Magnetic Properties of Fe/Cu Codoped ZnO NanocrystalsRanjani Viswanatha,†,# Doron Naveh,‡,∥ James R. Chelikowsky,§ Leeor Kronik,‡ and D. D. Sarma*,†,⊥,∇

†Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India‡Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel§Departments of Physics and Chemical Engineering, Center for Computational Materials, Institute for Computational Engineeringand Sciences, The University of Texas at Austin, Austin, Texas 78712, United States⊥Council of Scientific and Industrial Research − Network of Institutes for Solar Energy (CSIR-NISE), New Delhi, India

*S Supporting Information

ABSTRACT: Free-standing ZnO nanocrystals simultaneouslydoped with Fe and Cu with varying Fe/Cu compositions havebeen synthesized using colloidal methods with a mean size of∼7.7 nm. Interestingly, while the Cu-doped ZnO nanocrystalremains diamagnetic and Fe-doped samples show antiferro-magnetic interactions between Fe sites without any magneticordering down to the lowest temperature investigated, samplesdoped simultaneously with Fe and Cu show a qualitativedeparture in exhibiting ferromagnetic interactions, withsuggestions of ferromagnetic order at low temperature. XASmeasurements establish the presence of Fe2+ and Fe3+ ions,with the concentration of the trivalent species increasing in the presence of Cu doping, providing direct evidence of the Fe2+ +Cu2+ ⇌ Fe3+ + Cu+ redox couple being correlated with the ferromagnetic property. Using DFT, the unexpected ferromagneticnature of these systems is explained in terms of a double exchange between Fe atoms, mediated by the Cu atom, in agreementwith experimental observations.

SECTION: Physical Processes in Nanomaterials and Nanostructures

Electrical conductivity and magnetic properties in bulksemiconductor lattices are primarily controlled by the

incorporation of impurities. Transition-metal impurities havebeen found to alter optical, magnetic, and other physicalproperties of the host semiconductor significantly, leading tointense interest in dilute magnetic semiconductors (DMS).1−7

In particular, interest in FM semiconductors has spiked due totheir potential as spin-polarized carrier sources and the relativeease of their integration into semiconductor devices.8 Addi-tionally, the idea of magnetically doping wide-band-gapsemiconductors holds out the tantalizing possibility of formingtransparent magnets with interesting magneto-optical applica-tions. Elucidation of the origin of ferromagnetism in suchmaterials has turned out to be among the most importantproblems in magnetism to have emerged in several years,2,7 andidentification of the critical parameters governing DMSferromagnetism has been challenging.2,9

It is well-known that the physical properties of semi-conductor materials may also be tuned, in the absence of anydoping, by changing the grain size in the nanometer regime.Such semiconducting nanocrystals show a nearly continuoustuning of the band gap with decreasing size. This offers theexciting possibility of obtaining magnetic semiconductornanocrystals with any desired band gap by controlling thesize, if we are able to successfully dope semiconductornanocrystals with transition metal-ions and obtain magnetic

states. While it has been shown that even undoped semi-conductor nanocrystals may show magnetism, most probablydue to unpaired electrons in surface-related states,10 themagnetic moments are typically too small (∼6 × 10−4 emu/gfor 30 nm ZnO nanoparticles) in such cases to be of muchpractical use, particularly when compared with the largeunpaired spins associated with open 3d shell transition-metalions. Besides the possibility of engineering the band gap to anyvalue, including a gap beyond the visible range allowing fortransparent matter, there is another important consequence ofobtaining a magnetic nanocrystal of the dimension less than 10nm. Because the magnetic moment of each such nanocrystalmay be manipulated by the application of a magnetic field,these may constitute extremely small-sized magnetic storagebits in a future technology. Despite such exciting possibilities,we, however, note that both extensive doping of transition-metal ions and obtaining a magnetic state11−13 in suchsemiconductor nanocrystals have proven to be an even greaterchallenge than that faced in the case of bulk semiconductors.In order to realize this important, but difficult, goal of

attaining magnetic semiconductor nanocrystals, some of ushave investigated both experimentally and theoretically a large

Received: June 7, 2012Accepted: July 12, 2012Published: July 12, 2012

Letter

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number of doped systems.12−21 It turns out that all of ourcarefully doped and thoroughly characterized semiconductornanocrystal systems exhibited purely paramagnetic behaviordown to the lowest temperature with indications of smallantiferromagnetic interactions,14 belying our expectations.Prompted by a report of a magnetic state of the ZnO bulksemiconductor upon doping with Fe and Cu simultaneously,22

we wished to explore whether this route might be adopted toachieve the even more elusive magnetic, semiconductornanocrystal. In this Letter, we show that it is indeed possibleto dope Fe and Cu individually as well as together in smallquantities in free-standing ZnO nanocrystals. Interestingly,individually doped Fe−ZnO and Cu−ZnO systems show noevidence of a ferromagnetic order. In fact, there is clearevidence of antiferromagnetic interactions in such dopedsystems. Surprisingly, the simultaneously Fe- and Cu-dopedsystems show a clear signature of ferromagnetism. In order toobtain a clue to this unexpected behavior in terms ofmicroscopic interactions at the atomic level, we have carriedout X-ray absorption spectroscopy (XAS) measurements. XASresults clearly show the presence of both Fe2+ and Fe3+ species,the relative concentration being dependent on the presence ofCu dopant, thus providing a direct correlation betweenelectronic structure changes and ferromagnetic coupling usingXAS. In order to understand this phenomenon, we employcalculations based on density functional theory (DFT). Thesecalculations explain this phenomenon in terms of Cu-relatedelectronic structure changes that promote a ferromagneticconfiguration via a double-exchange mechanism, which isdistinct from the earlier suggestion of a double-exchangemechanism based solely on Fe d states.22

Zn1−x−yFexCuyO nanocrystals were synthesized by hydro-lyzing stoichiometric amounts of Zn, Cu, and Fe precursorswith NaOH. Further details are given in the Methods Section.Typical X-ray diffraction (XRD) patterns for differentconcentrations of Fe and Cu-doped ZnO nanocrystals areshown in Figure 1a. Comparing the XRD patterns with that ofthe bulk ZnO crystallizing in the wurtzite phase, we observethat both doped and undoped nanocrystals crystallize in thesame phase with similar lattice parameters. Similar to earlierliterature reports,12,23 the pattern was simulated (also shown inFigure 1a) by broadening the bulk XRD pattern using theScherrer formula, and quantitative information on the size ofthe nanocrystals was obtained. The size of the nanocrystalsobtained by this procedure for both doped and undopedsamples was found to be 7.7 nm. Nanocrystal size was alsoconfirmed using transmission electron microscopy (TEM),which showed an abundance of nearly spherical particles withan average size of 7.5 nm, in agreement with the valuesobtained from XRD measurements. A typical high-resolutionimage of the particle is shown in the inset to Figure 1b. The sizedistribution of these particles (Figure 1b) is quite broad due tothe absence of capping ligands during the growth of thenanocrystals. Selected area electron diffraction (SAED) of theparticles confirms a high degree of crystallinity. The variousrings that could be indexed to the diffraction planes of ZnOnanocrystals are shown in Figure 1c.Magnetic susceptibility measurements (Figure 2) show the

M−H curves at 10 K for various percentages of Fe/Cu-dopedZnO nanocrystals. Shown in inset I of Figure 2 is the M(H)curve of Cu-doped ZnO, exhibiting diamagnetic behavior asexpected. The inverse magnetic susceptibility plots as a functionof temperature (data not shown) of Fe-doped ZnO nano-

particles showed a negative intercept, suggesting the presenceof antiferromagnetic (AFM) interactions. Interestingly, themagnetic properties change qualitatively upon codoping withCu, as shown in the main panel of Figure 2. Attempts at fittingthe M(H) curve using the Brillouin function resulted in a poorfit in all regions, suggesting a deviation from the paramagneticcase for all concentrations of Fe and Cu. This suggests that theFe/Cu-doped ZnO behaves in a qualitatively different way thanboth individually doped Fe or doped Cu systems and couldshow signs of FM interactions in the nanocrystal. Importantly,the 5% Fe- and 1% Cu-doped ZnO nanocrystal showed anopening in the hysteresis loop when the M(H) was measured at2 K, as shown in inset II of Figure 2, whereas all curvesmeasured at higher temperature for the same sample werefound to scale with temperature. This provides direct evidencefor low-temperature FM coupling in ZnO free-standingnanoparticles when simultaneously doped with Fe and Cu.To test whether the enhancement of FM interaction is

directly related to the Cu codoping, we calculated from firstprinciples the total energy of doped Zn38O38 nanocrystals,constructed in a manner described in the Methods Section. Forthe Cu-free case, we found that the AFM alignment of the Fespin moments was lower in energy (by ∼275 mRy) than theFM alignment, in agreement with the experimental observation

Figure 1. (A) XRD patterns of free-standing ZnO (i) and Fe/Cu-doped ZnO nanocrystals (ii−iv), along with the simulated XRDpattern obtained for 7.7 nm (v) particles. The XRD pattern of bulkZnO (vi) is also shown for comparison. (B) Histogram of the sizedistribution of these nanocrystals obtained from the TEM images. Theinset shows a high-resolution image of the doped nanocrystal. (C)SAED pattern of Fe/Cu-doped ZnO nanocrystals.

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of antiferromagnetic interactions in the Fe-only-doped ZnOnanoparticles.In the presence of Fe and Cu codoping, we envisage redox-

like pairs, Fe2+ + Cu2+ ⇌ Fe3+ + Cu1+, via hopping. Thisanticipation is fully justified by our experimental results, asshown by probing the valence state of the Fe ions in the ZnOnanocrystals, using XAS measured for several typical Fe/Cucompositions at the Fe L3 edge corresponding to Fe 2p → 3dexcitations. The L3 edge of transition-metal ions is known to bedominated by the local electronic structure and, therefore, canprovide important information about the transition-metaloxidation state and point-group symmetry.24−27 The obtainedspectra are shown in Figure 3 as filled circles of different colorsfor different compositions. The signals show the L3 edge of Feto be centered around 712.5 eV, with a prominent shoulder atthe 710.9 eV photon energy, the relative intensity of thisshoulder depending on the dopant concentrations. Thesimultaneous presence of these two prominent features at712.5 and 710.9 eV cannot be explained by the presence of asingle oxidation state of Fe. It can only be explained by thepresence of both Fe3+, d5 and Fe2+, d6 oxidation states. Thevariation in the relative intensity of the shoulder at ∼710.9 eVindicates the presence of different percentages of two differentcomponents with changing compositions. In order todetermine the exact valence, symmetry, and crystal fieldstrengths, we carried out theoretical calculations (see theMethods Section for details) for various crystal field strengths.The calculated spectra for Fe2+ (d6) and Fe3+ (d5) are shown

at the bottom of Figure 3 by a thin black line (d6) and a thinred line (d5), respectively. The total spectrum for any givencomposition was calculated by adding different weights of thecomponent spectra to obtain the best fit with thecorresponding experimental spectrum. The good agreementbetween the experimental spectrum and the calculated result in

each case suggests that the doped Fe is present in the latticewith a tetrahedral FeO4 geometry, as both divalent and trivalentspecies. It is observed that the trivalent Fe component ishighest for the case of the 5% Fe- and 1% Cu-doped system,exactly where the saturation magnetization at 10 K is thehighest among all compositions investigated by us. Thiscomposition also exhibits a ferromagnetic order. In fact, wefind a monotonic relationship between the saturation magnet-ization at 10 K (Figure 2) and the relative contribution of theFe3+ signal in XAS (Figure 3) for all compositions, as observedin the inset to Figure 3, firmly establishing a correlationbetween the magnetic properties and the presence of the Fe3+

species. However, it is interesting to note that the averageconcentration of the dopant does not reflect the actualconcentration in each nanocrystal, including cases wherethere is no Cu or Fe in a particular nanocrystal. Moreover,even in the presence of simultaneously doped Fe and Cu inZnO nanocrystals, it is quite possible that the Cu dopant is notsufficiently close to a pair of Fe dopants to induce theferromagnetic interactions. Thus, it is clear that the magneticproperties that we have discussed are indeed an average overmany realizations of the dopant concentrations and positions.Interestingly, our work shows that it is still possible to induceferromagnetism in the average sense in such samples despitestatistical variations from one nanocrystal to another.While ref 22 suggested the absence of Cu2+ based on their

(unreported) XAS data in the context of the bulk sample, wenote that a very low concentration of overall Cu doping alongwith a lower intrinsic intensity of Cu XAS compared totransition-metal ions, such as Fe, makes it difficult to probeCu2+ species, if present, as a fraction of the total Cu doping. It isobvious that electron paramagnetic resonance (EPR) will be amore sensitive probe in such cases as the Cu2+ species will bethe only one that is EPR-active. Thus, we investigated the EPRof these compounds and obtained a clear Cu2+ signal, as shown

Figure 2. M−H curves of different percentages of Fe/Cu-doped ZnOnanocrystals, measured at 10 K. Inset I shows the M−H curve of Cu-doped ZnO, showing diamagnetism. Inset II shows M as a function ofH/T for the Zn0.94Fe0.05Cu0.01O sample at different temperatures,showing that the hysteresis loop at 2 K does not scale withtemperature.

Figure 3. XAS of different percentages of Fe/Cu-doped ZnOnanocrystals, where x and y correspond to Fe and Cu composition,respectively. The various scatter plots show the experimental spectra.The solid line shows a typical theoretically calculated spectrum, forboth d5 and d6 oxidation states, added in different ratios. Thecomponent spectra are shown at the bottom with a thin black line (d6)and a thin red line (d5). The inset shows the variation of d5

concentration as a function of saturation magnetization obtainedfrom Figure 2.

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in Figure S1 of the Supporting Information for the optimallydoped Fe0.05Cu0.1ZnO sample.In order to understand the role of Cu2+ in bringing about

magnetism, we have carried out density functional basedcalculations, as described in the Methods Section. In thepresence of simultaneous doping of Fe and Cu, twoprototypical atomistic models, corresponding to “high-symmetry” and “low-symmetry” configurations, were inves-tigated. In the high-symmetry case, the dopants replace nearest-neighbor Zn atoms on a plane perpendicular to the c axis, andin the low-symmetry case, the dopants replace second-nearest-neighbor Zn atoms on adjacent planes. Two low-energy stablespin configurations were found for both low-symmetry andhigh-symmetry cases. Carrying out computational calculationson all four configurations, we find that the lowest energy isobtained for the low-symmetry FM coupling case, which isstabilized by about 27 mRy compared to the AFM coupling.Both are shown in Figure 4 for the high-symmetry case, which

lends itself more naturally to visualization. In the firstconfiguration that is shown on the left of Figure 4, the Featoms were each coupled antiferromagnetically to the Cu atomand therefore ferromagnetically to each other. The origin of thisspecific magnetic configuration is easy to understand. Fe2+ ionshave a high-spin d6 (e↑

2t2↑3e↓

1) configuration. The d-electronhopping between the two sites would lower the total energy.Because the Cu d9 configuration (e4t2

5) can only supporthopping via partially filled t2 levels, this requires the spinconfiguration of the copper site to be necessarily e↑

2t2↓3e↓

2t2↑2,

such that the partially filled t2↑ state of Cu may allow hoppingof t2↑ electrons of iron, thereby making the Cu and the Feantiferromagnetically coupled. Exactly the same process ensuresan antiferromagnetic coupling between the Cu site and thesecond Fe ion, thereby leading to the ferromagnetic couplingbetween the two Fe sites. This implies that the magneticmoment on each Fe atom is +4 μB, arising from itse↑

2t2↑3e↓

1configuration, and the magnetic moment on the Cuatom is −1 μB, arising from its e↑

2e↓2t2↓

3t2↑2 configuration and

establishing the antiferromagnetic coupling between Fe and Cusites. It is important to note here that the origin of the Fe−Feferromagnetic coupling is a hopping mechanism between Feand Cu states, which is a form of the “double-exchange”process28 and not superexchange as suggested earlier for Co−Co ferromagnetic interaction mediated by Cu in Co−Cu

codoped in ZnO system29 or double exchange involving onlyFe states.22 This process of double exchange obtained due tothe presence of Cu in between two Fe atoms allowing for thehighest-energy electrons to delocalize via hopping between Feand Cu sites is similar to that observed in the literature earlier30

using theoretical ab initio calculations where the empty orbitalrequired for hopping is obtained from the process of anexcitation. In this scenario, the total magnetic moment of thetwo Fe and one Cu together is 7 μB. In the second competingconfiguration of comparable energy, shown on the right side inFigure 4, the magnetic moment on each atom is the same inmagnitude, but the two Fe atoms are coupled antiferromagneti-cally due to the superexchange mechanism, with the Cu atommoment aligned with one of the Fe atoms. In other words, thisAFM state carries the magnetic moment of the Cu atom, 1 μB.One may already anticipate this configuration to be destabilizedby the implicit frustration, where the Cu site cannotsimultaneously be antiferromagnetically coupled with both Fesites to lower the total energy; however, this configuration gainsfrom the lowering of the total energy by the superexchange-driven antiferromagnetic coupling of the two Fe sites. Otherspin configurations were found to be substantially higher inenergy and are ignored here. Thus, the computational resultsconfirm that Cu codoping strongly enhances the relativestability of Fe FM coupling with respect to that of Fe AFMcoupling. Again, this is in excellent agreement with theexperimental observation.Symmetry- and defect-position-related differences in the

relative stability of the FM and AFM phases are well-knownand, particularly for codoping of bulk ZnO with Co and Cu,have been studied thoroughly in ref 29. In addition to thisdoping geometry dependence, clearly, the exact numerical valueof the stabilization of the one magnetic phase over the otherwill also depend on the choice of density functional, residualatomic relaxations, nanocrystal size,31,32 and, for our specificcalculations, the choice of the on-site energy, U. Therefore, acomplete quantitative comparison with experiment is notattempted here. However, the salient point is that we findconsistently that Cu codoping can indeed explain thepromotion of FM in the Cu-codoped nanocrystals, whereasin the absence of any Cu, the Fe-doped sample is indeedexpected to manifest antiferromagnetic interactions, asobserved in the present experiments.In conclusion, we report the synthesis of free-standing Fe/

Cu-doped ZnO nanocrystals using a colloidal method withvarying Fe/Cu compositions. XRD patterns suggest theformation of wurtzite nanocrystals with a mean size of ∼7.7nm. While Fe-doped samples indicate antiferromagneticinteractions and Cu-doped samples are essentially diamagnetic,the magnetic susceptibility curves for these simultaneouslydoped systems surprisingly suggest possible ferromagneticinteractions as the curves could not be fitted with a Brilliounfunction. Moreover, at the extremely low temperature of 2 K,an opening of the hysteresis loop is also observed. XASmeasurements establish the presence of Fe2+ and Fe3+ indifferent ratios and expose a correlation between the trivalentspecies and the promotion of magnetism in these samples. This,in conjunction with the observation of a Cu2+ EPR signal,points to the possible importance of Fe2+ + Cu2+ ⇌ Fe3+ +Cu1+ redox pairs in stabilizing the unique magnetism of thecodoped system. Using DFT, the FM nature of these systems isascribed to a double-exchange mechanism between the Featoms, mediated by the Cu atom, while these calculations show

Figure 4. Isosurfaces of spin densities in high-spin (left) and low-spin(right) configurations. Red and gray balls represent oxygen and zincatoms of the ZnO nanocrystal, respectively. Iron and copper atoms arecovered by the spin density data, where yellow and cyan, respectively,correspond to isosurface values of ±0.2 × 10−8 μB bohr−3.

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that the antiferromagnetic coupling between Fe sites in theabsence of codoping is driven by the superexchangemechanism. This opens the road toward rational design ofthe magnetic properties of an oxide-based nanocrystalline dilutemagnetic semiconductor using controlled codoping.

■ METHODS SECTIONA typical synthesis of the Fe/Cu-doped ZnO nanocrystals wascarried out by hydrolyzing stoichiometric amounts of Cu-(OAc)2·H2O, Fe3(PO4)2, and Zn(OAc)2·2H2O in isopropanolwith NaOH solution under ultrasonic agitation formingZn1−x−yFexCuyO nanocrystals. The detailed synthesis procedureis discussed in the Supporting Information. The percentage ofFe/Cu, as determined by atomic absorption analysis, was foundto be close to the stoichiometric amounts added in each of thecases. Nanocrystal structure and size identification of theparticles was carried out using X-ray diffraction and trans-mission electron microscopy. The magnetic properties of thesesamples were measured using a Quantum Design MPMS XLsuperconducting quantum interference device magnetometer.X-ray absorption spectra at the Fe L3 edge were measured atthe BEAR beamline using the synchrotron radiation source,Elettra, Trieste.All density functional theory calculations were carried out by

solving the Kohn−Sham equations using the higher-orderfinite-difference pseudopotential method,33 as implemented inthe PARSEC software.34 An on-site implementation of theLSDA+U approximation35 was used to account for thetransition-metal d-orbital correlation. Further details andspecific parameters are provided in the Supporting Information.Theoretical 2p→3d XAS spectra were calculated using theLanczos iterative algorithm of a many-body Hamiltonian, basedon a fully coherent spectral function for the (FeO4)

6−

tetrahedral cluster corresponding to the wurtzite structure.The theoretical approach is discussed elsewhere,36 and detailsof the calculations and the parameters used are discussed in theSupporting Information.

■ ASSOCIATED CONTENT

*S Supporting InformationDetails of the synthesis procedure, measurement tools, anddiscussion of the theoretical methods along with parametersused in the calculations and the EPR spectrum of optimallycodoped Fe/Cu ZnO. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: sarma@sscu.iisc.ernet.in.

Present Addresses#Currently at International Centre for Materials Science andNew Chemistry Unit, Jawaharlal Nehru Centre for AdvancedScientific Research, Jakkur, Bangalore 560064.∥Currently at the Department of Electrical and ComputerEngineering, Carnegie Mellon University, Pittsburgh, PA15213, U.S.A.∇Also at Jawaharlal Nehru Centre for Advanced ScientificResearch, Jakkur, Bangalore 560064, and Department ofPhysics and Astronomy, Uppsala University, Sweden.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Work in Bangalore was supported by the Department ofScience and Technology, Government of India. R.V. and D.D.S.acknowledge the support of the International Centre forTheoretical Physics and Department of Science and Technol-ogy under their Users Programs for Synchrotron Radiation.R.V. and D.D.S. thank Debangshu Chaudhuri, Sameer Sapraand the beamline scientists at the BEAR beamline of Elettra,Trieste for help with the X-ray absorption study. Work inRehovoth was supported by the Minerva Foundation, the LiseMeitner Minerva Center for Computational Chemistry, and thehistorical generosity of the Perlman family. Work in Texas wassupported by the U.S. Department of Energy, Office of BasicEnergy Sciences and Office of Advanced Scientific ComputingResearch (Grant No. DE-FG02-06ER46286 on nanostructuresand Grant No. DE-SC0001878 on oxides). Computationalresources were provided by National Energy Research ScientificComputing Center (NERSC) and the Texas AdvancedComputing Center (TACC) under Grant TG-DMR090026.

■ REFERENCES(1) Jaworski, M. C.; Yang, J.; Mack, S.; Awschalom, D. D.; Heremans,J. P.; Myers, R. C. Observation of the Spin-Seebeck Effect in aFerromagnetic Semiconductor. Nat. Mater. 2010, 9, 898−903.(2) Dietl, T. A Ten-Year Perspective on Dilute Magnetic Semi-conductors and Oxides. Nat. Mater. 2010, 9, 965−974.(3) Le Gall, C. Optical Spin Orientation of a Single Manganese Atomin a Semiconductor Quantum Dot Using Quasiresonant Photo-excitation. Phys. Rev. Lett. 2009, 102, 127402.(4) Ohno, H. Making Nonmagnetic Semiconductors Ferromagnetic.Science 1998, 14, 951−956.(5) Furdyna, J. K. Diluted Magnetic Semiconductors. J. Appl. Phys.1988, 64, R29−R64.(6) Whitaker, K. M.; Ochsenbein, S. T.; Polinger, V. Z.; Gamelin, D.R. Electron Confinement Effects in the EPR Spectra of Colloidal N-Type ZnO Quantum Dots. J. Phys. Chem. C 2008, 112, 14331−14335.(7) Sato, K.; Bergqvist, L.; Kudrnovsky, J.; Dederichs, P. H.; Eriksson,O.; Turek, I.; Sanyal, B.; Bouzerar, G.; Katayama-Yoshida, H.; Dinh, V.A. First-Principles Theory of Dilute Magnetic Semiconductors. Rev.Mod. Phys. 2010, 82, 1633.(8) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.;von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M.Spintronics: A Spin-Based Electronics Vision for the Future. Science2001, 294, 1488−1495.(9) Zheng, W.; Strouse, G. F. Involvement of Carriers in the Size-Dependent Magnetic Exchange for Mn:CdSe Quantum Dots. J. Am.Chem. Soc. 2011, 133, 7482−7489.(10) Sundaresan, A.; Bhargavi, R.; Rangarajan, N.; Siddesh, U.; Rao,C. N. R. Ferromagnetism as a Universal Feature of Nanoparticles ofthe Otherwise Nonmagnetic Oxides. Phys. Rev. B 2006, 74, 161306.(11) Chelikowsky, J. R.; Alemany, M. M. G.; Chan, T. L.; Dalpian, G.M. Computational Studies of Doped Nanostructures. Rep. Prog. Phys.2011, 74, 046501.(12) Viswanatha, R.; Sapra, S.; Gupta, S. S.; Satpati, B.; Satyam, P. V.;Dev, B. N.; Sarma, D. D. Synthesis and Characterization of Mn-DopedZnO Nanocrystals. J. Phys. Chem. B 2004, 108, 6303−6310.(13) Pradhan, N.; Sarma, D. D. Advances in Light-Emitting DopedSemiconductor Nanocrystals. J. Phys. Chem. Lett. 2011, 2, 2818−2826.(14) Sarma, D. D.; Viswanatha, R.; Sapra, S.; Prakash, A.; Garcia-Hernandez, M. Magnetic Properties of Doped II−VI SemiconductorNanocrystals. J. Nanosci. Nanotechnol. 2005, 5, 1503−1508.(15) Viswanatha, R.; Chakraborty, S.; Basu, S.; Sarma, D. D. Blue-Emitting Copper-Doped Zinc Oxide Nanocrystals. J. Phys. Chem. B2006, 110, 22310−22312.

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The Journal of Physical Chemistry Letters Letter

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