Effect of iron doping concentration on magnetic properties of ZnO nanoparticles

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 2587–2591

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Effect of iron doping concentration on magnetic properties of ZnO nanoparticles Prashant K. Sharma a,, Ranu K. Dutta a, Avinash C. Pandey a, Samar Layek b, H.C. Verma b a b

Nanophosphor Application Centre, University of Allahabad, Allahabad 211002, India Department of Physics, Indian Institute of Technology, Kanpur 208016, India

a r t i c l e in fo

abstract

Article history: Received 2 February 2009 Received in revised form 18 February 2009 Available online 25 March 2009

The ZnO:Fe nanoparticles of mean size 3–10 nm were synthesized at room temperature by simple coprecipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe–Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mo¨ssbauer spectroscopy investigation showed Fe3+ nature of the iron atom in ZnO matrix. & 2009 Elsevier B.V. All rights reserved.

Keywords: DMS Ferromagnetism VSM Mo¨ssbauer Spectroscopy

1. Introduction In recent years the scientific community has paid much attention to the synthesis and characterization of II–VI semiconductor materials at nanometer scale, due to their great potential to test fundamental concepts of quantum mechanics [1,2] and because of their key role in various applications such as solid state lighting devices (LEDs), photonics [3], nanoelectronics [4], optoelectronics and data storage. ZnO is an important II–VI semiconductor having a wide and direct band gap (as wide as 3.37 eV), equivalent to that of GaN [5]. Besides this, ZnO is piezoelectric and optically transparent with a large exciton binding energy of 60 MeV. Recently, great progress has been made in ZnO device fabrication, especially in p-type doping, ultraviolet lasing and nanostructures. Various Group III metals such as Al, Mn, Fe, Co, Ni and rare earth elements such as Eu, Er and Tb have been doped in ZnO nanoparticles for various applications. Transition metal-doped ZnO has garnered special interest as diluted magnetic semiconductor (DMS) material. Progress in producing high-quality ZnO to obtain ferromagnetism at or above room temperature by doping with 3-d transition metals has been highlighted in several papers [6–8]. The effect of Fe doping as a magnetism activator and as a compensator of n-type material is of great importance for II–VI semiconductors [9–11]. It has been

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E-mail address: [email protected] (P.K. Sharma). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.03.043

found that the diluted magnetic semiconductors formed by replacing the cations of III–V or II–VI nonmagnetic semiconductors by ferromagnetic Mn, Fe and Co exhibit a number of unique magnetic, magneto-optical and magnetotransport properties, applicable for magneto-electronic and spintronic devices. In the present paper, we reveal a simple technique to synthesize ZnO:Fe nanoparticles with excellent magnetic properties.

2. Experimental details For synthesis of ZnO:Fe nanoparticles, the zinc nitrate hexahydrate (99.2%) Zn(NO3)2  6H2O, iron nitrate (99.4%) Fe(NO3)3  9H2O potassium hydroxide KOH, methanol and ethanol were, procured from E. Merck Limited, Mumbai 400018, India. All chemicals were of AR grade and were directly used without special treatment. Synthesis of ZnO:Fe (1–20% Fe doped) nanophosphors were carried out using the same technique followed by Jayakumar et al. [12] with little modification after optimization of reaction parameters and conditions. For synthesizing ZnO:Fe, appropriate amount of metal nitrates were taken in 100 ml of methanol and dissolved while continuous stirring for 2 hours at room temperature (Solution A). Simultaneously, 140 mmol KOH solution was prepared in 100 ml of methanol with refluxing through water condenser with constant stirring for 2 hours at 50 1C (Solution B). Now, mix the solutions A and B with constant stirring for 2 hours. This mixing was done while refluxing through water condenser at 50 1C. The final solution was allowed to cool at room temperature

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and aged overnight. This solution was centrifuged and washed several times with absolute ethanol and water in order to remove unnecessary impurities. The obtained product was placed in a vacuum oven for 24 h at 50 1C to get powders of ZnO:Fe. Similar procedure was followed for synthesis of all the varying contents of ZnO:Fe (1–20% Fe doping). We have named 1%, 2%, 3%, 5%, 8%, 10%, 15% and 20% Fe-doped samples as RF 1, RF 2, RF 3, RF 4, RF 5, RF 6, RF 7 and RF 8, respectively.

2.1. Characterization used The prepared ZnO:Fe nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), in order to elaborate structural properties in a precise manner for various doping percentage of Fe. XRD was performed on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using CuKa1 radiation with a wavelength of 1.54 A˚ in the wide angle region from 251 to 701 on 2y scale. The size and morphology of prepared nanoparticles were found using a transmission electron microscope (model Technai 30 G 2 S-Twin electron microscope) operated at 300 kV accelerating voltage by dissolving the assynthesized powder sample in ethanol and then placing a drop of this dilute ethanolic solution on the surface of copper grid. Room temperature magnetization measurement was carried out using a vibrating sample magnetometer (VSM, ADE Magnetics, USA) upto an applied field of 1.75 T with pressed pellets of prepared powdered samples. Room-temperature Mo¨ssbauer measurements were performed in the transmission geometry using a conventional 57Fe constant acceleration Mo¨ssbauer spectrometer employing 25 mCi 57Co (embedded in Rh matrix) source. All the spectra were analyzed using least square method assuming Lorentzian lineshapes of the spectra.

3. Results and discussion

Fig. 1. X-ray diffraction spectra of ZnO for different doping percentage of Fe. The XRD spectra showed crystalline nature having hexagonal wurtzite structure of ZnO having space group P63mc.

Fig. 1 shows the XRD pattern of the ZnO:Fe nanoparticles synthesized in the current work. XRD spectra show broad peaks at the positions of 31.631, 34.501, 36.251, 47.501, 56.601, 62.801, 66.361, 67.921 and 68.911, which are in good agreement with the standard JCPDS file for ZnO (JCPDS 36–1451, a ¼ b ¼ 3.249 A˚, c ¼ 5.206 A˚) and can be indexed as the hexagonal wurtzite structure of ZnO having space group P63mc. Furthermore, it can be seen that as the Fe doping percentages increases, the wurtzite

Fig. 2. TEM images of ZnO:Fe nanoparticles, for RF 1, RF 2, RF 4, RF 6 and RF 8 samples, respectively.

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structure of ZnO starts gradual degradation. The degradation in crystallinity was also observed from sample RF 1 to RF 8 and enhancement in the peak broadening was also observed indicating more nanonature of the higher Fe content samples. This was due to distortion in the host ZnO lattice because of the introduction of a foreign impurity i.e. Fe doping. This is mainly because of a decrease in nucleation and subsequent growth rate due to increasing Fe doping percentage. All the available reflections of the present XRD phases have been fitted with a Gaussian distribution. The broadening of XRD peaks (i.e.,

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Scherrer’s broadening) attributes nanosized formation of ZnO. Decrease in crystallinity was also observed with doping percentage. The variations in XRD results were well supported by TEM measurements. Fig. 2 represents the TEM image for all the samples. The morphology of all the samples was found to be spherical in nature having diameters ranging from 18 to 3 nm for different samples. Fig. 2 clearly shows that the diameters of these spherical nanoparticles were in good agreement with those obtained using XRD results for all the samples. Fig. 3(a) and (b)

Fig. 3. HRTEM images of ZnO:Fe nanoparticles for (a) RF 1 and (b) RF 8 samples, respectively.

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Fig. 4. Room temperature M–H loop for as-synthesized ZnO:Fe nanoparticles. Observed clear hysteresis indicates ferromagnetic nature of the prepared nanoparticles at room temperature.

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show high-resolution transmission electron microscopic (HRTEM) image of RF 1 and RF 8 samples respectively. The imaged lattice spacing 2.3 A˚ (Fig. 3a) for RF 1 sample, corresponds to the (0 0 2) planes of hexagonal wurtzite structure of ZnO while a remarkable deviation in the imaged lattice spacing 2.18 A˚ (Fig. 3b) was observed for RF 8 sample. HRTEM measurements showed a remarkable shrink in imaged d-spacing for higher doping percentage of Fe (samples RF 7 and RF 8), whereas for the other samples no significant change in imaged d-spacing were observed as compared to standard ZnO wurtzite structure, i.e., the effect of Fe doping is dominant and appeared only at higher doping percentages which is in good agreement with XRD and VSM results. Fig. 4 shows the dependence of magnetization with applied magnetic field (M–H loop) for all the ZnO:Fe samples at room temperature. A clear hysteresis loop, with noticeable coercivity, was observed. Initially, for 1% Fe doping ZnO nanoparticles (RF 1) shows diamagnetic character, while ferromagnetic nature was observed for 2% and 3% Fe-doped (RF 2 and RF 3) samples and again for further high doping of Fe (RF 4–RF 8) paramagnetic nature dominates. Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe–Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe.

Clear hysteresis loops with coercivities 10.0, 9.7, 0.7 and 0.5 mT were observed for RF 2, RF 4, RF 6 and RF 8 samples, respectively. Their corresponding magnetization of remanence were 8.98  103, 8.98  103, 6.48  103 and 5.0  103 emu/g, respectively. The noticeable coercivity of M–H loop could be attributed to strong ferromagnetism at room temperature. The narrow hysteresis implies a small amount of dissipated energy in repeatedly reversing the magnetization which is important for quick magnetization and demagnetization of the samples synthesized. The ferromagnetic behavior can be attributed to the presence of small magnetic dipoles located at the surface of nanocrystals, which interacts with their nearest neighbors inside the crystal. Consequently, the interchange energy in these magnetic dipoles making other neighboring dipoles oriented in the same direction. In nanocrystals, surface to volume ratio increases, so the population of magnetic dipoles oriented in the same direction will increase at the surface. Thus, the sum of the total amount of dipoles oriented along the same direction will increase subsequently. In short, the crystal surface will be usually more magnetically oriented. Fig. 5 shows the Mo¨ssbauer spectra for all the samples recorded at room temperature in order to probe local magnetic environment around the Fe sites and to determine the oxidation state of the Fe in ZnO matrix. Each spectrum shows a paramagnetic doublet with isomer shift (IS) ranging from 0.30 to 0.34 mm/s and quadrupole splitting (QS) 0.68–0.77 mm/s

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Fig. 5. Room temperature Mo¨ssbauer spectra of ZnO:Fe samples of different Fe concentration.

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indicating Fe3+ nature of the Fe atom in ZnO. No signature of Fe2+ has been found in any of this spectrum. Similar kind of behavior of the Fe environment has been found in Fe-doped ZnO nanoparticles synthesized using chemical pyrophoric reaction method with IS0.56 mm/s and QS0.73 mm/s reported recently [13]. Whereas coexistences of Fe3+ (QS0.81 mm/s) and Fe2+ (QS2.00 mm/s) state has been found in the Fe-doped ZnO polycrystalline sample prepared by solid state reaction method [14]. Relative transmission of the spectrum in our case is very much less than 2% reported for the sample prepared by the solid state reaction method and can be due to nanophase nature of the sample which is also seen in [13].

4. Conclusion We have successfully synthesized the ultrafine diluted magnetic semiconductors nanoparticles of ZnO:Fe of mean size 3–10 nm were synthesized at room temperature. Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe–Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room temperature Mo¨ssbauer spectroscopy investigation showed Fe3+ nature of the iron atom in ZnO matrix.

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Acknowledgements Authors are thankful to DST and CSIR, India for the financial assistance during the current work. ACMS facility at IITK is thankfully acknowledged for VSM measurements of all the samples. References [1] [2] [3] [4] [5]

[6] [7] [8] [9] [10] [11] [12] [13] [14]

S.M. Prokes, K.L. Wang, Mater. Res. Sci. Bull. 24 (1999) 13. J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. S. Nakamura, Science 281 (1998) 956. C.A. Mirkin, Science 286 (1999) 2095. V.A. Karpina, V.I. Lazorenko, C.V. Lashkarev, V.D. Dobrowolski, L.I. Kopylova, V.A. Baturin, S.A. Lytuyn, V.P. Ovsyannikov, E.A. Mauvenko, Cryst. Res. Technol. 39 (2004) 980–992. S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, J. Vac. Sci. Technol. B 22 (2004) 932. C. Liu, F. Yun, H. Moroc, J. Mater. Sci. 16 (2005) 555. R. Janisch, P. Gopal, N.A. Spaldin, J. Phys.: Condens. Matter. 17 (2005) R657. J. Hans, J.W. Song, C.-H. Yang, S.H. Park, J.-H. Park, Y.H. Jeong, K.W. Rhie, Appl. Phys. Lett. 81 (2002) 4212. X. Zhao, S. Komuro, H. Isshiki, Y. Aoyagi, Sugamo, J. Lumin. 87–89 (2000) 1254. T.G. Kryshtab, V.S. Khomchenko, V.P. Papsha, M.O. Mazin, Y.A. Tzykunov, Thin Solid Films 403/404 (2002) 76. O.D. Jayakumar, H.G. Salunke, R.M. Kadam, Manoj Mohapatra, G. Yaswant, S.K. Kulshreshtha, Nanotechnology 17 (2006) 1278–1285. D. Karmakar, S.K. Mandal, R.M. Kadam, P.L. Paulose, A.K. Rajarajan, T.K. Nath, A.K. Das, I. Dasgupta, G.P. Das, Phys. Rev. B 75 (2007) 144404. G.Y. Ahn, S.-I. Park, I.-B. Shim, C.S. Kim, J. Magn. Magn. Mater. 282 (2004) 166.