Exploring the Ce3+ ions doping effect on optical and magnetic properties of NiO nanostructures

Journal Pre-proofs Research articles Exploring the Ce3+ ions doping effect on Optical and Magnetic Properties of NiO Nanostructures M. Naseem Siddique, Ateeq Ahmed, S.K. Riyajuddin, Mohd Faizan, Kaushik Ghosh, P. Tripathi PII: DOI: Reference:

S0304-8853(19)33529-2 https://doi.org/10.1016/j.jmmm.2019.166323 MAGMA 166323

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

10 October 2019 14 December 2019 18 December 2019

Please cite this article as: M. Naseem Siddique, A. Ahmed, S.K. Riyajuddin, M. Faizan, K. Ghosh, P. Tripathi, Exploring the Ce3+ ions doping effect on Optical and Magnetic Properties of NiO Nanostructures, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.166323

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier B.V.

Exploring the Ce3+ ions doping effect on Optical and Magnetic Properties of NiO Nanostructures M. Naseem Siddiquea, Ateeq Ahmeda, S.K. Riyajuddinc, Mohd Faizanb, Kaushik Ghoshc and P. Tripathia* aDepartment

of Applied Physics, Faculty of Engg. and Technology, Aligarh Muslim University, Aligarh-202002, India bDepartment of Physics, Aligarh Muslim University, Aligarh-202002, India c Institute of Nano Science and Technology, Mohali, Punjab, India,160062 *Corresponding author. E-mail: [email protected] In this paper, pure and rare earth xCexO(x=0.00,

Abstract ion doped nickel oxide nanoparticles, i.e. Ni1-

Ce3+

0.01, 0.03 and 0.05) have been successfully prepared by sol-gel method and

qualitative studied the influence of Ce3+ ion on the optical and magnetic properties by the mean of Rietveld refined X-ray diffraction (XRD) analysis, Raman spectroscopy, Fourier transform

infrared

spectroscopy (FTIR),

transmission

electron

microscopy

(TEM),

Ultraviolet (UV) visible spectroscopy, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) as well as electron paramagnetic resonance (EPR) measurement. XRD analysis confirmed the single crystalline phase of the prepared samples without any impurity phase and the average crystallite size calculated from the Scherrer method follows the same trend as obtained from the Williamson- Hall method. Further, PL spectra of doped NiO nanoparticles exhibit green emission related to oxygen vacancies and the emissions at 398.78 nm and 466.70 nm (Blue emission) due to the Ce3+ ion transitions 2D3/2–2F5/2 and 2D3/2–2F7/2 which are also extremely supported by CIE1961 chromaticity diagram. Through magnetic measurement, room temperature weak ferromagnetism is observed for all samples. Moreover, EPR analysis reveals the presence of unpaired electron where the field resonance decreases along with increased g value as the Ce concentration increases when compared to pure NiO. The present studies also suggest that Ni vacancy might be responsible for inducing weak ferromagnetism in NiO samples through the collective localized magnetic moment.

1

1 Introduction The outstanding optical properties and room-temperature ferromagnetism (RTFM) have encouraged studies on the magneto-optical properties of some doped transition-metal (TM) oxide systems which lead to innovative magneto-optical devices [1]. There are broad studies of cation and oxygen vacancies in some oxides such as SrO, Al2O3, CaO, MgO, SnO2 and ZnO [2-3]. Among several TMs, nickel oxide (NiO) having excellent properties such as low cost, high thermal, chemical stability and enhanced chemical/physical characteristics via the transition from bulk to nanoscale materials. NiO is a p-type semiconductor with a wide bandgap in the range of 3.6 eV to 4.0 eV [4]. It is expected that several intrinsic structural defects such as Ni vacancy (VNi), Ni interstitial (Nii) and oxygen vacancy (Vo) etc. can be induced during the preparation of pure and doped NiO samples under the variable condition. Among several intrinsic defects, only oxygen vacancy and Ni vacancy show the localized moments while other defects are nonmagnetic [5]. Several percents of vacancy density provided the magnetic order in materials [6] because induced defects and vacancies in the lattice lead to localized magnetic moments which is generally known as defect induced magnetization (DIM). Theoretically, it is observed that extended tails of the defect wave functions assist the long-range coupling between the localized moments [7]. In several rare earth metals (REMs), cerium (Ce) has attained significant attention due to its unique and most significant characteristics caused by the production of Ce3+/Ce4+ based on oxidation/reduction [8]. Moreover, Ce3+ ions appropriately used in different applications like light-emitting diodes, high power laser and display purposes using light-emitting phosphors [9-14]. Many researchers reported the Ce incorporated metal oxide nanostructures to exhibit more efficient luminescence center [15] and enhanced visible emissions in PL analysis [16]. Nevertheless, intense green emission and ferromagnetic interaction generally associated with the concentration of oxygen vacancies and Ni vacancies that improve the alignment of magnetic moments associated with Ni2+ ion vacancies. Among several metal oxide nanoparticles, NiO shows antiferromagnetic behavior below 523 K. Over the past decades, room temperature magnetic properties of NiO nanoparticles have been extensively studied by the several researchers which may have many potential applications such as gas sensors, magnetic recording media, spintronics and biomedical applications [17]. Reported investigations demonstrate that the magnetic properties are very complex which may be caused by the role of intrinsic defects, surface effects, interface effects and finite-size etc. Patta Ravikumar et al. [18] reported a ferromagnetic phase in antiferromagnetic NiO which may be caused by the defect vacancies created by the dopant ions. Several synthesis methods 2

such as solid-state method, hydrothermal, pulsed laser deposition, chemical precipitation and solvothermal etc. were used to synthesize the nanoparticles [19, 20]. In these different methods, the sol-gel process is one of simplest, low cost and high yield methods to acquire homogeneous nanoparticles with a uniform structure and definite shape. Furthermost, defect vacancy induced FM and luminescence are extensively accepted mechanisms in NiO nanomaterials. Therefore, in the light of the above facts, we have discussed the influence of Ce3+ on the optical and magnetic properties in pure and Ce doped NiO nanoparticles. In this context, we have discussed the structural properties of prepared samples using the Rietveld refinement, FTIR and Raman spectroscopy. We found low transmittance and high transmittance in the ultraviolet and visible region, respectively. Simultaneously, the presence of some intrinsic defects in terms of oxygen vacancy (Vo) and Ni vacancy (𝑉2𝑁𝑖― ) exhibit a tuneable luminescence and magnetization such as multiemission centers like NBE, blue and green emissions and ferromagnetic behavior produced in all samples. 2. Experimental The sol-gel method, i.e. a chemical route prepared pure and Ce doped NiO nanoparticles. In this typical synthesis method, desired amount of Ni(NO3)2.6H2O and Ce(NO3)2.6H2O with desired mole ratio’s (Ni: Ce: 1.00: 0.00, 0.99: 0.01, 0.97: 0.03, 0.95: 0.05) were dissolved in 100 ml distilled water individually and continuously stirred for 3-4 hours through magnetic stirrer to obtain a transparent gel. To get a PH value 7 of a solution, we added some pallets of sodium hydroxide (NaOH) into a transparent solution. After the reaction process, the obtained precipitate was subjected to centrifuge and washed with both ethanol and distilled water for many times to remove soluble ions and then dried at 110 0C in an oven for 24 hours. Eventually, the as-prepared powder was calcined at 700 0C in an electric furnace for 5-6 hours to obtain Ce doped NiO nanostructures. A similar procedure was also applied to prepare pure NiO nanoparticles. The phase structure and crystallinity of the calcined samples were investigated by powder Xray diffraction (PXRD) (Rigaku Miniflex II) through the Rietveld analysis using Cu Kα (λ=0.15406 nm) radiation in the range 2θ = 200 -800. The morphology of samples has also been investigated by TEM analysis (JEOL, JEM-2010). Room temperature micro-Raman spectra were measured using LABRAM HR visible (400-1100 nm) model (632.8 nm excitation source of He-Ne laser). Also, UV visible spectrophotometer (Perkin Elmer 3

Lambda 35 UV–Vis spectrometer) and PL instrument (Perkin Elmer LS-55 fluorescent spectrometer) have been used to discuss the optical and luminescence properties of samples. The oxidation states of Ce such as Ce3+/Ce4+confirmed by the X-ray photoelectron spectroscopy (XPS). The magnetic measurement was also carried out using vibrating sample magnetometer (VSM) (14T-PPMS) as well as electron paramagnetic resonance (EPR). 3. Results and Discussion 3.1 Structural and morphological studies The structural analysis for all samples calcined at 700 0C has been carried out by using the XRD data with the standard JCPDS database and the data refined by Rietveld refinement technique using FULLPROF program. Figures 1(a-d) show the refined diffraction patterns of all samples and observed that there is no secondary phase which demonstrates the stoichiometric purity of all samples. The presence of multiple diffraction peaks from (111), (200), (220), (311) and (222) planes confirm the polycrystalline nature of all samples. In this study, the simulated XRD patterns for the Rietveld analysis produced with the cubic phase (FCC) of NiO having the space group Fm-3m. The obtained refined parameters such as Bragg R-factor (RB), structure factor (RF), lattice parameter (a), goodness factor (GOF) and Chi-square (𝜒2) for all samples are listed in table 1. The Bragg R-factor (RB) and structure factor (RF) defined as [21]

𝑅𝐹 =

𝑅𝐵 =

∑|(𝐼𝑘(𝑜𝑏𝑠)1/2 ― (𝐼𝑘(𝑐𝑎𝑙)1/2| ∑(𝐼𝑘(𝑜𝑏𝑠)1/2

∑|𝐼𝑘(𝑜𝑏𝑠) ― 𝐼𝑘(𝑐𝑎𝑙)|

(1)

(2)

∑𝐼𝑘(𝑜𝑏𝑠)

where 𝐼𝑘 is the integrated intensity for the kth reflection. As presented in table 1, it is seen that the value of 𝜒2 is less than six [22] which confirms the good structural refinement of the nanoparticles. Lattice parameter (a) for a cubic lattice with interplanar spacing (𝑑ℎ𝑘𝑙) and dislocation density (δ) [23] are given by the following relations

𝑑ℎ𝑘𝑙 =

𝑎 2

ℎ + 𝑘2 + 𝑙2

4

(3)

𝛿=

1 𝐷2

(4)

where h, k, and l represent the Miller indices Here, the crystallite size (D) was calculated using both Scherrer (SH) and Williamson – Hall (W-H) methods. The Scherrer [23] relation is given as: 0.9𝜆

𝐷 = 𝛽cos 𝜃

(5)

where 𝜃 is the diffraction angle, β is the Full width at half maximum (FWHM) and 𝜆 is the incident X-ray wavelength.

Figure 1(a-d): Refined XRD spectra of all samples.

5

The crystallite size of all samples determined from the (200) plane diffraction using Scherrer’s relation are listed in table 1. It is seen that with the increasing Ce concentration, full width at half maximum (FWHM) gradually increases and then corresponding crystallite size decreases. The reason for decreasing crystallite size with Ce insertion may be caused by small grain growth when compared with pure NiO nanoparticles [24]. Using the crystallite size (D), one can also determine the dislocation density (δ) which shows the respective increment with crystallite size and also mentioned in table 1. Unlike the SH method, W-H method follows tan𝜃 dependency and hence, discuss the effect of lattice strain on crystallite size based on the following relations [23] 𝛽𝑡𝑜𝑡𝑎𝑙 = 𝛽𝑠𝑖𝑧𝑒 + 𝛽𝑠𝑡𝑟𝑎𝑖𝑛 0.94𝜆

𝛽cos 𝜃 = 〈𝐷𝑋𝑅𝐷〉 +4𝜂𝑠𝑖𝑛𝜃

(6) (7)

where η represents the local lattice distortion parameter.

Figure 2: Size-strain (William- Hall) plots for all samples Figures 2 (a-d) show the plots of 𝛽𝑐𝑜𝑠𝜃 versus 4 sin𝜃 for all samples. The crystallite size (D) and lattice distortion parameter (η) can be determined using fitting and extrapolating the linear curve to the intercept of the y-axis. The slope of the linear line gives the value of η. The observed values of η for pure and doped NiO are well consistent with the previously reported results for NiO nanoparticles [25]. The intercept on the y-axis of the linear fitted line gives the value of inverse crystallite size (1/D). The obtained values of crystallite size using

6

the W-H method are well consistent with the calculated values from the SH method which are shown in table 1.

Figure 3: (a) Variation in lattice parameters/FWHM and (b) crystallite size/cell volume, as a the function of Ce concentration (c) XRD Peak shifting in samples (d) FTIR spectra for all samples Table1: Rietveld refined parameters for all samples Parameters Space group a=b=c (Å) α=β=γ Cell volume (Å3)

NiO F m -3 m 4.1642 90 72.209

Ni0.99Ce0.01O F m -3 m 4.1690 90 72.459

Ni0.97Ce0.03O F m -3 m 4.1692 90 72.469

Ni0.95Ce0.05O F m -3 m 4.1698 90 72.501

GOF(2) RBragg RF D (nm) (SH) D(nm) (WH) FWHM (β) 𝜼 x 10-3 Dislocation density (δ x1014) (Å-2)

3.48 3.561 % 3.988 % 40.69 38.80 0.2101 -2.00 6.03

3.53 4.42 % 3.88% 34.26 32.85 0.2495 -2.64 8.51

2.49 4.300 % 4.832% 33.24 31.08 0.2572 -2.50 9.05

2.30 7.420% 4.376 % 30.32 30.46 0.2819 -2.41 10.87

7

The lattice parameter confirms that prepared NiO exhibits face-centred-cubic (FCC) structure, as shown in fig.3 (a) (inset). Since the ionic radius of Ce3+ (1.03 Å) is larger than that of Ni2+ ions (0.72 Å). Thus, the cell volume and lattice parameter increases with Ce concentration as shown in figs. 3 (a) & 3(b). Also, the position of the peak (200) changes irregularly with the increase in Ce concentration. It is found that peak shifted towards lower angle for doped samples up to 43.3830 when compared to pure NiO sample. Hence, peaks shift towards lower angle with an increase in Ce doping concentrations exhibit the expansion in lattice parameters. Also, the XRD spectra (figs. 1(a-d)) exhibit the intensity of the XRD peak decreases with an increase in Ce concentration and the width of the XRD peaks shows a regular broadening with increasing doping concentration. These two synchronized remarks indicate the existence of non-uniform strain-induced and lattice disorder in the NiO lattice due to the substitution of relatively greater ionic radius of Ce3+ ions [26]. The morphology of calcined pure and 5% doped NiO samples using the TEM analysis as shown in figs. 4 (a, b). The TEM images of samples more confirm the good crystallinity and approximate spherical shaped nanoparticles. Simultaneously, we have also determined the average particle size of pure (x=0.00) and Ce doped NiO (x=0.05) nanoparticles using TEM analysis. The calculated values were found to be 32.69 nm and 29.12 nm, respectively which are good consistent with the estimated average crystallite size from the XRD patterns using Scherrer's analysis. As shown in figs.4(c, d), SAED patterns exhibit distinct bright rings which approve the polycrystalline nature and favoured orientation of nano crystals rather irregular [27].

8

Figure 4: (a, b) TEM images (c, d) SAED patterns of pure and Ce (5%) doped NiO samples Fourier-transform infrared spectroscopy (FTIR) spectra of calcined pure and Ce doped NiO samples presented in fig. 3(d). The spectra have various substantial absorption peaks which have been recorded in the range of 4000 cm-1 to 400 cm-1. It is seen that peaks centered at 3449. 25 cm-1 and 2929.23 cm-1 are assigned to OH stretching vibration and C-H stretching vibration (presence of ethanol during the washing). Also, a peak centered at 1641.85 cm-1 is assigned to H-O-H bending vibration. These peaks exhibit the presence of traces of water and ethanol in the samples. The absorption bands centered at 1462.67 cm-1 which may be attributed to the O-C=O symmetric and asymmetric stretching vibrations and the C–O stretching vibration mode. The absorption band around 831.12 cm−1 is assigned to the C=O 9

stretching vibrations. Simultaneously, one more band is also presently centered at 465.48 cm1

which attributed to Ni-O stretching vibration mode [28].

Figures 5(a-d) show room temperature Raman spectra for all calcined samples. In Raman spectra, the peaks including 1LO vibrational mode, 2LO vibrational mode and two magnons (2M) which were deconvoluted using the voigt function. Raman spectra of Ni1-xCexO (x= 0.0, 0.01, 0.03 and 0.05) demonstrate a strong peak ranging 512.36-529.23 cm-1 exhibits the 1LO vibrational mode, a peak ranging 1007.06 - 1065.06 cm-1 exhibits the 2LO mode. Besides this, in pure NiO sample, it can also be concluded that an additional peak visible at 1461.33 cm-1 corresponding to 2M band above the phonon mode (2LO) (represented as 2M in figure 5(a)) which is due to antiferromagnetic coupling strength i.e. Ni2+ -O2− -Ni2+ super-exchange interaction [29]. Except pure NiO, the 2M band disappeared in doped NiO samples due to disorder induced by defects or nickel or oxygen vacancies and decreased antiferromagnetic spin correlations. In this manner, Cazzanelli et al [30] also reported that 2M scattering decreased toughly upon the insertion of magnesium ions. In consequence, the disappearance of 1TO mode, 2TO mode and TO+LO mode demonstrating nano-crystalline nature of the pure and doped NiO samples. Table 2 shows the variation of Raman shift as well as line broadening (FWHM) of 1LO and 2LO modes as a function of Ce concentration in doped NiO samples. It can be seen that 1LO and 2LO modes shifted towards higher or lower wavenumber side, respectively, which may be caused by the surface relaxation, induced phonon confinement and defects.

10

Figure 5(a-d): Deconvoluted Raman spectra for all the samples Table 2: Voigt fitting Parameters for one and two-phonon modes of pure and Ce doped NiO nanoparticles Samples 1LO mode 2LO mode center (cm-1)

FWHM

center (cm-1)

FWHM

NiO

512.36

116.62

1007.06

203.07

Ni0.99Ce0.01O

529.23

202.60

1065.06

229.35

Ni0.97Ce0.03O

522.46

219.22

1055.75

243.76

Ni0.95Ce0.05O

522.15

226.69

1054.00

253.18

11

3.2 Optical studies Figure 6(a) shows the UV visible (UV) absorption spectra of calcined pure and Ce doped NiO samples. It is observed that a strong absorption found in the range of 300 - 340 nm for all samples. The strong absorption around 320 - 340 nm ascribed to NiO exciton transition. Using the absorption spectra, it can be observed that there is a sustained redshift for the absorption edge with increase of Ce concentration (0- 3%) into NiO lattice resulting in the bandgap decreases with rise in Ce concentration while further shift to lower value of wavelength, i.e. blueshift

at higher

concentration of Ce (5%). For a direct bandgap semiconductor, the optical bandgap energy (Eg) may be determined with the help of optical absorption using the Tauc’s method[31] 𝛼ℎ𝜐 = 𝐴(ℎ𝜐 ― 𝐸𝑔)1/2

(8) Figure 6: (a) absorption (b) extinction coefficient and (c) optical transmittance for where A and Eg are constant and bandgap energy, pure and Ce doped NiO nanoparticles whereas power 1/2 represents the direct bandgap transition while power 2 gives the indirect bandgap behavior of semiconductors[31].

12

Figure 7(a-d): Tauc’s plots for pure and Ce doped NiO nanoparticles Figures 7 (a-d) represent the Tauc’s plots for all samples. From these figures, the optical band gap energy for all samples are calculated and found that 3.58 eV, 3.51eV, 3.45 eV and 3.55 eV, respectively. Here, Eg first decreases with increasing Ce concentration (0-3%) and further increases at Ce concentration (5%) into NiO lattice, respectively. The reduction in the bandgap with Ce doping may be due to the existence of vacancies and the formation of energy states between the valance and conduction band induced by Ce3+ ions. Therefore, insertion of the Ce3+ ions at Ni sites creates vacancies/holes in the NiO lattice causing decreased the bandgap of NiO. Baraman et al. [32] and Dewan et al. [33] also reported similar results for bandgap reduction with the dopants whereas the blue shift in absorption band edge followed by the Burstein–Moss effect [34] which predicts that the bandgap increases at Ce (5%) and may be caused by the reduction in particle size. Extinction coefficient (k) for all samples in the wavelength range 200- 800 nm was also calculated using the relation [35] 𝛼𝜆

𝑘 = 4𝜋 where α is the absorption coefficient.

13

(9)

From fig.6 (b), it is observed that the extinction coefficient (k) increases rapidly with increase in wavelength from 300-400 nm and further achieves approximately constant value in the visible and infrared region (   nm). This behavior of increase /or decrease value of k directly associated with the absorption of light [35]. Also, the value of k increases with increasing Ce concentration into NiO sample, which is due to the increased absorption coefficient. In addition, with the help of absorbance data, we have also determined the transmittance for all the samples using the relation [35] 𝐴 = ―log 𝑇

(10)

where A is the absorbance. We have also studied the transmittance for all samples which is shown in fig. 6(c) and found that the transmittance decreases gradually with the wavelength in the ultraviolet region ( 300 nm - 350 nm) and rapidly increases at a higher wavelength (λ > 350 nm). This result suggests that the low value of transmittance in the ultraviolet and higher value of transmittance in the visible and infrared regions. Simultaneously, the value of T decreases with an increase in Ce concentration from x= 0.0 to 0.05, i.e. pure NiO sample shows maximum value (T= 0.75) and Ni0.95Ce0.05O shows very low transmittance (T=0.32). The decreasing behavior of T with an increase in Ce concentration may increase the localized state density which reduces the value of transmittance [36]. It is also seen that the involvement of the width of localized states in the bandgap of the materials disturbs the optical transitions and optical bandgap structure which is known as Urbach tail and directly related to an exponential tail for the localized states (density of states of band edges. The Urbach rule is given by the relation [37]

( )

𝛼 = 𝛼𝑜 𝑒𝑥𝑝

ℎ𝜈

𝐸𝑈

(11)

Urbach energy for all samples has been calculated by the slope of linear fitting of the plot (ln α versus hυ) as shown in fig.8 (a). In consequence, the values of the EU for all samples ranging from 0.218 to 0.416 meV. Urbach energy explains the disordering in materials. Therefore, a lower value of EU confirms the lesser disordering in material structures and vice versa.

14

Figure 8: (a) ln α versus hυ and (b) band diagram for all samples PL spectroscopy is a non-destructive tool for optical emission study which is also most suitable to estimate the defects/vacancies and crystal quality of the semiconducting materials. Figures 9 (a-c) exhibit the room temperature PL spectra for all investigated samples. Using the recorded PL spectra of NiO, a strong emission band observed at 375.13 nm with an excitation wavelength (270 nm) corresponds to the near band emission (NBE) through the direct recombination of free excitons. In addition to NBE, three more peaks were observed around at 452.45 nm, 550.05 nm and 606.39 nm which are related to the visible emissions such as blue, green and orange emission, respectively. The visible emissions assigned to the defect-induced emission instead direct recombination of conduction electron to the holes in valence. It is known that structural defects such as Ni vacancy (VNi), oxygen vacancy (Vo), oxygen interstitial (Oi) etc. are generally presented in the metal oxide nanostructures which lead to luminescence in the visible region [38]. Thus, the blue emission peak through the radiative transitions of Ni vacancy (𝑉2𝑁𝑖― ) to the valance band while the green emission emerged through the oxygen vacancies (Vo) [39, 40]. Furthermore, the observed orange emission at 606.39 nm is due to the transition between the conduction band and interstitial oxygen [41]. The low intensity of the orange band demonstrates the existence of a small quantity of interstitial oxygen in pure NiO sample. On the insertion of Ce into NiO lattice, it is seen that the position of UV emission shifts from 375.16 to 398.78 nm and hence, in PL spectra of Ce doped NiO nanoparticles, new emission spectra from 398.78 nm to 546.45 nm are established which leads to the redshift in the UV emission. In fig. 9 (c), the observed PL band centered at 398.78 nm and 466.70 nm (Blue emission) are due to the Ce3+ ion transitions 2D3/2–2F5/2 and 2D3/2–2F7/2, respectively [42]. However, it seems that the intensity of blue emission is higher than intrinsic NiO representing that an energy transfer process takes place from the NiO host to Ce3+ ion [43-45]. Here, the 15

NiO host absorbs some energy from the excitation energy leading to the formation of excitons and stimulates the excitation from the ground states (4f) to the excited states (5d) on the Ce3+. To well appreciate the energy transfer from NiO host and the possible transitions in Ce3+, we also sketched an energy-level diagram of Ce doped NiO, as shown in fig. 10.

Figure 9: (a-c) PL spectra and associated photographs of pure (x=0.00) and doped (x=0.01, 0.03 and 0.05) samples under 270 nm excitation wavelength (inset in figs. a & c) and (d) CIE chromaticity diagram for pure and Ce doped NiO nanoparticles

Figure 10: Schematic depiction of possible radiative transitions

16

coordinates TWO To TWPO codoped –Yb 3? calculated upon 980 know nm excitation glass glass laser the atsample, have different chromaticity sample, radiatio radiation atbeen CIE CIE pump colour colour in powers the Er

However, the decreased intensity of green emission at 546.45 nm with the increase of the Ce concentration may be attributed to the significant reduction of the oxygen vacancies in the NiO lattice. Otherwise, the reduced intensity of blue/green emission bands with increasing Ce concentrations may be responsible for the interaction between Ce atoms and oxide. In consequence, it is also seen that 1% of Ce is the optimum doping as found the maximum green emission (In fig.9 (c)) which may be significant in the applications of visible light emission such as laser diodes and light-emitting diodes etc. To study the color space chromaticity in pure and Ce doped NiO samples, we have determined CIE coordinates at excitation wavelength 270 nm. Fig. 9 (d) shows the CIE chromaticity diagram in the (x, y) coordinates. Color space CIE diagram shows the color emitted near the blue region (0.19, 0.17) for pure NiO sample. As the Ce concentration increases, the emitted color moves towards the green color but with increasing Ce concentration, the intensity of blue and green colors decreases so that the CIE coordinates falling near white region at (0.23,0.38), (0.23, 0.37) and (0.23, 0.35) for x =0.01, x=0.03 and x =0.05, respectively. The oxidation state of Ce such as Ce3+ and Ce4+ in NiO lattice has been further confirmed by X-ray photoelectron spectroscopy (XPS). Here, highly resolved scans of Ni 2p, Ce 3d, and O 1s are presented in figures 12(a-c), respectively. In fig.11 (a), peaks positioned at 852.78 and 859.12 eV correspond to Ni 2p3/2 levels while the peaks appearing at 869.78 and 877.9 belong to Ni 2p1/2 level [46]. The binding energies are located at 882.70 , 898.38 and 915.12 eV [47] belong to Ce3+ 3d5/2, Ce4+ 3d3/2, Ce4+ 3d5/2 whereas the binding energies such as 887.13 and 906.21 eV are correspond to Ce3+ 3d5/2 and Ce3+ 3d3/2 levels, respectively [48] as shown in fig. 11(b). As reported in the literature [47], two main oxidation states of Ce ion such as Ce4+ and Ce3+ were found in which Ce4+ is more stable than Ce3+ oxidation state in air. The high-resolution spectrum of O 1s is shown in fig.11(c). The strong peak observed at 529.67 eV corresponds to O2- associated with chemical bonding structure of Ni-O [49] and the peaks located at 531.39 eV and 532.21 eV belong to an absorbed hydroxyl group (O-H) and –CO bond, respectively [50]. Thus, the mixed oxidation valance states of Ce3+ and Ce4+ existed in Ce doped NiO nanoparticles.

17

Figure 11: Deconvoluted XPS spectra (a, b and c) of Ni 2p, Ce 3d and O1s of Ce (5%) doped NiO nanostructures Figures 12 (a-d) show the room temperature magnetic behavior of pure and Ce-doped NiO nanoparticles calcined at 700ºC. It is seen that the magnetization increases linearly with increasing applied magnetic field in a low field region and further show the non-saturated magnetization in the high magnetic field. These results reveal that there are two components of magnetization associated with this process (i) an easily magnetization in the low-field region and (ii) a linear variation of magnetization using nonsaturated magnetization in the high field region. By these facts, two magnetic phases are supposed to be related one with antiferromagnetic (AFM) and other with ferromagnetic (FM). The non-saturation magnetisation occurs due to the existence of an AFM phase [51]. However, the presence of AFM nature in NiO nanoparticles also confirmed by the 2M band in the Raman spectrum (discussed earlier) associated with Ni2+-O2−-Ni2+ super-exchange interaction. To separate paramagnetic (PM) or AFM contribution, the M-H curve can be defined by the relation [52]

18

𝑀(𝐻) = 𝑀𝐹𝑀(𝐻) + 𝜒𝐴𝐹𝑀𝐻

(12)

where MFM is the magnetization due to uncompensated surface spins and 𝜒𝐴𝐹𝑀 is the magnetic susceptibility of AFM which was calculated by fitting the linear portion of M-H curves in the high-field region. Figures 12(a-d) represent the measured M-H loop after subtracting the AFM component. Here, we extracted the FM phase after subtracting the AFM phase from field-dependent magnetization behavior. Therefore, we obtained the saturation magnetization (Ms) using the FM component. The extracted values of saturation magnetization are listed in table 3. The M-H curves show the nonsaturated behavior under 20 kOe applied field which may be caused by the ferromagnetic arrangement of uncompensated surface spins while the maximum value of saturation magnetization is about 1.24 emu/g for x=0.05 sample when compared to pure NiO sample. Non zero values of remanent magnetization and coercivity for small hysteresis loop in pure and doped samples calcined at 700 0C are shown in inset of figs. 12(a-d) which represent a weak ferromagnetic behavior in pure and doped NiO samples. Generally, Ce is incorporated in mixed oxidation states such as Ce3+ (paramagnetic) and Ce4+ (diamagnetic). Through the valance charge transfer from Ce4+ to Ce3+ ion, an unpaired electron is produced in Ce (4f) orbital and consequently, the exchange coupling between 4f1 unpaired electrons in Ce3+ ion may induce weak ferromagnetism in Ce doped NiO.

19

Figures 12(a-d): Room temperature M–H curves for pure and Ce doped NiO nanoparticles Table 3: Obtained magnetic parameters from the fitting of the M-H curve for pure and Ce doped NiO nanoparticles Samples

MS (emu/g) Mr (emu/g) x 10 -2

Hc (Oe)

χ (emu/g-Oe) x 10-6

x=0.00

0.0219

6.58

62.93

5.91

x=0.01

0.0213

0.34

156.89

1.20

x=0.03

0.0148

1.11

178.92

4.44

x=0.05

1.24

0.32

249.04

6.27

The existence of Ce3+ ions is related to huge number of Ni vacancies in NiO where the Ni vacancies show induced FM through polarizing the neighbouring ligand atoms of the vacancy to produce a localized magnetic moment [53, 54] and further interaction between these localized moments produce a collective ferromagnetic interaction through the concept of bound magnetic polaron (BMP). It is commonly known that carriers and defects such as Ni vacancies can result in BMP which may be responsible for ferromagnetism in materials [55]. Previous studies in this context have also been reported Ni vacancy induced FM in NiO nanostructures [56]. 20

To discuss the magnetic parameters associated with the ferromagnetic part such as saturation magnetization, remanent magnetization and coercivity values for all samples, we have used Stearns, and Cheng [57] suggested a relation as: 𝑀(𝐻) =

2𝑀𝑠

𝐻 ± 𝐻𝑐

𝜋

𝐻𝑐

𝑡𝑎𝑛 ―1[

( )] + 𝜒𝐻

𝑡𝑎𝑛

𝜋𝑠 2

(13)

𝑀𝑟

where 𝑀𝑠, 𝐻𝑐, s ( = 𝑀𝑠), H and 𝜒 are the saturation magnetization, coercivity, squareness of the FM loop, applied field and magnetic susceptibility, respectively. Here, we have fitted the M-H curve for NiO sample using the equation (13) as shown in fig.13 (a). A similar fitting method has been implemented to analyze the M-H curves for all Ce doped NiO samples. The obtained fit magnetic parameters such as Ms, Mr, Hc and 𝜒 are tabulated in table 3. Upon increasing concentration of Ce, more Ce3+ ions contribute to the FM interaction through BMP model, leading to a high value of saturation magnetization for x=0.05 sample as shown in fig.13(b). Table 3 presents the value of Mr decreases upon the Ce ion doping as compared with the pure NiO and increasing value of coercivity upon the Ce insertion from x=0.0 to x=0.05 which may be due to decreasing crystallite size [58].

Figure 13: (a) Fitted M-H curve for x=0.00 sample and (b) variation in saturation magnetization (Ms), and coercivity (Hc) with Ce (x%) concentration To discuss the macroscopic magnetic properties, we also performed EPR measurement for pure and Ce doped NiO samples at room temperature. Figures 14 (a-d) exhibit the room temperature EPR spectra of all samples. All the samples principally exhibited a resonance line of the paramagnetic phase in the EPR absorption spectra. As discussed in VSM measurement, Ce has the two mixed oxidation states such Ce3+ and Ce4+ wherein the presence of air, Ce3+ oxidation state is less stable than Ce4+ oxidation state [59]. Since Ce3+ ion is paramagnetic (spin ½) and can detect by EPR while Ce4+ ion is not detected by EPR due to 21

diamagnetic nature. Along with this, Ce4+ ions have a trend to attract electrons. Therefore, when an electron is trapped in Ce4+ ion, there is a probability of transition between 5d and 4f shells which produces electron signal in the EPR spectrum [60]. It seems that with the increased concentration of Ce from x=0.00 to x=0.05, the intensity of EPR signal decreases which predicts the decreased electron trapping in Ce4+ ions with the increasing Ce concentration. Hence, it is concluded that the magnetic state contribution of Ce3+ ions resulting in induced ferromagnetic phase in doped NiO samples. On the application of the external field, the Ce3+ ions aligned along Ni leading to weak ferromagnetism in Ce doped NiO nanoparticles. These results are also good consistent with the outcomes of VSM measurement. Table 4: Extracted EPR parameters for pure and Ce doped NiO nanoparticles Samples

Hr (mT)

ΔHPP (mT)

g value

X=0.00

317.12

47.195

2.20

X=0.01

302.36

265.66

2.30

X=0.03

270.02

191.01

2.58

X=0.05

286.63

236.14

2.43

With the help of EPR spectra, we have also determined the EPR parameters such as resonance field (Hr), g value and peak to the peak line width (ΔHPP) where g value is an important parameter to describe the spin system. The value of g factor determined using the following relation: ℎ𝜐

𝑔 = 𝛽𝐻𝑟

(14)

Here, ℎ is plank’s constant (6.67 × 10-34 Kg⋅m2⋅S-1), ?? is the Bohr magnetron (9.274 × 10-28 J⋅G-1) and 𝜐 is the microwave frequency (9.24 × 109 Hz). The calculated values of g, Hr and ΔHPP are summarized in table 4. As depicted in table 4, the g-value lies between 2.20-2.58 and found to increase with the Ce insertion into NiO lattice. This increment in g value may be due to shifting in resonance field (Hr) towards the lower field. The shifting of the resonance field may be explained based on two main sources. (i) Ferromagnetic ordering on the surface of nanoparticle and (ii) ferromagnetic or paramagnetic (Ce3+) particle surrounding a nanoparticle [61]. In consequence, all induced additional fields lead to a shift in resonance line position at the site 22

of a paramagnetic ion Ce3+ in the central area of a NiO nanoparticle. Hence, the shift in the EPR resonance field makes the difference in EPR spectra of NiO and Ce doped NiO nanoparticles. Simultaneously, peak to peak linewidth increases on the Ce concentration when compared to x=0.00 sample. These observed results can be discussed through some factors such as porosity, eddy currents, intrinsic properties and anisotropy etc. that affect the peak to peak line width [62]. However, sample x=0.00 shows a small value of HPP which has a significant application in minimum eddy current losses at high frequency [62].

Figure 14(a-d): EPR spectra of all investigated samples 4. Conclusion In this manuscript, we have discussed the role of Ce3+ ion on optical and magnetic properties of Ni1-xCexO (x= 0.0, 0.01, 0.03, 0.05) nanoparticles calcined at 7000 C which have been synthesized through chemical route such as sol-gel technique. The Rietveld refined XRD and FTIR spectra demonstrate no change in the structure of NiO after the incorporation of Ce into NiO lattice indicating single-phase polycrystalline nature of pure and doped NiO as confirmed by TEM and SAED images. We have also determined average particle size for pure (32.69 nm) and Ce (5%) (29.12 nm) doped NiO nanoparticles with the help of TEM which are good consistent with calculated values of crystallite size using Sherrer method. Moreover, we have also calculated the crystallite size through Scherrer as well as Williamson Hall methods and the results found in the range of 30.32- 40.69 nm and 30.46 - 38.80 nm, respectively. Deconvoluted Raman spectra through Voigt function demonstrate the existence 23

of 1LO vibrational mode, 2LO vibrational mode and 2M magnon mode due to antiferromagnetic coupling strength in pure NiO sample. On the insertion of Ce concentration, 2M magnon mode suppressed due to induced defects such as oxygen vacancies. In addition, Ce doping into the NiO lattice strongly affects the optical properties of the material and the sustained redshift found in absorption band edge with the increasing Ce concentration from 0% to 3% leads to decreasing bandgap whereas blue-shift upon the higher Ce concentration (5%) resulting bandgap increases. In the ultraviolet region (300-350 nm), the optical transmittance decreases with increasing wavelength and it rises in the visible region (λ > 350 nm). Therefore, low transmittance and high transmittance found in the ultraviolet and visible region, respectively. In this context, the extinction coefficient (k) increases gradually with increasing the wavelength and achieves wavelength - independent behavior in visible region which are in good agreement with the high value of transmittance in this region. Simultaneously, the incorporation of Ce ion into the NiO also confirmed the disorder in this sample which affects the optical band gap structures in terms of Urbach energy ranging from 0.218 to 0.416 meV. Moreover, PL spectra exhibited several luminescent centers (also presented in CIE1961 chromaticity diagram) such as blue and green emissions which may be caused by the formation of oxygen vacancies (Vo) and Ce3+ transitions such as 2D3/2–2F5/2 and 2D3/2–2F7/2. Consequently, 1% Ce optimum doping provides the maximum green emission which may be very imperative for the fabrication of lightemitting diodes and laser diodes etc. Also, the occurrence of mixed oxidation states of Ce ion (Ce3+ and Ce4+) confirmed from the XPS spectrum. The presence of Ni vacancy is found to be responsible for the observed weak ferromagnetism in pure and doped NiO samples. It is also observed that doping of Ce ions at the Ni site reduces the oxygen vacancy defect concentrations considerably. These reduced oxygen vacancies strongly confirmed by the decreased value of luminescence intensity, remanent magnetization as well as total magnetization with increasing Ce concentration. EPR analysis reveals the presence of unpaired electron where the field resonance decreases along with increased g value as the Ce concentration increases when compared to pure NiO. It is suggested that improved luminescence with Ce3+ ion transitions and ferromagnetism with the intrinsic defects may have many applications such as novel magneto-optical devices and spintronics.

24

Acknowledgements Authors are thankful to the Department of Applied Physics, AMU Aligarh, India for providing experimental facilities. One of the authors (M. Naseem Siddique) is also grateful to Prof. Absar Ahmad, Interdisciplinary Nanotechnology Centre, AMU, Aligarh (India) and Prof. Shabbir Ahmed Department of Physics AMU, Aligarh (India) for providing photoluminescence (PL) and FTIR facilities, respectively. In the same way, also thankful to Prof. Kaushik Ghosh, Institute of Nano Science and Technology, Mohali, Punjab, India,160062 for providing the VSM facility. This work financially supported by University Grants Commission (UGC), New Delhi, Government of India through Senior Research Fellowship (SRF) Grant No. 510591. References 1. M. Arshad, A. Azam, A.S. Ahmed, S. Mollah, A.H. Naqvi, J. Alloys Compd. 509, (2011) 8378–8381 2. Pablo Esquinazi, Wolfram Hergert, Daniel Spemann, Annette Setzer, and Arthur Erns, IEEE transaction on magnetics, 49 (8) (2013) 4668 3. Fernandes, V.; Schio, P.; de Oliveira, A. J. A.; Schreiner, W. H.; Varalda, J.; Mosca, D. H. J. Appl. Phys., 110 (2011) 113902 4. H.Y. Ryu, G.P. Choi, W.S. Lee, J.S. Park, J Mater Sci Lett 39 (2004) 4375 5. Bouzerar, G.; Ziman, T. Phys. Rev. Lett., 96 (2006) 207602 6. M. A. Ramos, J. Barzola-Quiquia, P. Esquinazi, A. Mu˜noz Martin, A. Climent-Font, and M. Garc´ıa-Hern´andez, “ Phys. Rev. B, vol. 81, p. (2010) 214404. 7. Y. Liu, G. Wang, S. Wang, J. Yang, L. Chen, X. Qin, B. Song, B. Wang, and X. Chen, ” Phys. Rev. Lett., vol. 106, p. (2011) 087205 8. Yousefi, M.; Amiri, M.; Azimirad, R.; Moshfegh, A. Z. J. Electroanal. Chem., 661, (2014) 106−112 9. Kai Li, Mengmeng Shang et al., Inorg. Chem. 2015, 54, 16, 7992-8002 10. Kai Li, Sisi Liang, Hongzhou Lian, Mengmeng Shang, Bengang Xing and Jun Lin, J. Mater. Chem. C, 2016,4, 3443-3453 11. Kai Li, Sisi Liang, Mengmeng Shang, Hongzhou Lian, Jun Lin, Inorg. Chem. 2016, 55, 15, 7593-7604, 12. KaiLi, HongzhouLian, YanqiuHan, MengmengShang, RikVan Deun, JunLin, Dyes Pig. 2017, 139, 701-707, 13. Bin Li, G. Annadurai, Liangling Sun, Jia Liang, Shaoying Wang, Qi Sun, and Xiaoyong Huang, Optics Letters, 2018, 43(20), 5138-5141., 14. HengGuo, Balaji Deva kumar BinLi Xiaoyong Huang, Dyes and Pigments, 2018, 151, 81-88 15. Jung, Y.; Noh, B. Y.; Lee, Y. S.; Baek, S. H.; Kim, J. H.; Park, I. K. Nanoscale Res. Lett., 7 (2012) 43−47 25

16. N. Sinha, G. Ray, S. Bhandari, S. Godara, and B. Kumar Ceram. Int. 40 (2014)12337 17. A. Roy, V. Srinivas, S. Ram, J. De Toro, U. Mizutani, Phys. Rev. B 71 (2005) 184443 18. Patta Ravikumar, Bhagaban Kisan, and A. Peruma, AIP ADVANCES 5(2015) 087116 19. L. Li, L. Chen, R. Qihe, G. Li, Appl. Phys. Lett. 89 (2006)134102,. 20. A.K. Ramasami, M.V. Reddy, G.R. Balakrishna, Mater. Sci. Semicond. Process. 40 (2015) 194–202. 21. H.M. Rietveld, Acta Crystallogr. 22 (1967)151–152. 22. M. Naseem Siddique, Ateeq Ahmed, P. Tripathi, Materials Chemistry and Physics 239 (2020) 121959 23. M. Naseem Siddique, Ateeq Ahmed, P. Tripathi, Journal of Alloys and Compounds 735 (2018) 516-529 24. V. D. Mote1, J. S. Dargad, B. N. Dole, Nanoscience and Nanoengineering 1(2) (2013) 116-122 25. Proença, M. P.; Sousa, C. T.; Pereira, A. M.; Tavares, P. B.; Ventura, J.; Vazquez, M.; Araujo, J. P. Size, Phys. Chem. Chem. Phys., 13 (2011) 9561−9567 26. P. Singh, A. Kaushal, D. Kaur, J Alloys and Compounds 471(2009) 11. 27. Thi Thanh Le Dang, Matteo Tonezzer, Procedia Engineering 120 ( 2015 ) 427 – 434 28. Anandan K., Rajendran V. Mater. Sci. Semicond. Process, 14 (2011) P. 43. 29. Grimsditch, M.; Kumar, S.; Goldman, R. S. A Brillouin. J. Magn. Magn. Mater., 129, (1994) 327−333 30. Cazzanelli, E.; Kuzmin, A.; Mariotto, G.; Mironova-Ulmane, N. J. Phys.: Condens. Matter, 15 (2003) 2045−2052. 31. M. Naseem Siddique, T. Ali, Ateeq Ahmed, P. Tripathi, Nano-Structures & NanoObjects 16 (2018) 156–166 32. M. Baraman, S. Paul, A. Sarkar, AIP Conf. Proc (2013) 1536 -427 33. S. Dewan, M. Tomar, R.P. Tandon, V. Gupta, J. Appl. Phys. 121 (2017) 215307. 34. P.M. Ponnusamy, S. Agilan a, N. Muthukumarasamy, Materials Characterization 114 (2016) 166–171 35. Nabeel A. Bakr, Sabah A. Salman, Ahmed M. Shano, International Letters of Chemistry, Physics and Astronomy, ISSN: 2299-3843(2014) Vol. 41, pp 15-30 36. Wasan Al-Taa’y, Mohammed Abdul Nabi et al. , International Journal of Polymer Science Volume, (2014) Article ID 697809, 6 pages 37. M. Naseem Siddique, Ateeq Ahmed and P. Tripathi, Optik - International Journal for Light and Electron Optics 185 (2019) 599–608 38. Zhu, W.; Wang, W.; Xu, H.; Shi, J. Mater. Chem. Phys, 99 (2006) 127 39. Ashish Chhaganlal Gandhi and Sheng Yun Wu, Nanomaterials 7 (2017) 231 40. Madhu, G., Biju, V., Physica E, 60 (2014) 200–205 41. Ashish Chhaganlal Gandhi and Sheng Yun Wu, Nanomaterials, 7(8) (2017)231 42. Sudheesh K. Shukla, Eric S. Agorku, Hemant Mittal, Ajay K. Mishra, Chemical Papers 68 (2) (2014) 217–222 43. Qiang Shi, Changzheng Wang, Shuhong Li, Qingru Wang, Bingyuan Zhang, Wenjun Wang, Junying Zhang and Hailing Zhu, Nanoscale Research Letters 2014, 9:480, 26

44. Luo Q, Wang LS, Guo HZ, Lin KQ, Chen Y, Yue GH, Peng DL: Appl Phys A 2012, 108:239–245 45. Liangling Sun, Balaji Devakumar, Jia Liang, Shaoying Wang, Qi Sun and Xiaoyong Huang J. Mater. Chem. C, 2019, 7, 10471-10480 46. Varghese, B.; Reddy, M. V.; Yanwu, Z.; Sheh Lit, C.; Hoong, T. C.; Subba Rao, G. V.; Chowdari, B. V. R.; Shen Wee, A. T.; Lim, C. T.; Sow, C. H. Chem. Mater. 20, (2008) 3360−3367 47. Jihui Lang, Qiang Han, Jinghai Yang, Changsheng Li, Xue Li, Lili Yang, Yongjun Zhang, Ming Gao, Dandan Wang, and Jian C, JOURNAL OF APPLIED PHYSICS 107 (2010) 074302 48. Ou, Y.; Li, G.; Liang, J.; Feng, Z.; Tong, Y. J. Phys. Chem. C, 114 (2010) 13509−13514. 49. Dan Liu, Dongsheng Li, a and Deren Yang, AIP ADVANCES 7(2017) 015028 50. Xinshan Rong, Fengxian Qiu, Jiao Qin, Hao Zhao, Jie Yan, Dongya Yang, Journal of Industrial and Engineering Chemistry 26 (2015) 354–363 51. Zhi-Yuan Chen, Yuqian Chen Q. K. Zhang et al., ECS Journal of Solid State Science and Technology, 6 (12) (2017) P798-P804 52. Patta Ravikumar, Bhagaban Kisan, and A. Perumal, AIP ADVANCES 5, 087116 (2015) 53. Hongfen Ji, Changlong Cai, Shun Zhou and Weiguo Liu, Journal of Materials Science: Materials in Electronics ,29 (2018) 12917–12926 54. S. Mandal, S. Banerjee, and K. S. R. Menon, Phys. Rev. B, 80 (2009) 214420 55. K.C. Verma, R.K. Kotnala, Phys. Chem. Chem. Phys. 18 (2016) 5647–5657 56. S. Mandal, K. S. R. Menon, S. K. Mahatha, and S. Banerjee, Appl. Phys. Lett., 99, (2011) 232507 57. S. R. Mohapatra et al., J. Appl. Phys, 121 (2017) 124101 58. Atefeh Jafaria, Siamak Pilban Jahromi et al. Journal of Magnetism and Magnetic Materials 469 (2019) 383–390 59. Nadia Febiana Djaja, Rosari Saleh, Materials Sciences and Applications, 4 (2013) 145-152 60. J. Iqbal, X. F. Liu, H. C. Zhu, C. C. Pan, Y. Zhang, D. P. Yu and R. H. Yu, Journal of Applied Physics, Vol. 106, (2009), p. 83515 61. S.K. Misra, S. Diehl, J. Magn. Reson. 219 (2012) 53–60. 62. G. Dixit · P. Negi · J.P. Singh · R.C. Srivastava, J Supercond Nov Magn 26 (2013) 1015–1019

Highlights  

Pure and Ce doped NiO nanoparticles have been successfully synthesized by chemical route such as sol-gel method. Average crystallite size has been calculated ranging from 30.32 to 40.69 nm.

27

   

Ce doping provides the blue and green emissions which may be very imperative for the fabrication of light-emitting diodes and laser diodes etc. The existence of mixed oxidation states of Ce ion (Ce3+ and Ce4+) confirmed from the XPS spectrum The presence of Ni vacancy is found to be responsible for the observed weak ferromagnetism in pure and doped NiO samples. EPR analysis reveals the presence of unpaired electron where the field resonance decreases along with increased g value as the Ce concentration increases when compared to pure NiO

Author Agreement

Dear Sir, Please find herewith a manuscript entitled “Exploring the Ce3+ ions doping effect on Optical and Magnetic Properties of NiO Nanostructures” for possible publication in your esteemed Journal.

The manuscript has not been previously published, is not currently submitted for review to any other journal and will not be submitted elsewhere before a decision is made by this journal. The authors declare no conflict of interest. Hopefully the manuscript meets the demands of the journal. We look forward to your positive response.

Thanking you,

Yours sincerely, Dr. P. Tripathi Department of Applied Physics, Aligarh Muslim University, Aligarh-202 002, India E-mail address: [email protected]

Conflict of interest 28

The authors declare no conflict of interest.

29