Yazar "Dahnovsky, Yuri" seçeneğine göre listele

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    The effect of spatial distribution of Zn vacancies in ZnS quantum dots on optical absorption spectra
    (2017-04-05) Pimachev, Artem; Proshchenko, Vitaly; Horoz, Sabit; Sahin, Omer; Dahnovsky, Yuri
    In this work we propose how to identify the location and spatial distribution of Zn vacancies in ZnS quantum dots (QDs). The experiments reveal a peak shift in absorption spectra towards the lower energies (about 0.1 eV) in the presence of Zn vacancies. To explain such a shift we computationally study the absorption spectra when Zn vacancies are localized in the core, on the surface, and both in the core and on the surface of a quantum dot. From the comparison with the experimental data we conclude that the most favorable configuration of the vacancies is an aggregation on a QD surface. We find that Zn vacancies localized in the core or uniformly distributed on the QD surface, do not explain the experimentally observed shift in the absorption curves.
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    Room temperature d0 ferromagnetism in ZnS nanocrystals
    (2016-05-27) Proshchenko, Vitaly; Horoz, Sabit; Tang, Jinke; Dahnovsky, Yuri
    Room temperature ferromagnetic semiconductors have a great deal of advantage because of their easy integration into semiconductor devices. ZnS nanocrystals (NCs), bulk, and surfaces exhibit d0 ferromagnetism at room temperature. The experiments reveal that NC ferromagnetism takes place at low and room temperatures only due to Zn vacancies (S vacancies do not contribute). To understand the mechanism of d0 ferromagnetism, we introduce the surface-bulk model of a nanocrystal, which includes both surface and bulk magnetizations. The calculations demonstrate that the surface has the higher than bulk magnetization. We find the mechanism of the ferromagnetism is due to sulfur s- and p-electrons in a tetrahedral crystal field. The bulk magnetic moment increases with Zn vacancy concentration at small concentrations and then goes down at larger concentrations. A surface magnetic moment behaves differently with the concentration. It is always a monotonically rising function. We find that the total NC magnetic moment increases with the size and concentration of Zn vacancies (only low concentrations). We also study the magnetization per unit cell where we find that it decreases for the surface and increases for bulk magnetism with the NC size.