**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 [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.

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 *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

**127**

Mn 2+ : 6 A1 (6

( 4

*Diluted Magnetic Semiconductors Nanocrystals: Saturation and Modulation*

for 10 hours to enhance the diffusion of Zn2+, Mn2+ and/or Co2+ and Te2− ions throughout the host PZABP matrix and induce the growth of Zn1-*x*MnxTe/Zn0.99 xMn0.01CoxTe 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

Co2+-doped Bi2S3 NCs were synthesized in a glass matrix with nominal composi-

tion 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

**2.2 A study in function of Mn concentration in ZnO nanocrystals: Solubility** 

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

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

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)**) [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

A1 (6

spin-orbit coupling were effectively described by the Racah B parameter (559 cm−1)

S) → <sup>4</sup>

T2 (4

G) (590 nm), 6

I) (410 nm) (**Figure 1b**) subtly permitted by

A1 (6

S) → <sup>4</sup> T2

substitutional and interstitial sites in the ZnO crystal structure.

G) (691 nm), 6

E (2

S) → <sup>2</sup>

and the crystalline field division (∆ = 5464 cm−1) [21, 22].

*DOI: http://dx.doi.org/10.5772/intechopen.96679*

~9.75 GHz and at Q-band ~34GHz).

**and Mn2+ ions localization**

of nanodevices electronics [16, 19].

S) → <sup>4</sup>

D) (482 nm), and 6

T1 (4

A1 (6

Bi2- xCoxS3 NCs.

#### *Diluted Magnetic Semiconductors Nanocrystals: Saturation and Modulation DOI: http://dx.doi.org/10.5772/intechopen.96679*

*Materials at the Nanoscale*

equilibrium.

tor nanocrystal.

**in glassy matrices**

**in glassy matrices**

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

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

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 semiconduc-

**2. Diluted magnetic semiconductors in Nanopowders and embedded** 

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

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 *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

**2.1 Synthesis of Nanopowders and nanocrystals and embedded** 

nearly pure and purchased from Sigma-Aldrich Company.

physical, chemical, and biological properties [11].

**126**

for 10 hours to enhance the diffusion of Zn2+, Mn2+ and/or Co2+ and Te2− ions throughout the host PZABP matrix and induce the growth of Zn1-*x*MnxTe/Zn0.99 xMn0.01CoxTe 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).

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 [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° 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)**) [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 A1 (6 S) → <sup>4</sup> T1 (4 G) (691 nm), <sup>6</sup> A1 (6 S) → <sup>4</sup> T2 (4 G) (590 nm), 6 A1 (6 S) → <sup>4</sup> T2 ( 4 D) (482 nm), and 6 A1 (6 S) → <sup>2</sup> 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].

#### **Figure 1.**

*(a) X-ray diffractograms (b) As adjusted curves represent the experimental energies of the transitions prohibited by spin 6 A1 (S)* → *<sup>4</sup> T1 (G), 4 T2 (G), 4 T2 (D), 2 A2 (I) e 2 E(I). (c) Tanabe-Sugano diagram for the electronic configuration of the Mn2+ (3d5) (C/B = 4.5) with vertical solid line in /B = 9.77, = 5464 cm−1, and B = 559 cm−1, PL spectra from 440 to 820 nm for concentrations of (d) 0.1 to 1.0 of Mn and (e) of 1.0 to 9.0 of Mn, (f) wurtzite structure of ZnO NCs with coordination geometry in which the Mn2+ ions are, represented by tetrahedral sites (td) and octahedral sites (od).*

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 [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 Mn2+ 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 T1 ← <sup>6</sup> A1) of Mn2+-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 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).

**129**

4

A2(F) → <sup>2</sup>

(2.33 eV), 4

A2(F) → <sup>4</sup>

oxygen centers (Eo ~ 1.91 eV) [14, 27].

luminescent devices.

*Diluted Magnetic Semiconductors Nanocrystals: Saturation and Modulation*

**2.3 Solubility of manganese and cobalt in ZnTe nanocrystals embedded in** 

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 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 semi-

**Figure 2** presents OA (a) and PL (c) spectra and photographs (b) of the samples containing Zn1-*x*MnxTe NCs, with Mn-concentrations ranging from *x* = 0.000 to *x* = 0.800, and with PZABP (65P2O5 · 14ZnO · 1Al2O3 · 10BaO · 10PbO (mol %)) glassy matrix doped only 80Mn (% wt of Zn). The OA spectra show absorption bands centered attributed to Zn1-*x*MnxTe NCs (quantum dots (QDs) and bulk NCs) and the incorporation of Mn2+ 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 Mn2+ ions (as detailed in the energy diagram)

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 [31]. The TEM images in **Figure 2(e)** (for x = 0.100), revealed interplanar distances corresponding to Zn0.9Mn0.1Te NCs (d ~ 0.347 nm) [32], MnO (d ~ 0.224 nm) [33] and MnO2 (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 Mn2+ ions in the Zn1-*x*MnxTe up to the nominal solubility limit of x = 0.100. Above this solubility limit, one can observe Mn′s saturation, forming MnO and MnO2 NCs, as represented in **Figure 2(d)**. Thus, one of the main motivations for studying semiconductors doped with transition metals, especially Mn2+ ions, is for applications in

**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 *x* = 0.000 to *x* = 0.800. **Figure 3(d)** presents the energy diagram of all transitions related to OA and PL data, and **Figure 3(e)** shows the transitions satisfying the selection rules ΔMS = ±1 and ΔMI = 0. The AO spectra, besides the bands attributed to Zn1-*x*MnxTe 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:

T1(P) (2.13 eV); and spin-forbidden transitions: 4

bands show the substitutional incorporation of Co2+ ions in tetrahedral Zn2+ ions sites. The PL spectra show emissions from two NC groups: QDs (Eexc ~ 2.53 eV) and bulk NCs (Eb ~ 2.21 eV); defects related to zinc vacancies (EVZn ~ 2.53 eV) and

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

A2(F) → <sup>2</sup>

T1(G) (1.95 eV) and 4

A2(F) → <sup>2</sup>

E(G) (1.91 eV) [14]. These

A1(G)

*DOI: http://dx.doi.org/10.5772/intechopen.96679*

conductor nanocrystal surface [30].

[26]. For x > 0, additional bands are observed.

**P2O5 - ZnO - Al2O3 - BaO - PbO glassy matrix**

*Materials at the Nanoscale*

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

*(a) X-ray diffractograms (b) As adjusted curves represent the experimental energies of the transitions* 

*T2 (D), 2*

*electronic configuration of the Mn2+ (3d5) (C/B = 4.5) with vertical solid line in /B = 9.77, = 5464 cm−1, and B = 559 cm−1, PL spectra from 440 to 820 nm for concentrations of (d) 0.1 to 1.0 of Mn and (e) of 1.0 to 9.0 of Mn, (f) wurtzite structure of ZnO NCs with coordination geometry in which the Mn2+ ions are, represented* 

*A2 (I) e 2*

*E(I). (c) Tanabe-Sugano diagram for the* 

*T2 (G), 4*

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 Mn2+ 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

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

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

T1 ← <sup>6</sup>

A1)

**128**

ZnO NCs.

gap [21, 22].

**Figure 1.**

*prohibited by spin 6*

of Mn2+-ions in ZnO NC host [23, 24].

*A1 (S)* → *<sup>4</sup>*

*by tetrahedral sites (td) and octahedral sites (od).*

*T1 (G), 4*

sites (Td) and/or octahedral sites (Od).
