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Doping dependent room-temperature ferromagnetism and structural properties of dilute magnetic semiconductor ZnO:Cu2+ nanorods
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 4001–4005
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Doping dependent room-temperature ferromagnetism and structural properties of dilute magnetic semiconductor ZnO:Cu2+ nanorods Prashant K. Sharma , Ranu K. Dutta, Avinash C. Pandey Nanophosphor Application Centre, University of Allahabad, Allahabad 211002, India
a r t i c l e in fo
abstract
Article history: Received 19 July 2009 Received in revised form 22 July 2009 Available online 5 August 2009
Copper doped ZnO nanoparticles were synthesized by the chemical technique based on the hydrothermal method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) for different doping percentages of Cu2+ (1–10%). TEM/SEM images showed formation of uniform nanorods, the aspect ratio of which varied with doping percentage of Cu2+. The wurtzite structure of ZnO gradually degrades with the increasing Cu2+ doping concentration and an additional CuO associated diffraction peak was observed above 8% of Cu2+ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Cu2+ doping concentrations was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong room-temperature ferromagnetic behavior, however at higher doping percentage of copper the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced. & 2009 Elsevier B.V. All rights reserved.
Keywords: ZnO nanorod DMS HRTEM Ferromagnetism VSM Raman 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]. Because of this, ZnO has attracted great interest in the recent past. Due to its excellent physical and chemical properties, ZnO has a wide range of applications in piezoelectric transducers, gas sensors, photonic crystals, light-emitting devices, photo-detectors, photodiodes, optical waveguides, transparent conductive films, varistors, solar cell windows, and transparent UV protection films, biological (drug delivery system) and chemical sensors [6–9]. Recently, great progress has been made in ZnO device fabrication, especially in p-type doping, ultraviolet lasing, and nanostructures. Doping is a widely used method to improve the electrical and optical properties of semiconductors. 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 [10–14]. The effect of Cu2+ doping as a luminescence
Corresponding author. Tel./fax: +91 532 2460675.
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.2009.07.066
activator and as a compensator of n-type material is already studied thoroughly [15–22]. In recent past, room-temperature (RT) ferromagnetism (FM) in transition metal (TM) doped p-type ZnO has been repeatedly predicted by several groups using various calculations/simulation studies [23,24]. This makes ZnO one of the most promising materials for potential applications in spintronics and as diluted magnetic semiconductor (DMS) material. Progress in producing high-quality ZnO to obtain ferromagnetism at or above room temperature by doping with 3d transition metals has been highlighted in several papers [11– 14]. In the same pursuit, we synthesized high-quality ZnO nanoparticles for series of Cu2+ doped (1–10%) and systematically studied the structural and magnetic properties.
2. Experimental details Synthesis of nanoparticles by the hydrothermal method involves formation of nuclei and subsequent particle growth under precise control of pressure and temperature in a hydrothermal reactor (autoclave). The hydrothermal method is a very effective and efficient way to synthesize large aspect ratio nanomaterials with uniform size distribution. In the hydrothermal method, special attention is needed to control the pressure and temperature of the hydrothermal vessel (autoclave) in order to achieve good chemical homogeneity of the final product. In the present work all the samples were prepared in a clean room of class 1000 under ambient conditions.
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2.1. Chemicals used The zinc acetate dihydrate (99.2%) Zn(CH3COO)2 2H2O, copper acetate 1-hydrate (99.4%)(CH3COO)2Cu H2O, oxalic acid and ethanol were used to synthesize ZnO:Cu2+. All chemicals were of analytical reagent grade and were directly used without special treatment. 2.2. Procedure In a typical hydrothermal synthesis process nominal composition of zinc acetate dihydrate Zn(CH3COO)2 2H2O, copper acetate 1-hydrate Cu(CH3COO)2 H2O were used to synthesize ZnO with different doping percentages of Cu2+. Appropriate amount of zinc acetate dehydrate, copper acetate 1-hydrate and 0.5 Molar oxalic acid was dissolved in double distilled water with vigorous stirring for 20 min. The clear solution obtained was put into a stainless steel autoclave, sealed, and maintained at the hydrothermal pressure of 150 kPa for 3 h. The external temperature of the autoclave was approximately 90 1C. The resulting white solid products were centrifuged, washed several times with double distilled water and finally with absolute ethanol to remove impurities remaining in the final products. The obtained product was dried at 60 1C for 3 h in air. The resulting white powders were then heated at 400 1C for 2 h, giving white ZnO:Cu2+ powders and named as RCu1, RCu2, RCu3, RCu5, RCu8 and RCu10 for 1%, 2%, 3%, 5%, 8% and 10% Cu doping, respectively. 2.3. Characterization used The prepared ZnO:Cu2+ nanoparticles were characterized by XRD, TEM, SEM and micro-Raman spectroscopy to elaborate structural properties in a precise manner for various doping percentages of Cu2+. XRD was performed on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using CuKa1 radiation with wavelength of 1.54 A˚ in the wide angle region from 301 to 701 on 2y scale. The size and morphology of prepared nanoparticles were investigated using TEM (model Technai 30 G2 S-Twin electron microscope) operated at 300 kV accelerating voltage by dissolving the as-synthesized powder sample in ethanol and then placing a drop of this dilute ethanolic solution on the surface of carbon-coated copper grid. The morphology of powder sample was further observed using a scanning electron microscope (model FEI Quanta 200 MK2 series). Room-temperature magnetization measurement was carried out using a vibrating sample magnetometer (VSM, ADE Magnetics, USA) with pressed pellets of prepared powdered samples. Raman spectra were taken with a Reinshaw micro-Raman spectroscope using 514 nm Ar+ laser as excitation source.
3. Results and discussion Fig. 1 shows the powder XRD pattern of the ZnO:Cu2+ nanoparticles synthesized by the hydrothermal method at a low hydrothermal pressure of 150 kPa. 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, ˚ and can be indexed as the hexagonal wurtzite c ¼ 5.206 A) structure of ZnO having space group P63mc. Furthermore, it can be seen that at higher doping percentages of Cu2+ (8% and 10% in present case), a new phase emerges at 38.631 in the XRD spectra. This new phase in the XRD spectra corresponds to CuO (111) (matched with JCPDS 05-0661), which may be due to the
Fig. 1. X-ray diffraction spectra of ZnO for different doping percentages of Cu2+.
formation of CuO from remaining un-reacted Cu2+ ions present in the solution. All available reflections of the present XRD phases have been fitted with a Gaussian distribution. The broadening of XRD peaks (i.e. Scherrer’s broadening [25]) attributes nanosized formation of ZnO. The particle size, d, of ZnO:Cu2+ nanoparticles was estimated by the Debye–Scherrer’s equation [25] d¼
0:9 l B cos y
ð1Þ
where, d is the particle size, l the wavelength of radiation used, y the Bragg angle and B the full width at half maxima (FWHM) on 2y scale. The average particle size using the Debye–Scherrer’s equation was 13 nm. From XRD pattern, it was also observed that there is no significant change in particle size for different doping percentages of Cu2+, while a decrease in crystallinity was observed with doping percentage. This was due to distortion in the host ZnO lattice because of the introduction of a foreign impurity i.e. Cu2+ doping. This is mainly because of decrease in nucleation and subsequent growth rate due to increasing Cu2+ doping percentage due to the size difference of Zn and Cu ions. Ionic radius of Cu2+ is 0.87 A˚ and is bigger than that of Zn2+ i.e. 0.72 A˚ and lowers the reaction rate. Similar observations were also observed using HRTEM analysis, as evident by typical HRTEM image for 10% Cu2+ doped ZnO shown in Fig. 2e, for higher doping percentages of copper. The morphology of hydrothermally synthesized ZnO:Cu2+ samples with 1% (Fig. 2a), 5% (Fig. 2b) and 10% (Fig. 2c) doping of copper is shown in Fig. 2. As evident from Fig. 2, nanorod-like structures were formed for all these samples. The diameter of these nanorods was around 15 nm and length was around 85 nm (aspect ratio 6:1) for 1% Cu doping (Fig. 2a), diameter 14 nm and length 70 nm (aspect ratio 5:1) for 5% Cu doping (Fig. 2b) and diameter around 13 nm and length 40 nm (aspect ratio 3:1) for 10% Cu doping (Fig. 2c), consisting of bunch of very thin nanorods (wire like) having diameter nearly 2 nm and length 40–85 nm (aspect ratio 40–85:2) for different doping percentages of copper. The morphology of these nanoparticles were further examined by scanning electron microscope (SEM) using powders of the prepared samples and a large number of very uniform nanorods having diameters 12–15 nm and length 40–80 nm were observed for all the samples under study. Fig. 2f and g shows typical SEM images for the 5% and 10% Cu2+ doped ZnO nanorods. Again these
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Fig. 2. TEM images of ZnO:Cu2+ nanoparticles, for (a) 1%, (b) 5%, and (c) 10% doping percentage of Cu2+. High resolution transmission electron microscopic image for (d) 5% (e) 10% Cu2+ doped ZnO, inset shows corresponding selected area electron diffraction (SAED) pattern. Fig. (f) and (g) shows typical SEM images for the 5% and 10% Cu2+ doped ZnO nanoparticles.
Zn Zn
2000 1500 Counts
results were in good agreement with the other structural characterization results. TEM/SEM results showed a decrease in aspect ratio of nanorods with increasing Cu2+ doping percentage. This is mainly because of a decrease in nucleation and subsequent growth rate due to increasing Cu2+ doping percentage. These structurally uniform nanorods indicated the growth of nanoparticles in a direction which was due to applied hydrothermal pressure and temperature at the time of synthesis process. Fig. 2d and e shows high resolution transmission electron microscopic (HRTEM) image of 5% and 10% Cu2+ doped ZnO nanoparticles respectively, inset of Fig. 2d shows the corresponding selected area electron diffraction (SAED) pattern. This SAED pattern showed that this ZnO nanorod was basically a single crystalline structure. The imaged lattice spacing 2.3 A˚ (Fig. 2d) for 5% Cu2+ doped ZnO nanorods, corresponds to the (0 0 2) planes of hexagonal wurtzite structure of ZnO while a remarkable deviation in the imaged lattice spacing 2.15 A˚ (Fig. 2e) was observed for 10% Cu2+ doped ZnO nanoparticles. HRTEM measurements showed a remarkable shrink in imaged d-spacing for 8% and 10% Cu2+ doped ZnO (Fig. 2e), whereas for the ZnO samples doped with 1%, 2%, 3% and 5% Cu2+ no change in imaged d-spacing was observed as compared to standard ZnO wurtzite structure i.e. the effect of Cu2+ doping is dominant and appeared only at higher doping percentages of copper 8% and 10% Cu2+ doped ZnO samples which is in good agreement with XRD and Raman studies. This result further supported our claim that this distortion in the host ZnO lattice arises due to the introduction of a foreign impurity i.e. Cu2+ doping at higher doping percentage as observed by XRD. In order to confirm the presence of Cu2+ in the synthesized ZnO nanorods, EDX measurements were performed. Fig. 3 shows the representative EDX spectra of 2% Cu doped ZnO samples. From the similarity of the Zn and Cu peak intensity line traces, it is clear that after the synthesis process, zinc and copper were homogenously distributed inside the nanorod. From the EDX line traces it can be also concluded that Cu2+ was successfully
Zn Zn
1000 O Zn
500 C
0
0
Cu
Si Si
Cu
2
4 6 Energy (keV)
8
Cu Zn
10
Fig. 3. Representative EDX spectra of 2% copper doped ZnO nanorods EDX measurements on single nanorod found that zinc and copper are homogeneously distributed throughout the ZnO:Cu2+ nanorods.
substituted into the crystal structure of ZnO nanoparticles. The estimated amount of Cu2+ ions was nearly 2.2% and is slightly greater than the expected value of 2%. This could be attributed due to the contribution arising from the copper grid used for the measurement. EDX measurements on single ZnO:Cu2+ nanorod found that zinc and copper were homogeneously distributed throughout the whole nanoparticle. Fig. 4 shows the dependence of magnetization at room temperature with applied magnetic field (M–H loop) for (a) combined M–H loop for all doping percentages, (b) RCu1 i.e. 1% copper doping, (c) RCu3 i.e. 3% copper doping, (d) RCu5 i.e. 5% copper doping, (e) RCu8 i.e. 8% Cu doping and (f) RCu10 i.e. 10% copper doping, respectively. A clear hysteresis loop, with noticeable coercivity, was observed. Initially, ferromagnetic nature with bigger hysteresis area was observed for 1–5% Cu (RCu1–RCu5) doped samples and again for further high doping of Cu (RCu8–RCu10) paramagnetic nature dominates and the hysteresis becomes much narrow. Initially these nanoparticles showed strong room-temperature ferromagnetic behavior, however at higher doping percentage of
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Fig. 4. Room-temperature M–H loop for as-synthesized ZnO:Cu2+ nanorods. (a) Combined M–H loop for all doping percentages, (b) RCu1 i.e. 1% copper doping, (c) RCu3 i.e. 3% copper doping, (d) RCu5 i.e. 5% copper doping, (e) RCu8 i.e. 8% Cu doping and (f) RCu10 i.e. 10% copper doping, respectively. Observed clear hysteresis indicates ferromagnetic nature of the prepared nanoparticles at room temperature.
copper the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced. The enhanced antiferromagnetic interaction between neighboring Cu–Cu ions suppressed the ferromagnetism at higher doping concentrations of Cu. Clear hysteresis loops with coercivities 12.0, 7.2, 5.0, 3.1 and 1.2 mT were observed for samples RCu1, RCu3, RCu5, RCu8 and RCu10, respectively. Their corresponding magnetizations of remanence were 12.0 105, 8.1 105, 6.2 105, 4.8 105 and 3.2 105 emu/g, respectively. The noticeable coercivity of M–H loop could be attributed to strong ferromagnetism at room temperature. 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. Raman spectroscopic studies were employed to understand the effect of copper doping on microscopic structure and vibrational properties of prepared ZnO:Cu2+ nanoparticles. Several literatures show that in hexagonal wurtzite ZnO following fundamental
optical bands should exist, as according to the group theory: E2 (low) at 101 cm1, E2 (high) at 437 cm1, A1 (TO) at 380 cm1, A1 (LO) at 574 cm1, E1 (TO) at 407 cm1 and E1 (LO) at 583 cm1. The low frequency E2 mode is associated with the vibration of heavy Zn sub-lattice and the high frequency E2 mode involves only the oxygen atoms. Raman spectra (Fig. 5) showed a strong Raman shift signal at 437 cm1, which is due to E2 (high) mode of ZnO nanoparticles. These results were consistent with already reported works [26–29]. The second order vibrations were at 208, 334, 1050–1200 and 3077 cm1. Presence of small peak at 583 and 378 cm1 confirmed that oxygen deficiency is quite low in the copper doped ZnO and are due to E1 (LO) and A1 (TO) modes of ZnO respectively, this peak diminishes gradually with increasing concentration of Cu, the dopant. The other peaks appearing in the low frequency region correspond to the second order vibrational modes of ZnO. Some other peaks at positions 1347, 1426, 2105 and 2169 cm1 are also observed for 10% Copper doped ZnO samples. Origins of these peaks are not clear at the present moment. These results, along with HRTEM results, further confirmed that the ZnO:Cu2+ nanoparticles were of hexagonal wurtzite structure with few defects at higher doping percentage of copper. For RCu1 sample, very strong and sharp Raman peaks were observed, indicating good wurtzite structure. The sharpest and strongest peak at about 437 cm–1 is the strongest Raman mode in
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netic interaction between neighboring Cu–Cu ions suppressed the ferromagnetism at higher doping concentrations of copper.
Acknowledgements Authors are thankful to DST and CSIR, India for supporting ‘Nanophosphor Application Centre’ under IRHPA, Nano-Mission, and NMITLI scheme. References [1] [2] [3] [4] [5]
[6] Fig. 5. Raman spectra of ZnO:Cu2+ nanoparticles (for 1%, 5% and 10% doping percentage of Cu2+).
[7] [8] [9] [10]
wurtzite crystal structure. However, as the Cu content increases to 5% and above, the Raman line of E2 (high) mode becomes broad and weak, which means that the wurtzite crystalline structure of ZnO might have been weakened by higher Cu doping concentration. It is well known that lattice defects and disorder could usually be introduced by exotic ions doping, defect-induced Raman modes would usually appear in defective crystals because the Raman selection rules are relaxed [30]. For high concentration doped crystal, the translational invariance of the crystal lattice is weakened and scattering events from the whole Brillouin zone are possible.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
4. Conclusion We have successfully synthesized the ultrafine diluted magnetic semiconductor nanoparticles of ZnO:Cu2+ in rod shape. XRD analysis and Raman study show wurtzite structures for all the ZnO:Cu2+ nanoparticles. With increasing Cu concentration, the wurtzite structures degrade gradually. Initially these nanorods showed strong ferromagnetic behavior, however at higher doping percentage of Cu the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced. The enhanced antiferromag-
[22] [23] [24] [25] [26] [27] [28] [29] [30]
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. A. Mendoza-Galvain, C. Trejo-Cruz, J. Lee, D. Bhattacharya, J. Metson, P.J. Evans, U. Pala, J. Appl. Phys. 99 (2006) 14306. Honma, S. Hirakowa, K. Yamada, J.M. Bae, Solid State Ionics 118 (1999) 29. T.S. Phely-Bobin, R.J. Muisener, J.T. Koberstein, F. Papadinmitrakopoulos, Synth. Met. 116 (2001) 439. P.K. Sharma, R.K. Dutta, M. Kumar, P.K. Singh, A.C. Pandey, J. Lumin. 129 (6) (2009) 605. 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. P.K. Sharma, R.K. Dutta, A.C. Pandey, S. Layek, H.C. Verma, J. Magn. Magn. Mater. 321 (2009) 2587. P.K. Sharma, R.K. Dutta, A.C. Pandey, J. Magn. Magn. Mater. (2009)10.1016/ j.jmmm.2009.06.055. T.G. Kryshtab, V.S. Khomchenko, V.P. Papsha, M.O. Mazin, Y.A. Tzykunov, Thin Solid Films 403/404 (2002) 76. P. Hoyer, H. Wellar, Chem. Phys. Lett. 221 (1994) 379. D.W. Bannemainn, C. Kormann, M.R. Hoffmann, J. Phys. Chem. 91 (1987) 3789. L. Spanhel, M.A. Anderson, J. Am. Chem. Soc. 113 (1991) 2826. J. Zhang, L. Sun, J. Yin, H. Su, C. Liao, C. Yon, Chem. Mater. 14 (2002) 4172. M.H. Huang, Y.Y. Wu, H.N. Feich, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113. A. Cetin, R. Kibar, M. Ayvacikli, N. Can, Ch. Buchal, P.D. Townsend, A.L. Stepanov, T. Karali, S. Selvi, NIMB 249 (2006) 474. K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallanl, J.A. Voigt, B.E. Gandu, J. Appl. Phys. 79 (1996) 7983. T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. M.H.F. Sluiter, Y. Kawazoe, P. Sharma, A. Inoue, A.R. Raju, C. Rout, U.V. Waghmare, Phys. Rev. Lett. 94 (2005) 187204. A.L. Patterson, Phys. Rev. 56 (1939) 972. Z. Wang, H. Zhang, L. Zhang, J. Yang, S. Yan, C. Wang, Nanotechnology 14 (2003) 11. T.C. Damen, S.P.S. Porto, B. Tell, Phys. Rev. 142 (1996) 570. Z. Zhang, B. Huang, Y. Yu, D. Guo, Mater. Sci. Eng. B 86 (2001) 109. M. Koyano, P. Quoc Bao, L.T. ThanhBinh, H.L. Hong, N. Ngoc Long, S.I. Katayama, Phys. Status Solidi A 193 (2002) 125. A. Chartier, P.D. Arco, R. Dovesi, V.R. Saunders, Phys. Rev. B 60 (1999) 14042.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Doping dependent room-temperature ferromagnetism and structural properties of dilute magnetic semiconductor ZnO:Cu2+ nanorods Prashant K. Sharma , Ranu K. Dutta, Avinash C. Pandey Nanophosphor Application Centre, University of Allahabad, Allahabad 211002, India
a r t i c l e in fo
abstract
Article history: Received 19 July 2009 Received in revised form 22 July 2009 Available online 5 August 2009
Copper doped ZnO nanoparticles were synthesized by the chemical technique based on the hydrothermal method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) for different doping percentages of Cu2+ (1–10%). TEM/SEM images showed formation of uniform nanorods, the aspect ratio of which varied with doping percentage of Cu2+. The wurtzite structure of ZnO gradually degrades with the increasing Cu2+ doping concentration and an additional CuO associated diffraction peak was observed above 8% of Cu2+ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Cu2+ doping concentrations was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong room-temperature ferromagnetic behavior, however at higher doping percentage of copper the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced. & 2009 Elsevier B.V. All rights reserved.
Keywords: ZnO nanorod DMS HRTEM Ferromagnetism VSM Raman 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]. Because of this, ZnO has attracted great interest in the recent past. Due to its excellent physical and chemical properties, ZnO has a wide range of applications in piezoelectric transducers, gas sensors, photonic crystals, light-emitting devices, photo-detectors, photodiodes, optical waveguides, transparent conductive films, varistors, solar cell windows, and transparent UV protection films, biological (drug delivery system) and chemical sensors [6–9]. Recently, great progress has been made in ZnO device fabrication, especially in p-type doping, ultraviolet lasing, and nanostructures. Doping is a widely used method to improve the electrical and optical properties of semiconductors. 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 [10–14]. The effect of Cu2+ doping as a luminescence
Corresponding author. Tel./fax: +91 532 2460675.
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.2009.07.066
activator and as a compensator of n-type material is already studied thoroughly [15–22]. In recent past, room-temperature (RT) ferromagnetism (FM) in transition metal (TM) doped p-type ZnO has been repeatedly predicted by several groups using various calculations/simulation studies [23,24]. This makes ZnO one of the most promising materials for potential applications in spintronics and as diluted magnetic semiconductor (DMS) material. Progress in producing high-quality ZnO to obtain ferromagnetism at or above room temperature by doping with 3d transition metals has been highlighted in several papers [11– 14]. In the same pursuit, we synthesized high-quality ZnO nanoparticles for series of Cu2+ doped (1–10%) and systematically studied the structural and magnetic properties.
2. Experimental details Synthesis of nanoparticles by the hydrothermal method involves formation of nuclei and subsequent particle growth under precise control of pressure and temperature in a hydrothermal reactor (autoclave). The hydrothermal method is a very effective and efficient way to synthesize large aspect ratio nanomaterials with uniform size distribution. In the hydrothermal method, special attention is needed to control the pressure and temperature of the hydrothermal vessel (autoclave) in order to achieve good chemical homogeneity of the final product. In the present work all the samples were prepared in a clean room of class 1000 under ambient conditions.
ARTICLE IN PRESS 4002
P.K. Sharma et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 4001–4005
2.1. Chemicals used The zinc acetate dihydrate (99.2%) Zn(CH3COO)2 2H2O, copper acetate 1-hydrate (99.4%)(CH3COO)2Cu H2O, oxalic acid and ethanol were used to synthesize ZnO:Cu2+. All chemicals were of analytical reagent grade and were directly used without special treatment. 2.2. Procedure In a typical hydrothermal synthesis process nominal composition of zinc acetate dihydrate Zn(CH3COO)2 2H2O, copper acetate 1-hydrate Cu(CH3COO)2 H2O were used to synthesize ZnO with different doping percentages of Cu2+. Appropriate amount of zinc acetate dehydrate, copper acetate 1-hydrate and 0.5 Molar oxalic acid was dissolved in double distilled water with vigorous stirring for 20 min. The clear solution obtained was put into a stainless steel autoclave, sealed, and maintained at the hydrothermal pressure of 150 kPa for 3 h. The external temperature of the autoclave was approximately 90 1C. The resulting white solid products were centrifuged, washed several times with double distilled water and finally with absolute ethanol to remove impurities remaining in the final products. The obtained product was dried at 60 1C for 3 h in air. The resulting white powders were then heated at 400 1C for 2 h, giving white ZnO:Cu2+ powders and named as RCu1, RCu2, RCu3, RCu5, RCu8 and RCu10 for 1%, 2%, 3%, 5%, 8% and 10% Cu doping, respectively. 2.3. Characterization used The prepared ZnO:Cu2+ nanoparticles were characterized by XRD, TEM, SEM and micro-Raman spectroscopy to elaborate structural properties in a precise manner for various doping percentages of Cu2+. XRD was performed on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using CuKa1 radiation with wavelength of 1.54 A˚ in the wide angle region from 301 to 701 on 2y scale. The size and morphology of prepared nanoparticles were investigated using TEM (model Technai 30 G2 S-Twin electron microscope) operated at 300 kV accelerating voltage by dissolving the as-synthesized powder sample in ethanol and then placing a drop of this dilute ethanolic solution on the surface of carbon-coated copper grid. The morphology of powder sample was further observed using a scanning electron microscope (model FEI Quanta 200 MK2 series). Room-temperature magnetization measurement was carried out using a vibrating sample magnetometer (VSM, ADE Magnetics, USA) with pressed pellets of prepared powdered samples. Raman spectra were taken with a Reinshaw micro-Raman spectroscope using 514 nm Ar+ laser as excitation source.
3. Results and discussion Fig. 1 shows the powder XRD pattern of the ZnO:Cu2+ nanoparticles synthesized by the hydrothermal method at a low hydrothermal pressure of 150 kPa. 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, ˚ and can be indexed as the hexagonal wurtzite c ¼ 5.206 A) structure of ZnO having space group P63mc. Furthermore, it can be seen that at higher doping percentages of Cu2+ (8% and 10% in present case), a new phase emerges at 38.631 in the XRD spectra. This new phase in the XRD spectra corresponds to CuO (111) (matched with JCPDS 05-0661), which may be due to the
Fig. 1. X-ray diffraction spectra of ZnO for different doping percentages of Cu2+.
formation of CuO from remaining un-reacted Cu2+ ions present in the solution. All available reflections of the present XRD phases have been fitted with a Gaussian distribution. The broadening of XRD peaks (i.e. Scherrer’s broadening [25]) attributes nanosized formation of ZnO. The particle size, d, of ZnO:Cu2+ nanoparticles was estimated by the Debye–Scherrer’s equation [25] d¼
0:9 l B cos y
ð1Þ
where, d is the particle size, l the wavelength of radiation used, y the Bragg angle and B the full width at half maxima (FWHM) on 2y scale. The average particle size using the Debye–Scherrer’s equation was 13 nm. From XRD pattern, it was also observed that there is no significant change in particle size for different doping percentages of Cu2+, while a decrease in crystallinity was observed with doping percentage. This was due to distortion in the host ZnO lattice because of the introduction of a foreign impurity i.e. Cu2+ doping. This is mainly because of decrease in nucleation and subsequent growth rate due to increasing Cu2+ doping percentage due to the size difference of Zn and Cu ions. Ionic radius of Cu2+ is 0.87 A˚ and is bigger than that of Zn2+ i.e. 0.72 A˚ and lowers the reaction rate. Similar observations were also observed using HRTEM analysis, as evident by typical HRTEM image for 10% Cu2+ doped ZnO shown in Fig. 2e, for higher doping percentages of copper. The morphology of hydrothermally synthesized ZnO:Cu2+ samples with 1% (Fig. 2a), 5% (Fig. 2b) and 10% (Fig. 2c) doping of copper is shown in Fig. 2. As evident from Fig. 2, nanorod-like structures were formed for all these samples. The diameter of these nanorods was around 15 nm and length was around 85 nm (aspect ratio 6:1) for 1% Cu doping (Fig. 2a), diameter 14 nm and length 70 nm (aspect ratio 5:1) for 5% Cu doping (Fig. 2b) and diameter around 13 nm and length 40 nm (aspect ratio 3:1) for 10% Cu doping (Fig. 2c), consisting of bunch of very thin nanorods (wire like) having diameter nearly 2 nm and length 40–85 nm (aspect ratio 40–85:2) for different doping percentages of copper. The morphology of these nanoparticles were further examined by scanning electron microscope (SEM) using powders of the prepared samples and a large number of very uniform nanorods having diameters 12–15 nm and length 40–80 nm were observed for all the samples under study. Fig. 2f and g shows typical SEM images for the 5% and 10% Cu2+ doped ZnO nanorods. Again these
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Fig. 2. TEM images of ZnO:Cu2+ nanoparticles, for (a) 1%, (b) 5%, and (c) 10% doping percentage of Cu2+. High resolution transmission electron microscopic image for (d) 5% (e) 10% Cu2+ doped ZnO, inset shows corresponding selected area electron diffraction (SAED) pattern. Fig. (f) and (g) shows typical SEM images for the 5% and 10% Cu2+ doped ZnO nanoparticles.
Zn Zn
2000 1500 Counts
results were in good agreement with the other structural characterization results. TEM/SEM results showed a decrease in aspect ratio of nanorods with increasing Cu2+ doping percentage. This is mainly because of a decrease in nucleation and subsequent growth rate due to increasing Cu2+ doping percentage. These structurally uniform nanorods indicated the growth of nanoparticles in a direction which was due to applied hydrothermal pressure and temperature at the time of synthesis process. Fig. 2d and e shows high resolution transmission electron microscopic (HRTEM) image of 5% and 10% Cu2+ doped ZnO nanoparticles respectively, inset of Fig. 2d shows the corresponding selected area electron diffraction (SAED) pattern. This SAED pattern showed that this ZnO nanorod was basically a single crystalline structure. The imaged lattice spacing 2.3 A˚ (Fig. 2d) for 5% Cu2+ doped ZnO nanorods, corresponds to the (0 0 2) planes of hexagonal wurtzite structure of ZnO while a remarkable deviation in the imaged lattice spacing 2.15 A˚ (Fig. 2e) was observed for 10% Cu2+ doped ZnO nanoparticles. HRTEM measurements showed a remarkable shrink in imaged d-spacing for 8% and 10% Cu2+ doped ZnO (Fig. 2e), whereas for the ZnO samples doped with 1%, 2%, 3% and 5% Cu2+ no change in imaged d-spacing was observed as compared to standard ZnO wurtzite structure i.e. the effect of Cu2+ doping is dominant and appeared only at higher doping percentages of copper 8% and 10% Cu2+ doped ZnO samples which is in good agreement with XRD and Raman studies. This result further supported our claim that this distortion in the host ZnO lattice arises due to the introduction of a foreign impurity i.e. Cu2+ doping at higher doping percentage as observed by XRD. In order to confirm the presence of Cu2+ in the synthesized ZnO nanorods, EDX measurements were performed. Fig. 3 shows the representative EDX spectra of 2% Cu doped ZnO samples. From the similarity of the Zn and Cu peak intensity line traces, it is clear that after the synthesis process, zinc and copper were homogenously distributed inside the nanorod. From the EDX line traces it can be also concluded that Cu2+ was successfully
Zn Zn
1000 O Zn
500 C
0
0
Cu
Si Si
Cu
2
4 6 Energy (keV)
8
Cu Zn
10
Fig. 3. Representative EDX spectra of 2% copper doped ZnO nanorods EDX measurements on single nanorod found that zinc and copper are homogeneously distributed throughout the ZnO:Cu2+ nanorods.
substituted into the crystal structure of ZnO nanoparticles. The estimated amount of Cu2+ ions was nearly 2.2% and is slightly greater than the expected value of 2%. This could be attributed due to the contribution arising from the copper grid used for the measurement. EDX measurements on single ZnO:Cu2+ nanorod found that zinc and copper were homogeneously distributed throughout the whole nanoparticle. Fig. 4 shows the dependence of magnetization at room temperature with applied magnetic field (M–H loop) for (a) combined M–H loop for all doping percentages, (b) RCu1 i.e. 1% copper doping, (c) RCu3 i.e. 3% copper doping, (d) RCu5 i.e. 5% copper doping, (e) RCu8 i.e. 8% Cu doping and (f) RCu10 i.e. 10% copper doping, respectively. A clear hysteresis loop, with noticeable coercivity, was observed. Initially, ferromagnetic nature with bigger hysteresis area was observed for 1–5% Cu (RCu1–RCu5) doped samples and again for further high doping of Cu (RCu8–RCu10) paramagnetic nature dominates and the hysteresis becomes much narrow. Initially these nanoparticles showed strong room-temperature ferromagnetic behavior, however at higher doping percentage of
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Fig. 4. Room-temperature M–H loop for as-synthesized ZnO:Cu2+ nanorods. (a) Combined M–H loop for all doping percentages, (b) RCu1 i.e. 1% copper doping, (c) RCu3 i.e. 3% copper doping, (d) RCu5 i.e. 5% copper doping, (e) RCu8 i.e. 8% Cu doping and (f) RCu10 i.e. 10% copper doping, respectively. Observed clear hysteresis indicates ferromagnetic nature of the prepared nanoparticles at room temperature.
copper the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced. The enhanced antiferromagnetic interaction between neighboring Cu–Cu ions suppressed the ferromagnetism at higher doping concentrations of Cu. Clear hysteresis loops with coercivities 12.0, 7.2, 5.0, 3.1 and 1.2 mT were observed for samples RCu1, RCu3, RCu5, RCu8 and RCu10, respectively. Their corresponding magnetizations of remanence were 12.0 105, 8.1 105, 6.2 105, 4.8 105 and 3.2 105 emu/g, respectively. The noticeable coercivity of M–H loop could be attributed to strong ferromagnetism at room temperature. 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. Raman spectroscopic studies were employed to understand the effect of copper doping on microscopic structure and vibrational properties of prepared ZnO:Cu2+ nanoparticles. Several literatures show that in hexagonal wurtzite ZnO following fundamental
optical bands should exist, as according to the group theory: E2 (low) at 101 cm1, E2 (high) at 437 cm1, A1 (TO) at 380 cm1, A1 (LO) at 574 cm1, E1 (TO) at 407 cm1 and E1 (LO) at 583 cm1. The low frequency E2 mode is associated with the vibration of heavy Zn sub-lattice and the high frequency E2 mode involves only the oxygen atoms. Raman spectra (Fig. 5) showed a strong Raman shift signal at 437 cm1, which is due to E2 (high) mode of ZnO nanoparticles. These results were consistent with already reported works [26–29]. The second order vibrations were at 208, 334, 1050–1200 and 3077 cm1. Presence of small peak at 583 and 378 cm1 confirmed that oxygen deficiency is quite low in the copper doped ZnO and are due to E1 (LO) and A1 (TO) modes of ZnO respectively, this peak diminishes gradually with increasing concentration of Cu, the dopant. The other peaks appearing in the low frequency region correspond to the second order vibrational modes of ZnO. Some other peaks at positions 1347, 1426, 2105 and 2169 cm1 are also observed for 10% Copper doped ZnO samples. Origins of these peaks are not clear at the present moment. These results, along with HRTEM results, further confirmed that the ZnO:Cu2+ nanoparticles were of hexagonal wurtzite structure with few defects at higher doping percentage of copper. For RCu1 sample, very strong and sharp Raman peaks were observed, indicating good wurtzite structure. The sharpest and strongest peak at about 437 cm–1 is the strongest Raman mode in
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netic interaction between neighboring Cu–Cu ions suppressed the ferromagnetism at higher doping concentrations of copper.
Acknowledgements Authors are thankful to DST and CSIR, India for supporting ‘Nanophosphor Application Centre’ under IRHPA, Nano-Mission, and NMITLI scheme. References [1] [2] [3] [4] [5]
[6] Fig. 5. Raman spectra of ZnO:Cu2+ nanoparticles (for 1%, 5% and 10% doping percentage of Cu2+).
[7] [8] [9] [10]
wurtzite crystal structure. However, as the Cu content increases to 5% and above, the Raman line of E2 (high) mode becomes broad and weak, which means that the wurtzite crystalline structure of ZnO might have been weakened by higher Cu doping concentration. It is well known that lattice defects and disorder could usually be introduced by exotic ions doping, defect-induced Raman modes would usually appear in defective crystals because the Raman selection rules are relaxed [30]. For high concentration doped crystal, the translational invariance of the crystal lattice is weakened and scattering events from the whole Brillouin zone are possible.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
4. Conclusion We have successfully synthesized the ultrafine diluted magnetic semiconductor nanoparticles of ZnO:Cu2+ in rod shape. XRD analysis and Raman study show wurtzite structures for all the ZnO:Cu2+ nanoparticles. With increasing Cu concentration, the wurtzite structures degrade gradually. Initially these nanorods showed strong ferromagnetic behavior, however at higher doping percentage of Cu the ferromagnetic behavior was suppressed and paramagnetic nature was enhanced. The enhanced antiferromag-
[22] [23] [24] [25] [26] [27] [28] [29] [30]
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