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Developing diluted magnetic semiconductors (DMSs) is an essential step toward the realization of spintronic devices where both the charge and the spin of the electrons are manipulated. However the mechanism governing the magnetic interactions in these materials challenges our understanding of magnetism in solids. This challenge mainly lies in the complex interplay between magnetic ions of the DMSs and the intrinsic and extrinsic defects of the host materials. Another contributing factor is the inhomogeneous distribution of the doped magnetic ions in the host materials which leads to the unintentional formation of hetrostructures and secondary phases which adversely affect the magnetic properties of DMSs. In this dissertation, the magnetic interactions in the DMSs based on Mn, Co and Eu doped ZnO were studied using density functional theory methodology. Additionally, the effect of most common point defect in ZnO, i.e. oxygen vacancy and hydrogen impurity in Co doped ZnO oxides was considered. It was found that in the Mn and Co doped ZnO, there is a strong tendency among the magnetic ions to aggregate via oxygen and form variety of highly stable M-O-M (M stands for transition metals) complexes. Such aggregation was not found to occur in Eu doped ZnO which implies more homogeneous doping pattern for Eu ions and possible achievement of intrinsic ferromagnetism in Eu doped ZnO. Magnetically, it was found that in all Co, Mn and Eu doped ZnO, no ferromagnetism can be induced without an aid from another agent. Specifically, p-type carrier doping by nitrogen proved to establish carrier mediated ferromagnetism in both Mn and Co doped ZnO. Hydrogen contamination in both substitutional and interstitial sites could also open a channel for long range spin-spin interaction and led to room temperature ferromagnetism in Co doped ZnO. However the most stable defect in ZnO, oxygen vacancy did not contribute significantly to magnetism in Co doped ZnO. In Co doped ZnO, Eu codoping resulted in room temperature ferromagnetic alignment between Co and Eu ions and thus enhanced magnetization. As a result Eu and Co codoped ZnO was found to be the potential candidate for technological applications.

Layered double hydroxides (LDHs) constitute a unique group of 2D materials that can deliver exceptional catalytic, optical, and electronic performance. However, they usually suffer from low stability compared to their oxide counterparts. Using density functional calculations, we quantitatively demonstrate the crucial impact of the intercalants (i.e., water, lactate, and carbonate) on the stability of a series of common LDHs based on Mn, Fe, and Co. We found that intercalation with the singly charged lactate results in higher stability in all these LDH compounds, compared to neutral water and doubly charged carbonate. Furthermore, we show that the dispersion effect aids the stability of these LDH compounds. This investigation reveals that certain intercalants enhance LDH stability and alter the bandgap favourably., Scientific and Technological Research Council of Turkey (TÜBİTAK); European Union (EU); Horizon 2020; European Commission (EC); Co-Funded Brain Circulation Scheme; COFUND; Marie Curie Action ; 7th. Framework Programme (FP7)