Undoped ZnO nanoparticles (NPs) with size ∼12 nm were produced using forced hydrolysis methods using diethylene glycol (DEG) [called ZnO-I] or denatured ethanol [called ZnO-II] as the reaction solvent; both using Zn acetate dehydrate as precursor. Both samples showed weak ferromagnetic behavior at 300 K with saturation magnetization Ms = 0.077 ± 0.002 memu/g and 0.088 ± 0.013 memu/g for ZnO-I and ZnO-II samples, respectively. Fourier transform infrared(FTIR) spectra showed that ZnO-I nanocrystals had DEG fragments linked to their surface. Photoluminescence (PL) data showed a broad emission near 500 nm for ZnO-II which is absent in the ZnO-I samples, presumably due to the blocking of surface traps by the capping molecules. Intentional oxygen vacancies created in the ZnO-I NPs by annealing at 450 °C in flowing Ar gas gradually increased Ms up to 90 min and x-ray photoelectron spectra (XPS) suggested that oxygen vacancies may have a key role in the observed changes in Ms. Finally, PL spectra of ZnO showed the appearance of a blue/violet emission, attributed to Zn interstitials,whose intensity changes with annealing time, similar to the trend seen for Ms. The observed variation in the magnetization of ZnO NP with increasing Ar annealing time seems to depend on the changes in the number of Zn interstitials and oxygen vacancies

Chess, J.

Eixenberger, J.

Hanna, C. B.

Punnoose, A.

Rainey, K.

Tenne, D. A.

Boise State University - ScholarWorks

Boise State UniversityScholarWorksPhysics Faculty Publications and Presentations Department of Physics3-26-2014Defect Induced Ferromagnetism in Undoped ZnONanoparticlesK. RaineyBoise State UniversityJ. ChessBoise State UniversityJ. EixenbergerBoise State UniversityD. A. TenneBoise State UniversityC. B. HannaBoise State UniversitySee next page for additional authorsCopyright 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of theauthor and the American Institute of Physics. The following article appeared in Journal of Applied Physics, 115(17), 17D727 and may be found at doi:10.1063/1.4867596AuthorsK. Rainey, J. Chess, J. Eixenberger, D. A. Tenne, C. B. Hanna, and A. PunnooseThis article is available at ScholarWorks: http://scholarworks.boisestate.edu/physics_facpubs/148Defect induced ferromagnetism in undoped ZnO nanoparticlesK. Rainey, J. Chess, J. Eixenberger, D. A. Tenne, C. B. Hanna, and A. Punnoose  Citation: Journal of Applied Physics 115, 17D727 (2014); doi: 10.1063/1.4867596 View online: http://dx.doi.org/10.1063/1.4867596 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/17?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Structural, optical, vibrational, and magnetic properties of sol-gel derived Ni doped ZnO nanoparticles J. Appl. Phys. 114, 033912 (2013); 10.1063/1.4813868  Unusual crystallite growth and modification of ferromagnetism due to aging in pure and doped ZnO nanoparticles J. Appl. Phys. 111, 07C319 (2012); 10.1063/1.3679147  Magnetic properties of ZnFe2O4 ferrite nanoparticles embedded in ZnO matrix Appl. Phys. Lett. 100, 122403 (2012); 10.1063/1.3696024  Enhanced indirect ferromagnetic p-d exchange coupling of Mn in oxygen rich ZnO:Mn nanoparticles synthesizedby wet chemical method J. Appl. Phys. 111, 033503 (2012); 10.1063/1.3679129  Effect of oxygen defects on ferromagnetic of undoped ZnO J. Appl. Phys. 110, 013901 (2011); 10.1063/1.3601107    [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:132.178.2.65 On: Thu, 24 Apr 2014 16:48:09Defect induced ferromagnetism in undoped ZnO nanoparticlesK. Rainey, J. Chess, J. Eixenberger, D. A. Tenne, C. B. Hanna, and A. Punnoosea)Department of Physics, Boise State University, Boise, Idaho 83725, USA(Presented 7 November 2013; received 23 September 2013; accepted 29 November 2013; publishedonline 26 March 2014)Undoped ZnO nanoparticles (NPs) with size 12 nm were produced using forced hydrolysismethods using diethylene glycol (DEG) [called ZnO-I] or denatured ethanol [called ZnO-II] as thereaction solvent; both using Zn acetate dehydrate as precursor. Both samples showed weakferromagnetic behavior at 300K with saturation magnetization Ms¼ 0.077 6 0.002 memu/g and0.088 6 0.013 memu/g for ZnO-I and ZnO-II samples, respectively. Fourier transform infrared(FTIR) spectra showed that ZnO-I nanocrystals had DEG fragments linked to their surface.Photoluminescence (PL) data showed a broad emission near 500 nm for ZnO-II which is absent in theZnO-I samples, presumably due to the blocking of surface traps by the capping molecules. Intentionaloxygen vacancies created in the ZnO-I NPs by annealing at 450 C in flowing Ar gas graduallyincreased Ms up to 90min and x-ray photoelectron spectra (XPS) suggested that oxygen vacanciesmay have a key role in the observed changes in Ms. Finally, PL spectra of ZnO showed the appearanceof a blue/violet emission, attributed to Zn interstitials, whose intensity changes with annealing time,similar to the trend seen for Ms. The observed variation in the magnetization of ZnO NP withincreasing Ar annealing time seems to depend on the changes in the number of Zn interstitials andoxygen vacancies.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4867596]Zinc Oxide (ZnO) nanoparticles (NPs) have been a largefocus in materials research. Because of their small size, NPsof many oxide materials such as ZnO display novel proper-ties, including room temperature ferromagnetism (RTFM).1Several types of lattice defects, including oxygen vacancies,metal interstitials, and metal vacancies, have been proposedas the source of the RTFM in undoped oxide nanopar-ticles.2,3 However, clear evidence on the role of surfacedefects on this unusual ferromagnetism in undoped oxide NPis lacking. To investigate the role and type of defects contrib-uting to the RTFM of ZnO NPs, their magnetic behavior isinvestigated as a function of defects introduced intentionallyby annealing in Ar atmosphere at 450 C.All undoped ZnO NPs used in this study were producedusing one of two forced hydrolysis methods, both using Znacetate dehydrate as precursor, with DEG (referred to asZnO-I) or denatured ethanol (ZnO-II) as the reaction solvent.Details of their synthesis and characterization are discussedin Ref. 4.As prepared samples were annealed in the 200–750 Crange in Ar atmosphere (flow rate 20ml/min) using aNetzsch Simultaneous Thermal Analysis (STA) 449 F1,capable of monitoring both mass and energy changes. ThisSTA unit was used in combination with a quadrupole massspectrometry (QMS) evolved gas analysis system to monitorelemental and molecular burn-off. It was determined that450 C was the optimal temperature to remove organic pre-cursors while causing the least grain growth. Samples wereannealed at 450 C for varying amounts of time (30, 45, 60,75, 90, and 120min). The as-prepared and annealed sampleswere analyzed using x-ray diffraction (XRD), XPS, vibratingsample magnetometry (VSM), FTIR, and low temperature(10K) photoluminescence (PL) and Raman spectra (with a325 nm UV excitation) following procedures described else-where.4 These studies confirmed the absence of contamina-tion in samples.XRD patterns confirm the samples to be single phasewurtzite ZnO with no detectable secondary phases. Lattice pa-rameters and crystallite size determined using Rietveldrefinement methods revealed an average crystallite size of12.36 0.35nm for both ZnO-I and ZnO-II. Lattice parameterswere around a¼ 3.256 0.0003 A˚ and c¼ 5.216 0.003 A˚ forthe as-prepared and annealed samples. Samples displayedgrowth up to 29 nm when the NPs were annealed at 450 C,attributed to the expected sintering process.The FTIR spectra of ZnO-I and ZnO-II are shown inFigs. 1(a) and 1(b). The data showed the major characteristicZn-O vibrations5 at 478 cm1 in ZnO-I and 444 cm1 inZnO-II. The broad absorptions around 3410–3420 cm1 dueto hydroxyl groups are relatively stronger in ZnO-I. Presenceof fragments of DEG is observed in ZnO-I NP. No such sur-factant layer is present on ZnO-II NP. The ZnO-I sampleexhibits two strong bands, commonly associated with thecarboxylate functional group, at 1412 cm1 [s(COO)] and1595 [as(COO)].6,7 Additionally, weaker bands consistentwith s(CH2) and (C-OH) at 907 and 1067 cm1 are alsoobserved.6–8 ZnO-II also showed many of these bands due tosurface adsorbed groups resulting from the acetate groups,but with somewhat lower intensity and at slightly differentfrequencies, presumably due to the absence of a surfactantlike DEG in this sample.The PL spectra of the samples recorded at 10K using a325 nm He:Cd laser are shown in Figures 1(c) and 1(d).a)Author to whom correspondence should be addressed. Electronic mail:apunnoos@boisestate.edu.0021-8979/2014/115(17)/17D727/3/$30.00 VC 2014 AIP Publishing LLC115, 17D727-1JOURNAL OF APPLIED PHYSICS 115, 17D727 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:132.178.2.65 On: Thu, 24 Apr 2014 16:48:09Higher order Raman peaks appear in the spectra caused byresonance Raman scattering due to the band gap approachingthe excitation laser line. The UV emission band at 3.37 eVcan be explained by a near band-edge transition of wideband gap ZnO NPs, the free exciton recombination throughan exciton–exciton collision process. It has been theoreti-cally proposed that the position of the Zn interstitials (Zni) inthe ZnO band structure is about 0.22 eV below the conduc-tion band.9 So a radiative transition from the acceptor statesdue to such Zni to the valence band will result in a blue/vio-let emission in the 400–450 nm range. Energy levels corre-sponding to oxygen interstitial (Oi) and oxygen vacancy (Vo)are located at 2.28 eV and 1.85 below the conduction band.Orange-red emissions centered at 600–760 nm are gener-ally attributed to deep level defects such as vacancies andinterstitials of oxygen. These orange-red emissions areexperimentally observed in oxygen-rich sample suggestingthat Oi may be the dominant player.10 Green emission ismost likely due to oxygen vacancies.10 In oxygen deficientsamples, both oxygen vacancies and zinc interstitials mightexist as potential defects. The oxygen vacancies can formtwo types of defect states: a shallow donor level and a deepdonor level. While the shallow donor level is very close tothe conduction band, the deep donor level is 0.7–0.8 eVbelow the conduction band.11 The energy difference betweenthe deep donor level and the top of the valence band is about2.43–2.53 eV, and this is consistent with the green emissionenergy. Zinc interstitials also can be formed in zinc-richsamples, which can act as electron donors. Both oxygenvacancies and zinc interstitials have low formation energies,but oxygen vacancies are expected to be the dominantdefects in nanoparticles due to their large surface to volume.According to the existing models,12 surface sites are pro-posed to be responsible for all non-excitonic relaxation proc-esses in NPs. There are two potential relaxation processesproposed in these models. In one case, a hole from the va-lence band can be trapped by surface O2 sites, thus produc-ing O sites. A green emission can occur if the hole tunnelsto the singly ionized oxygen vacancy in the neighborhood.Another possibility is that an electron from the conductionband gets trapped on a surface O to become O2. A greenemission can occur if this O2 traps a hole.Thus, the as-prepared ZnO-II NP with large green emis-sion might have large number of oxygen vacancies.Annealing it in Argon atmosphere at 450 C for 1 h producesa new blue/violet emission centered at 416 nm. It seems thatthe blue/violet emission and the exciton emission increasedin intensity at the expense of green emission (Figures 1(c)and 1(d)). It appears that the annealing process in Ar atmos-phere produced a Zn-rich sample with large number of Zni togenerate the blue/violet emission. Increased number of car-riers or larger crystallite size due to sintering might haveenhanced exciton emission also. Further increase in theannealing time to 2 h reduced the green emission while theintensity of the exciton emission increased significantly. Theblue/violet emission intensity also somewhat decreased.Interestingly, the ZnO-I NPs prepared using DEG did notshow any of the visible emissions and no significant changein the intensity of the exciton emission was observed. TheFTIR data clearly showed that DEG molecules are presenton the NP surface. Note that O2 surface trap in ZnO nano-crystals are actually OH- which is also seen in the FTIRdata. Capping ZnO-I nanocrystals with organic moleculessuch as DEG might have removed the surface hole traps.Absence of both green and blue/violet emissions in this sam-ples indicates that both these emissions are related to surfacestructure/defects.Magnetization (M) of the NPs as a function of appliedmagnetic field (H) was measured over the range of 61 T. Adiamagnetic susceptibility, v¼7.438 108 emu/Oe, fromthe straw was subtracted from all data. All samples showed aweak ferromagnetic component with open hysteresis loops atroom temperature, superimposed on an overall paramagneticsignal. The saturation magnetization (Ms) of the as-preparedZnO-II had a slightly higher Ms of 0.088 6 0.013 memu/gcompared to 0.077 6 0.002 memu/g. Interestingly, the Msvalues follow a clear trend in both samples as shown inFigure 2(a). Ms gradually increases with annealing time,reaching a maximum after 75min of annealing, followed bya decrease for 120min.The binding energy (BE) of Zn 2p3/2 peak of theas-prepared and annealed samples is centered around1021.2–1021.5 eV, with a spin-orbit splitting (DS) of23.1 eV, as expected for Zn2þ ions. The O 1s spectra showedbroad asymmetric features, which were fitted by two compo-nents, indicating the presence of two different sources of ox-ygen ions with different chemical environments in bothas-prepared as well as annealed ZnO samples. The relativelystronger component on the lower binding energy side, in therange of 530.2–529.9 eV, can be attributed to O2 ions onthe wurtzite structure surrounded by Zn2þ atoms in tetrahe-dral sites, located mainly in the core region of the NPs. Asannealing time increased from 0 to 120min, the BE showeda systematic increase from 529.69 to 529.79 eV. This mightbe the result of increasing formation of oxygen vacancies orZn interstitials. The second O 1s peak was observed at531.1–531.6 eV is ascribed to oxygen ions on the surfaceregion of the NP, probably related to -OH groupsFIG. 1. FTIR and PL spectra of annealed (60 and 120min) and as-preparedZnO-I [(a),(c)] and ZnO-II [(b),(d)].17D727-2 Rainey et al. J. Appl. Phys. 115, 17D727 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:132.178.2.65 On: Thu, 24 Apr 2014 16:48:09chemisorbed on the surface, as observed in FTIR data also,indicating the presence of loosely bound oxygen near thesurface region of ZnO nanoparticles. Changes in this compo-nent may be related to changes in the concentration of oxy-gen vacancies and/or oxygen interstitials on the surface ofZnO NPs. As annealing time increased from 0 to 120min,area of the larger peak (1021 eV) increased from 62.2% to72% while that of the smaller peak (1044 eV) decreasedfrom 37.8% to 28%. Furthermore, with increasing annealingtime, the smaller peak showed a systematic shift from 531.13to 531.45 eV. These gradual changes in intensity and peakposition suggest that annealing in Ar leads to more oxygenvacancies in the surface region of the NPs, while at the sametime, in the core of the nanocrystals, this process might havefilled any oxygen vacancies and possibly introduced some ox-ygen interstitials too. Figure 2(b) shows a plot of the relativeXPS intensity of Zn 2p and the main O 1s peaks obtainedfrom ZnO-II NPs plotted as a function of annealing time. Alsoplotted in this figure are Ms values measured at 300K. Theobserved similarity between the behavior of Ms and the rela-tive oxygen stoichiometry indicates that oxygen vacanciesmight be playing a role in the observed RTFM. In the begin-ning, increasing temperature breaks down surface attached ac-etate groups and hydroxyl groups, making more oxygen ionsavailable to the crystals and this might have caused theobserved difference between Ms and Zn:O ratio initially.FTIR data indicate that this process continues throughout theduration of annealing, although the rate of dissociation ofthese groups decreases significantly with increasing time.While this process adds oxygen ions, another simultaneousprocess taking place during this Ar annealing is the formationof oxygen vacancies and Zni, as evidenced from PL data.These competing processes may be responsible for the maxi-mum in Zn:O ratio and Ms, shown in Fig. 2(b).What is responsible for the observed RTFM in undopedZnO? Weak RTFM is observed in the as-prepared ZnO NPs.As shown in Fig. 2, Ms increased significantly after 1 h ofannealing in Ar and then it decreases when annealed for 2 h.The same trend is observed in both ZnO-I and ZnO-II sam-ples. Role of oxygen vacancies in the magnetic behavior ofZnO NPs is indicated from the XPS data (Figure 2(b)).Absence of capping molecules in ZnO-II allows it to emitgreen presumably due to the presence of large number of ox-ygen vacancies. However, its intensity decreases withincreasing Ar annealing time and a blue/violet emissionstarts appearing, suggesting significant changes in the defectcomposition and the formation of more Zni. The observedchanges in the intensity of the blue/violet emission correlatewith the observed variation of Ms suggesting that Zni mayalso be a key player in the observed magnetic properties ofZnO NPs. It seems that the blue emission grows at theexpense of green emission, indicating that the processes thatcreate Zni and Vo may be related.This work was supported in part by the National ScienceFoundation grants EAGER DMR-1137419, CBET 1134468,DMR 1006136, and DMR 1006136. We thank D. A.Hillsberry and Dr. K. M. Reddy for help with this article.1M. A. Garcia et al., Nano Lett. 7, 1489 (2007).2A. Sundaresan et al., Phys. Rev. B 74, 161306(R) (2006).3N. S. Norberg and D. R. Gamelin, J. Phys. Chem. B 109, 20810–20816(2005).4L. M. Johnson et al., Phys. Rev. B 82, 054419 (2010).5I. A. Farbun et al. Russ. J. Appl. Chem. 80, 1798–1803 (2007).6S.-W. Bian, I. A. Mudunkotuwa, T. Rupasinghe, and V. H. Grassian,Langmuir 27, 6059–6068 (2011).7J. J. Max and C. Chapados, J. Phys. Chem. A 108, 3324–3337 (2004).8I. A. Mudunkotuwa et al., Langmuir 28, 396–403 (2012).9V. Kumar et al., Mater. Lett. 101, 57 (2013).10L. Wu et al., Opt. Mater. 28, 418 (2006).11Q. P. Wang et al., Opt. Mater. 26, 23 (2004).12A. v. Dijken et al., J. Phys. Chem. B 104, 1715 (2000).FIG. 2. (a) Trend in Ms of annealed ZnO-I (black squares; left y-axis) andannealed ZnO-II (red circles; right y-axis) as a function of annealing time.(b) Ratio of Zn 2p area to O 1s and Ms trend of ZnO-II with annealing time.17D727-3 Rainey et al. J. Appl. Phys. 115, 17D727 (2014) [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:132.178.2.65 On: Thu, 24 Apr 2014 16:48:09

Defect Induced Ferromagnetism in Undoped ZnO Nanoparticles

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