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Doping effects of Co2+ ions on structural and magnetic properties of ZnO nanoparticles
Microelectronic Engineering 89 (2012) 129–132
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Doping effects of Co2+ ions on structural and magnetic properties of ZnO nanoparticles Faheem Ahmed, Shalendra Kumar, Nishat Arshi, M.S. Anwar, Bon Heun Koo, Chan Gyu Lee ⇑ School of Nano and Advanced Materials Engineering, Changwon National University, Changwon 641-773, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 7 April 2011 Keywords: Diluted magnetic semiconductor ZnO X-ray diffraction DC magnetization Ferromagnetism
a b s t r a c t In this paper, we report the synthesis of Zn1 xCoxO (0.0 6 x 6 0.10) nanoparticles by an auto-combustion method using glycine as a fuel. The prepared nanoparticles were characterized by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and DC magnetization measurements. XRD results showed that Co doped ZnO have a single phase nature with wurtzite structure and Co2+ ions were successfully incorporated into the lattice position of Zn2+ ions in ZnO matrix. FTIR spectra demonstrated that the values of absorption bands were blue shifted with the increase of Co content. From the Raman spectra, all the peaks observed in undoped and Co-doped samples can be assigned to the Raman active modes of ZnO crystal. However, in case of Co doped ZnO sample, additional modes were observed that can be related to the substitution of Co into the Zn site. Magnetic studies showed that Co doped ZnO nanoparticles exhibit room temperature ferromagnetism. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Diluted magnetic semiconductors (DMSs) are very interesting materials subjected to their promising applications to Spintronics (spin + electronics). There has been greatly attention in magnetic semiconductors which exploit both spin and charge carrier because of combination of two degree of freedom, which assure new functionality of memories detectors and light emitting source [1,2]. Theoretical studies [3] showed that transition metal (TM) doped wide band gap semiconductors are prospective candidates for the room temperature ferromagnetism (RTFM). In fact, RTFM has been observed in TM doped ZnO [4,5]. However, the results remain controversial and some reports showed a very low magnetic ordering temperature in TM doped ZnO [6] or even the absence of FM in these samples prepared by means of different methods. These controversial results give an indication that RTFM in DMSs is extremely sensitive to preparation methods and thus of preparation conditions. ZnO-based DMSs, especially Co-doped ZnO are a possible candidate for a high-TC ferromagnetic semiconductor and has attracted a lot of attention. In addition, Zn1 xCoxO may be an ideal material for short-wave magneto-optical devices because of the wide band gap of ZnO as well as the high thermal solubility of Co in ZnO. A variety of preparation of nanoscale ZnO-based DMSs have been in used such as solvothermal, hydrothermal, self assembly and template assisted sol–gel methods ⇑ Corresponding author. E-mail address: [email protected] (C.G. Lee). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.03.149
[4,5,7]. However, in this paper we report the synthesis of Zn1 xCoxO nanoparticles by a low-temperature combustion method. This method ensures high chemical homogeneity due to the aqueous solution mixture of initial reagents and, as a result, favors the production of desired phases or ceramics. In the present work, we have synthesized nanocrystalline Co-doped ZnO powders by auto combustion method and characterized using XRD, FESEM, FTIR, Raman spectroscopy and DC magnetization measurements. 2. Experimental details All the chemicals used in the experiment were of analytical grade purity and purchased from Sigma Aldrich. In a typical synthesis of Zn1 xCoxO (0.0 6 x 6 0.1) nanoparticles, the appropriate proportion of Zn(NO3)26H2O, Co(NO3)26H2O, and C2H5NO2 (glycine) were completely dissolved in a 1000 ml beaker to obtain a 200 ml aqueous solution. The aqueous solution was then stirred for about 1 h in order to mix the solution uniformly. The solution was evaporated on a hot plate under constant stirring. When the water was completely removed, the solution then converted into a ‘‘gel’’. The ‘‘gel’’ was subsequently swelling into foam like and undergo a strong self-propagating combustion reaction to give a fine powder. The calcined samples were characterized for crystal phase identification by X-ray diffraction (XRD) using Phillips X’pert (MPD 3040) X-ray diffractometer with Cu Ka radiations (k = 1.5406 Å) operated at voltage of 40 kV and current of 30 mA. Fourier transmission infrared (FTIR) spectra of the powders (as pellets in KBr) were recorded using a Fourier transmission infrared spectrometer
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F. Ahmed et al. / Microelectronic Engineering 89 (2012) 129–132
(Nicolet Impact 410 DSP) in the range of 4000 to 400 cm 1 with a resolution of 1 cm 1. The surface morphology of the fine synthesized powders was carried out using field emission scanning electron microscopy (FESEM) using a TESCAN; MIRA II LMH microscope. In order to get the phonon vibrational study of the pure and Co doped ZnO, we used a micro-Raman spectrometer (NRS-3100) with a 532 nm solid state primary laser as an excitation source at room temperature. Magnetization measurements were performed using a commercial Quantum Design physical property measurement system.
3. Results and discussions Fig. 1 shows the XRD patterns of Zn1 xCoxO (0.0 6 x 6 0.1) samples, which was indexed using POWDER-X software as the ZnO wurtzite structure and well matched with the standard data (JCPDS, 36-1451). It can be clearly seen from the XRD patterns that all the samples showed a single-phase nature with hexagonal wurtzite structure. No secondary phase was detected, thus indicated that the Co dopant ought to be incorporated into the lattice as a substitutional atom. In order to study the effect of Co doping, a careful analysis of the position of the XRD peaks indicate that there is a shifting in peak’s position towards lower 2h value (see inset (a) in Fig. 1) with increasing Co contents. The shifting of the peak’s position shows that the lattice parameters increase with the increase of Co doping. Refined value of the lattice parameters a and c as a function of Co content are shown in inset (b) of Fig. 1. These values obtained from the refinement are in good agreement with the standard data base [JCPDS, 36-1451]. The increasing trend of lattice parameters clearly indicates that Co ions are substituting Zn in ZnO matrix and these results are in a good agreement with those reported earlier [8]. The crystallite sizes of the synthesized powders were estimated from X-ray lines broadening using Scherer’s equation [9] were found to increase ranging from 32, 33, 34, 36, 39 and 41 nm for pure ZnO and Zn0.99Co0.010, Zn0.97Co0.030, Zn0.95Co0.050, Zn0.93Co0.070 and Zn0.90Co0.1O samples, respectively. The morphology and chemical composition of as synthesized powders were further investigated by FESEM and EDX analysis. Inset of Fig. 2(a) and (b) shows the FESEM images of undoped and 5%
Fig. 1. XRD patterns of pure and Co-doped ZnO nanoparticles. Inset (a) magnified spectrum of (1 0 0) peak and inset (b) the plot of lattice parameters vs. Co content.
Co doped ZnO nanoparticles which are homogeneous and agglomerated with diameters ranging from 40 to 50 nm. Due to the uniform distribution of oxidized metal anions in the threedimensional polymeric network structure, the agglomeration could be induced by densification resulting from the narrow space between particles [10]. It is clear from the FESEM images that the particles are nearly spherical in shape and Co doping results in an aggregation of nanoparticles. With the increase in Co doping, the spacing between the particles is expected to become narrower and also there is an increase in particle size, which leads to the agglomeration of nanoparticles. Fig. 2(a) and (b) depict the typical EDX spectra taken from pure and 5% Co-doped ZnO samples. The chemical analysis of Zn1 xCoxO with x = 0.05 measured by EDX analysis shows the presence of Zn, O and Co signals only; indicates that the nanoparticles are made up of zinc, oxygen and Co ions which shows that the Co ion is substituting the Zn ion in ZnO matrix. Only zinc and oxygen signals have been detected in undoped ZnO sample, suggesting that the nanoparticles are indeed made up of Zn and O. Thus resulting a high purity of ZnO nanopowder. To study the change in Zn–O bonding due to the Co substitution, FTIR measurements of Co doped ZnO has been carried out. FTIR measurements were performed in the wave number range 4000 to 400 cm 1 using KBr method at Room temperature as shown in Fig. 2 (c). The FTIR spectra show main absorption bands near 3400 cm 1 represent O–H mode, those at 2900 cm 1 are C–H mode, band arising from the absorption of atmospheric CO2 on the metallic cations at 2350 cm 1 and 1400–1600 cm 1 are the C@O stretching mode. The absorption band at 454 cm 1 is the stretching mode of ZnO [11]. However, in case of 1%, 3%, 5%, 7% and 10% Co doped samples, the value of absorption bands were found to be blue shifted at 449, 443, 438, 432, and 427 cm 1, respectively. The enlarged spectrum in the wave number range <1000 cm 1 is shown in the inset of Fig. 2(c). The change in the peak position of ZnO absorption bands reflects that Zn–O–Zn network is perturbed by the presence of Co in its environment. Therefore, the FTIR results also indicate that Co is occupying Zn position in ZnO matrix as observed in XRD measurements. The Raman spectra are sensitive to the crystal quality, structural defects and disorders of the grown products. ZnO has a Wurtzite structure that belongs to the C6v symmetry group. Fig. 2(d) shows the Raman spectra of un-doped and Co-doped ZnO nanoparticles. It is evident that Raman modes in the spectrum of undoped ZnO have similar positions as in the spectrum of bulk ZnO. However, several changes can be observed in the Raman spectrum of Co-doped ZnO in comparison with the spectrum of undoped ZnO. The mode E2high at 439 cm 1 that is related to the vibration of oxygen atoms in wurtzite ZnO [12] is the most prominent mode in undoped ZnO powder. Drastic decrease in its intensity and blue shift to 437, 435, 433 and 429 cm 1 for the 1%, 3%, 5% and 7% Co doped ZnO samples, respectively, observed in the Raman spectrum, indicate the changes in the defect structure due to mechanical activation. There are strong peaks appeared at 2B1low; 2LA (540, 525, 520 and 523 cm 1) for Co doping, as shown in Fig. 2(d). We deem that the peaks at 2B1low; 2LA (525, 523, 520 and 516 cm 1) were Co–O mode, which contributed to local vibrations of Co ions in ZnO lattice as a substitution of Co into Zn position [13]. In the present work, the additional mode appears after doping, which maybe due to Co2+ occupation at Zn2+ sites. Fig. 3 shows magnetization versus magnetic field (M–H) curves for Zn1 XCoxO (x = 0.03 and 0.07) samples measured at room temperature. The magnetization increased with increasing Co content, showed that Co-doped ZnO exhibit RTFM with a TC at or above room temperature. Calculated value of the coercive field (HC) and the remanent magnetization (Mr) for Zn1 XCoxO (x = 0.03 and 0.07) found to be 108.23 (Oe), 8.09 10 6 (emu) and 110.19 (Oe), 3.16 10 5 (emu) respectively. The origin of ferromagnetism
F. Ahmed et al. / Microelectronic Engineering 89 (2012) 129–132
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Fig. 2. (a) EDX spectrum of un-doped ZnO nanoparticles and inset shows corresponding FESEM image, (b) EDX spectrum of 5% Co-doped ZnO and inset shows corresponding FESEM image, (c) FTIR spectra of Zn1 xCoxO (0.0 6 x 6 0.1) nanoparticles and inset shows the enlarged spectra in the range < 1000 cm 1, (d) Raman spectra of Zn1 xCoxO (0.0 6 x 6 0.1) samples.
4. Conclusions In summary, pure and Co-doped ZnO nanoparticles have been successfully synthesized using an effective auto combustion route. XRD and FTIR results indicated that all the synthesized un-doped and Co-doped ZnO samples had the wurtzite structure and no secondary phase was detected which indicated that Co ions substituted for Zn ions. FESEM results revealed that the prepared Co-doped ZnO nanoparticles are nearly spherical in shape with particle size <50 nm, which is in good agreement with the size obtained from XRD. The blue shift of the major Raman mode (E2high) and the decrease in its intensity with the increase in Co doping, points to the incorporation of Co ions in the ZnO lattice. Magnetic measurements studies conclude that the all Co doped samples showed RTFM. The presented simple synthesis method using cheap precursors can be use to prepare other interesting metal oxides nanoparticles. Fig. 3. Magnetization curve of Zn1 Inset shows the magnified loop.
XCoxO
(x = 0.03 and 0.07) at room temperature.
in DMSs is still controversial; however, in the present study, there can be three possible origins of ferromagnetism. The first is Zn1 xCoxO, the carrier induced ferromagnetism (RKKY mechanism), which is often reported for DMSs [14]. The second is CoOx, but we did not find any secondary phase in the XRD of Co-doped ZnO. The third is micro Co clusters, we also could not find any signal of Co clusters in the XRD. In the present work, the singlephased XRD patterns, the slightly shifted ZnO (1 0 0) peak with Co doping and the evidence of Co incorporation into ZnO from FTIR and Raman studies suggested that the detected ferromagnetism could arise from the homogeneous doping of Co into ZnO which follow the RKKY mechanism.
Acknowledgment This research was supported by Grant No. RTI04-01-03 from the Regional Technology Innovation Program of the Ministry of Knowledge Economy, South Korea. References [1] [2] [3] [4]
S.S. Kim, J.H. Moon, B.T. Lee, O.S. Song, J.H. Je, J. Appl. Phys. 95 (2004) 454. K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. X. Liu, F. Lin, L. Sun, W. Cheng, X. Ma, W. Shi, Appl. Phys. Lett. 88 (2006) 062508. [5] L.Q. Liu, B. Xiang, X.Z. Zhang, Y. Zhang, D.P. Yu, Appl. Phys. Lett. 88 (2006) 063104. [6] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nature Mater 4 (2005) 173.
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F. Ahmed et al. / Microelectronic Engineering 89 (2012) 129–132
[7] H. Zhang, X.Y. Ma, J. Xu, J.J. Niu, D.R. Yang, Nanotechnology 14 (2003) 423. [8] S. Kumar, Y.J. Kim, B.H. Koo, H. Choi, C.G. Lee, JKPS 55 (2009) 1060–1064. [9] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, Prentice Hall, New Jersey, 2001. [10] J.H. Ryu, C.S. Lim, K.H. Auh, J. Kor. Ceram. Soc. 39 (2002) 321. [11] R.F. Silva, M.E.D. Zaniquelli, Colloid Surf. A 551 (2002) 198–200.
[12] K.A. Alim, V.A. Fonoberov, M. Shamsa, A.A. Balandin, J. Appl. Phys. 97 (2005) 124313. [13] K. Samanta, P. Bhattacharya, R.S. Katiyar, W. Iwamoto, P.G. Pagliuso, C. Rettori, Phys. Rev. B 73 (2006) 245. [14] S. Koshihara, A. Oiwa, M. Hirasawa, S. Katsumoto, Y. Iye, C. Urano, H. Takagi, H. Munekata, Phys. Rev. Lett. 78 (1997) 4617.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Doping effects of Co2+ ions on structural and magnetic properties of ZnO nanoparticles Faheem Ahmed, Shalendra Kumar, Nishat Arshi, M.S. Anwar, Bon Heun Koo, Chan Gyu Lee ⇑ School of Nano and Advanced Materials Engineering, Changwon National University, Changwon 641-773, Republic of Korea
a r t i c l e
i n f o
Article history: Available online 7 April 2011 Keywords: Diluted magnetic semiconductor ZnO X-ray diffraction DC magnetization Ferromagnetism
a b s t r a c t In this paper, we report the synthesis of Zn1 xCoxO (0.0 6 x 6 0.10) nanoparticles by an auto-combustion method using glycine as a fuel. The prepared nanoparticles were characterized by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and DC magnetization measurements. XRD results showed that Co doped ZnO have a single phase nature with wurtzite structure and Co2+ ions were successfully incorporated into the lattice position of Zn2+ ions in ZnO matrix. FTIR spectra demonstrated that the values of absorption bands were blue shifted with the increase of Co content. From the Raman spectra, all the peaks observed in undoped and Co-doped samples can be assigned to the Raman active modes of ZnO crystal. However, in case of Co doped ZnO sample, additional modes were observed that can be related to the substitution of Co into the Zn site. Magnetic studies showed that Co doped ZnO nanoparticles exhibit room temperature ferromagnetism. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Diluted magnetic semiconductors (DMSs) are very interesting materials subjected to their promising applications to Spintronics (spin + electronics). There has been greatly attention in magnetic semiconductors which exploit both spin and charge carrier because of combination of two degree of freedom, which assure new functionality of memories detectors and light emitting source [1,2]. Theoretical studies [3] showed that transition metal (TM) doped wide band gap semiconductors are prospective candidates for the room temperature ferromagnetism (RTFM). In fact, RTFM has been observed in TM doped ZnO [4,5]. However, the results remain controversial and some reports showed a very low magnetic ordering temperature in TM doped ZnO [6] or even the absence of FM in these samples prepared by means of different methods. These controversial results give an indication that RTFM in DMSs is extremely sensitive to preparation methods and thus of preparation conditions. ZnO-based DMSs, especially Co-doped ZnO are a possible candidate for a high-TC ferromagnetic semiconductor and has attracted a lot of attention. In addition, Zn1 xCoxO may be an ideal material for short-wave magneto-optical devices because of the wide band gap of ZnO as well as the high thermal solubility of Co in ZnO. A variety of preparation of nanoscale ZnO-based DMSs have been in used such as solvothermal, hydrothermal, self assembly and template assisted sol–gel methods ⇑ Corresponding author. E-mail address: [email protected] (C.G. Lee). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.03.149
[4,5,7]. However, in this paper we report the synthesis of Zn1 xCoxO nanoparticles by a low-temperature combustion method. This method ensures high chemical homogeneity due to the aqueous solution mixture of initial reagents and, as a result, favors the production of desired phases or ceramics. In the present work, we have synthesized nanocrystalline Co-doped ZnO powders by auto combustion method and characterized using XRD, FESEM, FTIR, Raman spectroscopy and DC magnetization measurements. 2. Experimental details All the chemicals used in the experiment were of analytical grade purity and purchased from Sigma Aldrich. In a typical synthesis of Zn1 xCoxO (0.0 6 x 6 0.1) nanoparticles, the appropriate proportion of Zn(NO3)26H2O, Co(NO3)26H2O, and C2H5NO2 (glycine) were completely dissolved in a 1000 ml beaker to obtain a 200 ml aqueous solution. The aqueous solution was then stirred for about 1 h in order to mix the solution uniformly. The solution was evaporated on a hot plate under constant stirring. When the water was completely removed, the solution then converted into a ‘‘gel’’. The ‘‘gel’’ was subsequently swelling into foam like and undergo a strong self-propagating combustion reaction to give a fine powder. The calcined samples were characterized for crystal phase identification by X-ray diffraction (XRD) using Phillips X’pert (MPD 3040) X-ray diffractometer with Cu Ka radiations (k = 1.5406 Å) operated at voltage of 40 kV and current of 30 mA. Fourier transmission infrared (FTIR) spectra of the powders (as pellets in KBr) were recorded using a Fourier transmission infrared spectrometer
130
F. Ahmed et al. / Microelectronic Engineering 89 (2012) 129–132
(Nicolet Impact 410 DSP) in the range of 4000 to 400 cm 1 with a resolution of 1 cm 1. The surface morphology of the fine synthesized powders was carried out using field emission scanning electron microscopy (FESEM) using a TESCAN; MIRA II LMH microscope. In order to get the phonon vibrational study of the pure and Co doped ZnO, we used a micro-Raman spectrometer (NRS-3100) with a 532 nm solid state primary laser as an excitation source at room temperature. Magnetization measurements were performed using a commercial Quantum Design physical property measurement system.
3. Results and discussions Fig. 1 shows the XRD patterns of Zn1 xCoxO (0.0 6 x 6 0.1) samples, which was indexed using POWDER-X software as the ZnO wurtzite structure and well matched with the standard data (JCPDS, 36-1451). It can be clearly seen from the XRD patterns that all the samples showed a single-phase nature with hexagonal wurtzite structure. No secondary phase was detected, thus indicated that the Co dopant ought to be incorporated into the lattice as a substitutional atom. In order to study the effect of Co doping, a careful analysis of the position of the XRD peaks indicate that there is a shifting in peak’s position towards lower 2h value (see inset (a) in Fig. 1) with increasing Co contents. The shifting of the peak’s position shows that the lattice parameters increase with the increase of Co doping. Refined value of the lattice parameters a and c as a function of Co content are shown in inset (b) of Fig. 1. These values obtained from the refinement are in good agreement with the standard data base [JCPDS, 36-1451]. The increasing trend of lattice parameters clearly indicates that Co ions are substituting Zn in ZnO matrix and these results are in a good agreement with those reported earlier [8]. The crystallite sizes of the synthesized powders were estimated from X-ray lines broadening using Scherer’s equation [9] were found to increase ranging from 32, 33, 34, 36, 39 and 41 nm for pure ZnO and Zn0.99Co0.010, Zn0.97Co0.030, Zn0.95Co0.050, Zn0.93Co0.070 and Zn0.90Co0.1O samples, respectively. The morphology and chemical composition of as synthesized powders were further investigated by FESEM and EDX analysis. Inset of Fig. 2(a) and (b) shows the FESEM images of undoped and 5%
Fig. 1. XRD patterns of pure and Co-doped ZnO nanoparticles. Inset (a) magnified spectrum of (1 0 0) peak and inset (b) the plot of lattice parameters vs. Co content.
Co doped ZnO nanoparticles which are homogeneous and agglomerated with diameters ranging from 40 to 50 nm. Due to the uniform distribution of oxidized metal anions in the threedimensional polymeric network structure, the agglomeration could be induced by densification resulting from the narrow space between particles [10]. It is clear from the FESEM images that the particles are nearly spherical in shape and Co doping results in an aggregation of nanoparticles. With the increase in Co doping, the spacing between the particles is expected to become narrower and also there is an increase in particle size, which leads to the agglomeration of nanoparticles. Fig. 2(a) and (b) depict the typical EDX spectra taken from pure and 5% Co-doped ZnO samples. The chemical analysis of Zn1 xCoxO with x = 0.05 measured by EDX analysis shows the presence of Zn, O and Co signals only; indicates that the nanoparticles are made up of zinc, oxygen and Co ions which shows that the Co ion is substituting the Zn ion in ZnO matrix. Only zinc and oxygen signals have been detected in undoped ZnO sample, suggesting that the nanoparticles are indeed made up of Zn and O. Thus resulting a high purity of ZnO nanopowder. To study the change in Zn–O bonding due to the Co substitution, FTIR measurements of Co doped ZnO has been carried out. FTIR measurements were performed in the wave number range 4000 to 400 cm 1 using KBr method at Room temperature as shown in Fig. 2 (c). The FTIR spectra show main absorption bands near 3400 cm 1 represent O–H mode, those at 2900 cm 1 are C–H mode, band arising from the absorption of atmospheric CO2 on the metallic cations at 2350 cm 1 and 1400–1600 cm 1 are the C@O stretching mode. The absorption band at 454 cm 1 is the stretching mode of ZnO [11]. However, in case of 1%, 3%, 5%, 7% and 10% Co doped samples, the value of absorption bands were found to be blue shifted at 449, 443, 438, 432, and 427 cm 1, respectively. The enlarged spectrum in the wave number range <1000 cm 1 is shown in the inset of Fig. 2(c). The change in the peak position of ZnO absorption bands reflects that Zn–O–Zn network is perturbed by the presence of Co in its environment. Therefore, the FTIR results also indicate that Co is occupying Zn position in ZnO matrix as observed in XRD measurements. The Raman spectra are sensitive to the crystal quality, structural defects and disorders of the grown products. ZnO has a Wurtzite structure that belongs to the C6v symmetry group. Fig. 2(d) shows the Raman spectra of un-doped and Co-doped ZnO nanoparticles. It is evident that Raman modes in the spectrum of undoped ZnO have similar positions as in the spectrum of bulk ZnO. However, several changes can be observed in the Raman spectrum of Co-doped ZnO in comparison with the spectrum of undoped ZnO. The mode E2high at 439 cm 1 that is related to the vibration of oxygen atoms in wurtzite ZnO [12] is the most prominent mode in undoped ZnO powder. Drastic decrease in its intensity and blue shift to 437, 435, 433 and 429 cm 1 for the 1%, 3%, 5% and 7% Co doped ZnO samples, respectively, observed in the Raman spectrum, indicate the changes in the defect structure due to mechanical activation. There are strong peaks appeared at 2B1low; 2LA (540, 525, 520 and 523 cm 1) for Co doping, as shown in Fig. 2(d). We deem that the peaks at 2B1low; 2LA (525, 523, 520 and 516 cm 1) were Co–O mode, which contributed to local vibrations of Co ions in ZnO lattice as a substitution of Co into Zn position [13]. In the present work, the additional mode appears after doping, which maybe due to Co2+ occupation at Zn2+ sites. Fig. 3 shows magnetization versus magnetic field (M–H) curves for Zn1 XCoxO (x = 0.03 and 0.07) samples measured at room temperature. The magnetization increased with increasing Co content, showed that Co-doped ZnO exhibit RTFM with a TC at or above room temperature. Calculated value of the coercive field (HC) and the remanent magnetization (Mr) for Zn1 XCoxO (x = 0.03 and 0.07) found to be 108.23 (Oe), 8.09 10 6 (emu) and 110.19 (Oe), 3.16 10 5 (emu) respectively. The origin of ferromagnetism
F. Ahmed et al. / Microelectronic Engineering 89 (2012) 129–132
131
Fig. 2. (a) EDX spectrum of un-doped ZnO nanoparticles and inset shows corresponding FESEM image, (b) EDX spectrum of 5% Co-doped ZnO and inset shows corresponding FESEM image, (c) FTIR spectra of Zn1 xCoxO (0.0 6 x 6 0.1) nanoparticles and inset shows the enlarged spectra in the range < 1000 cm 1, (d) Raman spectra of Zn1 xCoxO (0.0 6 x 6 0.1) samples.
4. Conclusions In summary, pure and Co-doped ZnO nanoparticles have been successfully synthesized using an effective auto combustion route. XRD and FTIR results indicated that all the synthesized un-doped and Co-doped ZnO samples had the wurtzite structure and no secondary phase was detected which indicated that Co ions substituted for Zn ions. FESEM results revealed that the prepared Co-doped ZnO nanoparticles are nearly spherical in shape with particle size <50 nm, which is in good agreement with the size obtained from XRD. The blue shift of the major Raman mode (E2high) and the decrease in its intensity with the increase in Co doping, points to the incorporation of Co ions in the ZnO lattice. Magnetic measurements studies conclude that the all Co doped samples showed RTFM. The presented simple synthesis method using cheap precursors can be use to prepare other interesting metal oxides nanoparticles. Fig. 3. Magnetization curve of Zn1 Inset shows the magnified loop.
XCoxO
(x = 0.03 and 0.07) at room temperature.
in DMSs is still controversial; however, in the present study, there can be three possible origins of ferromagnetism. The first is Zn1 xCoxO, the carrier induced ferromagnetism (RKKY mechanism), which is often reported for DMSs [14]. The second is CoOx, but we did not find any secondary phase in the XRD of Co-doped ZnO. The third is micro Co clusters, we also could not find any signal of Co clusters in the XRD. In the present work, the singlephased XRD patterns, the slightly shifted ZnO (1 0 0) peak with Co doping and the evidence of Co incorporation into ZnO from FTIR and Raman studies suggested that the detected ferromagnetism could arise from the homogeneous doping of Co into ZnO which follow the RKKY mechanism.
Acknowledgment This research was supported by Grant No. RTI04-01-03 from the Regional Technology Innovation Program of the Ministry of Knowledge Economy, South Korea. References [1] [2] [3] [4]
S.S. Kim, J.H. Moon, B.T. Lee, O.S. Song, J.H. Je, J. Appl. Phys. 95 (2004) 454. K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. X. Liu, F. Lin, L. Sun, W. Cheng, X. Ma, W. Shi, Appl. Phys. Lett. 88 (2006) 062508. [5] L.Q. Liu, B. Xiang, X.Z. Zhang, Y. Zhang, D.P. Yu, Appl. Phys. Lett. 88 (2006) 063104. [6] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nature Mater 4 (2005) 173.
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F. Ahmed et al. / Microelectronic Engineering 89 (2012) 129–132
[7] H. Zhang, X.Y. Ma, J. Xu, J.J. Niu, D.R. Yang, Nanotechnology 14 (2003) 423. [8] S. Kumar, Y.J. Kim, B.H. Koo, H. Choi, C.G. Lee, JKPS 55 (2009) 1060–1064. [9] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, Prentice Hall, New Jersey, 2001. [10] J.H. Ryu, C.S. Lim, K.H. Auh, J. Kor. Ceram. Soc. 39 (2002) 321. [11] R.F. Silva, M.E.D. Zaniquelli, Colloid Surf. A 551 (2002) 198–200.
[12] K.A. Alim, V.A. Fonoberov, M. Shamsa, A.A. Balandin, J. Appl. Phys. 97 (2005) 124313. [13] K. Samanta, P. Bhattacharya, R.S. Katiyar, W. Iwamoto, P.G. Pagliuso, C. Rettori, Phys. Rev. B 73 (2006) 245. [14] S. Koshihara, A. Oiwa, M. Hirasawa, S. Katsumoto, Y. Iye, C. Urano, H. Takagi, H. Munekata, Phys. Rev. Lett. 78 (1997) 4617.