; Mete, Ersen
    In this study, oxygen vacancies and adatoms have been considered on the surface of both hexagonal anssd triangular ZnO nanowires. Their effect on the electronic structure and optical spectra of the nanowires have been investigated using the exact exchange hybrid density functional theory calculations. A surface oxygen vacancy gives rise to appearance of a band gap state at almost 0.7 eV above the valence band of the both types of the nanowires while an oxygen adatom show bulk-like electronic properties. A shape dependence is also indicated by the calculated physical quantities of oxygen related point defects on ZnO nanowires. (C) 2018 Elsevier B.V. All rights reserved.
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    Theoretical investigation of the role of the organic cation in methylammonium lead iodide perovskite
    (Springer, 2023) Celik, Veysel
    The hybrid halide perovskite CH3NH3PbI3 is easy to manufacture and inexpensive. Despite these, its efficiency as a solar cell is comparable to today's efficient solar cells. For these reasons, it is attracting a lot of attention today. However, the effects of the CH3NH3+ (MA) cation in the perovskite structure on the electronic and structural properties are still a matter of debate. Previous studies have generally focused on the rotation of the MA cation. In this study, from a different perspective, the effects of the movement of the MA cation along the C-N axis are investigated. With this method, the effects of the MA cation were examined in a more controlled way. In this study, density functional theory that accounts for van der Waals interactions was used in the calculations for the cases. According to the data obtained, H-I bonds are formed between the MA cation and the inorganic framework. Although these bonds are predominantly hydrogen bonds, they also have ionic bond characteristics. Within the structure, the H-I-bond length tends to be preserved, although the position of the MA changes. In this mechanism, the I ion plays an important role by moving away from its place in the Pb-I-Pb alignment. The position of the I ion determines the nature of the band-gap transition. Another effect is on the value of the band gap. Depending on the position of the I ion, the band gap may narrow by about 0.26 eV. The separation of the I ion from the Pb-I-Pb alignment by the effect of the MA cation breaks the inverse symmetry. According to the data obtained from this study, this mechanism in the band gap is due to the breaking of the inverse symmetry in the crystal structure.
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    Öğe
    Use of local density approximation within range separated hybrid exchange-correlation functional to investigate Pb doped SnO2
    (Elsevier, 2021) Celik, Veysel
    In this study, local density approximation (LDA) was used instead of Perdew-Burke-Ernzerhof (PBE) within the standard Heyd-Scuseria-Ernzerhof (HSE) functional. Using a 29% mixing ratio, the electronic and structural properties of SnO2 were calculated in good agreement with the experimental data. Using this method, Pb doped SnO2 was investigated. The 12.5% Pb doping ratio narrows the band gap of SnO2 from 3.60 to 3.02 eV. The main reason for the band gap narrowing is the decrease in the conduction band minimum (CBM). The decrease in CBM is proportional to the Pb doping ratio. Doping with Pb does not cause a significant change in the valence band and the effective mass of electrons and holes. According to these results, doping with Pb can be a useful method to adjust the CBM of SnO2, which is used as an electron transport layer between the electrode and perovskite in perovskite solar cells.