Evidence for surface states in pristine and Co-doped ZnO nanostructures: magnetization and

nonlinear optical studies

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Nanotechnology 22 (2011) 095703 (5pp) doi:10.1088/0957-4484/22/9/095703

Evidence for surface states in pristine andCo-doped ZnO nanostructures:magnetization and nonlinear opticalstudiesRamakrishna Podila1, Benoy Anand2, J Palmer West3, Reji Philip4,S Siva Sankara Sai2, Jian He1, Malcolm Skove1, Shiou-Jyh Hwu3,Sumanta Tewari1 and Apparao M Rao1,5,6

1 Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA2 Department of Physics, Sri Sathya Sai Institute of Higher Learning, Prashanti Nilayam,Andhra Pradesh, 515134, India3 Department of Chemistry, Clemson University, Clemson, SC 29634, USA4 Light and Matter Physics Group, Raman Research Institute, C V Raman Avenue,Sadashivanagar, Bangalore 560080, India5 Center for Optical Materials Science and Engineering Technologies, Clemson University,Clemson, SC 29634, USA

E-mail: arao@clemson.edu

Received 5 October 2010, in final form 5 January 2011Published 24 January 2011Online at stacks.iop.org/Nano/22/095703

AbstractAn unexpected presence of ferromagnetic (FM) ordering in nanostructured ZnO has beenreported previously. Recently, from our detailed magnetization studies and ab initiocalculations, we attributed this FM ordering in nanostructured ZnO to the presence of surfacestates, and a direct correlation between the magnetic properties and crystallinity of ZnO wasobserved. In this study, through a systematic sample preparation of both pristine and Co-dopedZnO nanostructures, and detailed magnetization and nonlinear optical (NLO) measurements,we confirm that the observed FM ordering is due to the presence of surface states.

S Online supplementary data available from stacks.iop.org/Nano/22/095703/mmedia

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Wide interest in the scientific community to integrate bothcharge and spin into micro- and opto-electronic devices beganwith the observation of hole-mediated ferromagnetism (FM) in(Ga, Mn)As [1]. The low Curie temperature of (Ga, Mn)As(∼170 K) led to a search for alternate materials. Interestingly,high temperature FM in nanophase oxides was found in HfO2

and CuO, which are known to be diamagnetic (DM) in thebulk forms [2]. Subsequently, high temperature FM in severalnanophase oxides, such as CaO, MgO, ZnO, CeO2, Al2O3,In2O3 and SnO2, was reported in the literature [3–5].

6 Author to whom any correspondence should be addressed.

In this study, we investigate the magnetic propertiesof pristine and Co-doped ZnO nanostructures. Previously,homogeneous thin films of Zn1−xCox O were found to beanti-ferromagnetic (AFM) [6], while inhomogeneous filmsexhibited FM, implying that the FM results from the presenceof Co clusters [7–9]. In addition, it is widely believed that FMin many metal oxides originates from defects such as oxygenvacancies and is closely related to the electronic structure ofthe magnetic impurity ions [2–5, 10–12]. The nature andconcentration of defects are dependent on synthesis conditionsand therefore controversies regarding reproducibility arecommonplace [13–16].

In our earlier studies on the origin of FM in pristine micro-and nanostructured ZnO, we observed the changes in magnetic

0957-4484/11/095703+05$33.00 © 2011 IOP Publishing Ltd Printed in the UK & the USA1

Nanotechnology 22 (2011) 095703 R Podila et al

properties with varying crystallinity of ZnO. We attributed theFM to the presence of surface states [17]. Here, through asystematic sample preparation of both pristine and Co-dopedZnO nanostructures, and detailed magnetization and nonlinearoptical (NLO) measurements, we confirm that the observedFM results are due to surface defects.

2. Experimental details

For synthesizing pristine ZnO nanostructures, a 50 mMaqueous solution of ZnCl2 was injected (rate of 0.1 ml min−1)into a 1′′ diameter quartz tube reactor maintained at 550 ◦C.An appropriate amount of Co(CH3COO)2 was added to theZnCl2 to prepare the 2% Co-doped nanostructures. A constantflow of O2 and H2 (10:1) was maintained at 500 sccm.Detailed characterization using electron microscopy, x-raydiffraction and Raman spectroscopy is provided in thesupplementary section (available at stacks.iop.org/Nano/22/095703/mmedia) [17].

Magnetization studies were performed using a QuantumDesign SQUID MPMS (model 5S) and the following sampleswere measured: (i) as-prepared pristine and Co-doped ZnO(denoted AP-CVD and AP-CoZnO); (ii) pristine and Co-dopedZnO samples annealed in an O2 atmosphere at 100 ◦C for 5.5 h(O2-100-5.5 h and Co–O2-100-5.5 h); (iii) pristine and Co-doped ZnO samples annealed in an Ar atmosphere at 400 ◦Cfor 5 min (Ar-400-5 m and Co–Ar-400-5 m).

The NLO measurements were done using the openaperture Z -scan technique [18], with linearly polarized 5 nsoptical pulses from a Q-switched frequency-doubled Nd:YAGlaser at 532 nm. The sample solutions in isopropyl alcoholwere contained in 1 mm thick quartz cells for measuring thechange in transmittance.

3. Results and discussion

3.1. Electron microscopy

Figure 1 shows electron microscope images of pristine and2% Co-doped ZnO nanostructures prepared by the CVDtechnique. As shown in figure 1(a), pristine ZnO formsconical nanostructures that vary in diameter from 50 to 100 nmalong the length and are typically ∼1–2 μm long. Energydispersive x-ray spectroscopy (EDX) was employed to confirmthe presence of the Zn, Co and O atoms in the nanostructures(inset of figures 1(a) and (b)). The distribution of Co dopantswas observed to be fairly uniform for 2% Co-doped ZnOnanostructures (figure 1(b)).

3.2. Magnetization studies

As shown in figure 2, clear evidence for FM is observed in theAP-CVD samples. This FM saturates at low magnetic fields(∼1 T) and is embedded in a DM background response. Uponannealing in O2 at 100 ◦C (O2-100-5.5 h), the FM signatureis greatly enhanced as seen in figure 2(a). It is noteworthythat the sharp transition from FM to DM response in theinsets in figure 2(a) implies that the measured magnetizationis not due to a simple superposition of FM ordering and a DMresponse. The signs of saturation at relatively low magnetic

Figure 1. (a) SEM images of as-prepared CVD ZnO nanostructureson Si(100) substrates. The conical Co-doped ZnO nanostructuresvary in diameter from 50 to 100 nm along the length and aretypically ∼1–2 μm long. The inset shows EDX spectra of pristineZnO nanowires. (b) SEM images of 2% Co-doped ZnOnanostructures on Si(100) substrates. The insets show the EDX ofAP-CoZnO samples. Along with Zn and O, uniform distribution ofCo was confirmed, as seen in the lower right panel.

fields (∼1 T) indicate the presence of reasonably large spinclusters. This FM ordering may be understood by consideringchemisorption of O2 to form O−

2 . Since the growth occurs inan O-rich environment, it is probable that O2 is chemisorbedpreferentially near a cluster of Zn vacancies. Given such apossibility one may visualize a dynamic exchange of electronsbetween O2 and O− leading to FM ordering [17, 19–21].Furthermore, this FM ordering and hence the sharp transition isabsent in AP-CVD samples after annealing in Ar at 400 ◦C fora short time (Ar-400-5 m, figure 2(a)). In our previous studies,we observed that annealing in Ar presumably leads to (i) themerging of grains in our polycrystalline ZnO nanostructures,reducing the net surface/interfacial area [17], and (ii) reducingthe amount of chemisorbed O2 in the sample, resulting in a DMresponse.

As shown in figure 2(b), FM observed in AP-CoZnOsamples also saturates at low magnetic fields (∼1 T) and isembedded in a background DM response similar to pristineZnO. Unlike AP-CVD samples, a sharp transition from FM todiamagnetism is absent in AP-CoZnO samples (figure 2(b)).The absence of a sharp transition indicates that the FMordering and diamagnetism originate from two differentsources, namely, FM from Co dopants and diamagnetism

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Figure 2. (a) M–H curves of CVD prepared ZnO nanostructures.As-grown nanostructures show a weak, but clear (top inset), FMembedded in a dominant DM response. The bottom inset showsenhancement in FM ordering by at least an order of magnitude whenannealed in O2 at 100 ◦C. (b) M–H curves of CVD preparedCo-doped ZnO nanostructures at room temperature. As-grownCo-doped ZnO nanostructures (top inset) exhibit at least three timesstronger magnetization than pristine ZnO nanostructures. The bottominset shows enhancement in FM ordering when annealed in O2 at100 ◦C.

from ZnO. Furthermore, in the case of AP-CoZnO samples,the transition from FM to DM response does not occuruntil ∼2–3 T, suggesting that the robust FM is due to thepresence of Co atoms in the ZnO lattice. It is noteworthythat the net normalized magnetic moment for AP-CoZnO(∼25 memu g−1) samples is at least three times larger than thepristine ZnO nanostructures (∼7 memu g−1), confirming theenhancement of FM due to Co doping. Thus the FM orderingin AP-CoZnO samples may be understood in terms of Coatoms substitutionally doping the Zn sites. The formation ofCo clusters in our samples is highly unlikely, since the nominaldoping concentration (2 at.%) is so low. Also, EDX confirmsuniformly distributed Co in AP-CoZnO samples (figure 1(b)).In addition, no signs of Co clusters were observed in eitherTEM or x-ray diffraction.

Annealing effects on FM for AP-CoZnO samples werefound to be similar to AP-CVD samples. We observed anincrease in the net normalized magnetic moment from ∼25 to∼45 memu g−1 upon annealing the AP-CoZnO samples in O2

at 100 ◦C (Co–O2-100-5.5 h; figure 2(b)). We also observed aDM response in AP-CoZnO samples annealed at 400 ◦C in Arfor 5 min (Co–Ar-400-5 m; figure 2(b)).

3.3. Theoretical studies

Previously, we showed using ab initio calculations that thenet magnetization in ZnO nanostructures could arise from twopossible contributions: one due to localized O p orbitals andthe other due to delocalized O p orbitals in the ZnO lattice. Inour model, the localized orbitals (px ) are preferentially alignedperpendicular to the wire direction, whereas the delocalizedorbitals (pz) point in a direction parallel to the wire direction.This would imply that the localized orbitals contribute a localmoment, forming a paramagnetic center that can interact at alonger range with other such centers through the delocalizedorbitals. This results in a net magnetization. Thus, annealingin O2 leads to an enhancement in the FM signal observed in theO2-100-5.5 h and Co–O2-100-5.5 h samples (cf figures 2(a)and (b)). From our previous calculations, for pristine ZnOnanostructures, the net computed magnetic moment was foundto be strongly localized near the surface of the nanowires(FM ordering), while the interior of the wire did not show asimilar strong contribution (DM response; cf figure 4 in [17]).Also, the origin of magnetization in O p orbitals was stronglysupported by the presence of unpaired oxygen p levels at theFermi energy in the projected density of states (PDOS) ofpristine ZnO [17]. Earlier reports on Co-doped ZnO showedthat new surface states are introduced above the valence bandmaximum in the gap region of ZnO when the Co atoms areincorporated [22]. Such new surface states introduced by Cowould enhance the FM in ZnO nanostructures as observed infigure 2(b).

3.4. Nonlinear optical studies

Here we show that the presence of such surface statesin pristine and AP-CoZnO can be confirmed from anotherindependent measurement: the open aperture Z -scan. TheZ -scan is a nonlinear optical measurement, in which thelight-induced change in transmittance (�T ) of a mediumdue to optical nonlinearity is measured as a function ofinput light energy density (fluence) or intensity. From thisone can calculate the nonlinear absorption and refractioncoefficients. In the experiment, a continuous variation ofthe input fluence is achieved by translating the sample understudy through the focal region of a focused laser beam (thesample position is taken as z with z = 0 being the focalpoint for the incident laser beam; hence the name ‘Z -scan’).A detailed description of the Z -scan technique can be foundelsewhere [23]. The key point to note here is that, exceptwhen excited by ultrafast laser pulses, �T strongly depends onthe excited state population density. In view of this, we havemeasured the nonlinear transmission of the present samples inthe nanosecond excitation regime, using linearly polarized 5 ns

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Nanotechnology 22 (2011) 095703 R Podila et al

Figure 3. (a) Z -scan data for AP-CoZnO samples fitted to a three-photon absorption (3PA) equation. The inset is a schematic depictingeffective 3PA. When the intermediate states are virtual the effect is genuine 3PA, but when they are real it is an effective 3PA arising mostlyfrom excited state absorption. (b) Z -scan data for pristine ZnO samples fitted to a 3PA equation. (c) Z -scan data for AP-CoZnO samples atdifferent incident laser energies. (d) Variation of the 3PA coefficient (γ ) as a function of the incident laser pulse energy for AP-CVD,AP-CoZnO, O2-100-5.5 h and Ar-400-5 m samples.

optical pulses from a Q-switched frequency-doubled Nd:YAGlaser operating at 532 nm. The Z -scan data obtained forAP-CoZnO and pristine ZnO samples (figures 3(a) and (b)respectively) are found to be best fitted numerically by a three-photon absorption (3PA) process. Detailed NLO studies willbe reported elsewhere. The nonlinear absorption coefficient α

in the presence of 3PA is given by α(I ) = α0 + γ I 2, where Iis the intensity of the laser, α0 the unsaturated linear absorptioncoefficient, and γ the 3PA coefficient. The value of γ isobtained by fitting the Z -scan curve to the 3PA propagationequation, given by dI

dz′ = −α0 I − γ I 3, where z ′ is thepropagation distance within the sample.

The values of γ numerically obtained from the fits indicatethat nonlinear absorption in the present case arises not onlyfrom genuine 3PA, but also from ‘effective’ three-photonnonlinearity [23, 24]. Such effective 3PA originates fromsequential excited state absorption as shown in the inset offigure 3(a). In the case of genuine three-photon absorption(3PA), where the transition states involved are virtual, the3PA coefficient (γ ) is a constant and is independent of theincident laser fluence. All genuine ground state absorptioncoefficients (such as 2PA or 3PA) are dependent on the groundstate population N , since the absorption coefficient is obtainedby multiplying the corresponding absorption cross section (σ )by N . The absorption cross section is a microscopic parameter,

independent of N , that characterizes the average two-photon/three-photon absorbability per molecule. However, thechange in γ will be evident only when there is a substantialchange in the ground state population due to the absorption.Therefore, if the absorptions are weak, then the coefficientscan be considered to be almost constant, since there is anegligible change in N . Genuine 2PA and 3PA are usually veryweak phenomena, and hence the corresponding coefficientscan be considered as material constants at a given wavelengthand concentration. But strong excited state absorption (ESA)during an effective 3PA depletes the ground state populationsignificantly so that the absorption coefficient is no longer aconstant. Hence a change in γ w.r.t. incident laser intensitycan help confirm the presence of surface states in the forbiddengap of ZnO.

Both two-photon absorption and/or sequential absorptionof two photons at 532 nm (2.33 eV; inset figure 3(a)) willresult in a real terminal level which is above the lowestexcitonic state, that lies around 363 nm (3.4 eV) for ZnO,and another one-photon absorption to go from this level tothe band edge. Figures 3(c) and (d) show the variation ofγ for AP-CoZnO, AP-CVD, O2-100-5.5 h and Ar-400-5 mrespectively, as a function of incident laser energy. Whileγ remains almost a constant with input energy in AP-CVD,there is a pronounced increase in γ with incident energy in

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Nanotechnology 22 (2011) 095703 R Podila et al

AP-CoZnO and O2-100-5.5 h samples. Moreover, the 3PAcoefficient (and hence the optical limiting efficiency) for AP-CoZnO samples is obviously higher than that of pristine ZnO.These observations concur with the enhancement seen in themagnetic moment of AP-CoZnO and O2-100-5.5 h samplesand indicate the presence of enhanced surface state densitydue to Co atoms in the ZnO lattice. Thus the increase of γ

with the incident laser energy confirms effective 3PA due tothe existence of a wide range of surface states in AP-CoZnOand O2-100-5.5 h samples. Furthermore, the lowest γ valuesand a constant trend with input fluence were observed for Ar-400-5 m samples, indicating the absence of any surface states.Thus, we believe that an enhanced surface state density in AP-CoZnO due to Co doping results in stronger FM and betteroptical limiting compared to pristine ZnO nanostructures.

4. Conclusions

In summary, pristine ZnO nanostructure (AP-CVD) and 2%Co-doped ZnO nanostructure (AP-CoZnO) samples showedsimilar FM signatures. The magnitude of the observed changein magnetization was pronounced for AP-CoZnO samples andis presumably due to the presence of Co-induced intermediatestates. As expected, a sharp transition from FM to DM was notobserved in AP-CoZnO samples, indicating the presence of Codopants in the ZnO lattice. Lastly, we confirmed the presenceof intermediate states in the gap region of ZnO nanostructuresusing Z -scan nonlinear optical measurements.

Acknowledgments

RP and AMR thank Professor Amar Nath, University of NorthCarolina at Asheville, and are grateful for encouragement fromProfessor K Venkataramaniah of Sri Satya Sai Institute ofHigher Learning. SSSS and BA acknowledge funding fromNRB, DST and UGC, India. ST acknowledges funding fromDoE.

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