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Existence of ferromagnetism and structural characterization of nickel doped ZnO nanocrystals
Applied Surface Science 258 (2012) 7161–7165
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Existence of ferromagnetism and structural characterization of nickel doped ZnO nanocrystals R. Varadhaseshan, S. Meenakshi Sundar ∗ PG and Research Department of Physics, Sri Paramakalyani College, Alwarkurichi, Tirunelveli, 627412, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 18 March 2012 Received in revised form 3 April 2012 Accepted 3 April 2012 Available online 10 April 2012 Keywords: Metal oxide Solvothermal method Single phase DMS
a b s t r a c t Ni doped ZnO (Zn1−x Nix O, in which 0.1 ≤ x ≤ 0.4) diluted magnetic semiconductor nanoparticles are prepared by microwave irradiated solvothermal process. The structural properties are studied using X-ray diffraction. It is evident from the XRD spectrum that the Ni doped ZnO nanocompounds exhibit single phase hexagonal wurtzite structure with strong c-axis orientation. To improve the crystalline quality the samples are annealed at 400 ◦ C. The effects of annealing temperature and dopant concentration on the structural properties are also discussed. Unit cell expansion is clearly observed in Ni doped ZnO nanocrystals. The TEM images confirm that the particle size is 20 nm and the particles are well dispersed. The magnetic property of the nanocrystals was measured using vibrational sample magnetometer. According to the magnetization measurements ferromagnetic behavior was found in Zn0.9 Ni0.1 O combination. However, for the other higher dopant ratios paramagnetism is observed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a group II–VI wide bandgap semiconductor compound and has a direct bandgap around 3.2–3.37 eV in 300 K with a high exciton binding energy of 60 meV. ZnO has a stable hexagonal wurtzite structure with lattice spacing a = 0.325 nm and c = 0.521 nm and composed of a number of alternating planes with tetrahedrally coordinated O2− and Zn2+ ions, stacked alternately along the c-axis. Zinc oxide (ZnO) has wide range of technological applications as transparent conducting electrodes in solar cells, flat panel displays, surface acoustic wave devices, and sensors. Many techniques, such as PLD, CVD, MBE, sol–gel, etc. are used to synthesize nanocrystals. This paper focuses on the formation and structural characterization of Zn1−x Mx O (M = Ni) upto x = 0.4 using microwave irradiation technique. Since 1986, microwave irradiation has found a number of applications as a heating method in chemistry. As a quick, simple and energy efficient method, microwave synthesis has been developed. Microwave synthesis has the advantages of very short reaction times, production of small particles with a narrow particle size distribution, and high purity [1–4]. Jansen et al. [1] suggested that these advantages could be attributed to fast homogeneous nucleation and ready dissolution of the gel. Unfortunately the exact nature of the interaction of the microwaves with the reactants during the synthesis of materials
∗ Corresponding author. Tel.: +91 4634 283226; fax: +91 4634 283226. E-mail address: smsun [email protected] (S. Meenakshi Sundar). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.021
is somewhat uncertain and speculative. However, it is well known that the interaction between dielectric materials, liquids or solids, and the microwaves leads to what is generally known as dielectric heating in which electric dipoles in such materials respond to the applied electric field. In liquids, the constant reorientation leads to a friction between the molecules, which subsequently generates heat [2]. The synthesis and structural properties of these samples are discussed. Ferromagnetic doped and undoped oxides have recently attracted huge attention. Among them, diluted magnetic semiconductors (DMS) with a Curie temperature above room temperature exhibit a high application potential in spintronics [5]. Recently, Co, Mn and Ni doped ZnO have been investigated for possible applications as spintronic materials [6–13]. In most of these methods, the nature of the produced material is amorphous and an additional high-temperature processing step is required in order to obtain crystallinity. DMSs are formed by partial substitution of the cations of the host semiconductors with small amount of magnetic transition metal (TM) ions. The DMSs are with charge and spin degrees of freedom in a single substance. According to the ab initio studies on the magnetism in ZnO-based diluted magnetic semiconductors and materials design for ferromagnetic DMSs, it is proposed that high Curie temperature (TC) ferromagnetic DMSs can be realized with ZnO-based DMSs doped with V, Cr, Fe, Co or Ni. It is also shown that it is possible to raise the TC of Fe, Co or Ni-doped ZnO by electron doping [14–16]. Gamelin and his co-workers [17,18] have done a lot of researches on the colloidal diluted magnetic semiconductor quantum dots (DMS-QDs), such as Ni2+ :ZnO and Co2+ :ZnO DMS-QDs. Ferromagnetism with TC 350 K
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R. Varadhaseshan, S. Meenakshi Sundar / Applied Surface Science 258 (2012) 7161–7165
Fig. 1. Synthesis flow chart of Ni doped ZnO nanocrystal. Fig. 2. XRD patterns of as prepared Ni doped ZnO nanocrystals.
was observed by Schwartz et al. [17] in aggregated nanocrystals of Co2+ :ZnO. Ferromagnetism with TC 350K was also observed by Radovanovic and Gamelin [18] in the DMS Ni2+ :ZnO synthesized from solution. Colloidal Ni2+ :ZnO nanocrystals are paramagnetic, while their aggregation gave rise to robust ferromagnetism. We have synthesized of Zn1−x Nix O semiconductor nanocrystals by a simple microwave irradiated solvothermal process. The aim of the study is to see whether room temperature ferromagnetism can be obtained in Zn1−x Nix O nanocrystal. The origin of RT FM observed in such a system remains a controversial subject. It is still in dispute whether the observed phenomenon is truly intrinsic or related to secondary phases such as Ni clusters.
2. Experimental method The precursors used for the synthesis of ZnO:Ni, are zinc acetate dihydrate, nickel acetate tetra hydrate, and urea. Zinc and nickel are source, urea as a catalyst ethylene glycol as a solvent. All chemicals were purchased from Merck Company. All samples of ZnO:Ni nanostructured particles were prepared by the microwave irradiation technique. First at room temperature, zinc acetate and dopant nickel acetate with urea were dissolved in ethylene glycol solution. The microwave power was set to 650 watts and operated at 2 min per cycle with 1-min interval until the precipitates formed. The resulting powder was washed with distilled water and acetone and left to dry. Nickel doped ZnO nanocrystal preparation process flowchart is shown in Fig. 1. As prepared samples were characterized by X-ray diffraction (XRD) using a Rigaku diffractometer for the crystallographic measurements. The X-ray diffractometer was operated at 40 kV and ˚ The TEM images were 40 mA with Cu K␣ radiation ( = 1.540598 A). taken by Philips Technai10 equipment. The magnetic properties of the powder sample were performed by EG&G PARC VSM 155 vibrating sample magnetometer.
3. Result and discussion The XRD patterns of the powder sample were measured at room temperature with Rigaku diffractometer and Cu K␣ radia˚ as a source. The unannealed XRD of Ni doped tion ( = 1.5418 A) Zinc Oxide samples are highly c-axis oriented. All the peaks of unannealed samples shown in Fig. 2 correspond to standard ZnO diffraction pattern with wurtzite structure. It seems that the addition of Ni did not affect the structure of ZnO. It seems that the crystals are well oriented towards the c-axis, indicating that metal ions (Ni) tend to substitute for Zn sites without changing the crystal structure of ZnO (wurtzite structure) in the present composition region. The ‘d’ values of Ni doped ZnO crystals were in good agreement with those reported in the JCPDS file [PDF 79-206, ˚ possessing hexagonal wurtzite struca = 3.2499 A˚ and c = 5.2065 A], ture. To improve the crystallinity, the samples were annealed again at 400 ◦ C and the XRD spectra were taken for all the samples of Ni doped ZnO. The characteristic peaks with high intensities corresponding to the planes (1 0 0), (0 0 2), (1 0 1) and lower intensities of (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) indicate the annealed product is of high purity hexagonal ZnO wurtzite structure shown in Fig. 3. It is evident from the XRD spectra that there were no extra peaks due to Ni metal, other oxides or any Nickel phase, indicating that the as synthesized samples are of single phase. The lattice parameter values were calculated using the XRDA analysis software and tabulated (Table 1). The particle size was calculated using Debye–Scherer formula particle size d =
0.9 ˇ cos
where is the wavelength for the K␣ component of the employed ˚ ˇ is the corrected full width at half copper radiation (1.54056 A), maximum (FWHM) and is the Bragg’s angle. The average grain size of the nickel doped ZnO nanocrystal was calculated by using this formula is 25 nm to 40 nm respectively. From Table 1 the
R. Varadhaseshan, S. Meenakshi Sundar / Applied Surface Science 258 (2012) 7161–7165
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Table 1 Lattice parameters for different mole percentage of Ni doped ZnO nanocrystals annealed at 400 ◦ C. Lattice parameters ˚ (A)
Pure ZnO ˚ (A)
a c c/a ratio
3.2493 5.2073 1.6027
Different mol% of Ni doped ZnO 10% ˚ (A)
20% ˚ (A)
30% ˚ (A)
40% ˚ (A)
3.2851 5.2405 1.5952
3.2828 5.2323 1.5939
3.2703 5.2082 1.5926
3.2660 5.1806 1.5862
lattice constants of Zn1−x Nix O (x > 0) are slightly larger than those ˚ is larger than of pure ZnO, because the ionic radius of Ni2+ (0.69 A) ˚ [19,20]. The expansion of the lattice constants that of Zn2+ (0.60 A) of Zn1−x Nix O indicated that nickel is really, at least partially, doped into the ZnO structure. These results were plotted and it is shown in Fig. 4(a and b). TEM image of as prepared samples annealed at 400 ◦ C is measured by Philips Technai10 model instrument. For TEM measurements sample is dispersed in double distilled water and sonicated for 5 min. The dispersed solution is dropped on a copper grid and allowed to dry overnight in ambience. This is then analyzed with TEM. The grain size estimated using transmission electron microscope (using the 200 nm scale bar) is less than 20 nm, which is shown in Fig. 5. The origin of ferromagnetism at room temperature is still in dispute, because the substitution of equal valence ions is electrically equivalent, which is not consistent with Dietl’s hole induced ferromagnetism theory. DMS behavior has been observed in many reports [21–24]. Magnetic hysteresis (M–H) loop of the sample was obtained up to magnetic field range ±20 kOe at room temperature as the result is depicted in Fig. 6, indicating ferromagnetic behavior only in Zn0.9 Ni0.1 O combination [27,28]. The paramagnetic behavior, being linear has been fitted to a straight line with an intercept. The origin of ferromagnetism in Ni-doped ZnO has a number of possibilities. The first possibility is the formation
Fig. 3. XRD patterns of Ni doped ZnO nanocrystals annealed at 400 ◦ C.
of the secondary phase such as NiO, but this possibility can be easily ruled out, since bulk NiO is antiferromagnetic with a Neel temperature of 520 K [25]. Another possible origin of ferromagnetism is in Ni metal, which is a well-known ferromagnetic material. Concerning the intrinsic origin of RTFM in our Ni-doped ZnO nanocrystal, we can exclude the possibility that the observed FM originates from the secondary phase. First, metallic Ni is an unlikely source of the observed FM. From XRD analysis it is confirmed that there is a phase segregation in Zn1−x Nix O (x ≤ 0.4). We therefore conclude that the observed RTFM is an intrinsic property of Ni-doped ZnO nanocrystals. Ferromagnetism in dilute magnetic semiconductors is considered to originate from the exchange interaction between free delocalized carriers (holes or electrons from the valence band and the localized d spins on the TM ions [26]. Therefore, the presence of free carriers and localized d spins is a prerequisite for the appearance of ferromagnetism. According to Rudderman–Kittel–Kasuya–Yoshida
Fig. 4. (a) Ni concentration vs lattice constants a and c (b) Ni concentration vs c/a and volume of the unit cell.
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R. Varadhaseshan, S. Meenakshi Sundar / Applied Surface Science 258 (2012) 7161–7165 Table 2 Parameters calculated from VSM studies of different mol% Ni doped ZnO nanocrystals annealed at 400 ◦ C.
Fig. 5. TEM image showing the well dispersed particles of Zn0.9 Ni0.1 O nanocrystals.
Sample
Doping ratio
Coercivity (Hci) Oe
Retentivity (Hr) emu/g
Reference
Zn1−x Nix O
x = 0.1 x = 0.2 x = 0.3 x = 0.4
358.92 420.27 394.77 392.65
41 × 10−6 121 × 10−6 154 × 10−6 166 × 10−6
[27,28]
Unit cell expansion is clearly observed in the Ni doped ZnO samples. Ni2+ ions sit at Zn site in the wurtzite Structure. The particle sizes were calculated by using Debye–Scherer formula and it is between 25 nm and 40 nm. Room temperature ferromagnetic behavior was successfully achieved in solvothermal method synthesized Ni doped ZnO nanocrystals for lower dopant ratios (x = 0.1 mol). The results reveal that the ferromagnetism is induced by Ni2+ substituted for Zn2+ , not through secondary phases or magnetic clusters. Acknowledgement This work was supported by the University grants Commission (UGC- MRP), New Delhi, India. The authors thank the UGC for providing fund and the college management for continuous support to do this work. References
Fig. 6. VSM images of Zn1−x Nix O nanocrystals (0.1 ≤ x ≤ 0.4). The lower ratio (x = 0.1) only Ferromagnetic, shown in curve (a), the higher ratios (x = 0.2–0.4) are paramagnetic state, they are shown in curve (b–d).
(RKKY) theory, the magnetism is due to the exchange interaction between local-spin polarized electrons and conduction electrons. This interaction leads to the spin polarizations of conduction electrons. Subsequently, the spin polarization conductive electrons perform an exchange interaction with spin-polarized electrons of other Ni ions. Thus, after the long-range exchange interaction, almost all Ni moments align in the same direction. The conductive electrons are regarded as media to contact all Ni ions. As a result, the material exhibits ferromagnetism. So, the ZnO material doped with 10% Ni shows RTFM. Collectively, these results strongly indicate that the ferromagnetism is induced by doped Ni ions and is an intrinsic property of the powders. Table 2 shows the coercivity and retentivity value of Ni doped ZnO nanocrystals annealed at 400 ◦ C which is similar to the reported values. 4. Conclusion In summary, Nickel doped ZnO (Zn1−x Nix O, 0.1 ≤ x ≤ 0.5) diluted magnetic semiconductor nanoparticles are prepared by a solvothemal method of microwave irradiation technique. XRD analysis study shows wurtzite structures for all the Zn1−x Nix O nanocrystals.
[1] J.C. Jansen, A. Arafat, A.K. Barakat, H. Van Bekkum, in: M.L. Occelli, H. Robson (Eds.), Synthesis of Microporous Materials, Van Nostrand Reinhold, New York, 1992, p. 507. [2] K.J. Rao, B. Vaidhyanathan, M. Gaguli, P.A. Ramakrishnan, Chemistry of Materials 11 (1999) 882. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [4] K. Sato, H. Katayama-Yoshida, Japanese Journal of Applied Physics 39 (2000) L555. [5] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnár, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [6] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Science 291 (2001) 854. [7] S.A. Chambers, S. Thevuthasan, R.F.C. Farrow, R.F. Marks, J.W. Thiele, L. Folks, M.G. Samant, A.J. Kellock, N. Ruzycki, D.L. Ederer, U. Diebold, Applied Physics Letters 79 (2001) 3467. [8] Y. Matsumoto, R. Takahashi, M. Murakami, T. Koida, X.J. Fan, T. Hasegawa, T. Fukumura, M. Kawasaki, S.Y. Koshihara, H. Koinuma, Japanese Journal of Applied Physics Part 2 40 (2001) L1205. [9] W.K. Park, R.J. Ortega-Hertogs, J. Moodera, A. Punnoose, M.S. Seehra, Journal of Applied Physics 91 (2002) 8093. [10] P.A. Stampe, R.J. Kennedy, Y. Xin, J.S. Parker, Journal of Applied Physics 93 (2003) 7864. [11] A. Punnoose, M.S. Seehra, W.K. Park, J.S. Moodera, Journal of Applied Physics 93 (2003) 7867. [12] D.H. Kim, J.S. Yang, K.W. Lee, S.D. Bu, D.W. Kim, T.W. Noh, S.J. Oh, Y.W. Kim, J.S. Chung, H. Tanaka, H.Y. Lee, T. Kawai, J.Y. Won, S.H. Park, J.C. Lee, Journal of Applied Physics 93 (2003) 6125. [13] B.Z. Rameev, F. Yildiz, L.R. Tagirov, B. Aktas, W.K. Park, J.S. Moodera, Journal of Magnetism and Magnetic Materials 361 (2003) 258–259. [14] K. Sato, H. Katayama-Yoshida, Physica Status Solidi (B) 229 (2002) 673. [15] Y. Uspenskii, E. Kulatov, H. Mariette, H. Nakayama, H. Ohta, Journal of Magnetism and Magnetic Materials 248 (2003) 258–259. [16] K. Sato, H. Katayama-Yoshida, Semiconductor Science and Technology 17 (2002) 367. [17] D.A. Schwartz, N.S. Norberg, Q.P. Nguyen, J.M. Parker, D.R. Gamelin, Journal of the American Chemical Society 125 (2003) 13205. [18] P.V. Radovanovic, D.R. Gamelin, Physical Review Letters 91 (2003) 157202. [19] G. Pei, C. Xia, S. Cao, J. Zhang, F. Wu, J. Xu, Journal of Magnetism and Magnetic Materials 302 (July (2)) (2006) 340–342. [20] R.B. Zhao, D.L. Hou, J.M. Guo, C.M. Zhen, G.D. Tang, Journal of Superconductivity and Novel Magnetism 23 (2010) 1261–1265. [21] S.W. Jung, S.-J. An, G.-C. Yi, C.U. Jung, S.-I. Lee, S. Cho, Applied Physics Letters 80 (2002) 4561.
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[25] D.A. Schwartz, K.R. Littilatved, D.R. Gamelin, Applied Physics Letters 85 (2004) 1395. [26] T. Dietl, Semiconductor Science and Technology 17 (2002) 377. [27] J.B. Cui, U.J. Gibson, Applied Physics Letters 87 (2005) 133108. [28] L.-N. Tong, et al., Journal of Applied Physics 108 (2010) 023906.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Existence of ferromagnetism and structural characterization of nickel doped ZnO nanocrystals R. Varadhaseshan, S. Meenakshi Sundar ∗ PG and Research Department of Physics, Sri Paramakalyani College, Alwarkurichi, Tirunelveli, 627412, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 18 March 2012 Received in revised form 3 April 2012 Accepted 3 April 2012 Available online 10 April 2012 Keywords: Metal oxide Solvothermal method Single phase DMS
a b s t r a c t Ni doped ZnO (Zn1−x Nix O, in which 0.1 ≤ x ≤ 0.4) diluted magnetic semiconductor nanoparticles are prepared by microwave irradiated solvothermal process. The structural properties are studied using X-ray diffraction. It is evident from the XRD spectrum that the Ni doped ZnO nanocompounds exhibit single phase hexagonal wurtzite structure with strong c-axis orientation. To improve the crystalline quality the samples are annealed at 400 ◦ C. The effects of annealing temperature and dopant concentration on the structural properties are also discussed. Unit cell expansion is clearly observed in Ni doped ZnO nanocrystals. The TEM images confirm that the particle size is 20 nm and the particles are well dispersed. The magnetic property of the nanocrystals was measured using vibrational sample magnetometer. According to the magnetization measurements ferromagnetic behavior was found in Zn0.9 Ni0.1 O combination. However, for the other higher dopant ratios paramagnetism is observed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a group II–VI wide bandgap semiconductor compound and has a direct bandgap around 3.2–3.37 eV in 300 K with a high exciton binding energy of 60 meV. ZnO has a stable hexagonal wurtzite structure with lattice spacing a = 0.325 nm and c = 0.521 nm and composed of a number of alternating planes with tetrahedrally coordinated O2− and Zn2+ ions, stacked alternately along the c-axis. Zinc oxide (ZnO) has wide range of technological applications as transparent conducting electrodes in solar cells, flat panel displays, surface acoustic wave devices, and sensors. Many techniques, such as PLD, CVD, MBE, sol–gel, etc. are used to synthesize nanocrystals. This paper focuses on the formation and structural characterization of Zn1−x Mx O (M = Ni) upto x = 0.4 using microwave irradiation technique. Since 1986, microwave irradiation has found a number of applications as a heating method in chemistry. As a quick, simple and energy efficient method, microwave synthesis has been developed. Microwave synthesis has the advantages of very short reaction times, production of small particles with a narrow particle size distribution, and high purity [1–4]. Jansen et al. [1] suggested that these advantages could be attributed to fast homogeneous nucleation and ready dissolution of the gel. Unfortunately the exact nature of the interaction of the microwaves with the reactants during the synthesis of materials
∗ Corresponding author. Tel.: +91 4634 283226; fax: +91 4634 283226. E-mail address: smsun [email protected] (S. Meenakshi Sundar). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.021
is somewhat uncertain and speculative. However, it is well known that the interaction between dielectric materials, liquids or solids, and the microwaves leads to what is generally known as dielectric heating in which electric dipoles in such materials respond to the applied electric field. In liquids, the constant reorientation leads to a friction between the molecules, which subsequently generates heat [2]. The synthesis and structural properties of these samples are discussed. Ferromagnetic doped and undoped oxides have recently attracted huge attention. Among them, diluted magnetic semiconductors (DMS) with a Curie temperature above room temperature exhibit a high application potential in spintronics [5]. Recently, Co, Mn and Ni doped ZnO have been investigated for possible applications as spintronic materials [6–13]. In most of these methods, the nature of the produced material is amorphous and an additional high-temperature processing step is required in order to obtain crystallinity. DMSs are formed by partial substitution of the cations of the host semiconductors with small amount of magnetic transition metal (TM) ions. The DMSs are with charge and spin degrees of freedom in a single substance. According to the ab initio studies on the magnetism in ZnO-based diluted magnetic semiconductors and materials design for ferromagnetic DMSs, it is proposed that high Curie temperature (TC) ferromagnetic DMSs can be realized with ZnO-based DMSs doped with V, Cr, Fe, Co or Ni. It is also shown that it is possible to raise the TC of Fe, Co or Ni-doped ZnO by electron doping [14–16]. Gamelin and his co-workers [17,18] have done a lot of researches on the colloidal diluted magnetic semiconductor quantum dots (DMS-QDs), such as Ni2+ :ZnO and Co2+ :ZnO DMS-QDs. Ferromagnetism with TC 350 K
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R. Varadhaseshan, S. Meenakshi Sundar / Applied Surface Science 258 (2012) 7161–7165
Fig. 1. Synthesis flow chart of Ni doped ZnO nanocrystal. Fig. 2. XRD patterns of as prepared Ni doped ZnO nanocrystals.
was observed by Schwartz et al. [17] in aggregated nanocrystals of Co2+ :ZnO. Ferromagnetism with TC 350K was also observed by Radovanovic and Gamelin [18] in the DMS Ni2+ :ZnO synthesized from solution. Colloidal Ni2+ :ZnO nanocrystals are paramagnetic, while their aggregation gave rise to robust ferromagnetism. We have synthesized of Zn1−x Nix O semiconductor nanocrystals by a simple microwave irradiated solvothermal process. The aim of the study is to see whether room temperature ferromagnetism can be obtained in Zn1−x Nix O nanocrystal. The origin of RT FM observed in such a system remains a controversial subject. It is still in dispute whether the observed phenomenon is truly intrinsic or related to secondary phases such as Ni clusters.
2. Experimental method The precursors used for the synthesis of ZnO:Ni, are zinc acetate dihydrate, nickel acetate tetra hydrate, and urea. Zinc and nickel are source, urea as a catalyst ethylene glycol as a solvent. All chemicals were purchased from Merck Company. All samples of ZnO:Ni nanostructured particles were prepared by the microwave irradiation technique. First at room temperature, zinc acetate and dopant nickel acetate with urea were dissolved in ethylene glycol solution. The microwave power was set to 650 watts and operated at 2 min per cycle with 1-min interval until the precipitates formed. The resulting powder was washed with distilled water and acetone and left to dry. Nickel doped ZnO nanocrystal preparation process flowchart is shown in Fig. 1. As prepared samples were characterized by X-ray diffraction (XRD) using a Rigaku diffractometer for the crystallographic measurements. The X-ray diffractometer was operated at 40 kV and ˚ The TEM images were 40 mA with Cu K␣ radiation ( = 1.540598 A). taken by Philips Technai10 equipment. The magnetic properties of the powder sample were performed by EG&G PARC VSM 155 vibrating sample magnetometer.
3. Result and discussion The XRD patterns of the powder sample were measured at room temperature with Rigaku diffractometer and Cu K␣ radia˚ as a source. The unannealed XRD of Ni doped tion ( = 1.5418 A) Zinc Oxide samples are highly c-axis oriented. All the peaks of unannealed samples shown in Fig. 2 correspond to standard ZnO diffraction pattern with wurtzite structure. It seems that the addition of Ni did not affect the structure of ZnO. It seems that the crystals are well oriented towards the c-axis, indicating that metal ions (Ni) tend to substitute for Zn sites without changing the crystal structure of ZnO (wurtzite structure) in the present composition region. The ‘d’ values of Ni doped ZnO crystals were in good agreement with those reported in the JCPDS file [PDF 79-206, ˚ possessing hexagonal wurtzite struca = 3.2499 A˚ and c = 5.2065 A], ture. To improve the crystallinity, the samples were annealed again at 400 ◦ C and the XRD spectra were taken for all the samples of Ni doped ZnO. The characteristic peaks with high intensities corresponding to the planes (1 0 0), (0 0 2), (1 0 1) and lower intensities of (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) indicate the annealed product is of high purity hexagonal ZnO wurtzite structure shown in Fig. 3. It is evident from the XRD spectra that there were no extra peaks due to Ni metal, other oxides or any Nickel phase, indicating that the as synthesized samples are of single phase. The lattice parameter values were calculated using the XRDA analysis software and tabulated (Table 1). The particle size was calculated using Debye–Scherer formula particle size d =
0.9 ˇ cos
where is the wavelength for the K␣ component of the employed ˚ ˇ is the corrected full width at half copper radiation (1.54056 A), maximum (FWHM) and is the Bragg’s angle. The average grain size of the nickel doped ZnO nanocrystal was calculated by using this formula is 25 nm to 40 nm respectively. From Table 1 the
R. Varadhaseshan, S. Meenakshi Sundar / Applied Surface Science 258 (2012) 7161–7165
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Table 1 Lattice parameters for different mole percentage of Ni doped ZnO nanocrystals annealed at 400 ◦ C. Lattice parameters ˚ (A)
Pure ZnO ˚ (A)
a c c/a ratio
3.2493 5.2073 1.6027
Different mol% of Ni doped ZnO 10% ˚ (A)
20% ˚ (A)
30% ˚ (A)
40% ˚ (A)
3.2851 5.2405 1.5952
3.2828 5.2323 1.5939
3.2703 5.2082 1.5926
3.2660 5.1806 1.5862
lattice constants of Zn1−x Nix O (x > 0) are slightly larger than those ˚ is larger than of pure ZnO, because the ionic radius of Ni2+ (0.69 A) ˚ [19,20]. The expansion of the lattice constants that of Zn2+ (0.60 A) of Zn1−x Nix O indicated that nickel is really, at least partially, doped into the ZnO structure. These results were plotted and it is shown in Fig. 4(a and b). TEM image of as prepared samples annealed at 400 ◦ C is measured by Philips Technai10 model instrument. For TEM measurements sample is dispersed in double distilled water and sonicated for 5 min. The dispersed solution is dropped on a copper grid and allowed to dry overnight in ambience. This is then analyzed with TEM. The grain size estimated using transmission electron microscope (using the 200 nm scale bar) is less than 20 nm, which is shown in Fig. 5. The origin of ferromagnetism at room temperature is still in dispute, because the substitution of equal valence ions is electrically equivalent, which is not consistent with Dietl’s hole induced ferromagnetism theory. DMS behavior has been observed in many reports [21–24]. Magnetic hysteresis (M–H) loop of the sample was obtained up to magnetic field range ±20 kOe at room temperature as the result is depicted in Fig. 6, indicating ferromagnetic behavior only in Zn0.9 Ni0.1 O combination [27,28]. The paramagnetic behavior, being linear has been fitted to a straight line with an intercept. The origin of ferromagnetism in Ni-doped ZnO has a number of possibilities. The first possibility is the formation
Fig. 3. XRD patterns of Ni doped ZnO nanocrystals annealed at 400 ◦ C.
of the secondary phase such as NiO, but this possibility can be easily ruled out, since bulk NiO is antiferromagnetic with a Neel temperature of 520 K [25]. Another possible origin of ferromagnetism is in Ni metal, which is a well-known ferromagnetic material. Concerning the intrinsic origin of RTFM in our Ni-doped ZnO nanocrystal, we can exclude the possibility that the observed FM originates from the secondary phase. First, metallic Ni is an unlikely source of the observed FM. From XRD analysis it is confirmed that there is a phase segregation in Zn1−x Nix O (x ≤ 0.4). We therefore conclude that the observed RTFM is an intrinsic property of Ni-doped ZnO nanocrystals. Ferromagnetism in dilute magnetic semiconductors is considered to originate from the exchange interaction between free delocalized carriers (holes or electrons from the valence band and the localized d spins on the TM ions [26]. Therefore, the presence of free carriers and localized d spins is a prerequisite for the appearance of ferromagnetism. According to Rudderman–Kittel–Kasuya–Yoshida
Fig. 4. (a) Ni concentration vs lattice constants a and c (b) Ni concentration vs c/a and volume of the unit cell.
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R. Varadhaseshan, S. Meenakshi Sundar / Applied Surface Science 258 (2012) 7161–7165 Table 2 Parameters calculated from VSM studies of different mol% Ni doped ZnO nanocrystals annealed at 400 ◦ C.
Fig. 5. TEM image showing the well dispersed particles of Zn0.9 Ni0.1 O nanocrystals.
Sample
Doping ratio
Coercivity (Hci) Oe
Retentivity (Hr) emu/g
Reference
Zn1−x Nix O
x = 0.1 x = 0.2 x = 0.3 x = 0.4
358.92 420.27 394.77 392.65
41 × 10−6 121 × 10−6 154 × 10−6 166 × 10−6
[27,28]
Unit cell expansion is clearly observed in the Ni doped ZnO samples. Ni2+ ions sit at Zn site in the wurtzite Structure. The particle sizes were calculated by using Debye–Scherer formula and it is between 25 nm and 40 nm. Room temperature ferromagnetic behavior was successfully achieved in solvothermal method synthesized Ni doped ZnO nanocrystals for lower dopant ratios (x = 0.1 mol). The results reveal that the ferromagnetism is induced by Ni2+ substituted for Zn2+ , not through secondary phases or magnetic clusters. Acknowledgement This work was supported by the University grants Commission (UGC- MRP), New Delhi, India. The authors thank the UGC for providing fund and the college management for continuous support to do this work. References
Fig. 6. VSM images of Zn1−x Nix O nanocrystals (0.1 ≤ x ≤ 0.4). The lower ratio (x = 0.1) only Ferromagnetic, shown in curve (a), the higher ratios (x = 0.2–0.4) are paramagnetic state, they are shown in curve (b–d).
(RKKY) theory, the magnetism is due to the exchange interaction between local-spin polarized electrons and conduction electrons. This interaction leads to the spin polarizations of conduction electrons. Subsequently, the spin polarization conductive electrons perform an exchange interaction with spin-polarized electrons of other Ni ions. Thus, after the long-range exchange interaction, almost all Ni moments align in the same direction. The conductive electrons are regarded as media to contact all Ni ions. As a result, the material exhibits ferromagnetism. So, the ZnO material doped with 10% Ni shows RTFM. Collectively, these results strongly indicate that the ferromagnetism is induced by doped Ni ions and is an intrinsic property of the powders. Table 2 shows the coercivity and retentivity value of Ni doped ZnO nanocrystals annealed at 400 ◦ C which is similar to the reported values. 4. Conclusion In summary, Nickel doped ZnO (Zn1−x Nix O, 0.1 ≤ x ≤ 0.5) diluted magnetic semiconductor nanoparticles are prepared by a solvothemal method of microwave irradiation technique. XRD analysis study shows wurtzite structures for all the Zn1−x Nix O nanocrystals.
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