Diluted Magnetic Semiconductors Nanocrystals: Saturation and Modulation

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 Zn 1-x Mn x O NCs and Zn 1-x Mn x Te, Zn 0.99-x Mn 0.01 Co x Te, and Bi 2-x Co x S 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. We report the synthesis of the Zn 1-x Mn x O nanopowders and the Bi2-xCoxS3, Zn1-xMnxTe Zn 0.99-x Mn 0.01 Co x Te 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 TR 2+ ions (Mn 2+ and Co 2+ ) 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 TR 2+ ions (Mn 2+ and Co 2+ ) on the visible spectrum, altering the optical absorption and photoluminescence bands of undoped naocrystals. In this work, we hope to contribute signifi-cantly to the study of DMS NCs and their possible technological applications for society’s benefit.


Introduction
The doping of semiconductors with small concentrations of transition metals (TM) ions, such as Co 2+ , Cr 3+ , Fe 3+ , Mn 2+ , 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) [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 Zn 1-x Mn x O NCs [12] and Zn 1-x Mn x Te [13], Zn 0.99-x Mn 0.01 Co x Te [14], and Bi 2-x Co x S [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.

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 [15], 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.
Zn 1-x Mn x Te and Zn 0.99-x Mn 0.01 Co x Te 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 65P 2 O 5 · 14ZnO · 1Al 2 O 3 · 10BaO · 10PbO (mol %) adding 2Te (wt %), and Mn and/or Co at doping x content varying with Zn content from 0 to 80 (wt %), were synthesized by fusion in alumina crucibles at 1300°C for 30 minutes. Next, these melted mixtures were quickly cooled to room temperature forming a glass system doped with the precursor ions needed for nanoparticle growth. In the second step, the glass samples were thermally annealed at 500°C for 10 hours to enhance the diffusion of Zn 2+ , Mn 2+ and/or Co 2+ and Te 2− ions throughout the host PZABP matrix and induce the growth of Zn 1-xMn x Te/Zn 0.99x Mn 0.01 Co x Te NCs. The physical properties of the glass samples were studied by optical absorption (OA), recorded with a model UV-3600 Shimadzu UV-VIS-NIR spectrometer, operating between 190 and 3300 nm; Photoluminescence (PL), using a 405 nm (~ 3.06 eV) continuous wave laser; Transmission electron micrographs (TEM JOEL, JEM-2100, 200 kV) and electron paramagnetic resonance (EPR), using a high sensitivity Bruker ESP-300 spectrometer (operating at X-band ~9.75 GHz and at Q-band ~34GHz).
Co 2+ -doped Bi 2 S 3 NCs were synthesized in a glass matrix with nominal composition of 45SiO 2 . 30Na 2 CO 3 .1Al 2 O 3 . 24B 2 O 3 (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 Bi 3+ Co 2+ , and S 2− ions precursors within the hosting matrix, resulting in the formation of DMS Bi 2-x Co x S 3 NCs.

A study in function of Mn concentration in ZnO nanocrystals: Solubility and Mn 2+ 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 [16]. 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 [17]. The ZnO is a semiconductor of family II-VI that has a bandgap of 3.44 eV [18]. 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°  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 ZnMn 2 O 4 (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 Mn 2+ 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 Mn 2+ 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)) [20]. 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+ : 6 A 1 ( 6 S) → 4 T 1 ( 4 G) (691 nm), 6 A 1 ( 6 S) → 4 T 2 ( 4 G) (590 nm), 6 A 1 ( 6 S) → 4 T 2 ( 4 D) (482 nm), and 6 A 1 ( 6 S) → 2 E ( 2 I) (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 Mn 2+ dopants incorporated mainly at octahedral sites in the hexagonal structure of ZnO, for the sample with low concentration, and in the tetragonal structure of ZnMn 2 O 4 . Thus, the excited states of the Mn 2+ 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 [23] are shown in Figure 1(d, e). Luminescence intensification and blue shift to a concentration of 0.5 Mn. This result confirms the incorporation of Mn +2 ions into the tetrahedral sites of ZnO NCs (inside nanocrystal). However, there is a contribution to incorporating Mn 2+ ions into the octahedral sites (surface nanocrystal). It is noteworthy that the band widening around 570 nm is due an overlapping characteristics bands of oxygen vacancies and d-d ( 4 T 1 ← 6 A 1 ) of Mn 2+ -ions in ZnO NC host [23,24].
As the concentration increases from 0.5 to 1.0 Mn, the band's position remains, confirmed the half-height with x Mn -concentration, which is related to the presence of the formation of the ZnMn 2 O 4 NCs and that the Mn 2 + 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 Mn 2+ ions are in tetrahedral sites (Td) and/or octahedral sites (Od).

Solubility of manganese and cobalt in ZnTe nanocrystals embedded in P 2 O 5 -ZnO -Al 2 O 3 -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 [25]. Some possible applications require nanoparticles to be embedded in highly stable, robust, and transparent host materials, as glassy systems [26]. The present study reports on the investigation of the solubility/saturation of Mn 2+ ions in Zn 1-x Mn x Te [13,26,27] and competition between Co 2+ and Mn 2+ ions into Zn 0.99-x Mn 0.01 Co x Te [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 Mn 2+ /Co 2+ ion can be thermodynamically incorporated into II-VI semiconductors up to its solubility limit [29,30]. Above this limit, saturation occurs, and Mn 2+ /Co 2+ ions tend to be expelled toward the semiconductor nanocrystal surface [30]. Figure 2 presents OA (a) and PL (c) spectra and photographs (b) of the samples containing Zn 1-xMn x Te NCs, with Mn-concentrations ranging from x = 0.000 to x = 0.800, and with PZABP (65P 2 O 5 · 14ZnO · 1Al 2 O 3 · 10BaO · 10PbO (mol %)) glassy matrix doped only 80Mn (% wt of Zn). The OA spectra show absorption bands centered attributed to Zn 1-xMn x Te NCs (quantum dots (QDs) and bulk NCs) and the incorporation of Mn 2+ ions. This substitutional incorporation is confirmed from the EPR spectra (Figure 2(f)) that showed six lines associated to S = 5/2 spin half-filled d-state, characteristic of Mn 2+ ions (as detailed in the energy diagram) [26]. For x > 0, additional bands are observed.
The absorption band centered at 3.54 eV is attributed at MnO 2, and the bands centered at 3.02, and 2.43 eV is attributed at MnO [31]. The TEM images in Figure 2(e) (for x = 0.100), revealed interplanar distances corresponding to Zn 0.9 Mn 0.1 Te NCs (d ~ 0.347 nm) [32], MnO (d ~ 0.224 nm) [33] and MnO 2 (d ~ 0.582 nm) NCs [34]. 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 Mn 2+ ions in the Zn 1-xMn x Te up to the nominal solubility limit of x = 0.100. Above this solubility limit, one can observe Mn′s saturation, forming MnO and MnO 2 NCs, as represented in Figure 2(d). Thus, one of the main motivations for studying semiconductors doped with transition metals, especially Mn 2+ ions, is for applications in luminescent devices. shows the transitions satisfying the selection rules ΔM S = ±1 and ΔM I = 0. The AO spectra, besides the bands attributed to Zn 1-xMn x Te QDs and bulk NCs, as previously presented in Figure 3, four characteristic bands in the visible spectrum for all doped samples with cobalt. These four are due to the spin-allowed transitions: 4 A 2 (F) → 2 T 1 (P) (2.13 eV); and spin-forbidden transitions: 4 A 2 (F) → 2 A 1 (G) (2.33 eV), 4 A 2 (F) → 4 T 1 (G) (1.95 eV) and 4 A 2 (F) → 2 E(G) (1.91 eV) [14]. These bands show the substitutional incorporation of Co 2+ ions in tetrahedral Zn 2+ ions sites. The PL spectra show emissions from two NC groups: QDs (E exc ~ 2.53 eV) and bulk NCs (E b ~ 2.21 eV); defects related to zinc vacancies (E VZn ~ 2.53 eV) and oxygen centers (E o ~ 1.91 eV) [14,27].
The emission centered at approximately 2.03 eV and 1.89 eV corresponds to tetrahedrally (Td) and octahedrally (Oh) coordinated Mn 2+ ions. Already, when the spin-allowed transitions centred around 1.69 eV ( 4 T 1 ( 4 P) → 4 A 2 ( 4 F), named E 1Co2+ ) and 1.52 eV ( 4 T 1 ( 4 P) → 4 T 2 ( 4 F), named E 2Co2+ ) [14]. The energy diagram in Figure 3(d)shows these OA and PL spectra transitions. The EPR spectra, besides proving the substitutional incorporation of Mn 2+ in Zn 1-xMn x Te, as justified by Figure 3(f), also reveals that as the Co doping, an intense central line with g = 2.012 attributed to interacting Co 2+ [14]. For both Mn 2+ and Co 2+ ions the transitions observed satisfy the selection rules ΔM S = ±1 and ΔM I = 0 (Figure 3(e)). Thus, the EPR spectra suggest a kind of competition between Mn 2+ and Co 2+ ions to substitute Zn 2+ ions, controlled by the Co concentration.

Effects of symmetry and concentration of cobalt-doped Bi 2 S 3 nanocrystals embedded in host glass matrix
Bi 2 S 3 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 Bi 2 S 3 orthorhombic network by Co 2+ ions (0.72 A)  replacing Bi 3+ ions (1.03 A)  can reduce intrinsic point defects and consequently increase the efficiency of semiconductor nanocrystals, for applications in solar cells [38], photocatalytic devices of visible light [39], thermoelectric [40] and spintronic [41]. Therefore, the control of the magnetic saturation of the energy states located of the Co 2+ ions in the Bi 2 S 3 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 Co 2+ ions in cobalt-doped Bi 2 S 3 NCs embedded in a host glass matrix (SNAB -45SiO 2 · 30Na 2 CO 3 · 5Al 2 O 3 · 20B 2 O 3 ) (mol%). Figure 4(a) shows the UV-VIS-NIR optical absorption spectrum (OA) of the Bi 2-x Co x S 3 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 Bi 2-x Co x S 3 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 Co 2+ ions in the Bi 2-x Co x S 3 NCs. The structure of the absorption bands is due to that of the ligand field of S 2− ions in distorted tetrahedral sites around the Co 2+ ions. Such bands of electronic level transitions have increased due to the increasing concentration of Co 2+ ions in the Bi 2 S 3 NCs structure.
The redshift observed at the OA band edge for the Bi 2-x Co x S 3 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 Co 2+ ions [10]. No significant changes occur in the quantum size of the Bi 2-x Co x S 3 NCs with the incorporation of Co. The TEM image (Figure 4c) shows an average size  of 6 nm for Bi 2-x Co x S 3 NCs (x = 0.100) with quantum dot properties. The interplanar distance (d 211 = 0.310 nm) shows the crystal plane (211) characteristic of the orthorhombic structure of the mineral Bismuthinite. Figure 4(b) illustrates the Bi 2 S 3 orthorhombic structure unit cell doped with Co 2+ ions at distorted tetrahedral sites and the respective Bi 2-x Co x S 3 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 Bi 2-x Co x S 3 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 Bi 2 S 3 NCs [2,10]. The inset in Figure x  characteristic of the eight lines (hf 1 ; hf 2; hf 3 ; hf 4 ; hf 5 ; hf 6; hf 7 ; hf 8 ) of the hyperfine interaction between the electron spin (S = 3/2) and the nuclear spin (I = 7/2) and its similar shapes for all xCo concentrations confirms magnetic doping in distorted tetrahedral sites. The result is compatible with a region where there is a large, inhomogeneous crystal field due to the lack of interface between the host glass matrix and the NCs [42].

Conclusion
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 Zn 1-x Mn x O nanopowders and the Bi2-xCoxS3, Zn1-xMnxTe Zn 0.99-x Mn 0.01 Co x Te 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 TR 2+ ions (Mn 2+ and Co 2+ ) 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 TR 2+ ions (Mn 2+ and Co 2+ ) 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.