Thin Solid Films 519 (2011) 8199–8202

Contents lists available at ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r.com/ locate / ts f

Defect induced room temperature ferromagnetism in well-aligned ZnO nanorodsgrown on Si (100) substrate

Faheem Ahmed, Shalendra Kumar, Nishat Arshi, M.S. Anwar, Bon Heun Koo, Chan Gyu Lee ⁎School of Nano and Advanced Materials Engineering, Changwon National University, Changwon, Gyeongnam, 641-773, Republic of Korea

⁎ Corresponding author.E-mail address: chglee@changwon.ac.kr (C.G. Lee).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.03.062

a b s t r a c t

a r t i c l e i n f o

Available online 1 April 2011

Keywords:ZnONanorodsXRDPLMagnetization

In this work, we report the fabrication of high quality single-crystalline ZnO nanorod arrays which weregrown on the silicon (Si) substrate using a microwave assisted solution method. The as grown nanorods werecharacterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photo-luminescence (PL) and magnetization measurements. The XRD results indicated that the ZnO nanorods arewell oriented with the c-axis perpendicular to the substrate and have single phase nature with the wurtzitestructure. FE-SEM results showed that the length and diameter of the well aligned rods is about ~1 μm and~100 nm respectively, having aspect ratio of 20–30. Room-temperature PL spectrum of the as-grown ZnOnanorods reveals a near-band-edge (NBE) emission peak and defect induced green light emission. The greenlight emission band at ~583 nm might be attributed to surface oxygen vacancies or defects. Magnetizationmeasurements show that the ZnO nanorods exhibit room temperature ferromagnetism which may result dueto the presence of defects in the ZnO nanorods.

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Recently, extensive research on one dimensional (1D) ZnOnanostructures has received noteworthy attention due to their uniquephysical and chemical properties [1]. Because of its wide band gap(Eg~3.37 eV) and large excitonic binding energy (60 meV) larger thanthat of ZnSe (22 meV) and GaN (25 meV), ZnO is one of the mostpotential functional oxide material for diverse technological applica-tions at room temperature [1–3]. On the other hand, due to the roomtemperature ferromagnetism (RTFM) observed in the pristinesemiconductor in the absence of any magnetic ions [4,5], ZnO hasalso been studied extensively. The origin of ferromagnetism inundoped oxide semiconductors still remains unclear. It is generallyacknowledged that oxygen vacancies might play an important role forRTFM in wide band gap semiconductors such as ZnO, HfO2, TiO2, andIn2O3 [4,6]. Experimental and theoretical [5] results showed that theintrinsic ferromagnetism in undoped ZnO is attributed to theinteraction between the defects and 3d ions [7], grain boundaries[8], Zn interstitial [9], or exchange interactions between unpairedelectron spins arising from the oxygen vacancies at the nanoparticlesurfaces [4]. To understand the origin of ferromagnetism, we haveperformed both optical and magnetic measurements on the as-prepared ZnO nanorods. The PL spectra of ZnO usually exhibitultraviolet (UV) and visible bands. The UV band was identified in

terms of free and bound exciton complexes and the phonon replicas[10]. The visible emission is mainly related to deep-level emissionsintroduced by some defects, such as oxygen vacancies (VO), zincvacancies (VZn), zinc interstitials (Zni) and anti-site of OZn [11]. Up tonow, the origin of the green light emission is still a matter of debate,partly because of its sensitivity on the sample preparation conditions.

Diverse fabrication techniques, such as metal-organic chemicalvapor deposition (MOCVD) [12], catalyst or anodic aluminum oxide(AAO) template-assisted routes [13], epitaxial electrodeposition [14]and aqueous solution growth [15] had been used. Comparing withabove-mentioned various synthetic methods, the approach to therational fabrication of ZnO nanorod arrays based on microwaveassisted aqueous solution growth, is the most promising techniqueowing to its large-scale, low-cost, environmental-benign and mild-temperature advantages. Herein, we demonstrate an effectivemicrowave assisted technique to directly grow single crystal ZnOnanorods on Si (100) substrate with high density. With the help of PLand magnetization results, we have illustrated that oxygen vacanciesmight be the key element for the RTFM in ZnO nanorods.

2. Experimental details

High-quality ZnO nanorods were synthesized on NiO particles-deposited Si (100) substrate. The fabrication procedure consists of twosteps: (a) preparation of seed-layer and (b) growth of nanorod arrays. Inthe first step, a substrate waswetwith a droplet of 0.01 Mnickel nitrate(Sigma-Aldrich 99.999%) and ethanol, rinsed with clean ethanol after10 s, and then blown dry with a stream of argon. This coating step was

Fig. 2. Indexed XRD pattern of the ZnO nanorod arrays. The inset is the drawing plan ofthe c-axis elongated nanorods, highly oriented normal to the substrate.

8200 F. Ahmed et al. / Thin Solid Films 519 (2011) 8199–8202

repeated five times. The substrate, now covered with a film of nickelnitrate crystallites, was heated to 400 °C in air for 20 min to yield layersof NiO particles. The synthesis was carried out in a domestic microwaveoven (Samsung, 750 W). The seed-layer wafer was placed in round-bottom flask containing equamolar (0.01 M) aqueous solution of zincnitrate (Zn(NO)3·6H2O; 99.99%, Sigma Aldrich) and hexamethylene-tetramine (C6H12N4; 99.99%, Sigma Aldrich), then put into a domesticmicrowave oven.Microwave irradiation proceeded at 750W for 20 minto grow ZnO nanorods. Finally, the grown ZnO nanorods on seededsubstrates were thoroughly washed with deionized water to eliminateresidual salts, and dried in air at 80 °C for 2 h before characterization.The schematic diagram illustrating the ZnO nanorod formationprocesses is shown in Fig. 1.

The structure andmorphologywere characterized bymeans of X-raydiffraction using (Phillips X'pert; MPD 3040) X-ray diffractometer withCu Kα radiations (λ=1.5406 Å) and field-emission scanning electronmicroscope (FE-SEM, TESCAN, MIRA II LMH). The elemental composi-tionwasdeterminedby energydispersiveX-ray spectroscopy (EDS, IncaOxford, attached to the FE-SEM).Photo-luminescence (PL) spectroscopywas employed for optical characterization of the ZnO nanorod arrays.The PL measurements were carried out using a luminescencespectrometer (JASCO, FP-6500) with a Xenon lamp as the excitationsource at room temperature. The excitation wavelength used in themeasurement was 325 nm. Magnetization measurements were per-formed using a commercial Quantum Design physical propertymeasurement system. Only plastic tweezers were used throughoutthe experiments to avoid unintentional metal contamination.

3. Results and discussions

Fig. 2 shows the XRD pattern of the ZnO nanorod arrays grown onthe Si substrate. As shown in Fig. 2, the positions of the XRD peaks ofthe sample with lattice parameters a=3.249 and c=5.207 Å are verywell matched with the standard PDF values ((JCPDS, 89-0510), whilethe relative intensities of these peaks are distinct from those of ZnOpowders. The intensity of the (0 0 2) peak is very strong comparedwith that of the other peaks such as (1 0 0) and (1 01). The resultsindicate that the ZnO nanorod arrays are highly aligned perpendicularto the Si substrate with the c-axial growth direction. The degree oforientation can be illustrated by the relative texture coefficient [16],given by

TC002 =I002 = I

0002

I002 = I0002 + I101 = I

0101

ð1Þ

where TC002 is the relative texture coefficient of diffraction peaks (0 0 2)over (1 0 1), I002 and I101 are themeasured diffraction intensities due to(0 0 2) and (1 0 1) planes, respectively, I002

0 and I1010 are the

corresponding values of standard PDF measured from randomlyoriented powder samples. For materials with random crystallographic

Fig. 1. Schematic illustration of growth mechanism

orientations, e.g. powders, the texture coefficient is 0.5. The value ofsynthesized ZnO nanorod arrays is 0.97, which indicates an extremelyhigh c-axis orientation of the nanorods. The XRD results suggest thatprepared nanorod arrays are highly crystalline having wurtzitestructure with preferable c-axis orientation. No secondary phase wasdetected within the limit of XRD measurement.

Well-aligned single crystalline hexagonal nanorods, typically 100–150 nmwide and up to about 1–2 μm longwith an aspect ratio of 20–30,grewonto theSi (100) substrate is shown inFig. 3. The low-magnificationFESEM image (Fig. 3(a)) shows a large area uniform film-likematerial deposited on the substrate. From thehigh-magnification images(Fig. 3(b) and (c)), it can be clearly seen that a high density of ZnOnanorods with well-defined crystallographic phases, i.e., polar terminat-ed (001) andnon-polar low-symmetry (101) faces (Fig. 3(c), inset)weregrown vertically on the substrate and these correspond to the typicalcrystal habit and growth form of ZnO zincite (wurtzite structure) (Fig. 2,inset). We used NiO nanoparticles on the Si substrate to synthesize ZnOnanorods. It iswell-known that transitionmetal (TM)oxides, such asNiOand FeO, have a similar catalytic effect on the growth of semiconductornanorods as well as metal catalysts [17]. The reason for employing TMoxides as seed to control the growth of the ZnO nanorods is that the TMoxides play a key role when the synthesis was carried out at lowertemperature. Our experimental results show that the NiO nanoparticlesare fully covered with a laterally grown ZnO layer and no metal catalystparticles are observed on the ZnO nanorod tip. Fig. 3(d) shows thechemical composition of the nanorods determined by EDS. Only oxygenand zinc signals were detected which confirms that the nanorods areprimarily ZnO.

Fig. 4 shows the room-temperature PL spectra of the as-grown ZnOnanorod arrays. A sharp near-band-edge (NBE) emission at 381 nm

for the ZnO nanorod arrays on Si substrate.

Fig. 3. FE-SEM images of the well-aligned ZnO nanorod arrays on Si substrate. (a) Large-scale and low magnification, (b) and (c) fine structure of the nanorods under highmagnification, (d) EDS analysis of the highly oriented, hexagonal ZnO nanorod arrays.

8201F. Ahmed et al. / Thin Solid Films 519 (2011) 8199–8202

and a broad green light emission at 583 nm have been observed. Thenear band-edge emission is attributed to awell-known recombinationof free excitons of the wide band-gap ZnO [10] and the green lightemission is resulted from the recombination of photo-generated holewith a singly ionized charge state of specific defect [11]. Furthermore,the narrow full-width at half-maximum (FWHM) of the UV emissionindicates the high crystal quality of the ZnO nanorod arrays withuniform size distribution as deduced from FE-SEM images. It isgenerally complicated in experiments to distinguish PL bands causedby zinc interstitials (Zni) and oxygen vacancies (VO). Theoretically,Kohan et al. [18] and Van de Walle [19] calculated the formationenergies and electronic structure of native point defects in ZnO. Basedon their results, oxygen and zinc vacancies are the two most commondefects in ZnO. In zinc-rich conditions, the oxygen vacancies (VO)

Fig. 4. Room temperature PL spectra of the ZnO nanorod arrays.

have lower formation energy (1.2 eV) than the zinc interstitials (Zni)and will dominate in the defect, but in oxygen-rich conditions, zincvacancies (VZn) ought to dominate. In our microwave aqueous growthcondition, Zn source supplies from zinc salts and the O comes from theOH−. This aqueous growth method can be classified as Zn-richconditions due to the high solubility of the zinc salts. Therefore, PLspectra indicate that the low density of oxygen vacancies (VO) maypossibly responsible for the green light emission, while zincinterstitials (Zni) and zinc vacancies (VZn) may be excluded in thesynthesized ZnO rods. These results are in good agreement with theearlier report [20]. We predict that the rapid microwave heatingmight enhance the defects.

Fig. 5 depicts themagnetic hysteresis (M–H) curve of ZnO nanorodsat room temperature. It can be clearly seen from the (M–H) curve thatZnO rods exhibit room temperature ferromagnetism (RTFM). Theremanentmagnetization (Mr) and coercivity (Hc) are about 3.89 (μemu)and 114 Oe, respectively. The exact mechanism of intrinsic ferromag-netism in undoped semiconducting oxides is still under debate. Thefindings of RTFM without any transition metal doping have attractedsignificant attention and were called “d0 ferromagnetism” [21]. Theorigin of observed ferromagnetism at room temperature in thesenanorods could arise from a number of possibilities, such as NiO, Nimetal and Ni2O3 (seed material) and intrinsic property of the ZnOnanorods. The presence of a secondary phase such asNiO could be easilyruled out, as NiO in bulk shows antiferromagnetism with a TN of 520 K[22] and super-paramagnetism at nanocrystalline scale, anotherpossibility is Ni metal, which is a well-known ferromagnetic material.However, our nanorods are synthesized in air using ethanol, where Nimetal is unable to exist. Additionally, XRD results also clearlydemonstrate that the structure is in single wurtzite phase and there isno indication of extra phases like Ni metal and Ni2O3 in the preparednanorods. Therefore, ferromagnetism is expected to be raised from thedefect induced in the nanorods. PL study revealed a strong correlation

Fig. 5. Room temperature M–H loop of the ZnO nanorod arrays. The inset is the enlargedview of the loop.

8202 F. Ahmed et al. / Thin Solid Films 519 (2011) 8199–8202

between the ferromagnetism and the oxygen deficiency. The RTFM inZnO nanorods can be attributed to oxygen vacancy (VO), since they caninitiate defect-related hybridization at the Fermi level and establish along-range ferromagnetic ordering [13]. Although, the possibility ofother types of defects such as zinc interstitials (Zni) and zinc vacancies(VZn) as the origin of long range ferromagnetic ordering [23], have beenexcluded in ZnO nanorods. Because these defects often have higherformation energy than VO and can form only in oxygen richenvironments. As our experiments were carried out in oxygen poorconditions (or zinc rich conditions) where VO forever contributes to thenonstoichiometry. Therefore, magnetic study and the PL results suggestthat defects might be responsible for the origin of RTFM in ZnOnanorods. The observed intrinsic ferromagnetism makes these dilutedmagnetic semiconductor (DMS) nanorod potential for future spintronicdevices.

4. Conclusions

In summary, well-aligned ZnO nanorod arrays have been fabri-cated through a novel, simple microwave assisted aqueous solutionapproach. The diameter and length of the ZnO rods were 100–150 nm

and 1–2 μm, respectively with an aspect ratio of 20–30. The nanorodarrays are highly c-axis oriented with hexagonal wurtzite structureand perpendicular to the substrate with high crystalline quality. Theroom temperature PL measurements exhibit a prominent UV peak atabout 381 nm which is attributed to NBE emission and a broad greenlight emission at 583 nm which may be due to defects. The DCmagnetization measurements reflect that ZnO nanorods exhibit RTFMwhich may result from the defects present in the rods.

Acknowledgments

This research was supported by grant No. RTI04-01-03 from theRegional Technology Innovation Program of the Ministry of KnowledgeEconomy, South Korea.

References

[1] M.H. Huang, S. Mao, H.N. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D.Yang, Science 292 (2001) 1897.

[2] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nat. Mater. 4 (2005) 455.[3] H. Faber, M. Burkhardt, A. Jedaa, D. Kalblein, H. Klauk, M. Halik, Adv. Mater. 21

(2009) 3099.[4] A. Sundaresan, R. Bhargavi, N. Rangarajan, U. Siddesh, C.N.R. Rao, Phys. Rev. B 74

(2006) 161306.[5] S. Kumar, Y.J. Kim, B.H. Koo, S. Gautam, K.H. Chae, R. Kumar, C.G. Lee, Mater. Lett.

63 (2009) 194.[6] M. Venkatesan, C.B. Fitzgerald, J.M.D. Coey, Nature 430 (2004) 630.[7] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173.[8] B.B. Straumal, A.A. Mazilkin, S.G. Protasova, A.A. Myatiev, P.B. Straumal, G. Schütz,

P.A. van Aken, E. Goering, B. Baretzky, Phys. Rev. B 79 (2009) 205206.[9] K. Potzger, S. Zhou, J. Grenzer, M. Helm, J. Fassbender, Appl. Phys. Lett. 92 (2008)

182504.[10] J.J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215.[11] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J.

Appl. Phys. 79 (1996) 7983.[12] W.I. Park, D.H. Kim, S.W. Jung, G.C. Yi, Appl. Phys. Lett. 80 (2002) 4232.[13] M.H. Huang, S. Mao, H. Feick, H. Yan, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113.[14] R. Liu, A.A. Vertegel, E.W. Bohannan, T.A. Sorenson, J.A. Switzer, Chem. Mater. 13

(2001) 508.[15] Z.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, M.J. Mcdermott, M.A. Rodriguez, H. Konishi,

H.F. Xu, Nat. Mater. 2 (2003) 821.[16] Y. Kajikawa, S. Noda, H. Komiyama, Chem. Vapor Deposition 8 (2002) 99.[17] X.L. Chen, J.Y. Li, Y.G. Cao, Y.C. Lan, H. Li, M. He, C.Y. Wang, Z. Zhang, Z.Y. Qiao, Adv.

Mater. 12 (2000) 1432.[18] A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. Rev. B 61 (2000) 15019.[19] C.G. Van de Walle, Physica B 899 (2001) 308–310.[20] C.H. Hung, W.T. Whang, Mater. Chem. Phys. 82 (2003) 705–710.[21] J.M.D. Coey, Solid State Sci. 7 (2005) 660.[22] D.A. Schwartz, K.R. Kittilatved, D.R. Gamelin, Appl. Phys. Lett. 85 (2004) 1395.[23] J.A. Chan, S. Lany, A. Zunger, Phys. Rev. Lett. 103 (2009) 016404.