Room temperature synthesis of Cu incorporated ZnO

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CERAMICS INTERNATIONAL

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Room temperature synthesis of Cu incorporated ZnO nanoparticles with room temperature ferromagnetic activity: Structural, optical and magnetic characterization Özlem Altıntaş Yıldırıma,b,n, Caner Durucana a

Department of Metallurgical and Materials Engineering, Middle East Technical University, 06800 Ankara, Turkey b Department of Metallurgical and Materials Engineering, Selcuk University, 42030 Konya, Turkey Received 7 July 2015; received in revised form 22 October 2015; accepted 22 October 2015

Abstract Copper-incorporated zinc oxide (ZnO:Cu) nanoparticles with different amounts of Cu incorporation (Cu:Zn ratio at% of 1.25, 2.5 and 5) were synthesized for the first time by a room temperature precipitation technique without any subsequent post thermal treatment. Pure ZnO nanoparticles were also synthesized for direct structural and property-related comparison purposes. ZnO:Cu nanoparticles were thoroughly characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, vibrating sample magnetometer, UV–visible and photoluminescence spectroscopy. Detailed crystallographic investigation was accomplished through Rietveld refinement. ZnO:Cu nanoparticles exhibited room temperature ferromagnetic (RTFM) property. The origin of RTFM in ZnO:Cu nanoparticles has been investigated and this property was attributed to the substitutional incorporation of Cu ions into the ZnO lattice. The evidence for substitutional incorporation has been demonstrated with the positional changes in XRD peaks of ZnO and accompanying precise lattice parameter analyses by Rietveld refinement. Chemical analysis by XPS revealed incorporation of copper as Cu2 þ ions. The Rietveld-based percent occupancy values for Zn sites imply certain limited substitution of the Cu2 þ ions into the ZnO lattice with the increasing amount of Cu. Meanwhile, the magnetization enhanced with increasing Cu amount, due to the narrowing band gap, suggesting that RTFM characteristics are an intrinsic property of ZnO:Cu nanoparticles. This work gives insight into the origin of RTFM in ZnO nanoparticles and can be used to enhance ferromagnetism in diluted magnetic semiconductors. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Cu doped ZnO; Nanoparticles; RTFM; Precipitation; Room temperature synthesis

1. Introduction There has been intense research interest on various transition metal (TM) incorporated semiconductor metal oxides, given their room temperature ferromagnetism (RTFM). These semiconductors are referred to as diluted magnetic semiconductors (DMSs) and attract great attention for their potential applications in spintronics devices [1]. Most of the earlier work on n Corresponding author at: Department of Metallurgical and Materials Engineering, Selcuk University, 42030 Konya, Turkey. Tel.: þ 90 332 223 1997; fax: þ 90 332 241 0635. E-mail address: [email protected] (Ö. Altıntaş Yıldırım).

DMSs has been on III–V type semiconductors, which have low solubility for TM [2,3]. More recently, II–VI type TMincorporated semiconductors have been explored, as they allow high dopant concentrations and exhibit potential in achieving magnetic ordering at around ambient temperatures [4,5]. Among various II–VI type semiconductor metal oxides, zinc oxide (ZnO) has provoked great interest as a DMS due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV) [6]. The role of the TM incorporation in ZnO based DMSs is of considerable significance because the origin of the RTFM is claimed to be based on two controversies; the ferromagnetism is (i) an external property and comes from impurities like TM

http://dx.doi.org/10.1016/j.ceramint.2015.10.113 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Ö. Altıntaş Yıldırım, C. Durucan, Room temperature synthesis of Cu incorporated ZnO nanoparticles with room temperature ferromagnetic activity: Structural, optical and magnetic characterization, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.113

Ö. Altıntaş Yıldırım, C. Durucan / Ceramics International ] (]]]]) ]]]–]]]

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clusters [7,8] or (ii) an intrinsic property and originates from the spin-polarized carries [9,10]. Therefore, many studies have been performed to elucidate the mechanism of RTFM. Up to now, various TMs such as manganese [11], nickel [12], cobalt [13], and iron [14] have been incorporated into the ZnO structure to achieve RTFM property. The major drawback is that the dopant atoms can form clusters or precipitates directly affecting the ferromagnetic properties and such impurities impede the use of DMSs for spintronics applications [6]. Recently, researchers have focused on the copper incorporated ZnO (ZnO:Cu) nanostructures [15–17]; instead of traditional DMSs incorporated with TMs since metallic Cu is not magnetic, and neither Cu2O nor CuO is ferromagnetic. However, the origins of the RTFM behavior of the ZnO:Cu are still under debate due to conflicts in experimental and theoretical studies [18–20]. Therefore, there remains a need to understand the nature of the RTFM of ZnO:Cu nanoparticles with a detailed experimental study. The magnetic properties of the ZnO:Cu systems strongly depend on the synthesis procedure [21]. For practical applications, development of an efficient, cost-effective, and reproducible technique is desirable for the large-scale production of DMSs. Several researchers have studied the origin of the RTFM property of ZnO:Cu nanoparticles prepared with various synthesis methods [22–24]. However, to the best of our knowledge, no one has focused on the room temperature synthesis of ZnO:Cu nanoparticles as a DMSs material. The objectives of this paper is to synthesize ZnO:Cu nanoparticles at room temperature, to understand the ferromagnetic behavior of ZnO nanoparticles as a function of Cu amount, and to draw a conclusion for the origin of RTFM. In the present work, room temperature synthesis of ZnO:Cu nanoparticles is carried out for the first time by a simple precipitation method, based on our previous work regarding ZnO:Ag nanostructures [25]. 2. Experimental procedure 2.1. Materials The synthesis of ZnO and ZnO:Cu nanoparticles were achieved using zinc acetate dihydrate (C4H6O4Zn 2H2O, 99.5%, Fluka, Sigma-Aldrich Chemie GmbH, Steinheim, Germany), copper (II) acetate monohydrate ((CH3COO)2Cu H2O, 99%, Merck KGaA, Darmstadt, Germany), polyvinyl pyrrolidone ((C6H9NO)n, PVP, MW 55000, Aldrich, Sigma-Aldrich Chemie GmbH, Steinheim, Germany) and sodium hydroxide (NaOH pellets, 97%, Merck KGaA, Darmstadt, Germany). Ethylene glycol (C2H6O2, EG, 99%, Merck KGaA, Darmstadt, Germany) and ultra-pure deionized (DI) water were used as solvents. All reagents were used without further purification. 2.2. Experimental methods For synthesis of ZnO:Cu nanoparticles, first a zinccontaining solution (0.1 M, 100 mL) was prepared by dissolving zinc acetate dihydrate in EG. This solution was added

drop-wise into a PVP solution (0.5 g PVP, in 100 mL EG) using an automated injection pump (Top-5300 model syringe pump) at a rate of 50 mL/h and stirred for 15 min. Then, 2 M NaOHaq was added (at a rate of 20 mL/h) into this mixture until the pH reached to 12.7. Meanwhile, three 50 mL aqueous copper acetate solutions of varying Cu concentration, corresponding to a final solution concentration of Cu:Zn ratio of 1.25, 2.5 and 5 at% Cu, were prepared. Each Cu-solution was added into a separate zinc-containing parent solution at a rate of 20 mL/h right after pH adjustment. Once injection was completed, the final solutions were homogenized for 15 min at 900 rpm. The powder products were collected by centrifugation at 10000 rpm (using Eppendorf 5805 model (Eppendorf AG, Hamburg, Germany)) for 10 min and subsequently washed thoroughly (five times) with DI-water. The powders were then dried at 70 1C for 3 h under vacuum and no further thermal treatment was performed. In addition to ZnO:Cu nanoparticles, pure ZnO nanoparticles were also synthesized according to the same experimental procedure, by substituting aqueous copper acetate solution (in 50 mL DI-water) with pure DI-water of same amount. 2.3. Materials characterization The phase identification was conducted by x-ray diffraction (XRD) analyses using a Rigaku D/Max-2000 PC (Rigaku Corporation, Tokyo, Japan) diffractometer. Typical data acquisition was performed in a diffraction angle (2θ) range of 25– 921 with a step size of 0.51/min, using Cu-Kα radiation. A detailed crystal structure investigation was conducted by Rietveld refinement analyses using General Structural Software [26] with the EXPGUI graphical user interface [27]. The chemical identification and evaluation of the chemical state of the constituent elements was accomplished by x-ray photoelectron spectroscopy (XPS) analyses using a PHI 5000 VersaProbe (Physical Electronics, MN, USA). The quantitative analyses for chemical identification were performed using high-resolution scans of the Cu(2p), O(1s) and Zn(2p) spectral regions. The binding energies and charge corrections have been calibrated for C(1s) signal (285.0 eV). JEOL 2100F model (JEOL Ltd., Tokyo, Japan) transmission electron microscope (TEM) at an accelerating voltage of 200 kV was used to determine the size and morphology of the nanoparticles. The representative TEM samples were prepared by drying out ultrasonically dispersed aqueous powder suspensions on holey carbon coated copper grids. The magnetic properties of the samples were investigated using a vibrating sample magnetometer (ADE Magnetics EV9, MicroSense LLC, MA, USA). All measurements were performed at room temperature with a maximum field of 4 kOe. Saturation magnetization and coercivity values of the each sample were obtained from the hysteresis curves. The optical properties were determined with ultraviolet– visible (UV–vis) absorption spectroscopy performed using a double beam spectrophotometer (Cary100 Bio; Agilent Technologies Pty Ltd., Mulgrave, VIC, Australia) in a wavelength range of 250–800 nm; room temperature photoluminescence



(PL) spectrum was conducted with the Perkin-Elmer LS 55 spectrophotometer. Samples were dispersed in DI-water by 10 min ultrasonication. The background contribution for UV– vis and PL spectroscopies were evaluated using DI-water as a reference. The direct band gap energy (Eg) of the ZnO nanoparticles is determined by plotting the absorption coefficient (α) versus photon energy (hυ) graph and extrapolating the straight-line portion of this plot to the hυ axis. 3. Results and discussion 3.1. Crystal structure analyses and properties The XRD analyses results of the ZnO:Cu nanoparticles have been mainly employed in investigating the effects of Cu addition/incorporation on crystal structure of ZnO and in interpreting the relevant changes. Fig. 1(a–d) show the XRD diffractograms of pure ZnO and 1.25, 2.5 and 5 at% Cu incorporated ZnO:Cu nanoparticles prepared at room temperature without any subsequent post thermal treatment. The XRD patterns for pure and Cu incorporated samples match a

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wurtzite type ZnO crystal structure (JCPDS card no: 36– 1451). This data indicates no trace of any additional phase of copper, copper oxide(s) or any binary Zn–Cu phase. Although the XRD technique has limited sensitivity for the detection of very small amount of a given phase, it is reasonable to assume absence of secondary phases due to the small amount of Cu dopant, which is lower than the solubility limit of Cu in ZnO as determined by others [28–30]. According to XRD patterns, highly crystalline ZnO and ZnO:Cu nanoparticles can be synthesized by room temperature precipitation. Fig. 1(a–d) also indicate the results of Rietveld refinements of pure and Cu- incorporated ZnO nanoparticles. The diffractogram for each sample is composed of two overlapping sets; the actual experimental data, Rexp, presented with solid lines and also the simulated counterpart, Rwp, obtained from the Rietveld refinement indicated with cross signs. The graycolored patterns (on the bottom) show the differences between the experimental data and simulated results of the Rietveld analyses. Rexp and Rwp values for all samples are listed in Table 1. The Chi-squared values, X2 (X2 ¼ (Rwp/Rexp)2)), were obtained as 1.34, 1.40, 1.45 and 1.42; for pure, 1.25, 2.5 and

Fig. 1. (a–d) The experimental XRD diffractograms, Rietveld refinement simulation results and differences between experimental and simulated data of pure ZnO and ZnO:Cu nanoparticles with different Cu contents (1.25, 2.5 and 5 at% Cu), respectively. Please cite this article as: Ö. Altıntaş Yıldırım, C. Durucan, Room temperature synthesis of Cu incorporated ZnO nanoparticles with room temperature ferromagnetic activity: Structural, optical and magnetic characterization, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.113


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Table 1 Rexp and Rwp values, 10̅1 1 ZnO peak positions, lattice parameters (c), Zn occupancy values, XRD crystallite size obtained from Rietveld analyses and average crystalline size determined from TEM analyses for pure ZnO and ZnO:Cu nanoparticles with different Cu contents (1.25, 2.5 and 5 at% Cu). Cu:Zn ratio (at.%)

Rexp Rwp (%)

10̅1 1 Peak Positiona (degree of 2θ)

Lattice Parameter (c, Å)

Occupancy, Zn (%) XRD Crystallite Size (nm)

TEM Particle Size (nm)

0 1.25 2.5 5

5.587.50 5.567.78 5.588.07 5.597.94

36.25 36.27 36.28 36.29

5.26 5.25 5.21 5.20

100.0 99.2 98.3 96.8

17.372.5 17.172.7 16.172.5 13.972.5

a

20.9 19.6 16.8 13.7

peak position of JCPDS card of ZnO (36-1451); 36.251.

5 at% Cu incorporated ZnO powders. The low R factors and X2 figures at around a value of 1 suggest that refinements are relatively reliable [31]. All the diffraction events for the ZnO nanoparticles fit to that of wurtzite structure; P63mc space group with atomic position of (1/3, 2/3, 0) for Zn and (1/3, 2/3, 0.3817) for O, respectively [32]. Fig. 2 exhibits the enlarged regional XRD patterns of the samples within the 2θ range of 30–381. The full width at half maximum of the diffraction peaks increase with Cu addition, meanwhile the peak positions slightly shift to higher 2θ values. The 2θ positions for 10̅1 1 diffractions extracted from Fig. 2 are listed in the Table 1. For ZnO:Cu particles with increasing Cu amount, a 2θ shift in the range 0.02–0.041 was observed. Table 1 also lists the precise figures for the lattice parameter (c) as determined by the Rietveld refinement analyses. The lattice parameters decrease with Cu incorporation and reach a minimum value of 5.20 Å in the case of 5 at% Cu- incorporated sample, which is 5.26 Å for pure ZnO. A change in the lattice size with Cu substitution is expected due the ionic size differences between Cu (Cu2 þ : 0.57 Å) and Zn (Zn2 þ : 0.60 Å) ions for the same coordination number (four-fold coordination) [33]. Thus, the Rietveld refinement results are in good agreement with the conventional XRD data and relevant interpretations indicate a possible lattice contraction due to selective/specific substitution of Cu2 þ with Zn2 þ in the lattice as also reported by others [29,34]. The Rietveld refinement analyses of XRD data also allow determination of quantitative occupancy values for Zn sites [35]. The percent occupancy values of Zn sites were listed in Table 1. The calculated Zn occupancy values were slightly lower than the theoretical values. The marginal differences between initially available Cu content and actual substitution extent suggest that most of the Cu ions in solution readily substitute into ZnO during precipitation of ZnO crystals. The deviation in the occupancy values is more significant for higher amounts of Cu-containing precipitation formulations, suggesting an increasing incorporation of the copper ions into Zn2 þ sites with the higher amount of the Cu addition. Both experimental [36] and theoretical [37] studies indicate that substitution of Cu2 þ ions into the ZnO lattice is more favorable than that of Cu1 þ and Cu0, as there would not be any change in the net charge upon substitution of Cu2 þ for Zn2 þ for stoichiometric ZnO. Furthermore, Ye et al. proposed that Cu has a tendency to occupy the Zn sites in the crystal structure, instead of interstitial positions [9]. Therefore,

Fig. 2. Enlarged region (2θ¼ 30.5–37.51) of the experimental XRD diffractograms of pure ZnO and ZnO:Cu nanoparticles with different Cu contents (1.25, 2.5 and 5 at% Cu).

substitutional incorporation of Cu occurs with replacement of host Zn2 þ with Cu2 þ ions. The average crystallite size for the ZnO nanoparticles estimated by Rietveld refinements are also presented in Table 1. The particle size decreases from 20.9 nm (pure ZnO) to 13.7 nm in the case of 5 at% Cu incorporation. The observed smaller crystallite size can be attributed to the combined effects of (i) lattice distortion of the host ZnO crystal because of the substitution of smaller ionic-sized copper atoms sitting in the ZnO lattice [38] and (ii) hindering of the crystal growth due to the formation of a thin layer of Cu–O–Zn on the surface of the doped samples due to the presence of excessive copper ions in the precipitation solution [39]. 3.2. Microstructural analyses and properties Fig. 3 shows low magnification TEM micrographs of ZnO nanoparticles. All ZnO nanoparticles have an equiaxed/spherical morphology. The average particle sizes determined from TEM analyses (given in Table 1) are in agreement with the crystallite sizes obtained from Rietveld refinement. Highresolution TEM images of selected ZnO nanoparticle(s) are shown in Fig. 4. The fringes of individual planes are clearly visible in the micrographs suggesting the single crystal nature for ZnO nanoparticles. The lattice spacing values of pure ZnO



Fig. 3. Low magnification TEM images of (a) pure ZnO and (b) 1.25, (c) 2.5 and (d) 5 at% Cu incorporated ZnO:Cu nanoparticles.

Fig. 4. High resolution TEM images of (a) pure ZnO and (b) 1.25, (c) 2.5 and (d) 5 at% Cu incorporated ZnO:Cu nanoparticles.


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Fig. 5. XPS spectra for pure ZnO and ZnO:Cu nanoparticles with different Cu contents (1.25, 2.5 and 5 at% Cu); (a) survey analyses, and high resolution regional spectra of (b) Cu(2p), (c) Zn(2p) and (d) O(1s) signals.

and ZnO:Cu nanoparticles are also shown in Fig. 4. The spacing between adjacent fringes was measured as 0.281 nm for pure ZnO nanoparticles which is in good proximity for the interplanar spacing of 10̅1 0 . The spacing between adjacent fringes of the ZnO:Cu samples were measured as 0.278, 0.276 and 0.273 nm for the 1.25, 2.5 and 5 at% Cu-added ZnO:Cu samples, respectively. This indicates a lattice contraction due to replacement of smaller sized Cu2 þ with Zn2 þ . 3.3. Chemical analyses and properties The chemical composition and chemical state of the constituents for ZnO:Cu nanoparticles were investigated by XPS analyses. The survey spectra of pure ZnO and ZnO:Cu nanoparticles are shown in Fig. 5(a). The major signals for pure ZnO nanoparticles were assigned to Zn, O and C elements and no signal for other elements was detected. The low intensity C signal is most likely due to remaining PVP or simply due to the adsorption of organic contaminants during handling. In the survey spectra of the ZnO:Cu samples, additional signals for Cu(2p) were detected. The informative

spectral regions for Cu(2p), Zn(2p) and O(1s) are also presented with the accompanying high resolution regional spectra in Fig. 5(b), (c) and (d), respectively. The corresponding binding energies (BE) in eV for the Cu(2p), Zn(2p) and O (1s) signals determined from the high resolution scans are summarized in Table 2. The high resolution Cu(2p) signals (Fig. 5(b)) provide additional insight regarding the chemical state of Cu in the ZnO lattice. The XPS can clearly distinguish the state of Cu due to measurable differences in BE of the signals for metallic and ionic entities. Metallic Cu (Cu0) generates sharp and welldefined Cu(2p3/2) and Cu(2p1/2) signals centered at 932.6 eV and 952.5 eV, respectively. Monovalent Cu (Cu1 þ ) produces also sharp and well-defined Cu(2p) signal centered at the same BE values, but slightly narrower signal peak widths. Meanwhile, divalent Cu ion (Cu2 þ ) characteristically exhibits considerably broader Cu(2p) signals with an 1.0 eV shift to higher BE and centered at 933.6 eV and 953.5 eV for Cu (2p3/2) and Cu(2p1/2), respectively [40,41]. The high resolution XPS spectra of ZnO:Cu, with Cu(2p3/2) and Cu(2p1/2) binding energies corresponding that of Cu2 þ oxidation state, indicate



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Table 2 Binding energy values (in eV) of Cu(2p3/2), Cu(2p1/2), Zn(2p3/2), Zn(2p1/2) and O(1 s) (lattice and chemisorbed oxygen) for pure ZnO and ZnO:Cu nanoparticles with different Cu contents (1.25, 2.5 and 5 at% Cu).

energy values with increasing Cu addition also supports the extensive substitution of Cu2 þ ions into ZnO lattice and is in agreement with the Rietveld refinement results.

Cu:Zn ratio Binding energy (eV) (at%) Cu(2p3/2) Cu(2p1/2) Zn (2p3/2) Zn (2p1/2) O(1s)a O(1s)b

3.4. Magnetic analyses and properties

0 1.25 2.5 5 a

– 933.46 933.69 933.92

– 953.21 953.36 954.16

1021.19 1021.32 1021.45 1021.58

1044.21 1044.32 1044.46 1044.58

529.91 530.08 530.13 530.37

531.51 531.58 531.61 531.74

Lattice oxygen. Chemisorbed oxygen.

b

that Cu is mainly incorporated as Cu2 þ . Furthermore, the positions of Cu(2p) signals shift toward the higher BE’s values with the increasing Cu addition. This can be explained with a decrease in Cu–O bond separation as a result of lattice parameter change with increasing dopant amount. Fig. 5(c) shows the features of Zn(2p) signal for all samples. For pure ZnO nanoparticles, Zn(2p3/2) and Zn(2p1/2) signals, originating from strong spin orbit coupling, appear at 1021.19 and 1044.21 eV, respectively. These energy values agree well with the previously reported BE values for stoichiometric ZnO [42]. The positions of these signals shift slightly to higher BE values for ZnO:Cu nanoparticles. This change can be explained by the differences in O(1s) signal characteristics for pure and Cu-incorporated ZnO as shown in Fig. 5(d). The O(1s) signals were deconvoluted using Gaussian fit with XPS PEAK41 software. For pure ZnO, the lower BE side of the O (1s) signal centered at 529.91 eV is attributed to lattice oxygen ions neighboring with Zn2 þ ions in the hexagonal ZnO lattice. The higher BE component of the O(1s) signal centered at 531.51 eV is associated with either presence of loosely bound oxygen or chemisorbed oxygen, such as surface hydroxyl groups [43]. The O(1s) BE values are also in agreement with the those reported for stoichiometric ZnO [44]. For Cu-containing ZnO nanoparticles; the lower BE O(1s) signal is attributed to lattice oxygen neighbors with Zn2 þ or Cu2 þ ions. This signal appears at relatively higher BE values than pure ZnO, as can be seen by the vertical drop line placed in Fig. 5(d) for visual aid and also by BE values listed in the Table 2. This positional shift may be due to change in the ionic character of O–X bonds (X ¼ Zn2 þ for pure ZnO, and X ¼ Zn2 þ or Cu2 þ for Cu-incorporated ZnO); originating from electronegativity differences for the specific ion associating with oxygen. According to the Pauling principle, the negativity difference of a Cu–O bond is smaller than that of a Zn–O bond [45]. Therefore, in presence of Cu, the relatively poor ionic association of Cu–O pairs compared to those for Zn–O bonds, results in a higher valance electron density [42]. As a result of higher valance electron density with increasing Cu amount, enhanced electron transfer from the conduction band of ZnO to 3d band level of Cu provides BE of Zn(2p) and O(1s) signals to shift towards to higher value. Therefore, the chemical shift in the BE of Zn(2p) and O(1s) signals to higher

Fig. 6 shows magnetization vs. magnetic field (M–H) curves of ZnO and ZnO:Cu nanoparticles. The diamagnetic terms arising from the sample holder have been subtracted in plotting these loops. Pure ZnO nanoparticles exhibit diamagnetic property at room temperature. M–H curves of ZnO:Cu nanoparticles indicate RTFM ordering with distinct S shaped hysteresis loops. Therefore, the observed RTFM can be attributed to incorporation of Cu ions into the ZnO structure. Although the coercivity (Hc) values for all Cu-incorporated ZnO nanoparticles were nearly same, the saturation magnetization (Ms) values increase with increasing incorporated Cu amount. Ms and Hc values for ZnO:Cu samples are given in Table 3. The largest Ms value was observed as 0.86 μB/Cu for 5 at% Cu incorporated sample. This value is higher than previously reported experimental values [46], but smaller than the theoretically calculated value, 0.96 μB/Cu [47]. The reason of the RTFM for transition metal-incorporated DMSs still remains controversial in the relevant literature. The discussion is mainly on the origin of ferromagnetism; the ferromagnetism either is originated from the magnetic interactions between the dopant atoms or is an intrinsic property originated from substitutional spin-polarized Cu atoms sitting

Fig. 6. M–H curves of ZnO and ZnO:Cu nanoparticles with different Cu contents (1.25, 2.5 and 5 at% Cu).

Table 3 Saturated magnetization (Ms) and coercive fields (Hc) for pure ZnO and ZnO: Cu nanoparticles with different Cu contents (1.25, 2.5 and 5 at% Cu). Cu:Zn ratio (at%)

Saturated magnetization (Ms, μB/ Coarcive fields (Hc, Cu) Oe)

1.25 2.5 5

0.64 0.72 0.86

100 100 90


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in the ZnO lattice. In regard to the former possibility, although theoretical predictions support that Cu atoms tend to occupy well-separated positions in ZnO crystal [9], some experimental investigations report the formation of Cu precipitates or clusters with increasing Cu amount. In this case, antiferromagnetic matchup between Cu pairs destroys the ferromagnetic ordering and results in lower magnetic moments [5,48]. However, the results of current study indicated an increase in Ms value upon Cu incorporation. Similar findings have been also reported for Co- and Mn-incorporated ZnO nanostructures [49,50]. Therefore, in our study, formation of the Cu clusters is not expected even at the highest amount of the Cu addition (here 5 at% Cu). The analytical finding of this study- XRD and TEM- confirmed that Cu ions do not form any clusters or additional phases, and copper doping takes place by ionic substitution of Cu into ZnO crystal. This means that the distance between adjacent magnetically active ions in all Cuincorporated ZnO samples is still in the range of ferromagnetic ordering. The conservation of short range magnetic ordering through the magnetic interaction provides improvement of the RTFM with higher amount of Cu addition. Thus, the apparent ferromagnetic nature of the ZnO:Cu nanoparticles does not seem to commence due the magnetic interactions of the dopant atoms. 3.5. Optical analyses and properties The main objective of this work was to draw a conclusion for the origin of RTFM in ZnO:Cu systems. G.Z. Xing et al. reported that the observed magnetism is originated from the change in the electronic band structure generated by the impurities and defects [51]. In that respect, the band gap and electronic configuration of ZnO:Cu systems become critical informative merits. Fig. 7(a) shows UV–vis absorption spectra from pure and Cu-incorporated ZnO. For the ZnO nanoparticles, the main absorption band edge related to the wurtzite structure appeared at 340.0 nm. However, the absorption band edges for the ZnO:Cu particles show a red shift to higher wavelength with increasing Cu amount. An important consequence of the red shift in the absorption band edge is the reduction in the Eg values of ZnO nanoparticles shown as an inset in the Fig. 7(a). For pure ZnO, Eg value was determined as 3.341 eV and it decreases to 3.276, 3.268, and 3.256 eV for the samples incorporated with 1.25, 2.5 and 5 at% Cu, respectively. A smaller Eg value with increasing Cu incorporation is somewhat contradictory with particle size characteristics, which decrease with increasing Cu amount. The red shift in the absorption band edge and Eg values of the ZnO nanostructures is associated with sp-d exchange interactions between the band electrons of ZnO and localized d electrons of Cu2 þ . Eg narrowing occurs as a result of the negative or positive corrections of the conduction or valance band edges originating from s–d or p–d interactions [52–54]. The observed Eg narrowing also confirms substitution of Cu2 þ ions into the ZnO crystal structure. To further understand the excitation process, PL spectra of pure and 2.5 at% Cu incorporated ZnO nanoparticles was performed (Fig. 7(b)). The shape of the PL spectra for the ZnO

Fig. 7. (a)The UV–vis spectra of pure and Cu incorporated ZnO nanoparticles with different Cu contents (1.25, 2.5 and 5 at% Cu). Band edge locations of samples are also given in the figure. The insets show plots of (αhυ)2 as a function of photon energy (Eg) for all samples. (b) PL spectra of pure and 2.5 at% Cu incorporated ZnO nanoparticles.

nanoparticles is similar to those reported by others [55,56] and is dominated by a strong UV-emission peak (3.26 eV) attributed to the near-band-edge transition, and a second order diffraction of UV emission [55] which appears only in the nanostructures with a strong UV emission peak and shows higher crystallite structure [56,57]. The absence of green (broad peak between 1.6–2.8 eV), violent (3.13 eV) and red (2.1 eV) emissions show that defect free structure of the pure and 2.5 at% Cu incorporated ZnO nanoparticles [55]. Therefore, it can be concluded that the origin of RTFM in ZnO:Cu systems is not originated from defects. With the half full shell electronic configuration (3d9, 4s1), Cu2 þ ions form an impurity level, which is located near the Fermi energy level and just below the conduction band of ZnO. This impurity level behaves as a deep acceptor and provides carrier transformation of non-equilibrium holes or electrons by hybridization of band electrons with half full Cu (3d) electrons. Therefore, they result in the formation of bound



magnetic polarization and responsible for RTFM behavior [52,58]. As a summary, a proposed room temperature precipitation system can provide useful insights for large-scale production of ZnO:Cu systems with RTFM property. From the above analyses, it can be concluded that Cu2 þ ions substitute into the ZnO lattice and change the crystal and band structures of the host ZnO by varying incorporated Cu amount. Therefore, the RTFM property in ZnO:Cu nanoparticles is proposed to be an intrinsic behavior due to sp-d spin carrier exchange interaction. 4. Conclusions Pure and Cu-incorporated ZnO nanoparticles were synthesized through a room temperature precipitation technique. The effects of the Cu amount on the crystal structural, microstructural and magnetic properties of the ZnO nanoparticles have been reported. The results indicated that the Cu doping occurs by substitutional incorporation of Cu þ 2 ions in the ZnO lattice as revealed by extensive crystallographic analyses of the XRD data by Rietveld refinement. The chemical state analysis by XPS also supports such chemical modification in the ZnO structure. The magnetic measurements showed that Cu addition obviously enhances the RTFM property of ZnO nanoparticles. In addition to a reduction of the absorption band edge, smaller band gap energies were observed for Cu-incorporated ZnO, evidenced by optical spectroscopy. All the findings show that the RTFM behavior for the Cu-incorporated ZnO nanoparticles seems to be an intrinsic behavior and originated from substitution of Cu2 þ into ZnO crystal. This work describes a correlation between crystal structure, band structure, and RTFM behavior of the ZnO nanostructures. This cost-effective, simple and rapid room temperature doping strategy may be adapted to a variety other TM elements to obtain DMSs with RTFM property. Acknowledgments This work was funded by METU-BAP through grant BAP03-08-2012-007. OAY thanks The Scientific and Technological Research Council of Turkey (TUBITAK) for the support by the national scholarship program for PhD student and also METUÖYP Program. The assistance of Mehmet Yıldırım in Rietveld refinement analyses and magnetic measurements is kindly appreciated. In addition, the authors also thank to METU Central Laboratory for analytical characterization support. 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, Spintronics: a spin-based electronics vision for the future, Science 294 (2001) 1488–1495. [2] S. Koshihara, A. Oiwa, M. Hirasawa, S. Katsumoto, Y. Iye, C. Urano, H. Takagi, H. Munekata, Ferromagnetic order induced by photogenerated carriers in magnetic III–V semiconductor heterostructures of (In,Mn)As/ GaSb, Phys. Rev. Lett. 78 (1997) 4617–4620.

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