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.
- Diluted magnetic semiconductor
- Transition metal
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, 2, 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) , spin transistors , lasers , supercapacitor , among others.
DMS materials, when developed under the quantum confinement regime, form nanocrystals (NCs) with smaller dimensions than the bulk material . 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 .
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 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.
2. Diluted magnetic semiconductors in Nanopowders and embedded in glassy matrices
2.1 Synthesis of Nanopowders and nanocrystals and embedded in glassy matrices
ZnO NCs were synthesized by precipitation method using zinc nitrate and sodium hydroxide (NaOH, ≥98%) as precursors. In this work, the aqueous solution (1 M) of zinc nitrate (Zn(NO3)2. 6H2O, 98%) and the solution (2 M) of NaOH were mixed at room temperature. The NaOH solution was slowly added into zinc nitrate solution under vigorous stirring, which resulted in the formation of a white suspension. The white product was centrifuged at 6000 rpm for 5 min and washed several times with distilled water until the pH of the solution is around 7. The obtained samples were dried at 100°C for 24 hours. The samples were not subjected to temperatures above 100oC to avoid diffusion of Mn2+ ions from the nucleus to the surface of the ZnO NCs , since this is not the focus of this work. The ZnO:x Mn NCs were synthesized using the same procedure as ZnO NCs, but with the manganese (II) chloride (MnCl2, 98%) solution during the synthesis process. The xMn- concentration was determined based on the mass percentage of Zn present in ZnO (wt%), for x = 0.1; 0.3; 0.5; 0.7; 0.9; 1.0; 3.0; 5.0; 7.0 and 9.0. All reagents are nearly pure and purchased from Sigma-Aldrich Company.
Zn1-xMnxTe and Zn0.99-xMn0.01CoxTe were synthesized by fusion method in a glass matrix and annealed post-growth. The fusion method consists of two sequential melting-nucleation processes that produce ensembles of nearly spherical nanoparticles embedded in a glass matrix. In the first step, the PZABP glass matrix with a nominal composition of 65P2O5 · 14ZnO · 1Al2O3 · 10BaO · 10PbO (mol %) adding 2Te (wt %), and Mn and/or Co at doping
Co2+-doped Bi2S3 NCs were synthesized in a glass matrix with nominal composition of 45SiO2. 30Na2CO3.1Al2O3. 24B2O3 (mol%) adding 2S (wt%), 2Bi (wt%) and x = 0, 1 and 5% of Cobalt (Co), as a function of Bismuth (Bi) concentration. Samples were produced with the powder mixture added in an alumina crucible, and placed in a furnace at 1200 C for 30 min, followed by a fast cooling of the melted mixture down to room temperature, which formed a host glass. Thermal annealing was the next step: The previously-melted glass matrix, as-growth, was then heated at 500 C for 2, 10 and 24 h to enhance the diffusion of Bi3+ Co2+, and S2− ions precursors within the hosting matrix, resulting in the formation of DMS Bi2- xCoxS3 NCs.
2.2 A study in function of Mn concentration in ZnO nanocrystals: Solubility and Mn2+ ions localization
The zinc oxide (ZnO) is a semiconductor with several interesting physical and biological properties such as ultraviolet absorption and bactericidal and antitumor properties . Depending on the ions incorporated in the crystalline structure of ZnO nanocrystals (NCs) one can intensify or generate new and interesting properties. Dantas et al. demonstrated that doped ZnO nanocrystals’ bactericidal and antitumor properties are potentiated or inhibited with the concentration and type of dopant . The ZnO is a semiconductor of family II-VI that has a bandgap of 3.44 eV . Due to its wide bandgap range, ZnO is suitable for technological applications of photonic devices operating in the blue and ultraviolet region, fabrication of nanodevices electronics [16, 19].
The structural and optical properties are shown in Figure 1. The characteristic diffraction patterns of ZnO crystals with wurtzite structure (JCPDS n° 36–1451) were observed in the X-ray patterns of samples (Figure 1a). The presence of additional diffraction peaks for concentrations above 3.0 Mn (%wt) corresponding to the phase ZnMn2O4 (JCPDS n° 24–1133), indicating the formation of the second phase (open circles). The closed circles are identified by the aluminum sample holder. Thus, this new crystal formation is associated with the saturation of the substitutional and interstitial sites in the ZnO crystal structure.
In order to identify the symmetrical environment in which the Mn2+ ions may be found in the ZnO NCs, adjustments will be made to the optical absorption spectra of the samples (ZnO, 0.5Mn, 1.0Mn and 9Mn) shown in Figure 1(b). These adjustments are made in the positions of absorption of Mn2+ ions, making possible the use of the crystalline field theory (CFT) based on the crystalline field strength parameters ∆ and Racah B, both calculated with help of Tanabe– Sugano diagram (Figure 1(c)) . From the OA spectra of the NCs of Zn: xMn, based on the Tanabe-Sugano diagram, the energies of the characteristic electronic transitions of Mn 2+: 6A1 (6S) → 4T1 (4G) (691 nm), 6A1 (6S) → 4T2 (4G) (590 nm), 6A1 (6S) → 4T2 (4D) (482 nm), and 6A1 (6S) → 2E (2I) (410 nm) (Figure 1b) subtly permitted by spin-orbit coupling were effectively described by the Racah B parameter (559 cm−1) and the crystalline field division (∆ = 5464 cm−1) [21, 22].
The results from ∆ and B show that the Mn2+ dopants incorporated mainly at octahedral sites in the hexagonal structure of ZnO, for the sample with low concentration, and in the tetragonal structure of ZnMn2O4. Thus, the excited states of the Mn2+ in the binder field reside in the host semiconductor energy gap [21, 22].
Fluorescence spectra of the nanocrystals of ZnO: x Mn where the band at 570 nm are assigned to vacancies  are shown in Figure 1(d, e). Luminescence intensification and
As the concentration increases from 0.5 to 1.0 Mn, the band’s position remains, confirmed the half-height with xMn-concentration, which is related to the presence of the formation of the ZnMn2O4 NCs and that the Mn2 + are incorporated in the octahedral sites of the crystal structure saturation of the sites present in the ZnO NCs.
In order to visually check how these are coordination geometry, Figure 1(f) shows the wurtzite structure of ZnO NCs, which the Mn2+ ions are in tetrahedral sites (Td) and/or octahedral sites (Od).
2.3 Solubility of manganese and cobalt in ZnTe nanocrystals embedded in P2O5 - ZnO - Al2O3 - BaO - PbO glassy matrix
The incorporation of magnetic ions into the crystal structure of ZnTe semiconductor nanocrystals, which grow in glassy systems, has become very interesting due to the various applications in spintronics are governed by sp–d spin interaction between carriers and magnetic ions . Some possible applications require nanoparticles to be embedded in highly stable, robust, and transparent host materials, as glassy systems . The present study reports on the investigation of the solubility/saturation of Mn2+ ions in Zn1-xMnxTe [13, 26, 27] and competition between Co2+ and Mn2+ ions into Zn0.99-xMn0.01CoxTe [14, 28] nanosized DMS embedded in a glassy system with a wider Mn/Co concentration range (x = 0.000–0.800). According to the literature, the Mn2+/Co2+ ion can be thermodynamically incorporated into II-VI semiconductors up to its solubility limit [29, 30]. Above this limit, saturation occurs, and Mn2+/Co2+ ions tend to be expelled toward the semiconductor nanocrystal surface .
Figure 2 presents OA (a) and PL (c) spectra and photographs (b) of the samples containing Zn1-
The absorption band centered at 3.54 eV is attributed at MnO2, and the bands centered at 3.02, and 2.43 eV is attributed at MnO . The TEM images in Figure 2(e) (for x = 0.100), revealed interplanar distances corresponding to Zn0.9Mn0.1Te NCs (d ~ 0.347 nm) , MnO (d ~ 0.224 nm)  and MnO2 (d ~ 0.582 nm) NCs . This result can be confirmed by the PL spectra, on what the observed redshift shows that it was possible to tune Mn emission energy from orange to near-infrared as a function of concentration. These results confirm the successful inclusion of Mn2+ ions in the Zn1-
Figure 3 presents OA (a), PL (b) and EPR (c) spectra and photographs (between OA and PL spectra) of the samples containing Zn0.99-xMn0.01CoxTe NCs, with Co-concentrations ranging from
The emission centered at approximately 2.03 eV and 1.89 eV corresponds to tetrahedrally (Td) and octahedrally (Oh) coordinated Mn2+ ions. Already, when the spin-allowed transitions centred around 1.69 eV (4T1(4P) → 4A2(4F), named E1Co2+) and 1.52 eV (4T1(4P) → 4 T2(4F), named E2Co2+) . The energy diagram in Figure 3(d)shows these OA and PL spectra transitions. The EPR spectra, besides proving the substitutional incorporation of Mn2+ in Zn1-
2.4 Effects of symmetry and concentration of cobalt-doped Bi2S3 nanocrystals embedded in host glass matrix
Bi2S3 semiconductor nanocrystals (NCs) present radioactive recombination of electron–hole carriers adjustable according to the temperature, time, and concentration of doping ions in the system [15, 35, 36, 37]. The occupation of vacancies in the Bi2S3 orthorhombic network by Co2+ ions (0.72replacing Bi3+ ions (1.03can reduce intrinsic point defects and consequently increase the efficiency of semiconductor nanocrystals, for applications in solar cells , photocatalytic devices of visible light , thermoelectric  and spintronic . Therefore, the control of the magnetic saturation of the energy states located of the Co2+ ions in the Bi2S3 NCs bandgap allows assigning new magnetic and optoelectronic properties. In this context, we investigated the effect of the molar fraction and the coordination symmetry of Co2+ ions in cobalt-doped Bi2S3 NCs embedded in a host glass matrix (SNAB - 45SiO2 · 30Na2CO3 · 5Al2O3 · 20B2O3) (mol%).
Figure 4(a) shows the UV–VIS–NIR optical absorption spectrum (OA) of the Bi2-xCoxS3 NCs embedded in a SNAB glass matrix, at temperature (300 K), as a function of the increasing concentration of doping xCo ions (x = 0.000; 0.005; 0.010; 0.050; 0.100). The OA spectrum of the SNAB matrix is transparent in the spectral region where the Bi2-xCoxS3 NCs can absorb and emit a photon. The OA spectrum of the samples shows energy bands of the exciton charge carrying states and the d-d transitions of the Co2+ ions in the Bi2-xCoxS3 NCs. The structure of the absorption bands is due to that of the ligand field of S2− ions in distorted tetrahedral sites around the Co2+ ions. Such bands of electronic level transitions have increased due to the increasing concentration of Co2+ ions in the Bi2S3 NCs structure.
Furthermore, the p-d and spin-orbit interactions in an asymmetric tetrahedral coordination complex [CoS4]6− provide a higher intensity for 3d-3d transitions [14, 15, 20, 42]. Therefore, the exotic properties of these new materials called diluted magnetic semiconductors (DMS) NCs are linked to the nature of the sp-d exchange interaction that occurs between the charge carriers of the Bi2S3 semiconductor bands around the electrons in the central metal ion orbital (Co2+) with 3d7 electronic configuration (spin) 3d7 (S = 3/2) [15, 20, 36, 42].
For Bi2-xCoxS3 NCs, the spin allowed transitions 4A2 (4F) → 4T1 (4F) and 4A2 (4F) → 4T1 (4P) are obtained with the energy value of the barycenter for a normal distribution of the bands observed in the spectrum absorption in the range of 0.62–1.24 eV and 1.70–2.60 eV. The bands observed in the OA spectrum of Figure 4(a) correspond to the spin allowed and forbidden transitions, 4A2 (4F) → 4T1 (4F) (0.84 eV), 4A2(4F) → 2E (2G) (1.93 eV), 4A2(4F) → 2T1(2G) (1.99 eV), 4A2(4F) → 4T1(4P) (2.08 e V), 4A2(4F) → 2A1(2G) (2.19 eV) and 4A2(4F) → 2T2(2G) (2.29 eV). Such identified 3d-3d transitions are attributed to the crystal field strength (∆ = 3882 cm−1) and the electronic repulsion parameter (Racah, B = 772 cm−1), based on the crystal field theory and the Tanabe-Sugano diagram d7, tetrahedral for C/B = 4.5 [20, 43, 44]. The wide absorption band in the infrared is due to a spin-orbit coupling interaction that split the excited state 4T1 (4F) into three energy sub-states:(0.97 eV),(0.84 eV) e (0.72 eV) [12, 13]. Therefore, the excited states of the Co2+ ions are located in the energy gap (2.65 eV) of electrons and holes confined between the ground state and the conduction band of the Bi2-xCoxS3 NCs. The energy level diagram of Co2+ ions (3d7) in a ligand field of S2− ions at tetrahedral coordination sites [CoS4]6− is shown in the inset in Figure x (a). The energy band associated with the spin allowed transition 4A2 (4F) → 4T1 (4F) (0.48 eV) does not appear in the OA spectrum (Figure 4a) due to the low intensity and energy [15, 43].
The redshift observed at the OA band edge for the Bi2-xCoxS3 NCs with increasing concentration xCo, from x = 0.000 (2.75 eV) to x = 0.100 (2.56 eV) (see Figure 4a), are consequences of sp-d exchange interactions between sp. electrons confined in states of quantum dots and localized states partially filled with 3d electrons of the Co2+ ions . No significant changes occur in the quantum size of the Bi2-xCoxS3 NCs with the incorporation of Co. The TEM image (Figure 4c) shows an average size of 6 nm for Bi2-xCoxS3 NCs (x = 0.100) with quantum dot properties. The interplanar distance (d211 = 0.310 nm) shows the crystal plane (211) characteristic of the orthorhombic structure of the mineral Bismuthinite. Figure 4(b) illustrates the Bi2S3 orthorhombic structure unit cell doped with Co2+ ions at distorted tetrahedral sites and the respective Bi2-xCoxS3 quantum dots function of the xCo concentration.
Figure 4(d) shows the EPR spectra obtained in the best conditions at 77 K and 9.75 GHz for samples of Bi2-xCoxS3 NCs embedded in SNAB glass matrix, with xCo concentration ranging from x = 0.005 to 0.100. The intense central signal at (g ≈ 2.005; ΔH ≈ 8 mT) corresponds to free Co+2 ions, dispersed in the glass matrix and not incorporated in the Bi2S3 NCs [2, 10]. The inset in Figure x (d) presents a diagram with the allowed hyperfine lines Δ
The synthesis and investigation of the DMS NC have allowed the production of new types of materials with possible technological applications in Engineering, Medicine, Environment, Telecommunications, and others. We report the synthesis of the Zn1-xMnxO nanopowders and the Bi2-xCoxS3, Zn1-xMnxTe Zn0.99-xMn0.01CoxTe NCs in glass by fusion method. The formation of DMS NCs is observed by SEM and MET images. The EPR spectra confirm the incorporation of TR2+ ions (Mn2+ and Co2+) in the doped nanocrystals’ crystalline structure. The optical properties of these materials were investigated by photoluminescence and Uv–Vis spectroscopy techniques, which show the influence of TR2+ ions (Mn2+ and Co2+) on the visible spectrum, altering the optical absorption and photoluminescence bands of undoped naocrystals. In this work, we hope to contribute significantly to the study of DMS NCs and their possible technological applications for society’s benefit.