ELECTRONIC AND MAGNETIC PROPERTIES OF SPINEL Co 3 O 4 (111) SURFACE IN GGA + U APPROXIMATION PACS 73.20.At, 75.70.Rf The atomic structure and electronic properties of the spinel Co3O4 (111) surface are calculated within the methods of density functional theory. Possible types of the surface are analyzed, and their formation energies are calculated. Electron states formed at the surface by broken bonds are studied in detail, and their partial density of states is calculated. It is shown that, unlike the bulk of spinel, its surface has nontrivial magnetic properties, because Co atoms acquire an additional magnetic moment under near-surface conditions. K e y w o r d s: spinel, cobalt oxide, magnetic surface structure.

Structural and electronic characteristics of neutral and charged vacancies of cadmium and sulfur in CdS nanocrystals have been performed using the density functional method with hybrid exchange-correlation functional. Total and partial density of states, formation energies and the energies of thermodynamic transitions were calculated. Based on these theoretical findings and available experimental data, we can confirm the assumption, that the singly charged vacancies of cadmium are the centers of radiative recombination in such the structures.

We have studied the structural, electronic and magnetic properties of spinel Co 3 O 4 (111) surfaces and their interfaces with ZnO (0001) using density functional theory (DFT) within the Generalized Gradient Approximation with on-site Coulomb repulsion term (GGA+U). Two possible forms of spinel surface, containing Co 2+ or Co 3+ ions and terminated with either cobalt or oxygen ions were considered, as well as their interface with zinc oxide. Our calculations demonstrate that Co 3+ ions attain non-zero magnetic moments at the surface and interface, in contrast to the bulk, where they are not magnetic, leading to the ferromagnetic ordering. Since heavily Co-doped ZnO samples can contain Co 3 O 4 secondary phase, such a magnetic ordering at the interface might explain the origin of the magnetism in such diluted magnetic semiconductors (DMS). PACS numbers: 73.20.-r, 75.70.-i Acknowledgments 16References 17 I. INTRODUCTIONMagnetic semiconductors (MS) and diluted magnetic semiconductors (DMS) exhibit both ferromagnetic and semiconducting properties. Therefore, they are promising materials for spintronics, which utilizes for information processing not only the electron charge but also its spin. Historically, the first DMS with a high Curie temperature up to about 200 K was GaAs doped with Mn ions. 1,2 In that compound, the ferromagnetism is promoted by hole carriers, which align along the local Mn magnetic moments and called carrier-induced ferromagnetism or Zener p − d exchange. It is crucial for this mechanism that Mn at the Ga site becomes Mn 2+ instead of Ga 3+ , thus providing at the same time a local spin and a hole charge carrier. Extension of the mechanism, proposed in a very influential paper 3 of Dietl and co-workers, allows a prediction that the above room-temperature ferromagnetism in ZnO:Co and GaN:Mn is due to the same carrier-induced mechanism. This would be responsible for the ferromagnetism with a sufficiently high number of hole charge carriers.First experiments after that prediction 4 seemed to confirm the mechanism proposed and has also been supported by ab-initio calculations. 5 However, it soon turned out that the Co impurity is in fact isovalent to the Zn ion 6 and provides no charge carriers at all, while the situation in GaN:Mn is similar. 7We are going to concentrate here on ZnO:Co, where the experimental reports demon-2 strate that the above room-temperature ferromagnetism in ZnO:Co persist. Even though its origin is still not clarified, there are clear indications in more recent experiments that the magnetism in the ZnO:Co system is attributed to the formation of the Co 3 O 4 phase in ZnO. 8-12 Therefore, we will focus here on the role of the Co 3 O 4 phase, although several attempts to explain the mechanism of the ferromagnetism in realistic ZnO:Co systems exist including, for instance, spinodal decomposition 13 or Lieb-Mattis ferrimagnetism, 14 to cite just two ideas. The typical doping level of Co in ZnO can be relatively high (in the range between 10% and 30%). This leads to the secondar...

Дефекти у квантових точках можуть впливати на їх оптичні характеристики, створюючи додаткові канали випромінювальної та безвипромінювальної рекомбінації. Методом функціоналу густини проведено розрахунки структурних та електронних характеристик вакансій металу (кадмій, цинк) та міжвузлової сірки, як найбільш імовірних типів дефектів у нанокристалах Cd 1-x Zn x S. Розраховано повну та парціальну густину електронних станів, проведено аналіз структурної релаксації. Виходячи з отриманих теоретичних результатів та наявних експериментальних даних зроблено висновок, що з усіх розглянутих типів дефектів лише вакансії кадмію можуть бути центрами випромінювальної рекомбінації у квантових точках Cd 1-x Zn x S. Ключові слова: квантові точки, дефекти, вакансія, CdZnS, метод функціоналу густини NATIVE DEFECTS IN CdZnS QUANTUM DOTS

Structural and electronic characteristics of metal vacancies in nanocrystals Cd 1-x Zn x S have been studied within the density functional method. The electron state density and vacancy formation energy have been calculated, and an analysis of structural relaxation has been performed. With these theoretical findings and available experimental data, we have concluded that the vacancies of cadmium are the possible centers of radiative recombination in these structures.