Microelectronic Engineering 89 (2012) 129–132

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Microelectronic Engineering

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Doping effects of Co2+ ions on structural and magnetic propertiesof 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 a b s t r a c t

Article history:Available online 7 April 2011

Keywords:Diluted magnetic semiconductorZnOX-ray diffractionDC magnetizationFerromagnetism

0167-9317/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.mee.2011.03.149

⇑ Corresponding author.E-mail address: chglee@changwon.ac.kr (C.G. Lee)

In this paper, we report the synthesis of Zn1�xCoxO (0.0 6 x 6 0.10) nanoparticles by an auto-combustionmethod 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 dopedZnO have a single phase nature with wurtzite structure and Co2+ ions were successfully incorporated intothe lattice position of Zn2+ ions in ZnO matrix. FTIR spectra demonstrated that the values of absorptionbands were blue shifted with the increase of Co content. From the Raman spectra, all the peaks observedin 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 substitutionof Co into the Zn site. Magnetic studies showed that Co doped ZnO nanoparticles exhibit room temper-ature ferromagnetism.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Diluted magnetic semiconductors (DMSs) are very interestingmaterials subjected to their promising applications to Spintronics(spin + electronics). There has been greatly attention in magneticsemiconductors which exploit both spin and charge carrier be-cause of combination of two degree of freedom, which assurenew 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 candidatesfor the room temperature ferromagnetism (RTFM). In fact, RTFMhas been observed in TM doped ZnO [4,5]. However, the results re-main controversial and some reports showed a very low magneticordering temperature in TM doped ZnO [6] or even the absence ofFM in these samples prepared by means of different methods.These controversial results give an indication that RTFM in DMSsis extremely sensitive to preparation methods and thus of prepara-tion conditions. ZnO-based DMSs, especially Co-doped ZnO are apossible candidate for a high-TC ferromagnetic semiconductorand has attracted a lot of attention. In addition, Zn1�xCoxO maybe an ideal material for short-wave magneto-optical devices be-cause of the wide band gap of ZnO as well as the high thermalsolubility of Co in ZnO. A variety of preparation of nanoscaleZnO-based DMSs have been in used such as solvothermal, hydro-thermal, self assembly and template assisted sol–gel methods

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[4,5,7]. However, in this paper we report the synthesis ofZn1�xCoxO nanoparticles by a low-temperature combustion meth-od. This method ensures high chemical homogeneity due to theaqueous solution mixture of initial reagents and, as a result, favorsthe production of desired phases or ceramics. In the present work,we have synthesized nanocrystalline Co-doped ZnO powders byauto 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 analyticalgrade purity and purchased from Sigma Aldrich. In a typical syn-thesis of Zn1�xCoxO (0.0 6 x 6 0.1) nanoparticles, the appropriateproportion of Zn(NO3)2�6H2O, Co(NO3)2�6H2O, and C2H5NO2 (gly-cine) were completely dissolved in a 1000 ml beaker to obtain a200 ml aqueous solution. The aqueous solution was then stirredfor about 1 h in order to mix the solution uniformly. The solutionwas evaporated on a hot plate under constant stirring. When thewater was completely removed, the solution then converted intoa ‘‘gel’’. The ‘‘gel’’ was subsequently swelling into foam like and un-dergo a strong self-propagating combustion reaction to give a finepowder. The calcined samples were characterized for crystal phaseidentification by X-ray diffraction (XRD) using Phillips X’pert (MPD3040) X-ray diffractometer with Cu Ka radiations (k = 1.5406 Å)operated at voltage of 40 kV and current of 30 mA. Fourier trans-mission 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 aresolution of 1 cm�1. The surface morphology of the fine synthe-sized powders was carried out using field emission scanning elec-tron microscopy (FESEM) using a TESCAN; MIRA II LMHmicroscope. In order to get the phonon vibrational study of thepure and Co doped ZnO, we used a micro-Raman spectrometer(NRS-3100) with a 532 nm solid state primary laser as an excita-tion source at room temperature. Magnetization measurementswere performed using a commercial Quantum Design physicalproperty measurement system.

3. Results and discussions

Fig. 1 shows the XRD patterns of Zn1�xCoxO (0.0 6 x 6 0.1) sam-ples, which was indexed using POWDER-X software as the ZnOwurtzite structure and well matched with the standard data(JCPDS, 36-1451). It can be clearly seen from the XRD patterns thatall the samples showed a single-phase nature with hexagonalwurtzite structure. No secondary phase was detected, thus indi-cated that the Co dopant ought to be incorporated into the latticeas a substitutional atom. In order to study the effect of Co doping,a careful analysis of the position of the XRD peaks indicate thatthere is a shifting in peak’s position towards lower 2h value (see in-set (a) in Fig. 1) with increasing Co contents. The shifting of thepeak’s position shows that the lattice parameters increase withthe increase of Co doping. Refined value of the lattice parametersa and c as a function of Co content are shown in inset (b) ofFig. 1. These values obtained from the refinement are in goodagreement with the standard data base [JCPDS, 36-1451]. Theincreasing trend of lattice parameters clearly indicates that Co ionsare substituting Zn in ZnO matrix and these results are in a goodagreement with those reported earlier [8]. The crystallite sizes ofthe synthesized powders were estimated from X-ray lines broad-ening using Scherer’s equation [9] were found to increase rangingfrom 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 synthesizedpowders were further investigated by FESEM and EDX analysis. In-set 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) magnifiedspectrum of (1 0 0) peak and inset (b) the plot of lattice parameters vs. Co content.

Co doped ZnO nanoparticles which are homogeneous and agglom-erated with diameters ranging from 40 to 50 nm. Due to the uni-form distribution of oxidized metal anions in the three-dimensional polymeric network structure, the agglomerationcould be induced by densification resulting from the narrow spacebetween particles [10]. It is clear from the FESEM images that theparticles are nearly spherical in shape and Co doping results in anaggregation of nanoparticles. With the increase in Co doping, thespacing between the particles is expected to become narrowerand also there is an increase in particle size, which leads to theagglomeration of nanoparticles. Fig. 2(a) and (b) depict the typicalEDX spectra taken from pure and 5% Co-doped ZnO samples. Thechemical analysis of Zn1�xCoxO with x = 0.05 measured by EDXanalysis shows the presence of Zn, O and Co signals only; indicatesthat the nanoparticles are made up of zinc, oxygen and Co ionswhich shows that the Co ion is substituting the Zn ion in ZnO ma-trix. Only zinc and oxygen signals have been detected in undopedZnO sample, suggesting that the nanoparticles are indeed made upof 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. FTIRmeasurements were performed in the wave number range 4000to 400 cm�1 using KBr method at Room temperature as shown inFig. 2 (c). The FTIR spectra show main absorption bands near3400 cm�1 represent O–H mode, those at 2900 cm�1 are C–Hmode, band arising from the absorption of atmospheric CO2 onthe metallic cations at 2350 cm�1 and 1400–1600 cm�1 are theC@O stretching mode. The absorption band at 454 cm�1 is thestretching mode of ZnO [11]. However, in case of 1%, 3%, 5%, 7%and 10% Co doped samples, the value of absorption bands werefound 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 thepeak position of ZnO absorption bands reflects that Zn–O–Zn net-work is perturbed by the presence of Co in its environment. There-fore, the FTIR results also indicate that Co is occupying Zn positionin ZnO matrix as observed in XRD measurements.

The Raman spectra are sensitive to the crystal quality, structuraldefects and disorders of the grown products. ZnO has a Wurtzitestructure that belongs to the C6v symmetry group. Fig. 2(d) showsthe Raman spectra of un-doped and Co-doped ZnO nanoparticles. Itis evident that Raman modes in the spectrum of undoped ZnO havesimilar positions as in the spectrum of bulk ZnO. However, severalchanges can be observed in the Raman spectrum of Co-doped ZnOin comparison with the spectrum of undoped ZnO. The mode E2

high

at 439 cm�1 that is related to the vibration of oxygen atoms inwurtzite ZnO [12] is the most prominent mode in undoped ZnOpowder. 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 ZnOsamples, respectively, observed in the Raman spectrum, indicatethe changes in the defect structure due to mechanical activation.There are strong peaks appeared at 2B1

low; 2LA (540, 525, 520and 523 cm�1) for Co doping, as shown in Fig. 2(d). We deem thatthe peaks at 2B1

low; 2LA (525, 523, 520 and 516 cm�1) were Co–Omode, which contributed to local vibrations of Co ions in ZnO lat-tice as a substitution of Co into Zn position [13]. In the presentwork, the additional mode appears after doping, which maybedue to Co2+ occupation at Zn2+ sites.

Fig. 3 shows magnetization versus magnetic field (M–H) curvesfor Zn1�XCoxO (x = 0.03 and 0.07) samples measured at room tem-perature. The magnetization increased with increasing Co content,showed that Co-doped ZnO exhibit RTFM with a TC at or aboveroom temperature. Calculated value of the coercive field (HC) andthe remanent magnetization (Mr) for Zn1�XCoxO (x = 0.03 and0.07) 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

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 correspondingFESEM 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.

Fig. 3. Magnetization curve of Zn1�XCoxO (x = 0.03 and 0.07) at room temperature.Inset shows the magnified loop.

F. Ahmed et al. / Microelectronic Engineering 89 (2012) 129–132 131

in DMSs is still controversial; however, in the present study, therecan be three possible origins of ferromagnetism. The first isZn1�xCoxO, the carrier induced ferromagnetism (RKKY mecha-nism), which is often reported for DMSs [14]. The second is CoOx,but we did not find any secondary phase in the XRD of Co-dopedZnO. The third is micro Co clusters, we also could not find any sig-nal of Co clusters in the XRD. In the present work, the single-phased XRD patterns, the slightly shifted ZnO (1 0 0) peak withCo doping and the evidence of Co incorporation into ZnO from FTIRand Raman studies suggested that the detected ferromagnetismcould arise from the homogeneous doping of Co into ZnO whichfollow the RKKY mechanism.

4. Conclusions

In summary, pure and Co-doped ZnO nanoparticles have beensuccessfully synthesized using an effective auto combustion route.XRD and FTIR results indicated that all the synthesized un-dopedand Co-doped ZnO samples had the wurtzite structure and nosecondary phase was detected which indicated that Co ionssubstituted for Zn ions. FESEM results revealed that the preparedCo-doped ZnO nanoparticles are nearly spherical in shape withparticle size <50 nm, which is in good agreement with the size ob-tained from XRD. The blue shift of the major Raman mode (E2

high)and the decrease in its intensity with the increase in Co doping,points to the incorporation of Co ions in the ZnO lattice. Magneticmeasurements studies conclude that the all Co doped samplesshowed RTFM. The presented simple synthesis method using cheapprecursors can be use to prepare other interesting metal oxidesnanoparticles.

Acknowledgment

This research was supported by Grant No. RTI04-01-03 from theRegional Technology Innovation Program of the Ministry of Knowl-edge Economy, South Korea.

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