Origin of FM Ordering in Pristine Micro- andNanostructured ZnOR. Podila,† W. Queen,‡ A. Nath,§ Jeverson T. Arantes,| Aline L. Schoenhalz,| A. Fazzio,|Gustavo M. Dalpian,| J. He,† Shiou J. Hwu,‡ Malcolm J. Skove,† and Apparao M. Rao*,†

†Department of Physics and Astronomy, ‡Department of Chemistry, Clemson University,Clemson, South Carolina 29634, §Department of Chemistry, University of North Carolina,Asheville, North Carolina 28804, and |Centro de Ciencias Naturais e Humanas, Universidade Federal do ABC,Santo Andre, SP 09210-200, Brazil

ABSTRACT An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reportedpreviously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpointits origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisinglywe find that annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes)the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetizationin micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initiocalculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions.For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealingconditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations.

KEYWORDS Ferromagnetism, para-magnetism, diamagnetism, vacancies and ab initio calculations.

A debate regarding observation of high-temperatureFM in nanophase oxides started with reports ofHfO2 and CuO exhibiting FM.1 Then followed a

cascade of communications reporting high temperatureFM in other oxides, such as CaO, MgO, ZnO, CeO2, Al2O3,In2O3 and SnO2.2-4 Interestingly, it was reported that asthe thickness of ZnO films increases, the FM per unitvolume decreases, implying that the FM is due to surfacedefects.5,6 FM observed in nanophase TiO2 was alsoattributed to surface defects.7 Furthermore, it is widelybelieved that the FM in many metal oxides originates fromoxygen vacancies.2-11 The nature and concentration ofdefects vary with conditions of preparation andtherefore controversies regarding reproducibility arecommonplace.12-15 Importantly, systematic studies aimedat unraveling the origin of FM in these oxides are lacking.ZnO, which exhibits unique fundamental properties thatare best suited for applications,16 is an ideal system forgaining insights into the origin of FM since it can bereadily synthesized in various forms at different lengthscales such as nanocombs, nanorings, nanosprings, nano-belts, nanowires, nanospheres, and micrometer-sizedcubes. In this study, we correlate the changes in themagnetic behavior of pristine ZnO prepared using chemi-cal vapor deposition (CVD) and pusled laser vaporization(PLV) techniques as a function of thermal treatments in

Ar, O2, and/or Zn vapors. Finally, we present ab initiocalculations which suggest that extended defects areresponsible for the observed FM.

Experimental Section. Figure 1 shows electron micro-scope images of ZnO prepared by the CVD (Figures 1a-c) and

* To whom correspondence should be addressed. E-mail: arao@clemson.edu.Received for review: 01/15/2010Published on Web: 03/02/2010

FIGURE 1. (a) SEM images of as prepared CVD microstructures onpyrex substrates. A combination of micrometer-scale and nanoscaleZnO structures is observed. (b) SEM images of as prepared CVD ZnOnanostructures on Si (100) substrates. Inset figures depict theelemental analysis that shows no impurity phases in the samples.(c) Typical electron diffraction pattern for samples depicted in panelsa and b. (d) (i) HRTEM image of an individual PLV grown ZnOnanowire, (ii) a magnified view of the boxed area in (i), and (iii)electron diffraction pattern obtained from a ZnO nanowire shownin (i).

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© 2010 American Chemical Society 1383 DOI: 10.1021/nl1001444 | Nano Lett. 2010, 10, 1383–1386

PLV (Figure 1d) methods. Sample synthesis details areprovided in the Supporting Information. Predominantly,highly crystalline ZnO nanowires are obtained in the PLVprocess (Figure 1d) whereas both micrometer and nanoscaleZnO structures are obtained by the thermal CVD method(Figure 1a-c). The as-prepared conical nanostructures varyin diameter from 50 to 100 nm along the length and aretypically ∼1-2 µm long (Figure 1a). Detailed characteriza-tion using electron microscopy, X-ray diffraction, and Ra-man spectroscopy is provided in the Supporting Information.The CVD grown samples are relatively more defective thanthe PLV grown nanowires.

Magnetization studies were performed using a QuantumDesign SQUID MPMS (Model 5S) and the following sampleswere measured: (i) as prepared (AP-CVD, AP-PLV); CVDgrown samples annealed in (ii) an O2 atmosphere at 100 °Cfor 5.5 h (O2-100-5.5 h); (iii) an O2 atmosphere at 500 °Cfor one hour (O2-500-1 h) and subsequently exposed to Zn

vapors at 500 °C for two hrs (O2-Zn-500-2 h); (iv) an Ar flowat 400 °C for 5 min (Ar-400-5 m) and subsequently exposedto Zn vapors for 2 h at 500 °C (Ar-Zn-500-2 h); and (v) Znvapors at 500 °C for two hours (Zn-500-2 h). As discussedlater in Figure 3, since room temperature FM was notobserved in the PLV prepared samples, these samples werenot subjected to the annealing process.

Results and Discussion. As shown in Figure 2, a clearevidence for FM is observed in the AP-CVD samples. ThisFM saturates at low magnetic fields (∼1 T) and is embeddedin a diamagnetic background response. Interestingly, uponannealing in O2 at 100 °C (O2-100-5.5 h), the FM signatureis greatly enhanced as seen in Figure 2a. It is noteworthythat the sharp transition from FM to diamagnetic responsein the insets in Figure 2a implies that the measured magne-tization is not due to a simple superposition of a ferromag-netic ordering and a diamagnetic response. To the best ofour knowledge, such a sharp transition in the low field

FIGURE 2. (a) M-H curves of CVD prepared ZnO nanostructures. Asgrown nanostructures show a weak but clear (top inset), FM embed-ded in a dominant diamagnetic response. The bottom inset showsenhancement in FM ordering by at least an order of magnitude whenannealed in O2 at 100 °C. (b) O2 chemisorbs near a cluster of Znvacancies and a dynamic exchange of electrons between the oxideions and O2 could lead to FM. (c) HRTEM image showing thepolycrystalline nature of CVD grown ZnO nanostructures. The insetshows schematically the effect of how thermal treatment might alterthe grain size in these nanostructures.

FIGURE 3. (a) CVD prepared ZnO nanostructures annealed in oxygenat 500 °C for 1 h and reannealed in Zn vapors at 500 °C for 2 hexhibit a diamagnetic response. Further, a much suppressed FM isseen in as prepared ZnO nanostructures when annealed in Zn vaporsat 500 °C for 2 h. (b) Magnetization data for AP-PLV ZnO nanowiresdo not exhibit FM ordering at 300 K and show negligible FM at 5 K.In both panels, the inset figures show an expanded view in the low-field regime.

© 2010 American Chemical Society 1384 DOI: 10.1021/nl1001444 | Nano Lett. 2010, 10, 1383-–1386

regime has not been reported previously. The signs ofsaturation at relatively low magnetic fields (∼1 T) indicatethe presence of reasonably large spin clusters. In addition,FC and ZFC magnetization measurements (data not shown)showed the absence of superparamagnetism in thesesamples. This ferromagnetic ordering may be understoodby chemisorption of O2 to form O2

-. Since the growth occursin an O-rich environment, it is probable that O2 is chemi-sorbed preferentially near a cluster of Zn vacancies. Givensuch a possibility one may visualize a dynamic exchange ofelectrons, as seen in Figure 2b, between O2 and O- leadingto FM ordering.17-19 Furthermore, this FM ordering andhence the sharp transition is absent in AP-CVD sampleswhen annealed in Ar at 400 °C for a short period of time(Ar-400-5 m, Figure 2a). The HRTEM image (Figure 2c)shows the polycrystalline nature of the AP-CVD samples.Annealing in Ar presumably leads to (i) the merging of grainsthat consequently reduces the net surface/interfacial area,20

and (ii) reduced amount of chemisorbed O2 in the samplethat results in a diamagnetic response.

Independent experiments to further confirm the loss inthe FM ordering as a result of high temperature-inducedmerging of grains were performed. An AP-CVD sampleannealed in O2 at 500 °C for 1 h (O2-500-1 h) showed anabsence of ferromagnetic ordering (Figure 3a). Similarly, thePLD grown ZnO samples did not show ferromagnetic order-ing (Figure 3b), and this observation is consistent with theab initio calculations discussed later in Figure 4d. Alterna-

tively, since ZnO may have Zn defects, the AP-CVD sampleswere annealed in Zn vapor at 500 °C. The magnetizationdata for this sample (Zn-500-2 h) shows a much suppressedresponse as compared to the AP-CVD sample (Figure 3a).

Alternatively, our ab initio calculations show that the netmagnetization could arise from two possible contributions:one due to localized O p-orbitals and the other due todelocalized O p-orbitals. In our model, the localized orbitals(px) are preferentially aligned perpendicular to the wiredirection, whereas the delocalized orbitals (pz) point in adirection parallel to the wire direction. This suggests that thelocalized orbitals contribute a local moment, forming aparamagnetic center that can interact with other suchcenters through the delocalized orbitals and thus result in anet magnetization. In Figure 4a, we show the spin chargedensity, that is, spin up charge density minus the spin downcharge density, for a ZnO nanowire along the [1-210]direction. It should be mentioned that although we pick awire direction in our model calculations, unlike the PLVnanowires, the AP-CVD samples do not have a uniquegrowth direction due to their polycrystalline nature. There-fore, we have computed net magnetization for several wiredirections (Figure 4d). A strongly localized moment is ob-served near the surface of the nanowires (ferromagneticordering), while the interior of the wire does not show asimilar strong contribution (diamagnetic response).

The origin of such magnetization in O p-orbitals is furthersupported by the presence of unpaired oxygen p levels at

FIGURE 4. (a) Cross sectional view of a (1-210) nanowire, as obtained after relaxing all forces in our calculations. The figure in the right showsthe spin charge density for the (1-210) nanowire. (b) Calculated projected density of states for ZnO nanowires. The upper panel is for spin-upand the bottom for spin down. The dotted line represents the Fermi energy. (c) A schematical view of the denstity of states. The bands in lightand dark green are respectively the valence and conduction bands. The red bands represent delocalized surface states and the dotted/dashedlines represent the Fermi energy. (d) The table shows magnetization obtained per atom in our calculations for different wire directions andstructures.

© 2010 American Chemical Society 1385 DOI: 10.1021/nl1001444 | Nano Lett. 2010, 10, 1383-–1386

the Fermi energy in the projected density of states (PDOS)(Figure 4b). The existence of spin polarization for a certainkind of defects is not sufficient to warrant a macroscopicmagnetization. In other words, intrinsic point defects mayalso lead to spin-polarization, but not necessarily to macro-scopic magnetization. For instance, Zn vacancies in bulkZnO are spin polarized. However, their population will neverbe large enough to allow a magnetic interaction between twoimpurity sites. In AP-CVD samples, we observe the forma-tion of extended defects, for example, the surface (cf.HRTEM image in Figure 2c). Such extended defects lead tothe formation of surface states in the band structure that isshown schematically in Figure 4c.

Conclusions. To summarize, unlike AP-PLV nanowiresthe AP-CVD samples showed a weak but clear FM signature,thus confirming the role of surface defects in FM ordering.Low-temperature (∼100 °C) annealing of AP-CVD samplein flowing oxygen enhances the ferromagnetic ordering; thisenhancement is attributed to an increase in the amount ofchemisorbed oxygen. However, annealing at high temper-atures (∼500 °C), irrespective of the environment, leads toa diamagnetic response due to an increase in the crystalgrain size. A model based on the dynamic exchange ofelectrons between chemisorbed O2 and O- near a cluster ofdefects was proposed. Possible explanations based on,macroscopic magnetization observed in simulations of ZnOnanowires, using ab initio calculations were also discussed.

Acknowledgment. We thank Pooja Puneet (Dept. ofPhysics, Clemson University), Haijun Qian (Microscopy facil-ity, Clemson University) and Jennings W. Palmer (Dept. ofChemistry, Clemson University) for their help with TEM andSQUID. The work in Brazil was supported by FAPESP, CNPqand CAPES.

Supporting Information Available. Supporting Informa-tion contains the following details: (1) synthesis, (2) charac-terization, (3) magnetic measurements, and (4) ab initiocalculations. This material is available free of charge via theInternet at http://pubs.acs.org.

REFERENCES AND NOTES(1) Mishra, S. R; Losby, J.; Dubenko, I.; Roy, S.; Ali, N.; Marasinghe,

K. Magnetic properties of mechanically milled nanosized cupricoxide. J. Magn. Magn. Mater. 2004, 279, 111.

(2) Osorio-Guillen, J.; Lany, S.; Barabash, S. V.; Zunger, A. Magnetismwithout Magnetic Ions: Percolation, Exchange, and FormationEnergies of Magnetism-Promoting Intrinsic Defects in CaO. Phys.Rev. Lett. 2006, 96, 107203.

(3) Sundaresan, A; Bhargavi, R; Rangarajan, N.; Siddesh, U.; Rao,C. N. R. Ferromagnetism as a universal feature of nanoparticlesof the otherwise nonmagnetic oxides. Phys. Rev. B 2006, 74,161306.

(4) Belghazi, Y.; Schmerber, G; Colis, S.; Rehspringer, J. L; Dinia, A.;Berrada, A. Extrinsic origin of ferromagnetism in ZnO andZn0.9Co0.1O magnetic semiconductor films prepared by sol-geltechnique. Appl. Phys. Lett. 2006, 89, 122504.

(5) Hong, N. H; Sakai, J.; Brize, V. Observation of ferromagnetism atroom temperature in ZnO thin films. J. Phys.: Condens. Matter2007, 19, 036219.

(6) Gacie, M.; Jakob, G.; Herbort, C.; Adrian, H. Magnetism of Co-doped ZnO thin films. Phys. Rev. B 2007, 75, 205206.

(7) Wei, X; Skomski, R; Balamurugan, B; Sun, Z. G; Ducharme, S.;Sellmyer, D. J. Magnetism of TiO and TiO2 nanoclusters. J. Appl.Phys. 2009, 105, 07C517.

(8) Straumal, B. B.; Mazilkin, A. A.; Protasova, S. G.; Myatiev, A. A.;Straumal, P. B.; Schutz, G.; van Aken, P. A.; Goering, E.; Baretzky,B. Magnetization study of nanograined pure and Mn-doped ZnOfilms: Formation of a ferromagnetic grain-boundary foam. Phys.Rev. B 2009, 79, 205206.

(9) Sundaresan, A.; Rao, C. N. R. Implications and consequences offerromagnetism universally exhibited by inorganic nanoparticles.Solid State Comm. 2009, 149, 1197.

(10) Venkatesan, M.; Fitzgerald, C. B.; Coey, J. M. D. Thin films:Unexpected magnetism in a dielectric oxide. Nature 2004, 430,630.

(11) Kittelstved, K. R.; Liu, W. K.; Gamelin, D. R. Electronic structureorigins of polarity-dependent high-TC ferromagnetism in oxide-diluted magnetic semiconductors. Nat. Mater. 2006, 5, 291.

(12) Diebold, U. Structure and Properties of TiO2 surfaces: a briefreview. Appl. Phys. A 2003, 76, 681.

(13) Ozgur, U.; Alivov, I. Y; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan,S.; Avrutin, V; Cho, S. J.; Morkoc, A. Comprehensive review ofZnO materials and devices. J. Appl. Phys. 2003, 98, 041301.

(14) McCluskey, M. D.; Jokela, S. J. Defects in ZnO: Focused Review.J. Appl. Phys. 2009, 106, 071101.

(15) Schmidt-Mende, L.; MacManus-Driscoll, J. L. ZnO nanostructures,defects, and devices. Mater. Today 2007, 10, 40.

(16) Wang, Z. L. Zinc oxide nanostructures: growth, properties andapplications. J. Phys.: Condens. Matter 2004, 16, R829.

(17) Tam, K. H.; Cheung, C. K.; Leung, Y. H.; Djurisic, A. B.; Ling, C. C.;Beling, C. D; Fung, S.; Kwok, W. M.; Chan, W. K.; Phillips, D. L.;Ding, L.; Ge, W. K. Defects in ZnO nanorods prepared by ahudrothermal method. J. Phys. Chem. B 2006, 110, 20865.

(18) Fan, Z; Chang, P. C; Lu, J. G; Walter, E. C; Penner, R. M; Lin, C.;Lee, H. P. Photoluminescence and polarized photodetection ofsingle ZnO nanowires. Appl. Phys. Lett. 2004, 85, 6128.

(19) Lee, H. J.; Helgren, E.; Hellman, F. Gate-controlled magneticproperties of the magnetic semiconductor (Zn, Co)O. Appl. Phys.Lett. 2009, 94, 212106.

(20) Harper, J. M. E.; Rodbell, K. P. Microstructure control in semi-conductor metallization. J. Vac. Sci. Technol., B 1997, 15, 763.

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