*3.1.4 UV-Visible absorption spectra and Tauc plot*

*l*Zn-O(Å) = 1.9808, 1.9825, 1.9829, and 1.9837; and V(Å3

*Magnetic Materials and Magnetic Levitation*

Ms(emu g<sup>1</sup>

vacancies.

T = 300–5 K show AF-FM transitions. At 5 K, the measured value of

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

ZNiO, ZNiO/Ce, ZCuO, and ZCuO/Ce. The enhancement in the oxygen vacancies and ferromagnetism with Ce doping might depend on mixed valence

*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

high Tc. At 300 K, the values of Ms(emu g<sup>1</sup>

and at 10 K Ms(emu g<sup>1</sup>

state Ce3+/Ce4+ ions.

to the interstitial O(Oi).

**116**

the weak RTFM is formed due to the mixed valance states Fe2+/Fe3+ via oxygen

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

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

) = 0.339 and 0.478 for ZFCeO and ZCCeO, respectively. For ZFCeO,

) = 47.834, 47.796, 47.987,

) = 0.073, 0.085, 0.053, and 0.132,

) = 0.096, 0.198, 0.136, and 0.251, respectively, for

**Figure 3d**<sup>0</sup> shows UV-visible absorption spectra measured at room temperature 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* 2 ¼ *K hν* � *Eg* ; and for an indirect transition:ð Þ *<sup>α</sup>h<sup>ν</sup>* <sup>1</sup>*=*<sup>2</sup> <sup>¼</sup> *K h<sup>ν</sup>* � *Eg* . The symbols in 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 present in the forbidden region.
