Materials Chemistry and Physics 113 (2009) 749–755
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Structural, optical and magnetic properties of nanocrystalline Zn0.9 Co0.1 O-based diluted magnetic semiconductors Y. Kalyana Lakshmi a , K. Srinivas a , B. Sreedhar b , M. Manivel Raja c , M. Vithal d , P. Venugopal Reddy a,∗ a
Department of Physics, Osmania University, Hyderabad 500007, India I&PC Division, Indian Institute of Chemical Technology, Hyderabad 500007, India c Defence Material Research Laboratory, Hyderabad 500058, India d Department of Chemistry, Osmania University, Hyderabad 500007, India b
a r t i c l e
i n f o
Article history: Received 1 December 2007 Received in revised form 28 May 2008 Accepted 3 August 2008 PACS: 75.50.Pp 74.25.Gz 75.30.Hx 75.60.Ej Keywords: Diluted magnetic semiconductor Nanostructures Optical properties Magnetic properties
a b s t r a c t With a view to understand the influence of nano size on various properties of cobalt-doped ZnO-based diluted magnetic semiconductors, a series of materials were prepared by the citrate gel route. The phase and morphology studies have been carried out by X-ray diffraction and transmission electron microscopy, respectively. All the samples of the present investigation are found to have hexagonal wurtzite structure and crystallite sizes are found to vary from 25 nm to 65 nm. From the optical absorption measurements it has been observed that upon doping with cobalt, the energy band gap is found to shift towards lower energy side (red shift) while it shifts towards higher energy side (blue shift) when the crystallite size is increased continuously. It has been observed from the XPS results that oxidation state of Cobalt is +2 and that the difference in binding energies of Co 2p3/2 and Co 2p1/2 is found to increase continuously with increasing crystallite size. Finally, all the samples are found to exhibit room temperature ferromagnetism and the specific magnetization decreases with increasing crystallite size. Published by Elsevier B.V.
1. Introduction Diluted Magnetic semiconductors (DMS) have recently attracted a great deal of attention due to possibility of manipulating charge and spin degrees of freedom. They exhibit unique magnetic, magneto-optical and magneto-electrical effects and can be exploited as spintronic devices [1]. Efforts are going on to meet some of the deficiencies of these materials such as intrinsic ferromagnetism, high ferromagnetic Curie-temperature, large magnetization and a precise controllable spin properties, etc. In search of a new DMSs, lead the attention towards wide band gap oxide-based DMSs such as Zn1−x TMx O, Ti1−x TMx O2 , Sn1−x TMx O2 and Hf1−x TMx O2 (TM: Co, Ni, Mn, Fe) [2–5], etc., because of their stable ferromagnetism near or above room temperature. Among oxide-based DMSs, ZnO doped with small amount of Co2+ have attracted considerably. Moreover, ZnO has a large exciton binding energy of 60 meV, with wide band gap energy (∼3.3 eV) resulting in efficient excitonic emission at room temperature as well as suitable in making transparent devices. ∗ Corresponding author. Tel.: +91 40 27682287; fax: +91 40 27090020. E-mail addresses:
[email protected],
[email protected] (P.V. Reddy). 0254-0584/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.matchemphys.2008.08.021
It is well known that nano size has great influence on the performance of several material systems. Moreover, the role of dimensionality in shaping the spin-polarized electronic structure of nanocrystalline DMSs is important in understanding their ferromagnetic behavior [6–12]. Apart from this, nanocrystalline Zn0.9 Co0.1 O may be exploited for ferro-fluids, magnetic recording, and biomedical applications because quantum confinement may result in intriguing magnetic properties. As the structural, magnetic and electronic properties of Zn0.9 Co0.1 O are sensitive to the nano size of the materials, an effort has been made to understand the influence of nano size on their structural, optical and ferromagnetic behavior and the results of such an investigation are presented here. 2. Experimental procedure The nanocrystalline Zn0.9 Co0.1 O samples were prepared using citrate gel route by taking the corresponding starting materials as nitrates. More details are given in an earlier publication [9]. In this method, first the nitrates were converted into citrates and by adding suitable chemicals, pH was adjusted to 6.5. After getting a solution on slow evaporation, a gelating reagent ethylene glycol was added and heated at about 180 ◦ C to get a gel. After a few more steps dry fluffy porous mass (precursor), was calcined at 250 ◦ C for 3 h. Finally, the samples in the form of pellets were sintered at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C temperatures for 3 h in order to obtain the materials with varying crystallite size.
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The crystal structure data was determined using X-ray diffraction (XRD) using Phillips (Xpert) diffractometer with Cu K␣ radiation ( = 1.5406 Å) by recording at a 2 value of 20–80◦ in a step of 0.02◦ . Transmission electron micrographs (TEM) and selected area electron diffraction (SAD) patterns were recorded using a Phillips Tecnai G2 FEI F12 electron microscope. With a view to determine the optical band gap, the optical absorption spectra measurements were undertaken in the range of 200–800 nm using a GBC Cintra 10e UV–Vis-DRS spectrometer. For this purpose, all the samples were palletized by mixing the sample powder with KBr. As the powders are not highly transparent, KBr was used as a reference for the absorbance measurements. In order to make the base line corrections for the band gap values due to free excitons near the band gap, the authors recorded the absorbance spectra of all the Co-doped samples using ZnO pellet as a reference. The thicknesses of all the pellets were found to be about 3 mm. X-ray photoelectron spectroscopic (XPS) measurements were also performed on a Kratos X-ray photoelectron spectrometer. The X-ray excitation energy was 1253.6 eV (Mg K␣), and the spectra were recorded with a pass energy of 80 eV. The angle between the detector and the X-ray flux direction was constant and equal to 90◦ , while the measurements were made at an electron take off angle of 70◦ . The calibration of the spectrometer and the charging correction were verified by determining the binding energy of C 1s level with a graphite substrate (at 285 eV). Finally, in order to determine ferromagnetism of these samples, magnetization measurements were undertaken using a vibrating sample magnetometer (Model: DMSADE-1660 MRS) at room temperature by applying 15 kOe magnetic field.
3. Results and discussion 3.1. X-ray diffraction measurements The XRD patterns of Zn0.9 Co0.1 O samples sintered at different temperatures along with undoped ZnO sample sintered at 600 ◦ C and prepared under similar experimental conditions are shown in Fig. 1(a). All the patterns are found to have hexagonal wurtzite structure, without any additional impurity phases thereby indicating that the wurtzite structure might have not affected due to the substitution of Cobalt. Further, as no excess peaks were detected, it has been concluded that all the starting organic precursors might have been completely decomposed. The diffraction peaks along with their relative intensities are found to be in agreement with those from JCPDS [card no. 36-1451] and the patterns were indexed to ZnO. The XRD data were also analyzed by Reitveld refinement technique and using the data, lattice parameters (Table 1) were obtained and are found to be in agreement with the reported ones [7]. Reitveld refined pattern of one of the samples viz; Zn0.9 Co0.1 O sintered at 300 ◦ C is shown in Fig. 1(b). Further, the average crystallite size values of all the samples were estimated using peak broadening technique (Table 1) and are found to be in the range of 25–65 nm. The lattice parameter “a” obtained from the Reitveld data indicate that there is no systematic variation.
However, “c” parameter is found to decrease continuously with increasing nano size of the materials except in the case of ZCO-600. In fact a similar behavior was reported earlier [7]. It is also interesting to note that c/a value of these materials is found to decrease continuously. 3.2. Transmission electron microscopy studies The morphological and structural studies were investigated by TEM and the morphology images are shown in Fig. 2 and the corresponding selected area electron diffraction (SAED) patterns are shown in Fig. 3. The average particle sizes of the aggregated nanocrystalline samples were estimated by considering the minimum and maximum diameter of large number of particles and are given in Table 1 and the particle size are found to be in the range of 20–60 nm. It can be seen from Fig. 3 that the SAED patterns clearly indicate the improvement of the crystallinity due to the removal of the residual organic matter with increasing sintering temperature. Further, as the SAED patterns also do not exhibit any additional diffraction spots and rings of Co, CoO, Co2 O3, Co3 O4 or ZnCo2 O4 phases [6,7,12] one may conclude the existence of only ZnO wurtzite structure in the samples of the present investigation. 3.3. X-ray photoelectron spectroscopy studies Samples of the present investigation sintered at different temperatures were analyzed using XPS measurements, mainly to study the influence of nano size on the binding energies and to determine the electronic state of cobalt. Further, Co(II) contributions were determined from the Co 2p peak deconvolution. The core level binding energies of Co 2p3/2 and Co 2p1/2 are shown in Fig. 4. Further, the experimentally observed binding energies of Co 2p3/2 and Co 2p1/2 are comparable with those from the literature [13,14]. Comparing the binding energies of the cobalt with those observed in the case of Co2+ in CoO, and Co3+ in ␥-Co2 O3 [13,15], it has been concluded that the oxidation state of Co in Zn0.9 Co0.1 O samples might be Co2+ and might have not bonded to oxygen as CoO or Co3 O4 . These measurements also indicate that cobalt atoms might be occupying the substitutional zinc sites in ZnO. Further, the difference between the two binding energies of Co 2p3/2 and Co 2p1/2 peaks (Table 2) are found to vary from 15.10–15.29 eV. This confirms the absence of Co cluster formation as the literature value of the binding energy of Co 2p3/2 in Co metal cluster is 778.3 eV and the difference between Co 2p3/2 and Co 2p1/2 core
Fig. 1. XRD pattern for (a) undoped and Co-doped Zn0.9 Co0.1 O nanocrystalline powders sintered at different temperatures and (b) Reitveld-fitted XRD pattern of nanocrystalline Zn0.9 Co0.1 O powder sintered at 300 ◦ C.
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Table 1 Reitveld refinement results obtained from XRD and average particle sizes obtained TEM for the prepared samples Sample code
ZCO-300 ZCO-400 ZCO-500 ZCO-600 ZnO-600
Sintering temperature (◦ C)
300 400 500 600 600
Lattice parameters a = b (nm)
c (nm)
3.2511 3.2525 3.2495 3.2574 3.2530
5.2137 5.2108 5.2058 5.2179 5.2130
levels for metallic Co is 14.97 eV [13]. Further, as BE value is found to increase with increasing crystallite size one may conclude that nano size influences the core level energy spectra of Co 2p in the compound.
Volume (%)
c/a
Average crystallite size from XRD (nm)
Average particle size from TEM (nm)
47.78 47.74 47.61 47.96 47.78
1.60367 1.60209 1.60203 1.60186 1.60252
25 35 45 65 60
20 34 40 60 58
3.4. Optical absorption studies In order to determine the optical band gap, optical absorption measurements were carried out at room temperature using
Fig. 2. TEM bright field images of surface morphology for Zn0.9 Co0.1 O nanocrystalline powders sintered at (a) 300 ◦ C, (b) 400 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C and (e) undoped ZnO sintered at 600 ◦ C temperatures respectively.
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Fig. 3. TEM bright field images of SAED patterns for Zn0.9 Co0.1 O nanocrystalline powders sintered at (a) 300 ◦ C, (b) 400 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C and (e) undoped ZnO sintered at 600 ◦ C temperatures, respectively.
UV–Vis-DR spectrometer and the absorption spectra are shown in Fig. 5(a). The optical absorption coefficient (˛), has been calculated form the optical absorption spectra using the relation, ˛ =
A
d
=−
1 d
ln
I
(1)
I0
Table 2 XPS peak deconvolution data of Co 2p of the sample Zn0.9 Co0.1 O powders sintered at (a) 300 ◦ C (b) 400 ◦ C (c) 500 ◦ C and (d) 600 ◦ C, respectively Sample name
ZCO-300 ZCO-400 ZCO-500 ZCO-600
Area (%)
Binding energy (eV)
Co 2p3/2
Co 2p1/2
Co 2p3/2
Co 2p1/2
79.1 69.3 82.5 73.8
20.9 30.7 17.5 26.2
779.57 779.66 779.59 779.47
794.67 794.81 794.84 794.76
BE (eV)
15.10 15.15 15.27 15.29
where A is the absorbance for the corresponding wavelength, I is intensity of transmitted light, I0 is the intensity of incident light, d is the path length of the sample and ˛ is the Napierian absorption coefficient. The energy band gap values were estimated using the relation, ˛h = A(h − Eg )
n
(2)
where h is the photon energy, ˛ is the absorption coefficient, Eg is the energy band gap, A is the constant and the exponent n takes values of 1/2, 1, 2 and 3 depending on the types of electronic transition in k-space. For different n values, n = 1/2 was found to give the best fit of Eq. (2) as shown in Fig. 5(b). These measurements clearly indicate the band gap corresponding transition as direct electronic transition [16]. The energy band gap values have been obtained from the extrapolation of the straight line to (˛h)2 = 0 axis and
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Fig. 4. XPS spectra of Co of Zn0.9 Co0.1 O nanocrystalline powders sintered at (a) 300 ◦ C (b) 400 ◦ C (c) 500 ◦ C and (d) 600 ◦ C, respectively.
Fig. 5. (a) The optical absorption spectra and (b) plot of (˛h)2 versus h for the Co-doped ZnO samples sintered at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C temperatures and undoped ZnO sintered at 600 ◦ C.
are given in Table 3 [Baseline corrections have eliminated free carrier contributions to determining optical band gaps]. The UV–vis absorption spectra of the nanocrystalline undoped ZnO powders calcined at 600 ◦ C having average crystallite size ∼50 nm show an absorption edge observed at ∼3.2117 ± 0.03 eV ( = 380 nm). The reduction in the band gap compared with bulk ZnO band gap
(∼3.35 eV) might be due to the enhanced band bending at the grain boundaries [17,18] and this value is in agreement with the reports [7,17,19,20]. It is interesting to note that the band gap value of ZnO600 is found to shift from 3.211 eV to 3.11 ± 0.02 eV due to the substitution of cobalt, clearly indicating a red shift of the band edge and a similar shift was reported earlier [6,7,21]. The observed red
Table 3 The optical band gaps with corresponding crystallite sizes for the prepared samples along with Ms and Hc values Sample name
Crystallite size from XRD (nm)
Band gap Eg (eV)
Saturation magnetization (emu g−1 )
Coercive field Hc (Oe)
ZCO-300 ZCO-400 ZCO-500 ZCO-600 ZnO-600
25 35 45 66 60
3.053 3.071 3.091 3.116 3.211
0.128585 0.084370 0.049671 0.0479365 –
163.297 130.658 104.646 255.774 –
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Fig. 6. Room temperature B–H curves of Zn0.9 Co0.1 O samples sintered at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C temperatures, respectively.
shift may be due to induced band gap renormalization effect [22]. The band gap shrinkage and free carrier screening are responsible for the band gap renormalization effect. The s–d and p–d exchange interactions give rise to a negative and a positive correction to the conduction-band and valence-band edges, respectively, resulting in shrinkage of the band gap. The optical band gap plots of Zn0.9 Co0.1 O samples sintered at different temperatures are shown in Fig. 5(b). The estimated direct fundamental optical band gap (Eg ) values for the Zn0.9 Co0.1 O samples listed in Table 3. These results are in agreement with the reported literature [23–26]. It can be observed from the figures that there is a shift in the absorption edge to higher energy side (blue shift) with increasing crystalline size and the origin of this blue shift may be explained using Burstein-Moss effect [27,28]. Further, it has been argued that the change in band gap values influences the local structural environment of Co-doped ZnO lattice. In order to investigate the modifications in the electronic structure, the slope of the linear part of (˛h)2 versus h plot (Fig. 5(b)) has been determined. It has been observed that there is a small change in the slope of the plot and may be attributed to the changes in the conduction-band and valence-band edges, which itself might have arisen due to the addition of cobalt. 3.5. Magnetic properties Properties of nanocrystalline Zn0.9 Co Fig. 6 shows, room temperature magnetization versus magnetic field (B–H) curves at an
applied field of 15 kOe for the samples sintered at different temperatures. It can be seen from Fig. 6 that all the samples exhibiting clear magnetic hysteresis loops with low coercivties and the magnetization values are well saturated for the all samples indicating room temperature ferromagnetism. The saturation magnetization (Ms ) and coercive field (Hc ) of all the samples are given in Table 3. The observed magnetic behavior may be attributed to the exchange interaction between the localized magnetic dipole moments of the magnetic ions (the localized d-spins of Co ions) and the free delocalized charge of current carriers (holes or electrons from the valence band). The impurities/defects play a major source of these free carriers, and strongly related to the oxygen vacancies and processing conditions. It can be seen that Ms values are found to be decrease with increasing crystallite size and this may be due to the trapping of free charge carriers in the grain boundaries leading to the variation of charge carriers occupying states. The variation in the values of charge carriers in turn influences the magnetic ordering in the samples thereby decreasing the values of saturation magnetization. 4. Conclusions Nanocrystalline Zn0.9 Co0.1 O materials were synthesized using citrate gel route and the crystallite sizes are found to be in the range of 25–65 nm. It has been concluded from the XPS studies that Co is incorporated in ZnO host with an oxidation state of Co2+ without forming cobalt clusters and might have not bonded to oxygen either as CoO or as Co3 O4 . It has been found that the optical band gap of
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these materials is increasing with increasing crystallite size of the materials. From optical studies, a small change in the slope of the linear part of (˛h)2 versus h also implies the presence of strong Coulomb interaction between Co, Zn and O sites, and supports the observed ferromagnetism in these samples. All the samples exhibited room temperature ferromagnetism and decreases with increasing crystallite size of the materials. As the high value of saturation magnetization is exhibited by a sample with lowest crystalline size, one may conclude that nano size might be beneficial to the ferromagnetism in the samples of the present investigation. Acknowledgement The authors are very much thankful to DRDO New Delhi, for providing financial assistance and research facilities through research project. References [1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [2] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. [3] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S.Y. Koshihara, H. Koinuma, Science 29 (2001) 854. [4] H. Kimura, T. Fukumura, H. Koinuma, M. Kawasaki, Appl. Phys. Lett. 80 (2001) 94. [5] I.B. Shim, C.S. Kim, J. Magn. Magn. Mater. 272 (2004) e1571. [6] S. Maensiri, P. Laokul, S. Phokha, J. Magn. Magn. Mater. 305 (2006) 381–387. [7] S. Maensiri, J. Sreesongmuang, C. Thomas, J. Klinkaewnarong, J. Magn. Magn. Mater. 301 (2006) 422.
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