**Abstract**

Diluted Magnetic Semiconductor (DMS) nanocrystals are a new class of materials formed by doping the semiconductor with transition metals (TM), which gives interesting magneto-optical properties. These properties are attributed to the exchange interaction between the pure semiconductor's sp-electrons and the localized TM d-electrons. This book chapter shows exciting results of new DMS developed by the group, both in powder form and embedded in glassy systems. Depending on the concentration of doping ions, saturation of the incorporation of substitutional and interstitial sites in the nanocrystal structure may occur, forming other nanocrystals. In this context, we investigated the doping saturation limit in nanopowders of DMS Zn1-xMnxO NCs and Zn1-xMnxTe, Zn0.99-xMn0.01CoxTe, and Bi2-xCoxS NCs synthesized in glassy matrices. Thus, the sites' saturation into the crystalline lattice of nanocrystals is a topic little reported in the literature, and we will comment on this work. Therefore, we will show results from the group about the modulation and saturation in diluted magnetic semiconductors nanocrystals in this work.

**Keywords:** Diluted magnetic semiconductor, Nanocrystals, Saturation, Modulation, Doping, Transition metal

### **1. Introduction**

The doping of semiconductors with small concentrations of transition metals (TM) ions, such as Co2+, Cr3+, Fe3+, Mn2+, allows the formation of new types of materials that have interesting spin-dependent electrical, optical, magnetic, and structural properties [1–3]. These new properties are attributed to the sp-d exchange interactions that involve the d-sub-levels of transition metal (TM) ions and the sp-electrons of the conduction band and/or holes in the host semiconductor valence band [4, 5]. These materials are called diluted magnetic semiconductors (DMS) and present great possibilities for technological applications such as the production of light-emitting diodes (LEDs) [6], spin transistors [7], lasers [8], supercapacitor [9], among others.

DMS materials, when developed under the quantum confinement regime, form nanocrystals (NCs) with smaller dimensions than the bulk material [10]. These DMS NCs have chemistry and physical properties dependent on their shape and size. These materials are obtained from a controlled process known as thermal diffusion of precursor ions to form DMS NC under the requirement of thermodynamic equilibrium.

The substitutional and interstitial sites' saturation in the nanocrystals structure may occur depending on the dopant concentration, and other nanocrystals types are formed. This system is called a nanocomposite and can have several exciting physical, chemical, and biological properties [11].

In this context, we investigated the doping saturation limit in nanopowders of DMS Zn1-xMnxO NCs [12] and Zn1-xMnxTe [13], Zn0.99-xMn0.01CoxTe [14], and Bi2-xCoxS [15] NCs synthesized in glassy matrices by the fusion method. The properties of the nanomaterials were investigated by experimental techniques of photoluminescence (PL), UV–Vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (MET). The theoretical study applying the crystalline field theory and Uv–Vis spectroscopy data allows identifying the tetrahedral (Th) or octahedral (Oh) location that the TM ions occupy in the crystalline structure of the semiconductor nanocrystal.
