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Structural, optical and magnetic properties of nanoparticles of ZnO:Ni—DMS prepared by sol–gel method
Materials Chemistry and Physics 123 (2010) 450–455
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structural, optical and magnetic properties of nanoparticles of ZnO:Ni—DMS prepared by sol–gel method R. Elilarassi, G. Chandrasekaran ∗ Department of Physics, School of Physical Chemical and Applied Sciences, Pondicherry University, R.V. Nagar, Kalapet, Puducherry 605 014, India
a r t i c l e
i n f o
Article history: Received 28 November 2009 Received in revised form 19 March 2010 Accepted 28 April 2010 Keywords: Diluted magnetic semiconductor Sol–gel growth X-ray diffraction Optical property
a b s t r a c t Ni-doped ZnO nanoparticles having 0%, 2%, 4%, 6%, 8% and 10% of Ni are synthesized by means of low temperature sol–gel (auto-combustion) method. The effects of Ni doping on the structural and optical properties of ZnO:Ni particles in powder sample are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible spectroscopy and photoluminescence (PL) spectroscopy. The XRD analyses on Ni doped and undoped samples reveal the formation of single phase, polycrystalline and hexagonal-wurtzite structure. The SEM images show clusters of particles in nanosizes and some particles in the form of thin rods. The FTIR spectra confirm the formation of tetrahedral coordination of the oxygen ions surrounding the zinc ions and a shift in the frequency of bands brings out Ni doping in ZnO. The optical absorption spectra show a shift in the position of band edge towards lower energy. The estimated band gap is found to decrease with higher nickel doping. The room temperature PL measurements illustrate UV emission centered on 392 nm (3.16 eV), which is ascribed to the near-band-edge (NBE) emissions of ZnO, violet emission at 411 nm (3.01 eV) and blue emission at 450 nm (2.75 eV). The cause of decrease in intensity of these emission lines when content of Ni is increased in Ni-doped ZnO nanoparticles is explained on the basis of relative changes in the distribution of radiative and non-radiative defect sites as Ni chooses its sites. The nanoparticles of ZnO:Ni exhibit room temperature ferromagnetic phase in them. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Among the wide band semiconductors, oxide semiconductors are studied widely because of their promising role in photonic and spintronic applications. ZnO is one of the wide band gap semiconductors having a value of band gap, Eg = 3.37 eV and exciton binding energy = 60 meV [1,2]. Dietl et al. [3] and Sato and KatayamaYoshida [4] have predicted theoretically that ZnO can behave like a ferromagnet at room temperature when a dilute amount of transition element is doped in it. Ultimately a subsequent logical step of the experimentalists is to confirm those theoretical predictions about diluted magnetism in semiconductor. The researches on the synthesis and experimental characterizations of the nanostructures for diluted magnetic semiconductor (DMS) at room temperature have gained momentum. Wakano et al. [5] have observed ferromagnetism at 2 K in the Ni-doped ZnO films, but it has become superparamagnetism at 300 K and retained it up to room temperature [7]. Yin et al. [6] have reported that thin film of Ni-doped ZnO shows paramagnetism [8]. On the other hand, ferromagnetism is
∗ Corresponding author. Tel.: +91 413 2654408. E-mail address: [email protected] (G. Chandrasekaran). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.04.039
observed at room temperature in Ni/ZnO nanorods [11] and Nidoped ZnO films [10]. Cui et al. reported the room temperature ferromagnetism anisotropy of Co and Ni-doped ZnO nanowires synthesized by an electrochemical process [9]. Quantum dots of Ni-doped ZnO are proved to be good DMS materials with ferromagnetism up to 350 K [12–14]. Cong et al. [15,16] have studied the nanoparticles of Ni-doped ZnO prepared by a rheological phase reaction-precursor method and given experimental demonstration of magnetic phases in them. The present work envisages to prepare nanoparticles of ZnO:Ni by a wet chemistry synthesis i.e., sol–gel route of synthesis and study the structural, optical and magnetic properties due to nickel doping. 2. Experimental 2.1. Synthesis The synthesis of ZnO:Ni samples is done by dissolving the stoichiometric amount of Zn(NO3 )2 ·6H2 O, Ni(NO3 )2 ·6H2 O, and C2 H5 NO2 (glycine) to get 100 ml aqueous solution. The nitrates and glycine are in highly pure analytical reagent grade quality. “Glycine” serves as fuel for the combustion reaction [17]. The aqueous solution is stirred well for about 1 h to yield a uniform mixture of precursor. The precursor solution is continuously stirred and heated at 100 ◦ C so that the excess free water evaporates. Then, the solution is converted into a “Gel”. The “Gel” subsequently swells as foam which thereafter proceeds to a self-propagating combustion reaction leaving a fine powder. The powder so obtained appears moss green for ZnO
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Table 1 Particle size and concentration of nickel in ZnO:Ni nanoparticles. Concentration (x)
Particle size (nm)
0 0.02 0.04 0.06 0.08 0.1
18 22 21 20 18 15
3. Results and discussion
The observed values of lattice parameters are plotted with respect to concentration as shown in Fig. 2. It is clear from Fig. 2 that the enhancement of Ni doping has reduced the size of lattice parameters a and c which is attributed to the ionic size mismatch between Zn2+ (0.074 nm) and Ni2+ (0.069 nm) [19]. In order to understand surface deformation caused by any doping in the nanoparticles a systematic analysis of the values of parameters a, c, and d spacing in (0 0 2) and (1 0 0) planes using least square fit has been done. The least square fit lines could reveal that the spacing between (0 0 2) planes shows a shallow widening when Ni replaces Zn while the parameters a, c and interplanar spacing between (1 0 0) planes decrease. Such differing trends may be considered as an influence of nanoregime. It means that when Ni ions replace the sites of Zn, defects are also created and they preferably exist in the surface of the nanoparticles. The defects may have larger sizes than Ni ions and hence a shallow increase in the value of d spacing in (0 0 2) plane. The size of the crystallites lies in the range from 15 nm to 22 nm. The Ni content definitely enhances of size of particles as seen from Table 1. It can happen as Ni ions get substituted in more number on the surface i.e., along ‘c’ axis, in the beginning. The surface Ni ions accelerate the growth by inter particle attraction during nucleation. The excess Ni would tend to occupy down the surface i.e. along ‘a’ and ‘b’ axes. The later action causes reduction of lattice constant of crystal structure. It is considered as intra particle contribution arising out of different coordination of ions. For further addition of Ni, the later action offsets the earlier one and hence a gradual decrease caused by the resultant of forces. Thus different trends in particle size and lattice constant are attributed to the intra and inter nucleating forces forming the nanocrystals.
3.1. XRD study
3.2. SEM–EDAX study
The X-ray diffraction patterns of undoped and nickel doped ZnO samples are shown in Fig. 1. The well resolved peaks observed in the X-ray diffraction patterns indicate that the powder samples possess single phase and polycrystalline particles of ZnO:Ni. The XRD patterns show good matching with hexagonal P63mc structure of ZnO indexed using Joint Council for Powder Diffraction Standards file (JCPDS 36-1451, a = b = 3.249 A and c = 5.206 A). The manual calculation of lattice parameters (a and c) of the samples is done using the formula [18] for the structure:
The morphology of the particles of Ni-doped ZnO investigated using SEM is shown in Fig. 3 (representative cases). A glance at the SEM images of the samples shows a group of dark and bright regions. Fig. 3(c) is taken for a resolution of 500 nm; a comparison of least count of graticule in this SEM micrograph reveals the presence of the cluster formations surrounded by the pores. It is clearly observed that the cluster comprises of some tiny needle shaped particles. The SEM Fig. 3(a)–(d) shows that the particles are in nanosizes. The EDAX analysis of ZnO:Ni nanoparticles is carried out to establish purity of the samples and confirm presence of Ni in them. The observed EDAX patterns are shown in Fig. 4. It is noticed from Fig. 4 that the samples prepared contain only Zn, Ni and O. Thus the nanocrystallites are found to contain no spurious contamination. The ratio of the atomic percentage of the elements present in the samples complies with the quantity taken for their preparation.
Fig. 1. X-ray diffraction patterns of ZnO:Ni powder samples.
and it becomes darker with increasing Ni content. The powder samples are then ground well using agate mortar and pestle for an hour and made available for further characterization. 2.2. Characterization techniques The structure and morphology of the powder samples are estimated by X-ray powder diffraction analysis (XRD) (PAnalytical Model: X’Pert PRO) and scanning electron microscope (SEM) (HITACHI Model: S-3400N) respectively. The direct evidence for the substitution of Ni is brought out using energy density X-ray analysis (EDAX) (Thermo S-3400N). The vibration frequency of bonds in the sample is observed using Fourier transform infrared (FTIR) spectrometer (SHIMADSU) in the range 400–1000 cm−1 . The optical absorption spectra are recorded at room temperature using ultraviolet–visible (UV–vis) spectrophotometer (Model: CARY-5000) in the wavelength range 300–800 nm. The room temperature photoluminescence (PL) measurement is carried out using a Spectroflourimeter (Fluorolog-3) and magnetic study is done using vibrating sample magnetometer (VSM) (Lakeshore-7404).
2 sin = 4
2
4 3
h2 + hk + k2 a2
l2 + 2 c
(1)
where is the diffraction angle, is incident wavelength ( = 0.154056 nm) and h, k and l are all Miller’s indices. The average grain size of the pure ZnO:Ni samples is estimated using the data of full width half maximum (FWHM) for all persistent peaks found in XRD pattern. The data are substituted in Scherrer formula [18] i.e. D=
0.9 cos
(2)
where D is crystallite diameter, is the wavelength of X-ray radiation, is FWHM and is the Bragg angle.
3.3. FTIR study Infrared transmittance spectra are employed to study the vibration bands due to Zn–O bond and the changes due to Ni substitution in its structure. Normally the band frequencies within 1000 cm−1 should be attributed to the bonds between inorganic elements. The
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Fig. 2. Lattice parameters of ZnO:Ni nanoparticles. (a) Lattice spacing d0 0 2 and d1 0 0 ; (b) lattice constants a and c.
Fig. 3. SEM micrographs of ZnO:Ni nanoparticles for representative concentrations (a), (b) for Zn0.98 Ni0.02 O, (c) and (d) for Zn0.92 Ni0.08 O.
FTIR spectra for different amount of Ni dilution in ZnO are shown in Fig. 5. The persistent bands which show a shift as a response to the incorporation of Ni in the sample are given in Table 2. The absorption bands observed in the ranges from 613 to 649 cm−1 Table 2 FTIR vibration band frequencies of ZnO:Ni nanoparticles. Concentration (x)
Wave number (cm−1 )
0.00 0.02 0.04 0.06 0.08 0.10
412 410 414 416 412 418
460 457 458 460 464 462
647 649 648 652 618 613
762 766 762 764 763 760
832 825 834 847 850 849
and from 457 to 462 cm−1 are attributed to the stretching modes of Zn–O [20,21] in the tetrahedral and octahedral coordinations respectively. A band observed close to 412 cm−1 is attributed to the motion of mass content of ions in tetrahedral ligand field. It should also be mentioned that the vibration frequency of Zn–O bonds shows shifts. The shift in the values of vibration frequencies from 412, 460 and 647 cm−1 is suggestive of incorporation of Ni in the octahedral and tetrahedral sites existing in hexagonal-wurtzite structure. The shift in frequency is caused by the difference in the bond lengths that occurs when Ni ion replaces Zn ion. The bands occurring near 762 and 832 cm−1 are attributed to the vibrations of Zn–O–Ni local bonds and defect states respectively. A change in the density of defect states surrounding Ni ions is confirmed [22,23] as these later band frequencies sincerely vary with Ni concentration.
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Fig. 6. Optical absorption spectra of ZnO:Ni samples.
Fig. 4. EDAX spectra for (a) Zn0.98 Ni0.02 O and (b) Zn0.92 Ni0.08 O nanoparticles.
Fig. 7. The absorbance (˛h)2 versus photon energy (h) for ZnO:Ni nanoparticles.
Fig. 5. FTIR spectra of ZnO:Ni nanoparticles.
This inference is supported by the explanation given in the XRD study. 3.4. UV–visible study The room temperature optical absorption spectra for the various compositions of ZnO:Ni nanoparticles are shown in Fig. 6. Pure ZnO sample shows absorption band edge at 400 nm but the band
edge shows a shift towards higher wavelength side for the nickel doped samples. The red shift of band edge for the nickel doped samples clearly indicates that Ni2+ ions are incorporated into the ZnO lattice [13]. The inherent reason for red shift in band edge is due to the change of the sp–d exchange interactions between the band electrons and the localized d-electrons of the Ni2+ ions [24] which is also considered as the blossoming of magnetic phase. A wide strong absorption band around 450–550 nm and a weak absorption band in the range 600–700 nm are also observed in visible region for the nickel doped samples. These absorption bands are assigned to the spin–orbit split 3 T1 (F) → 3 T1 (P) ligand field transitions [25] of the Ni2+ ions in tetrahedral coordination. The values of band gap energy of the samples are estimated from the plots of (˛h)2 versus photon energy (h) shown in Fig. 7. It involves a standard procedure of extrapolating the vertical and linear part of the curve to cut the energy axis. The value of energy at the point where the extrapolated line cuts x-axis is taken as band gap energy. The measured values of optical band gap energy of ZnO:Ni nanoparticles range between 3.24 and 3.05 eV. The variation of experimental and theoretical band gap energy values for substitution of Ni in ZnO is shown in Fig. 8. The experimental values of band gap are the ones obtained from the optical study. The theoretical ones are calculated as per the procedure fol-
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Fig. 8. Concentration of Ni versus band gap energy of ZnO:Ni nanoparticles. Fig. 10. Nickel concentration versus normalized FWHM of emission peaks of ZnO:Ni nanoparticles.
Fig. 9. Photoluminescence spectra of ZnO:Ni nanoparticles.
lowed in literature using the data of particle size, effective mass values of electron and hole [26–30]. Two curves seen in Fig. 8 show a good agreement and a small reduction in band gap in ZnO:Ni nanoparticles. It is explained on the basis of creation of energy levels of 5 D4 state of Ni2+ below 3 F4 state of Zn2+ i.e., just below the conduction band of ZnO. 3.5. Photoluminescence study The PL spectra for the dilution of nickel in ZnO shown in Fig. 9 are obtained employing a laser light of 350 nm wavelength as the excitation source in spectroflourimeter. The PL spectra in Fig. 9 show three strong peaks occurring around 392 nm (3.16 eV), 411 nm (3.01 eV) and 450 nm (2.75 eV); the first one is in the ultraviolet (UV) region, while other two correspond to violet and blue respectively in visible region. They are certainly due to ZnO as they are present in x = 0 case in Fig. 9. It is well known that the UV emission peak at 3.16 eV is ascribed to the near-band-edge emission of ZnO which originates from the recombination of free excitons through an exciton–exciton collision process [31–33]. The violet emission around 3.01 eV is related to oxygen vacancies. The blue emission around 2.75 eV may be attributed to negatively charged Zn vacancies [34,35]. The studies on the present set of samples using XRD and FTIR have already brought out the presence of
defect states in ZnO:Ni nanoparticles and their variation with Ni content. An analysis of the FWHM of the peaks is done in order to understand an effect of Ni substitution in ZnO:Ni nanoparticles. It is done so because the emissions of violet and blue bands are due to vacancy sites which will be varying during the substitution of Ni. It is also known that the respective FWHM is related to the particle size or band width or the number of vacancy sites near the bottom of conduction band. The variation of normalized FWHM (ratio of FWHM of the violet or blue band to the ultraviolet band) with respect to the concentration of Ni in PL spectra is shown in Fig. 10. It is clearly observed from Fig. 10 that the FWHM of the emitted spectral bands show a faster upward bowing for blue, where as, a slow trend for violet with the increase of Ni content. It reveals a correlation of relative Ni contents in ZnO:Ni nanoparticles to the defect states very clearly [36]. Normally the defect-related emission in ZnO is due to the several relaxation processes accompanied by the band edge excitation. Further the recombination processes of electrons and holes arise from different radiative and non-radiative defect centers present in a photonic material. Considering the information from the XRD and FTIR studies, it can be said that the formation of complexes near Ni enhances the zinc vacancies along with some anomalous situations. The anomaly is due to the creation of the octahedral complexes near Ni besides the tetrahedral complexes of Zn. It is these combinations which statistically distribute and enhance the zinc vacant and oxygen defect sites in ZnO:Ni. Legitimately the difference in crystal field stabilization surrounding Ni is responsible for the shallow structural deformations during the growth of nanoparticles. A reasonable mechanism may be the interactions of Ni ions with lattice through some relaxation processes. It is so because Ni2+ ions possess the magnetically active spin states. Thus it can be explained that the present set of samples may be ferromagnetic as they support carrier-mediated exchange interactions as predicted [3,4] for several transition-metal-doped ZnO DMS. 3.6. Magnetic study The magnetic study on ZnO:Ni nanoparticles would unambiguously throw more light on the role of spin dependent magnetic interaction and the formation of ferromagnetic phase. The mag-
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magnetic semiconductors which have a platform for the carriermediated d–d exchange interaction and will be used in spintronics. Acknowledgements The authors are thankful to Central Instrumentation Facility, Pondicherry University for permitting us to use SEM, EDAX, FTIR, PL, VSM facilities, DST-FIST for funding XRD facility in Department of Physics, Pondicherry University and Department of Chemistry, Pondicherry University for UV–visible spectroscopic analysis. References
Fig. 11. Magnetic hysteresis curves of ZnO:Ni nanoparticles.
netic hysteresis curves traced at room temperature for the samples with Ni substitution in ZnO are shown in Fig. 11. The bent natures of curves clearly exhibit a shallow ferromagnetism in our samples. The observed values of coercive field vary between 13.0 and 24.7 mT. The magnetization ranges from 1.2 × 10−3 to 5.0 × 10−3 emu/gm at a maximum applied magnetic field of 600 mT. Undoubtedly the presence of Ni in ZnO nanoparticles and hence the magnetic d–d exchange interaction between the magnetic moments of Ni2+ contribute for the ferromagnetic state. The changes in magnetization and coercive field are explained on the basis of the distribution of Ni2+ ions in the structure of ZnO nanoparticles. The observed order of coercive field in our samples is attributed to the differences in defect states and anisotropy contribution by the clusters of crystallites. The ZnO:Ni nanoparticles of the present work having considerably low magnetization could form the useful diluted magnetic semiconductors for spintronic applications. 4. Conclusion Nickel doped ZnO nanoparticles have been successfully prepared by sol–gel auto-combustion route. The nanoparticles of Ni-doped ZnO continue to have a similar wurtzite phase as that of ZnO. The XRD and FTIR analyses confirm the formation of defect centers in the structure. Interestingly, the optical study reveals the encroachment of Ni energy levels in forbidden region near the bottom of conduction band. PL emission property establishes that magnetic interactions developed by the spins of Ni2+ ions substantially contribute for vacancy sites. The magnetic study reports that ZnO:Ni nanoparticles of sol–gel method show a shallow ferromagnetism. Thus the study of structural, optical and magnetic properties of ZnO:Ni nanoparticles establishes that they are diluted
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structural, optical and magnetic properties of nanoparticles of ZnO:Ni—DMS prepared by sol–gel method R. Elilarassi, G. Chandrasekaran ∗ Department of Physics, School of Physical Chemical and Applied Sciences, Pondicherry University, R.V. Nagar, Kalapet, Puducherry 605 014, India
a r t i c l e
i n f o
Article history: Received 28 November 2009 Received in revised form 19 March 2010 Accepted 28 April 2010 Keywords: Diluted magnetic semiconductor Sol–gel growth X-ray diffraction Optical property
a b s t r a c t Ni-doped ZnO nanoparticles having 0%, 2%, 4%, 6%, 8% and 10% of Ni are synthesized by means of low temperature sol–gel (auto-combustion) method. The effects of Ni doping on the structural and optical properties of ZnO:Ni particles in powder sample are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible spectroscopy and photoluminescence (PL) spectroscopy. The XRD analyses on Ni doped and undoped samples reveal the formation of single phase, polycrystalline and hexagonal-wurtzite structure. The SEM images show clusters of particles in nanosizes and some particles in the form of thin rods. The FTIR spectra confirm the formation of tetrahedral coordination of the oxygen ions surrounding the zinc ions and a shift in the frequency of bands brings out Ni doping in ZnO. The optical absorption spectra show a shift in the position of band edge towards lower energy. The estimated band gap is found to decrease with higher nickel doping. The room temperature PL measurements illustrate UV emission centered on 392 nm (3.16 eV), which is ascribed to the near-band-edge (NBE) emissions of ZnO, violet emission at 411 nm (3.01 eV) and blue emission at 450 nm (2.75 eV). The cause of decrease in intensity of these emission lines when content of Ni is increased in Ni-doped ZnO nanoparticles is explained on the basis of relative changes in the distribution of radiative and non-radiative defect sites as Ni chooses its sites. The nanoparticles of ZnO:Ni exhibit room temperature ferromagnetic phase in them. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Among the wide band semiconductors, oxide semiconductors are studied widely because of their promising role in photonic and spintronic applications. ZnO is one of the wide band gap semiconductors having a value of band gap, Eg = 3.37 eV and exciton binding energy = 60 meV [1,2]. Dietl et al. [3] and Sato and KatayamaYoshida [4] have predicted theoretically that ZnO can behave like a ferromagnet at room temperature when a dilute amount of transition element is doped in it. Ultimately a subsequent logical step of the experimentalists is to confirm those theoretical predictions about diluted magnetism in semiconductor. The researches on the synthesis and experimental characterizations of the nanostructures for diluted magnetic semiconductor (DMS) at room temperature have gained momentum. Wakano et al. [5] have observed ferromagnetism at 2 K in the Ni-doped ZnO films, but it has become superparamagnetism at 300 K and retained it up to room temperature [7]. Yin et al. [6] have reported that thin film of Ni-doped ZnO shows paramagnetism [8]. On the other hand, ferromagnetism is
∗ Corresponding author. Tel.: +91 413 2654408. E-mail address: [email protected] (G. Chandrasekaran). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.04.039
observed at room temperature in Ni/ZnO nanorods [11] and Nidoped ZnO films [10]. Cui et al. reported the room temperature ferromagnetism anisotropy of Co and Ni-doped ZnO nanowires synthesized by an electrochemical process [9]. Quantum dots of Ni-doped ZnO are proved to be good DMS materials with ferromagnetism up to 350 K [12–14]. Cong et al. [15,16] have studied the nanoparticles of Ni-doped ZnO prepared by a rheological phase reaction-precursor method and given experimental demonstration of magnetic phases in them. The present work envisages to prepare nanoparticles of ZnO:Ni by a wet chemistry synthesis i.e., sol–gel route of synthesis and study the structural, optical and magnetic properties due to nickel doping. 2. Experimental 2.1. Synthesis The synthesis of ZnO:Ni samples is done by dissolving the stoichiometric amount of Zn(NO3 )2 ·6H2 O, Ni(NO3 )2 ·6H2 O, and C2 H5 NO2 (glycine) to get 100 ml aqueous solution. The nitrates and glycine are in highly pure analytical reagent grade quality. “Glycine” serves as fuel for the combustion reaction [17]. The aqueous solution is stirred well for about 1 h to yield a uniform mixture of precursor. The precursor solution is continuously stirred and heated at 100 ◦ C so that the excess free water evaporates. Then, the solution is converted into a “Gel”. The “Gel” subsequently swells as foam which thereafter proceeds to a self-propagating combustion reaction leaving a fine powder. The powder so obtained appears moss green for ZnO
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451
Table 1 Particle size and concentration of nickel in ZnO:Ni nanoparticles. Concentration (x)
Particle size (nm)
0 0.02 0.04 0.06 0.08 0.1
18 22 21 20 18 15
3. Results and discussion
The observed values of lattice parameters are plotted with respect to concentration as shown in Fig. 2. It is clear from Fig. 2 that the enhancement of Ni doping has reduced the size of lattice parameters a and c which is attributed to the ionic size mismatch between Zn2+ (0.074 nm) and Ni2+ (0.069 nm) [19]. In order to understand surface deformation caused by any doping in the nanoparticles a systematic analysis of the values of parameters a, c, and d spacing in (0 0 2) and (1 0 0) planes using least square fit has been done. The least square fit lines could reveal that the spacing between (0 0 2) planes shows a shallow widening when Ni replaces Zn while the parameters a, c and interplanar spacing between (1 0 0) planes decrease. Such differing trends may be considered as an influence of nanoregime. It means that when Ni ions replace the sites of Zn, defects are also created and they preferably exist in the surface of the nanoparticles. The defects may have larger sizes than Ni ions and hence a shallow increase in the value of d spacing in (0 0 2) plane. The size of the crystallites lies in the range from 15 nm to 22 nm. The Ni content definitely enhances of size of particles as seen from Table 1. It can happen as Ni ions get substituted in more number on the surface i.e., along ‘c’ axis, in the beginning. The surface Ni ions accelerate the growth by inter particle attraction during nucleation. The excess Ni would tend to occupy down the surface i.e. along ‘a’ and ‘b’ axes. The later action causes reduction of lattice constant of crystal structure. It is considered as intra particle contribution arising out of different coordination of ions. For further addition of Ni, the later action offsets the earlier one and hence a gradual decrease caused by the resultant of forces. Thus different trends in particle size and lattice constant are attributed to the intra and inter nucleating forces forming the nanocrystals.
3.1. XRD study
3.2. SEM–EDAX study
The X-ray diffraction patterns of undoped and nickel doped ZnO samples are shown in Fig. 1. The well resolved peaks observed in the X-ray diffraction patterns indicate that the powder samples possess single phase and polycrystalline particles of ZnO:Ni. The XRD patterns show good matching with hexagonal P63mc structure of ZnO indexed using Joint Council for Powder Diffraction Standards file (JCPDS 36-1451, a = b = 3.249 A and c = 5.206 A). The manual calculation of lattice parameters (a and c) of the samples is done using the formula [18] for the structure:
The morphology of the particles of Ni-doped ZnO investigated using SEM is shown in Fig. 3 (representative cases). A glance at the SEM images of the samples shows a group of dark and bright regions. Fig. 3(c) is taken for a resolution of 500 nm; a comparison of least count of graticule in this SEM micrograph reveals the presence of the cluster formations surrounded by the pores. It is clearly observed that the cluster comprises of some tiny needle shaped particles. The SEM Fig. 3(a)–(d) shows that the particles are in nanosizes. The EDAX analysis of ZnO:Ni nanoparticles is carried out to establish purity of the samples and confirm presence of Ni in them. The observed EDAX patterns are shown in Fig. 4. It is noticed from Fig. 4 that the samples prepared contain only Zn, Ni and O. Thus the nanocrystallites are found to contain no spurious contamination. The ratio of the atomic percentage of the elements present in the samples complies with the quantity taken for their preparation.
Fig. 1. X-ray diffraction patterns of ZnO:Ni powder samples.
and it becomes darker with increasing Ni content. The powder samples are then ground well using agate mortar and pestle for an hour and made available for further characterization. 2.2. Characterization techniques The structure and morphology of the powder samples are estimated by X-ray powder diffraction analysis (XRD) (PAnalytical Model: X’Pert PRO) and scanning electron microscope (SEM) (HITACHI Model: S-3400N) respectively. The direct evidence for the substitution of Ni is brought out using energy density X-ray analysis (EDAX) (Thermo S-3400N). The vibration frequency of bonds in the sample is observed using Fourier transform infrared (FTIR) spectrometer (SHIMADSU) in the range 400–1000 cm−1 . The optical absorption spectra are recorded at room temperature using ultraviolet–visible (UV–vis) spectrophotometer (Model: CARY-5000) in the wavelength range 300–800 nm. The room temperature photoluminescence (PL) measurement is carried out using a Spectroflourimeter (Fluorolog-3) and magnetic study is done using vibrating sample magnetometer (VSM) (Lakeshore-7404).
2 sin = 4
2
4 3
h2 + hk + k2 a2
l2 + 2 c
(1)
where is the diffraction angle, is incident wavelength ( = 0.154056 nm) and h, k and l are all Miller’s indices. The average grain size of the pure ZnO:Ni samples is estimated using the data of full width half maximum (FWHM) for all persistent peaks found in XRD pattern. The data are substituted in Scherrer formula [18] i.e. D=
0.9 cos
(2)
where D is crystallite diameter, is the wavelength of X-ray radiation, is FWHM and is the Bragg angle.
3.3. FTIR study Infrared transmittance spectra are employed to study the vibration bands due to Zn–O bond and the changes due to Ni substitution in its structure. Normally the band frequencies within 1000 cm−1 should be attributed to the bonds between inorganic elements. The
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Fig. 2. Lattice parameters of ZnO:Ni nanoparticles. (a) Lattice spacing d0 0 2 and d1 0 0 ; (b) lattice constants a and c.
Fig. 3. SEM micrographs of ZnO:Ni nanoparticles for representative concentrations (a), (b) for Zn0.98 Ni0.02 O, (c) and (d) for Zn0.92 Ni0.08 O.
FTIR spectra for different amount of Ni dilution in ZnO are shown in Fig. 5. The persistent bands which show a shift as a response to the incorporation of Ni in the sample are given in Table 2. The absorption bands observed in the ranges from 613 to 649 cm−1 Table 2 FTIR vibration band frequencies of ZnO:Ni nanoparticles. Concentration (x)
Wave number (cm−1 )
0.00 0.02 0.04 0.06 0.08 0.10
412 410 414 416 412 418
460 457 458 460 464 462
647 649 648 652 618 613
762 766 762 764 763 760
832 825 834 847 850 849
and from 457 to 462 cm−1 are attributed to the stretching modes of Zn–O [20,21] in the tetrahedral and octahedral coordinations respectively. A band observed close to 412 cm−1 is attributed to the motion of mass content of ions in tetrahedral ligand field. It should also be mentioned that the vibration frequency of Zn–O bonds shows shifts. The shift in the values of vibration frequencies from 412, 460 and 647 cm−1 is suggestive of incorporation of Ni in the octahedral and tetrahedral sites existing in hexagonal-wurtzite structure. The shift in frequency is caused by the difference in the bond lengths that occurs when Ni ion replaces Zn ion. The bands occurring near 762 and 832 cm−1 are attributed to the vibrations of Zn–O–Ni local bonds and defect states respectively. A change in the density of defect states surrounding Ni ions is confirmed [22,23] as these later band frequencies sincerely vary with Ni concentration.
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Fig. 6. Optical absorption spectra of ZnO:Ni samples.
Fig. 4. EDAX spectra for (a) Zn0.98 Ni0.02 O and (b) Zn0.92 Ni0.08 O nanoparticles.
Fig. 7. The absorbance (˛h)2 versus photon energy (h) for ZnO:Ni nanoparticles.
Fig. 5. FTIR spectra of ZnO:Ni nanoparticles.
This inference is supported by the explanation given in the XRD study. 3.4. UV–visible study The room temperature optical absorption spectra for the various compositions of ZnO:Ni nanoparticles are shown in Fig. 6. Pure ZnO sample shows absorption band edge at 400 nm but the band
edge shows a shift towards higher wavelength side for the nickel doped samples. The red shift of band edge for the nickel doped samples clearly indicates that Ni2+ ions are incorporated into the ZnO lattice [13]. The inherent reason for red shift in band edge is due to the change of the sp–d exchange interactions between the band electrons and the localized d-electrons of the Ni2+ ions [24] which is also considered as the blossoming of magnetic phase. A wide strong absorption band around 450–550 nm and a weak absorption band in the range 600–700 nm are also observed in visible region for the nickel doped samples. These absorption bands are assigned to the spin–orbit split 3 T1 (F) → 3 T1 (P) ligand field transitions [25] of the Ni2+ ions in tetrahedral coordination. The values of band gap energy of the samples are estimated from the plots of (˛h)2 versus photon energy (h) shown in Fig. 7. It involves a standard procedure of extrapolating the vertical and linear part of the curve to cut the energy axis. The value of energy at the point where the extrapolated line cuts x-axis is taken as band gap energy. The measured values of optical band gap energy of ZnO:Ni nanoparticles range between 3.24 and 3.05 eV. The variation of experimental and theoretical band gap energy values for substitution of Ni in ZnO is shown in Fig. 8. The experimental values of band gap are the ones obtained from the optical study. The theoretical ones are calculated as per the procedure fol-
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Fig. 8. Concentration of Ni versus band gap energy of ZnO:Ni nanoparticles. Fig. 10. Nickel concentration versus normalized FWHM of emission peaks of ZnO:Ni nanoparticles.
Fig. 9. Photoluminescence spectra of ZnO:Ni nanoparticles.
lowed in literature using the data of particle size, effective mass values of electron and hole [26–30]. Two curves seen in Fig. 8 show a good agreement and a small reduction in band gap in ZnO:Ni nanoparticles. It is explained on the basis of creation of energy levels of 5 D4 state of Ni2+ below 3 F4 state of Zn2+ i.e., just below the conduction band of ZnO. 3.5. Photoluminescence study The PL spectra for the dilution of nickel in ZnO shown in Fig. 9 are obtained employing a laser light of 350 nm wavelength as the excitation source in spectroflourimeter. The PL spectra in Fig. 9 show three strong peaks occurring around 392 nm (3.16 eV), 411 nm (3.01 eV) and 450 nm (2.75 eV); the first one is in the ultraviolet (UV) region, while other two correspond to violet and blue respectively in visible region. They are certainly due to ZnO as they are present in x = 0 case in Fig. 9. It is well known that the UV emission peak at 3.16 eV is ascribed to the near-band-edge emission of ZnO which originates from the recombination of free excitons through an exciton–exciton collision process [31–33]. The violet emission around 3.01 eV is related to oxygen vacancies. The blue emission around 2.75 eV may be attributed to negatively charged Zn vacancies [34,35]. The studies on the present set of samples using XRD and FTIR have already brought out the presence of
defect states in ZnO:Ni nanoparticles and their variation with Ni content. An analysis of the FWHM of the peaks is done in order to understand an effect of Ni substitution in ZnO:Ni nanoparticles. It is done so because the emissions of violet and blue bands are due to vacancy sites which will be varying during the substitution of Ni. It is also known that the respective FWHM is related to the particle size or band width or the number of vacancy sites near the bottom of conduction band. The variation of normalized FWHM (ratio of FWHM of the violet or blue band to the ultraviolet band) with respect to the concentration of Ni in PL spectra is shown in Fig. 10. It is clearly observed from Fig. 10 that the FWHM of the emitted spectral bands show a faster upward bowing for blue, where as, a slow trend for violet with the increase of Ni content. It reveals a correlation of relative Ni contents in ZnO:Ni nanoparticles to the defect states very clearly [36]. Normally the defect-related emission in ZnO is due to the several relaxation processes accompanied by the band edge excitation. Further the recombination processes of electrons and holes arise from different radiative and non-radiative defect centers present in a photonic material. Considering the information from the XRD and FTIR studies, it can be said that the formation of complexes near Ni enhances the zinc vacancies along with some anomalous situations. The anomaly is due to the creation of the octahedral complexes near Ni besides the tetrahedral complexes of Zn. It is these combinations which statistically distribute and enhance the zinc vacant and oxygen defect sites in ZnO:Ni. Legitimately the difference in crystal field stabilization surrounding Ni is responsible for the shallow structural deformations during the growth of nanoparticles. A reasonable mechanism may be the interactions of Ni ions with lattice through some relaxation processes. It is so because Ni2+ ions possess the magnetically active spin states. Thus it can be explained that the present set of samples may be ferromagnetic as they support carrier-mediated exchange interactions as predicted [3,4] for several transition-metal-doped ZnO DMS. 3.6. Magnetic study The magnetic study on ZnO:Ni nanoparticles would unambiguously throw more light on the role of spin dependent magnetic interaction and the formation of ferromagnetic phase. The mag-
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magnetic semiconductors which have a platform for the carriermediated d–d exchange interaction and will be used in spintronics. Acknowledgements The authors are thankful to Central Instrumentation Facility, Pondicherry University for permitting us to use SEM, EDAX, FTIR, PL, VSM facilities, DST-FIST for funding XRD facility in Department of Physics, Pondicherry University and Department of Chemistry, Pondicherry University for UV–visible spectroscopic analysis. References
Fig. 11. Magnetic hysteresis curves of ZnO:Ni nanoparticles.
netic hysteresis curves traced at room temperature for the samples with Ni substitution in ZnO are shown in Fig. 11. The bent natures of curves clearly exhibit a shallow ferromagnetism in our samples. The observed values of coercive field vary between 13.0 and 24.7 mT. The magnetization ranges from 1.2 × 10−3 to 5.0 × 10−3 emu/gm at a maximum applied magnetic field of 600 mT. Undoubtedly the presence of Ni in ZnO nanoparticles and hence the magnetic d–d exchange interaction between the magnetic moments of Ni2+ contribute for the ferromagnetic state. The changes in magnetization and coercive field are explained on the basis of the distribution of Ni2+ ions in the structure of ZnO nanoparticles. The observed order of coercive field in our samples is attributed to the differences in defect states and anisotropy contribution by the clusters of crystallites. The ZnO:Ni nanoparticles of the present work having considerably low magnetization could form the useful diluted magnetic semiconductors for spintronic applications. 4. Conclusion Nickel doped ZnO nanoparticles have been successfully prepared by sol–gel auto-combustion route. The nanoparticles of Ni-doped ZnO continue to have a similar wurtzite phase as that of ZnO. The XRD and FTIR analyses confirm the formation of defect centers in the structure. Interestingly, the optical study reveals the encroachment of Ni energy levels in forbidden region near the bottom of conduction band. PL emission property establishes that magnetic interactions developed by the spins of Ni2+ ions substantially contribute for vacancy sites. The magnetic study reports that ZnO:Ni nanoparticles of sol–gel method show a shallow ferromagnetism. Thus the study of structural, optical and magnetic properties of ZnO:Ni nanoparticles establishes that they are diluted
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