*3.1.1 X-ray diffraction of Zn0.94Fe0.03Ce0.03O and Zn0.94Co0.03Ce0.03O nanoparticles*

The Zn0.94Fe0.03Ce0.03O (ZFCeO) and Zn0.94Co0.03Ce0.03O (ZCCeO) nanoparticles were synthesized by a sol-gel process [40]. **Figure 3a** shows the X-ray diffraction (XRD) results for ZFCeO and ZCCeO nanoparticles using Rietveld refinement (space group *P*63*mc*). The Rietveld refinement initiated with Zn2+ and O2� atoms is located at (1/3, 2/3, 0) and (1/3, 2/3, z), respectively. The XRD reflections result into a hexagonal wurtzite ZnO phase. The refined lattice parameters are *a*(Å) = 3.259(1) and 3.262(3) and *c*(Å) = 5.215(3) and 5.218(2); unit cell volume, V(Å<sup>3</sup> ) = 47.9682(3) and 48.0828(2); bond length, *l*Zn-O(Å) = 1.9826 and 1.9842; Rp(%) = 6.57 and 6.95; Rwp(%) = 9.0 and 9.8; and χ<sup>2</sup> = 1.97 and 2.05, respectively, for ZFCeO and ZCCeO. Lattice parameters for the hexagonal wurtzite ZnO structure is also calculated using the relation

$$\frac{1}{d^2} = \frac{4}{3} \frac{\left(h^2 + hk + k^2\right)}{d^2} + \frac{l^2}{c^2} \tag{1}$$

where *a*, *c*, *h*, *k*, *l*, and *d* have their usual meaning. The value of bond length is calculated [40]:

*Diluted Magnetic Semiconductor ZnO: Magnetic Ordering with Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.90369*

$$d\_{Zn-O} = \sqrt{\frac{a^2}{3} + \left(\frac{1}{2} - u\right)^2 \* c^2} \tag{2}$$

where

magnetic anisotropy and tailoring the coupling between dopants and defects should be a general approach toward stable ferromagnetic order in ZnO nanomaterials. Among RE ions, Sm3+ with five 4f electrons offers a unique possibility to induce the bifunctional properties for RTFM as well as visible luminescence in ZnO, making

The DMS ZnO materials are synthesized by different methods such as thermal evaporation method [30], chemical vapor deposition [31], sol-gel spin-coating technique [32], spray pyrolysis [33], hydrothermal synthesis [34], solid state reac-

Recently, a lot of research work has been reported on RE ion-based DMS ZnO [36]. Sun et al. [37] reported La-doped ZnO quantum dots in which luminescent behavior is greatly enhanced by introducing defects and oxygen vacancies (VO). This is due to larger ionic size of La3+ in ZnO lattice that induces stress. However, Bantounas et al. [38] suggested the weak magnetic coupling in Gd/ZnO and the material remain paramagnetic at room temperature. Aravindh et al. [39] gives origin of ferromagnetism in Gd/ZnO in which oxygen vacancies play an important role. Using DFT calculation, it is analyzed that the RE Ce atoms replaced those Zn sites in the wurtzite structure, which is the nearest neighbor to TM/Fe or Co atoms [40]. The 4f electrons in Ce are tightly bound around the nucleus and shielded by

6s<sup>2</sup> electrons, leading to strong local spin. For Ce-doped TM/ZnO, the larger

ratio of dopant cation to cation radius structure causes more defects, leading to a

*3.1.1 X-ray diffraction of Zn0.94Fe0.03Ce0.03O and Zn0.94Co0.03Ce0.03O nanoparticles*

nanoparticles were synthesized by a sol-gel process [40]. **Figure 3a** shows the X-ray diffraction (XRD) results for ZFCeO and ZCCeO nanoparticles using Rietveld refinement (space group *P*63*mc*). The Rietveld refinement initiated with Zn2+ and O2� atoms is located at (1/3, 2/3, 0) and (1/3, 2/3, z), respectively. The XRD reflections result into a hexagonal wurtzite ZnO phase. The refined lattice parameters are *a*(Å) = 3.259(1) and 3.262(3) and *c*(Å) = 5.215(3) and 5.218(2); unit cell

) = 47.9682(3) and 48.0828(2); bond length, *l*Zn-O(Å) = 1.9826 and

*a*<sup>2</sup> þ

*l* 2

*<sup>c</sup>*<sup>2</sup> (1)

The Zn0.94Fe0.03Ce0.03O (ZFCeO) and Zn0.94Co0.03Ce0.03O (ZCCeO)

1.9842; Rp(%) = 6.57 and 6.95; Rwp(%) = 9.0 and 9.8; and χ<sup>2</sup> = 1.97 and 2.05,

respectively, for ZFCeO and ZCCeO. Lattice parameters for the hexagonal wurtzite

*<sup>h</sup>*<sup>2</sup> <sup>þ</sup> *hk* <sup>þ</sup> *<sup>k</sup>*<sup>2</sup>

where *a*, *c*, *h*, *k*, *l*, and *d* have their usual meaning. The value of bond length is

suitable material in spin transport properties and spin-LEDs [29].

**2. Experimental methods**

*Magnetic Materials and Magnetic Levitation*

**3. Results and discussion**

5s2 p6 d1

volume, V(Å<sup>3</sup>

calculated [40]:

**114**

tion [29], coprecipitation method [35], etc.

larger concentration of electrons and holes.

ZnO structure is also calculated using the relation

1 *<sup>d</sup>*<sup>2</sup> <sup>¼</sup> <sup>4</sup> 3

**3.1 Wurtzite structure and defect calculation in DMS ZnO**

$$
\mu = \frac{a^2}{3c^2} + 0.25\tag{3}
$$

where u is a positional parameter. The volume per unit cell for the hexagonal system is calculated using.

$$\mathbf{V} = \mathbf{0}.866 \times \mathbf{a}^2 \times \mathbf{c} \tag{4}$$

The calculated values of the lattice parameters are *a*(Å) = 3.257, 3.256, 3.260, and 3.261; *c*(Å) = 5.207, 5.206, 5.214, and 5.217; c/a = 1.5987, 1.5988, 1.5994, and 1.5998;

#### **Figure 3.**

*(a) XRD pattern of Zn0.94Fe0.03Ce0.03O (ZFCeO) and Zn0.94Co0.03Ce0.03O (ZCCeO) nanoparticles. (b, d) Raman and UV-visible absorption (for energy band calculation) spectra of Zn0.95Ni0.05O (ZNiO), Zn0.91Ni0.05Ce0.04O (ZNiO/Ce), Zn0.95Cu0.05O (ZCuO), and Zn0.91Cu0.05Ce0.04O (ZCuO/Ce) nanoparticles. (c) Photoluminescence spectra for ZnO with Co, Mn, and Fe nanoparticles (adapted from [27, 40, 41]).*

*l*Zn-O(Å) = 1.9808, 1.9825, 1.9829, and 1.9837; and V(Å3 ) = 47.834, 47.796, 47.987, and 48.044, calculated for Zn0.97Fe0.03O (ZFO), Zn0.97Co0.03O (ZCO), ZFCeO, and ZCCeO, respectively. The calculated values of the c/a ratio of ZFO and ZCO are slightly increased over pure ZnO (c/a = 1.598) due to the shape/size effect of the nanorods. But, it is again enhanced with RE ions due to the ionic size effect. Therefore, the observed variation in lattice parameters with doping indicated displacement of atoms in wurtzite lattice to create defects, i.e., vacancies or interstitials. It is also reported that the average size, D, of nanoparticles is 97 4 nm and 106 3 nm for ZFCeO and ZCCeO, respectively. The lattice defects are also evaluated with Raman and photoluminescence spectra. The zero-field cooling (ZFC) and field cooling (FC) magnetization measurement at H = 500 Oe and T = 300–5 K show AF-FM transitions. At 5 K, the measured value of Ms(emu g<sup>1</sup> ) = 0.339 and 0.478 for ZFCeO and ZCCeO, respectively. For ZFCeO, the weak RTFM is formed due to the mixed valance states Fe2+/Fe3+ via oxygen vacancies.

*3.1.4 UV-Visible absorption spectra and Tauc plot*

*DOI: http://dx.doi.org/10.5772/intechopen.90369*

; and for an indirect transition:ð Þ *<sup>α</sup>h<sup>ν</sup>*

present in the forbidden region.

**3.2 Microstructural study of DMS ZnO**

*3.2.1 SEM image of Mn-doped ZnO nanowires*

*K hν* � *Eg*

**Figure 4.**

**117**

*for pure ZnO (adapted from [21, 27, 44, 45]).*

**Figure 3d**<sup>0</sup> shows UV-visible absorption spectra measured at room temperature

<sup>1</sup>*=*<sup>2</sup> <sup>¼</sup> *K h<sup>ν</sup>* � *Eg*

. The symbols in

2 ¼

for Ni-, Cu-, and Ce-substituted ZnO nanoparticles. The absorption peaks are observed corresponding to violet emission, i.e., ZNiO (409 and 426 nm), ZNiO/Ce (433 nm), ZCuO (407 and 427 nm), and ZCuO/Ce (401 and 429 nm). In order to evaluate the effect of dopant on Ni, Cu, Ce, on ZnO, the energy band gap, Eg, is calculated using the Tauc relation [27] used for a direct transition using: ð Þ *αhν*

*Diluted Magnetic Semiconductor ZnO: Magnetic Ordering with Transition Metal…*

these equations have their usual meanings. In **Figure 3d**, the value of direct band energy, Eg(direct) = 3.38, 3.42, 3.41, and 3.44 eV, and from **Figure 7d**00, the indirect energy band gap, Eg(indirect) = 3.13, 3.21, 3.19 and 3.24 eV, respectively measured, for ZNiO, ZNiO/Ce, ZCuO, and ZCuO/Ce. These values of Eg show small variation with bulk sample of pure ZnO [27]. However, a significant change in Eg value from direct and indirect measurement clearly indicates that some of the defect states are

The ZnO nanowires were synthesized by a thermal evaporation method with 1 atom % Mn doping [44]. **Figure 4a** is a SEM image of Mn/ZnO nanowires of several micrometer lengths and 70 nm diameters. The reported work given TC to be 437 K

*(a) Scanning electron microscopy (SEM) pattern for ZnO/Mn nanowires. (b) HRTEM for*

*Zn0.92Fe0.05La0.03O nanoparticles. (c) TEM pattern for Zn0.91Ni0.05Ce0.04O nanoparticles. (d) AFM pattern*

#### *3.1.2 Lattice structure and defect/vacancy evaluation by Raman spectra*

The Zn0.95Ni0.05O (ZNiO), Zn0.91Ni0.05Ce0.04O (ZNiO/Ce), Zn0.95Cu0.05O (ZCuO), and Zn0.91Cu0.05Ce0.04O (ZCuO/Ce) nanoparticles were synthesized by sol-gel process [27]. XRD pattern found wurtzite structure with lattice distortion to perform lattice defects. The average particles size is D = 27, 81, 57 and 159 nm, respectively, measured for ZNiO, ZNiO/Ce, ZCuO, and ZCuO/Ce. The Raman modes observed at room temperature for these pure Ni-, Cu-, and Ce-doped ZnO are shown in **Figure 3b**. The presence of E2 mode in all samples indicates that the doping does not change the wurtzite phase. It is observed that Ni and Cu doping on ZnO gradually decreases the intensity of E2(high) mode as compared with pure ZnO [22]. But, it is again strengthen with Ce co-doping. This type of change in E2(high) mode with dopant ions might induce structural defects and local lattice distortions of wurtzite lattice [32]. The E2(high)-E2(low) modes indicate oxygen defects or vacancy formation. The peak position of E2(high) mode also changes with Ni, Cu, and Ce doping that is ascribed with the change in the level of oxygen vacancies [33]. The magnetic results also reported low temperature ZFC/FC magnetic measurement that show AF-FM ordering and the doping of Ce ions results to high Tc. At 300 K, the values of Ms(emu g<sup>1</sup> ) = 0.073, 0.085, 0.053, and 0.132, and at 10 K Ms(emu g<sup>1</sup> ) = 0.096, 0.198, 0.136, and 0.251, respectively, for ZNiO, ZNiO/Ce, ZCuO, and ZCuO/Ce. The enhancement in the oxygen vacancies and ferromagnetism with Ce doping might depend on mixed valence state Ce3+/Ce4+ ions.

#### *3.1.3 Photoluminescence spectra for Fe-, Co-, and Mn-doped ZnO nanoparticles*

The photoluminescence spectra for Fe (0.15%)-, Co (0.20%)-, and Mn (0.20%) doped ZnO nanoparticles are given in **Figure 3c** [41]. Pure ZnO nanoparticles show emission maxima at 385 nm along with blue (424 nm, 468 nm) and green (521 nm) luminescence. Transitions from Zn interstitials to valence band are attributed with blue emission (424 nm). Oxygen vacancies are related with blue (468 nm) and green emission (521 nm). The green emission is understood to be due to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. The blue emission is caused by two defect levels, either transition from Zni to the valance band or transition from bottom of the conduction band to the interstitial O(Oi).

*Diluted Magnetic Semiconductor ZnO: Magnetic Ordering with Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.90369*
