1D Morphology Stabilization and Enhanced Magnetic Properties of Co

O. D Jayakumar,† C. Sudakar,‡ Clas Persson,§ V. Sudarsan,† T Sakuntala,. ⊥. Ratna Naik,‡ and A. K. Tyagi*,†. †Chemistry Division, Bhabha Atomic Resear...
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DOI: 10.1021/cg900481m

1D Morphology Stabilization and Enhanced Magnetic Properties of Co:ZnO Nanostructures on Codoping with Li: A Template-Free Synthesis

2009, Vol. 9 4450–4455

O. D Jayakumar,† C. Sudakar,‡ Clas Persson,§ V. Sudarsan,† T Sakuntala,^ Ratna Naik,‡ and A. K. Tyagi*,† †

Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India, ‡Department of Physics & Astronomy, Wayne State University, Detroit, Michigan 48201, §Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and ^Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Received May 1, 2009; Revised Manuscript Received July 15, 2009

ABSTRACT: 1D nanostructures of Zn1-xCoxO (x = 0, 0.03 and 0.05) and Co and Li codoped ZnO (Zn0.85Li0.10Co0.05O) were prepared by a soft chemical method. We report a very interesting observation of morphological control and transformation of ZnO nanorods to spherical particles induced by Co substitution. It is also remarkable to note that the morphology completely reverts back to rod shape by Li incorporation. In addition to this unusual observation, the Li incorporation enhances the roomtemperature ferromagnetic (RTFM) properties. These experimental observations are well-supported by theory work as well. These results are significant, as the 1D RTFM will have implications in spintronic devices.

1. Introduction One-dimensional (1D) nanomaterials have attracted considerable attention because of various remarkable physical and chemical properties distinctive from conventional bulk materials. ZnO-based nanomaterials, because of their wide band gap (Eg = 3.37 eV), excellent chemical and thermal stability, and specific electrical and optoelectronic property of being II-VI semiconductors with a large exciton binding energy, can be used for a broad range of high technology applications.1 In addition, transition metal (TM)-doped ZnO is a potential candidate material for spintronic applications.2-5 Nanoscale dilute magnetic semiconductors (DMS), including quantum wires and rods, are envisioned as pivotal architectural elements in several spintronics devices. This has triggered significant research worldwide on the synthesis and characterization of various wide band gap semiconductor nanorods/nanowires.6-11A few studies have reported synthesis of 1D ZnO-based materials by various methods.12-17 There is also a considerable interest in understanding the spin effects in nanoscale magnetic semiconductors, and many essential advances in this field would get impetus by the development of facile methods for the preparation of highquality DMS nanostructures. Various approaches like ion implantation18,19 and solution chemical synthesis at low temperature20-23 have been employed to obtain magnetic TM doped ZnO nanowires/nanorods. Further, Li doping has shown to be an effective way to improve many properties like dielectric constant, ferroelctricity and magnetic properties of undoped and TM doped ZnO films and powders.24-27 Lin et al.26 and Jayakumar et al.27observed enhanced room temperature ferromagnetism (RTF) in Co-doped ZnO when codoped with Li. Several of these approaches to fabricate nanostructures in bulk and thin films have motivated us to explore the possibility of template-free solution-based synthetic routes for preparation of 1D Co-doped ZnO with Li *To whom all the correspondence should be addressed. E-mail: aktyagi@ barc.gov.in. Tel: þ91 22-2559 5330. Fax: þ91 22-2550 5151. pubs.acs.org/crystal

Published on Web 08/26/2009

codoping. Direct chemical synthesis of doped ZnO DMS may allow a better control over chemical composition and dopant speciation. Furthermore, solution chemical synthesis offers attractive advantages for scaling up and further processing. Fabrication of 1D TM-doped ZnO nanostructures, exhibiting RTF by direct chemical synthesis is scarce. In this work, we report synthesis of Zn1-xCoxO and Li doped Zn1-xCoxO nanorods through a template free soft chemical synthesis. Most significantly, we show the control on the morphology of the ZnO nanoparticles by Co and Li doping. Co doping reduces the high aspect ratio of the ZnO particles, whereas Li codoping retains the nanorod shape. We also show that Li codoping enhances the RTF signature of the C-doped ZnO significantly. 2. Experimental Section In a typical synthesis to prepare ZnO nanowires/rods, zinc acetate dihydrate (10 mmol) (99.99%) was mixed with 15 mL of trioctylamine in a round-bottomed flask. The mixture was rapidly heated to 320 C and maintained at that temperature for 2 h with refluxing and cooled to room temperature. The length of the wire/rod changed depending on the duration of heating. The obtained white precipitate was washed several times with acetone and absolute ethanol and dried by a rotoevaporator. The same procedure has been followed to prepare Co-doped ZnO and Co-doped Li codoped ZnO, by taking Co acetate and Li acetate in the appropriate proportion. The phase purity and crystal structure of the samples were analyzed in a Philips Diffractometer (model PW 1071) using Cu KR radiation fitted with graphite crystal monochromator. High-resolution transmission electron microscopy (HRTEM) imaging and selected area electron diffraction (SAED) studies were carried out with a JEOL JEM 2010 transmission electron microscope. DC magnetization measurements were carried out using an EG&G PAR vibrating sample magnetometer (model 4500). PL studies were carried out using a Hitachi instrument (F-4010) with 150 W Xenon lamp as the excitation source. All the emission spectra were corrected for the detector response. Raman spectra of pristine and doped ZnO have been recorded using 532 nm line from a diode-pumped Nd:YAG laser (power 20 mW) and the scattered light detected using CCD-based (Andor Technology) home-built 0.9 m monochromator coupled with a supernotch filter. Entrance slits were kept at 50 μm which gives a resolution-limited width of 3 cm-1. r 2009 American Chemical Society

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Theoretical Methodology. Total energies are obtained from the first-principles projector augmented wave method28,29 within the local density approximation (LDA) in the density functional theory. The Zn1-x-yCoxLiyO compounds are modeled by a 3  3  2 wurtzite bulk structure consisting of 72 atoms. Calculations of bulk systems are justified by the primarily bulk-like property of the relatively large wires. Experimental lattice constants are used, and ions are fully relaxed by means of both the conjugate-gradient and quasi-Newton algorithms with a 3  3  3 k-mesh. The densityof-states (DOS) are obtained using the LDAþU method with Ud(Co)=4 eV and Ud(Zn)=6 eV, muffin-tin radii of 0.9 A˚, and a 6  6  6 k-mesh. The correction of the Zn d-states within LDAþU have been found30 to significantly improve the Zn d-O p hybridization at ∼7 eV below the valence band maximum.

3. Results and Discussion ZnO, Zn1-xCoxO, and Li-doped Zn1-xCoxO nanorods were synthesized through a soft chemical method. X-ray diffraction (XRD) patterns of undoped ZnO and Co and Li doped samples showed that they are monophasic with wurtzite structure in the concentration range we have studied (Supplementary Figure S1). The lattice parameters of these samples are given alongside of each XRD pattern and it can be seen that the cell parameters of the (Li-Co)-doped ZnO and Li-doped ZnO are slightly increased compared to the Codoped and pristine ZnO nanorod samples. We also carried out microstructural and morphological studies using TEM on these samples. Pure ZnO samples show nanorod morphology having length around 0.5-3 μm with a diameter of around 100-150 nm (Figure 1a). These pure ZnO nanorods exhibit high aspect ratio in the range of 15-20. HRTEM and SAED images shown in the insets of Figure 1a indicate that the particles are highly crystalline and devoid of defects and impurity phases. Figure 1b depicts the TEM and SAED of Zn0.97Co0.03O. It is clear from the image that the aspect ratio of Zn0.97Co0.03O nanorod has reduced significantly, while retaining the crystallinity of the particles. When, the concentration of Co is increased to 5 atom %, hardly any particles with nanorod shape is seen, as majority of them become spherical like irregular particles (Figure 1c). This clearly indicates the influence of Co to destabilize the nanorod shape of the ZnO particles. We also carried out the compositional analyses by energy dispersive spectroscopy (EDS) to ascertain the uniformity of the Co dopant in the nanoparticles. EDS analysis was carried out using a focused electron probe at a number of locations throughout the specimens, which indicated the homogeneity of the samples and found that Co is uniformly distributed in ZnO. A typical EDS spectrum in the inset of Figure.1c for Zn0.95Co0.05O sample showed that the Co concentration is 4.8 at %, which is consistent with the 5 at % nominal concentration. Despite the uniform doping of Co in ZnO, it is noteworthy that complete conversion of nanorods to spherical particles has not taken place in Zn0.95Co0.05O. Unlike cobalt (Co) dopant, adding Li (10 at %) in ZnO did not change the morphology of ZnO particles and in fact retained the nanorod shape with slightly higher aspect ratio of ∼20-25 (Figure 2a). The morphology is also not affected when Li is codoped with 5 at % of Co in ZnO, as revealed by the TEM (Figure 2b). Cobalt doping, which significantly affected the morphology of the ZnO particles, does not affect the shape in presence of Li dopants, which implies that Li stabilizes the shape and morphology of the ZnO particles. The HRTEM and SAED images further confirm that the sample is highly crystalline without any impurity phases, though occasional planar defects are ob-

Figure 1. Transmission electron micrographs of (a) pristine ZnO, (b) Zn0.97Co0.03O, and (c) Zn0.95Co0.05O. The insets in (a) show the SAED pattern (top left) and HRTEM image (bottom right) for one of the crystallites. The inset in (b) shows the SAED of the nanorod assembly. The inset in (c) shows a typical EDS spectrum obtained from Zn0.95Co0.05O nanoparticles.

served in the Zn0.85Co0.05Li0.10O nanoparticles. We believe that in spite of the planar defects that can potentially break the long particles (possibly the case in Co-doped ZnO), Li doping helps to stabilize the nanorod shape in Zn0.85Co0.05Li0.10O. We carried out first-principle theoretical modeling to understand the dopant distribution in ZnO and its influence on the morphology of the particles. Our first-principles theoretical modeling shows that interstitial Li atom (Lii) is relatively mobile in ZnO; the energy barrier is 2.5 eV for diffusion of Lii along the (001) direction between unoccupied hexagonal sites, whereas the corresponding energy barrier for interstitial Zn is

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Figure 2. Transmission electron micrographs of (a) Zn0.90Li0.10O and (b) Zn0.85Co0.05Li0.10O. The inset in (a) shows the HRTEM image for one of the crystallites. The insets in (b) show the SAED (top left) pattern and HRTEM (bottom right) image.

Figure 4. Raman spectrum of (a) pristine ZnO nanorod, (b) Zn0.97Co0.03O, (c) Zn0.95Co0.05O, (d) Zn0.85Co0.05Li0.10O, and (e) Zn0.90Li0.10O. The broadband around 540 cm-1 in the Co-doped sample, marked as L in (b), is understood as being due to the local mode related to Co.

Figure 3. Photoluminescence emission spectra obtained after 280 nm excitation of nanorods of (a) pure ZnO, (b) Zn0.90Li0.10O, (c) Zn0.95Co0.05O, and (d) Zn0.85Co0.05Li0.10O.

higher (4.8 eV). Thus, Li can localize at Zn sites close to the Co dopants during the growth of the nanorods, and thereby stabilize the morphology. The details of these calculations will be discussed subsequently. To investigate the effect of dopants on the optical properties, photoluminescence studies are carried out at room temperature (Figure 3). Pure ZnO nanorods and Zn0.90Li0.10O nanorods are characterized by band edge luminescence around 380 nm and a broad defect emission around 510 nm characteristic of the recombination of conduction electron with the hole trapped at the oxygen vacancies.31 For the Zn0.90Li0.10O sample, the relative intensity of band edge luminescence (380 nm) is less compared to the defect emission (550 nm) because incorporation of Liþ ions creates additional anion vacancies in the lattice. With incorporation of 3 and 5 at % Co2þ in ZnO, the band edge and defect emission got completely quenched and instead new bands around 697 and 747 nm are observed. These bands are attributed to the transitions between localized d-levels, [4T1(P), 2T1(G), 2 E(G) f 4A2(F)] of Co2þ occupying tetrahedral environment.31,32 These results very clearly establish that the Co2þ ions exist in the tetrahedral position of ZnO lattice. However, on codoping the Zn0.95Co0.05O sample with 10 at % Liþ ions, the relative intensity of the 747 nm peak decreases with respect to 697 nm peak, along with a reduction in the signal-to-noise ratio of the whole spectrum. From these results, it has been inferred that codoping Liþ ions in Zn0.95Co0.05O results in the

distortion around Co2þ luminescence center due to the additional anion vacancies created by Liþ ions incorporation in the lattice. The Co 2P3/2 XPS spectrum of Zn0.85Co0.05Li0.10O (see the Supporting Information, Figure S2) shows that Co is in Co2þ oxidation state with binding energy 780.9 eV, comparable to that of the energies of the corresponding photoelectrons of Co2þ in CoO.33-35 Figure 4 shows the Raman spectra of pristine as well as Co and Li-doped ZnO samples. The Raman mode at 438 cm-1 (Figure 4a) is the nonpolar mode of E2 symmetry typical of the wurtzite phase and is reported to have the strongest intensity under nonresonant conditions.36 Raman spectrum of the ZnO nanorod in the present work agree very well with that reported in literature confirming the high quality of the pristine samples. Two more bands at 379 and 327 cm-1 (Figure 4a) are also noted, which are due to A1(TO) and overtone of the acoustic modes, respectively.37 Another broad feature centered around 470 cm-1 is also observed. This broad feature centered around 470 cm-1 in the Raman spectra in Figure 4 has also been found in (Co, Mn):ZnO38 and (Co, N):ZnO films,39 which is assigned as surface or interface phonon mode. There are some reports40 that oxygen vacancy defect complexes in ZnO give rise to a band around 504 cm-1; however it is not very clear if the broadband around 470 cm-1 is associated with oxygen defects or due to overtones. The frequency of the E2 mode is noted to shift to lower frequencies with increasing Co concentration as may be seen from Figure 4c (from a value of 437.4 cm-1 in pure ZnO to a value of 435.84 and 434.1 cm-1 in 3 and 5% Co-doped ZnO samples, respectively). Full width at half-maximum of this band increases from a value of 8.5 cm-1 in the pristine sample (Figure 4a), to about 18 cm-1 in the Codoped samples spectra b and c in Figure 4). More noticeably, two broad overlapping bands centered around 540 and

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Figure 5. Magnetization versus field measurements at room temperature for ZnO, Zn0.90Li0.10O, Zn0.97Co0.03O, Zn0.95Co0.05O, and Zn0.85Co0.05Li0.10O.

565 cm-1 appear in the spectrum, the latter is due to the A1(LO) of ZnO.41 The broad feature around 540 cm-1 has been reported by several other groups also,42,43 which is understood as a local mode related to cobalt. The appearance of a band at 565 cm-1 is sometimes attributed to oxygen vacancies also. This Raman signal is noted to be very weak and broad in samples that have both Li and Co doping (Figure 4d), indicating the possible distortions close to the dopant atoms. Our theoretical calculations points toward a probable scenario where Li is stabilized in the ZnO host lattice close to the Co-O bond because Li energetically prefers LiZn sites near Co. Spectrum of the Zn0.90Li0.10O (Figure 4e) closely resembles to that of the pristine ZnO in the line shape of the E2 mode as well as in the absence of the broad feature around 565 cm-1. This agrees well with the earlier report that Li-doping does not lead to noticeable changes in the Raman spectrum of ZnO.44 Dhanajay et al. have conjectured that the observed ferroelectricity in Li-doped ZnO could be possibly due to lattice distortion consequent to Li-doping.25 However, our present Raman results do not indicate any significant change in the phonon behavior in the Li-doped sample as compared to the pristine ZnO nanorod. In the literature, Fourier Transform Spectroscopic studies and time-resolved photoluminescence studies have been shown to exhibit a definite correlation between the surface and bulk defects to particle size, and consequently to the photoluminescence properties.37 In light of this, the changes in the Raman line shape consequent to Co and Li substitution in the present work appear to correlate well with the quality and size of the nanorods as observed from TEM pictures. DC magnetization loops recorded at room temperature for Zn1-xCoxO (x = 0, 0.03 and 0.05) and Zn0.85Co0.05Li0.10O are depicted in Figure 5. Magnetization loops for pure ZnO and Zn0.90Li0.10O are also included for comparison. Cobalt doped ZnO (x = 0.03, 0.05) and Zn0.85Co0.05Li0.10O showed RTF behavior, whereas pristine ZnO and Li-doped ZnO (Zn0.90Li0.10O) showed diamagnetic nature, as expected. The Zn0.95Co0.05O sample showed marginally increased saturation magnetization value compared to Zn0.97Co0.03O at the applied field, with the coercive field and remanence more or less the

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same. When codoping Zn0.95Co0.05O with Li (10 at %), a significant increase in the saturation magnetization is observed. Recently the enhanced RTF has been reported by the Li codoping in Co doped ZnO nanoparticles27 and the increase in magnetization is explained by the assumption that the injection of additional carriers are required in addition to the randomization of the magnetic ions and defects. This was in agreement with the studies of Sluiter et al.45 and Kittilstved et al.46 on cobalt-doped ZnO. Using first-principles density functional calculations, Lee and Chang47 have shown that substitutional Li and Na are better acceptors in ZnO with shallow acceptor levels. However, local density functional calculations by Wardle et al.48 have shown that interstitial Lii is more stable than substitutional LiZn in ZnO. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) experiments on Li/Na doped ZnO nanoparticles by Orlinskii et al.49 showed the presence of shallow donors that are related to interstitial Li and Na atoms, thus supporting the theoretical predictions of Wardle et al.48 that Li and Na can enter the ZnO lattice interstitially and can act as shallow donors. This shows that codoping with Li generates donors rather than acceptors. However, site preference of dopants depends both on the doping concentration as well as on the chemical potentials and Fermi level.50 Also, one should distinguish between doping in the dilute limit and doping in the percentage regime. We modeled Zn1-x-yCoxLiyO by 72 atoms, 3  3  2 super cells with and without metal dopants (i.e., x = 0 and 0.03 and y = 0, 0.03, and 0.06), which qualitatively mimics the present nanorod compositions. The calculations of Zn0.97Co0.03O reveal that Co energetically prefers the Zn sites forming substitutional CoZn dopants with a bond length of δCo-O = 1.91 A˚ to the four neighboring O atoms in the tetrahedral coordination. This bond length is smaller than that of host ZnO: δZn-O = 1.98 A˚, which is a consequence of the even much shorter bond length δCo-O = 1.86 A˚ in a four-coordinated tetrahedral CoO crystal. Thus, although Co2þ and Zn2þ are 4s2 isovalent, there is a local strain around the Co dopant in Zn0.97Co0.03O. We find that additional Li doping compensates this by forming LiZn close to the CoZn dopant, partly because of δLi-O = 2.01 A˚ in Zn0.97Li0.03O, which is larger than the Zn-O bond length. Moreover, in Zn0.94Co0.03Li0.03O, the codopant Li is energetically favored to form LiZn close to the CoZn dopant, rather forming Lii plus a well-separated VZn that has ∼3.3 eV higher total energy. Also in Zn0.91Co0.03Li0.06O with high Li concentrations, LiZn close to CoZn is favored, although a LiZn dopant easily can be located away from CoZn (with only ∼0.3 eV higher total energy). The bond lengths of δCo-O in Zn0.91Co0.03Li0.06O are 1.83-1.87 A˚, which agree better with tetrahedral CoO. Figure 6 shows that the relaxation of the neighboring O atoms of Co is much stronger in Zn0.91Co0.03Li0.06O compared to Zn0.97Co0.03O, demonstrating that LiZn stabilizes the bond lengths of Co-O in the Co- and Li-doped compounds. This explains the improved aspect ratio and morphology in the Li-rich nanorods, and thereby partly explains the enhanced ferromagnetic characteristics. The calculated spin dependent density-of-states (DOS) of ferromagnetic Zn0.97Co0.03O reveals that (see Figure 7) Co forms the t2 and e states above the host ZnO Fermi energy, and that the e band is partially filled. The magnetic moment is 3.0 μB per Co atom as expected. Additional Li doping has a strong effect on the DOS. The t2 and e states in Zn0.97Co0.03O are split by the Li dopants into five one-dimensional representations (at the Γ-point) and only the

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Figure 6. Crystal structure of (a) Zn0.97Co0.03O and (b) Zn0.91Co0.03Li0.06O demonstrating the strong relaxation effects due to the presence of Li substitutional dopants.

lowest band is partially filled. The origin of this is 2-fold: first, the relaxation effects enhance a split of the bands. Second, and more important, both Co 3d and the surrounding O 2p compensate the missing LiZn s contribution in the cation-anion bond. Thereby, Co induces a magnetic moment of the O p-like states of the four neighboring anions. This charge-transfer induced magnetic moment is ∼0.3 μB per O atom, whereas the magnetic moment of Co decreases slightly to 2.9 μB. Thus, the enhanced ferromagnetism in the Co and Li codoped ZnO is an effect of the magnetization of surrounding O atoms. Several theoretical models have been proposed to explain the development of room temperature ferromagnetism in transition-metal-doped semiconducting oxides51-56 One of the models is the carrier-induced Ruderman- KittelKasuya-Yoshida (RKKY) interaction between local magnetic moments and the conduction band electrons.52 Carriers present in the host lattice interact with the local magnetic moment of the dopant ions resulting in long-range ferromagnetic order. However, in these carrier-mediated models, the ferromagnetic transition temperature rarely exceeds 300 K, and the materials under investigation typically have insufficient carrier densities for this mechanism. The other models51,53 postulate the formation of bound magnetic polarons associated with oxygen vacancies in a dielectric matrix with magnetic impurities. Recent reports of ferromagnetism in undoped HfO2 films have led to the development of models for dilute magnetic semiconductor oxides (DMSO) materials.54-56 The intrinsic ferromagnetism in HfO2 is thought to arise from lattice defects located near the film/ substrate interface. Lin et. al26 showed that the ferromagnetic properties of Co-doped ZnO films can be enhanced by the Li codoping and they ascribed the enhancement to the indirect exchange via Li related defects. Because ZnO is in general highly insulating, the percolative transition proposed in the bound magnetic polaron model is more relevant for our results. The magnetization of 3% Co-doped ZnO is ∼0.48 μB/Co, whereas the magnetization significantly increases to 2.7 μB/Co for Li codoped sample. For 3% of Co in ZnO, the average distance between the Co atoms is expected to be around 9 A˚ assuming homogeneous isotropic distribution of Co in ZnO. The predicted polaron size in ZnO51 is 7.6 A˚. Therefore, in the absence of defects (for example, because of Liþ substitution) the percolation of the polaron is limited. However, with Li substitution, the defects can develop the percolative network for the polarons. In addition,

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Figure 7. Spin-dependent valence DOS of ferromagnetic Zn0.97Co0.03O and Zn0.91Co0.03Li0.06O, where the energy is referred to the valence-band maximum of ZnO. The dotted lines indicate the Fermi energy of the two doped systems. The calculations are based on LDA, with on-site Coulomb correction of the d-like states. The exact energy position of the defect states may be slightly shifted by additional correction of the LSDAþU band gap; however, the states will retain in the gap region.

the structural defects can play significant role in controlling the ferromagnetic properties. Pan’s group has systematically demonstrated the important role of structural defects in room temperature ferromagnetism in Co:ZnO films without and with additional doping.57-59 They also observed that, ZnObased films exhibit ferroelectric behavior, and ferroelectric properties can be tuned by the dopants. Although the origin of ferromagnetism due to secondary phase nanoclusters are a big concern, our present results on the enhancement of the Ms value on codoping with Li indicates that the observed ferromagnetism cannot be attributed to any impurity phases involving Co oxides or Co metal clusters. From the magnetization versus temperature curve (Figure 5 inset), measured at an applied field of 200 Oe, it is clear that the Curie temperature Tc is well above room temperature, because the field cooled (FC) and zero field cooled (ZFC) did not converge at room temperature. 4. Conclusions In conclusion, 1D nanostructures of Zn1-xCoxO (x = 0, 0.03, and 0.05) and Co and Li codoped ZnO (Zn0.85Li0.10Co0.05O) were prepared by a soft chemical method and characterized using, X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL), X-ray photo electron spectroscopy (XPS), Raman spectroscopy, and DC magnetization studies. The morphological studies strongly indicate that cobalt doping in ZnO destabilizes the nanorod microstructure, whereas codoping Li in Co-ZnO favors the stabilization of the nanorod shape. Theoretical calculations shows that substitutional Li stabilizes the strained Co-O bond in Zn1-x-yCoxLiyO, thereby stabilizing the nanorod morphology. Room-temperature ferromagnetism is observed in the Co doped ZnO samples, whereas pure ZnO and Li-doped ZnO are diamagnetic. The magnetization of Co-doped ZnO is increased by an order of magnitude by codoping with Li because of a charge-induced magnetic moment of neighboring O atoms. Our microstructural and magnetic study point toward the important role played by the Li ion doping in stabilizing both the nanorod

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shape of ZnO particle as well as significantly enhancing the magnetization of Co doped ZnO. Supporting Information Available: XRD patterns and XPS spectra of Co in Zn0.85Co0.05Li0.10O nanorod. This material is available free of charge via the Internet at http://pubs.acs.org.

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