Magnetic Properties of Ni-doped ZnO Nanocombs by CVD Approach

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1 Nnoscle Res Lett (2010) 5: DOI /s NANO EXPRESS Mgnetic Properties of Ni-doped ZnO Nnocoms y CVD Approch Zhou ShoMin Yun HongLei Liu LiSheng Chen XiLing Lou ShiYun Ho YoMing Yun RuiJin Li Ning Received: 15 April 2010 / Accepted: 5 My 2010 / Pulished online: 18 My 2010 The Author(s) This rticle is pulished with open ccess t Springerlink.com Astrct The serch for ove room temperture ferromgnetism in dilute mgnetic semiconductors hs een intense in recent yer. Arrys of perpendiculr ferromgnetic nnowire/rods hve recently ttrcted considerle interest for their potentil use in mny res of dvnced nnotechnology. We report simple low-temperture chemicl vpor deposition (CVD) to crete self-ssemled com-like Ni-/undoped ZnO nnostructure rrys. The phses, compositions, nd physicl properties of the studied smples were nlyzed y different techniques, including high-resolution X-ry diffrction/photoelectron spectroscopy/trnsmission electron microscopy, photoluminescence, nd MPMS. In prticulr, the Ni-doped ZnO nnocoms (NCs) with ferromgnetic nd superprmgnetic properties hve een oserved wheres undoped ZnO NCs dispper. The corresponding ferromgnetic source mechnism is discussed, in which defects such s O vcncies would ply n importnt role. Keywords Mgnetism II VI semiconductors Nnostructures Zinc oxide Introduction An rry of hierrchicl ssemly of nnoscle uilding locks such s nnocom/cntilevers (NCs) is crucil Z. ShoMin (&) Y. HongLei L. LiSheng C. XiLing L. ShiYun H. YoMing Y. RuiJin L. Ning Key L for Specil Functionl Mterils of Ministry of Eduction, Henn University, Kifeng, People s Repulic of Chin e-mil: smzhou@henu.edu.cn step towrd reliztion of functionl nnosystems nd represents significnt chllenge in the field of nnoscle science [1 5]. Dilute mgnetic semiconductors (DMSs) re ttrcting intense interest for potentil new device pplictions in spin-sed informtion-processing technologies [6 22]. The development of prcticl semiconductor spintronic devices will require new DMSs with Curie temperture ove room temperture [6 21]. Therefore, DMSs of ove room temperture ferromgnetism (ARTF) doping NCs provide very good opportunity to integrte new functionlity into the existing semiconductor nnodevices [6]. Ni is n excellent resistnce to corrosion, ferromgnetic, nd resonle conductor of het nd electricity. In ddition, ion rdius of Ni (*0.69 Å) is smller thn tht of Zn (*0.74 Å) nd the ZnO compound cn e esily sustituted y Ni 2?. Motivted y this pproch, possily, Ni ions sustitute prt of Zn 2? in ZnO, which turn ZnO nnoprticles into Ni-doped ZnO NCs sed on vpor chemicl rection. Recently, room temperture mgnetic properties of Ni-doped ZnO simple nnostructures such s nnoprticles nd nnowire/rods hve widely een reported [11 20]. To dte, however, ARTF Ni-doped ZnO hierrchicl nnostructures such s NCs hve not een limited though ferromgnetism of Co-doped ZnO nnorrys hs een reported y our reserch group [6]. Previously, we hve studied on the friction of Ni-doped ZnO NCs nd investigted their opticl properties [3]. In this pper, we systemticlly reserched on their mgnetic nd opticl properties, which revel tht the s-studied Ni-doped ZnO NC smples exhiit ARTF ehviors with sturtion mgnetiztion of 0.62 emu/g nd coercivity vlue up to 88 Oe, s well s, the ferromgnetic ordering in Ni-doped ZnO NCs is very sensitive to the nneling temperture.

Nnoscle Res Lett (2010) 5:1284 1288 1285 Experimentl Section Smples of un/ni-doped ZnO NCs were synthesized on Si sustrtes in horizontl tue furnce system, which is similr to our erlier reports [2 4,

2 Nnoscle Res Lett (2010) 5: Experimentl Section Smples of un/ni-doped ZnO NCs were synthesized on Si sustrtes in horizontl tue furnce system, which is similr to our erlier reports [2 4, 6]. In typicl process, solution of nickel nitrte (nominl rte: 0, 0.1, or 0.2 M) nd ethnol ws dropped onto the Si sustrte. After drying the Si sustrte t 373 K in mient ir (to form uniform Ni(NO 3 ) 2 film), some prticles were evenly formed on the Si sustrte. Then, the sustrte ws put on the top of qurtz ot loded with commercil zinc powders (5.0 g, %) nd inserted into qurtz tue furnce. The zinc source ws physiclly vporized to chemiclly synthesize the Zn 1-x Ni x O(x = 0, 0.1, 0.2) smples in tmospheric pressure under N 2 (99.999%) flow rte of 50 sccm for 120 min t the temperture of 723 K. After the rection, the sustrte surfce ppered s lyer of wx-like mteril. The morphologies nd microstructure/compositions of these s-fricted specimens were investigted y scnning/ trnsmission electron microscopy (SEM/TEM, XL 30-S- FEG/JEOL 2010), selected re electron diffrction (SAED), the energy dispersive X-ry spectroscopy (EDS), nd high-resolution (HR) TEM (HRTEM). The ulk crystl structures nd chemicl constitutions were detected y HR X-ry diffrction/photoelectron spectroscopy [HRXRD/ HRXPS (X pert MRD-Philips diffrctometer with Cu K rdition, k = nm)/(mkii XPS, Mg K)], nd inductively coupled plsm spectrometer (ICP-96B), respectively. Room temperture photoluminescence (RTPL) mesurements were crried out y fluorescence spectrophotometer (SPEX F212) with s the excittion light source (330 nm). Mgnetic properties were crried out y using Quntum Design Superconductor Quntum Interference Device (SQUID) (MPMS XL5) t tempertures from 5 to 400 K with the pplied mgnetic field perpendiculr or prllel (500 Oe, in-plne or out-plne) to the studied ulk specimens. Results nd Discussion A typicl SEM imge of the Zn 0.8 O smple is shown in Fig. 1, which revels tht the s-otined products re composed of lrge quntities of high-qulity com-like nnostructures with the typicl length up to severl ten microns. In fct, the morphologies nd sizes of the others (Zn 0.9 Ni 0.1 O nd ZnO specimens) re similr to those of 20%-Ni-doped ZnO NCs (not shown). A representtive wide-scn (20 80 ) XRD pttern (not reveled) contins HRXRD spectr to indicte hexgonl wurtzite-structured ZnO. Figure 1 shows three explored HRXRD ptterns, tken from the (002) spcing of Zn 0.8 O, Zn 0.9 Ni 0.1 O, nd ZnO NCs, respectively. Both of the (002) (002) Zn 0.8 O, Zn 0.9 Ni 0.1 O, ZnO thet Fig. 1 A typicl low mplifiction SEM imge () ofzn 0.8 O NCs; Three HRXRD ptterns (002) () from Zn 0.8 O NCs, Zn 0.9 Ni 0.1 O NCs, nd ZnO NCs diffrction peks of Ni-doped NCs hve slight shift to low two thet ngle compred with undoped NCs. Unlike our erlier report [3], Zn 0.8 O NCs formed in the work shift the pek position to low ngle reltive to the undoped ZnO (out 0.21 ). The reson my e different doping types from dissimilr conditions, including the low temperture chemicl rection nd gs glow, s well s lot of distortions in the host ZnO lttices. A representtive HRSEM imge of the Zn 0.8 Ois shown in Fig. 2. It cn e seen tht ll nnorod rnches hve uniform dimeters nd re evenly distriuted t only one side of the stem with periodicity of nm. In ddition, dimeter nd length of the rnched chins re out nm nd *1 lm, respectively. As shown Fig. 2, corresponding low enlrgement TEM imge of Zn 0.8 O is demonstrted. It is more ovious tht the products hve rry morphology, uniform dimeter, nd periodic structures, which re excellent in greement on these results tken from SEM imges. These hierrchiclly ordered nnowire/rod rrys re monolithiclly single crystlline s evidenced y the SAED pttern nd HRTEM s exposed in two inset imges of Fig. 2. The SAED model (right inset) shows tht the s-synthesized smple is indexed to hexgonl wurtzite phse nd the com ckone grows long [2 1 10], its top/ottom surfces re

1286 Nnoscle Res Lett (2010) 5:1284 1288 853.7 ev(ni,2p3/2) 890 885 880 875 870 865 860 855 850 BE(eV) 1021.9 ev (Zn 2p3/2) Fig. 2 HRSEM imge () nd TEM imge () for Zn 0.

3 1286 Nnoscle Res Lett (2010) 5: ev(ni,2p3/2) BE(eV) ev (Zn 2p3/2) Fig. 2 HRSEM imge () nd TEM imge () for Zn 0.8 O(left inset: SAED, right one: HRTEM) BE(eV) (01 10), nd the nnornches grow long [0001] direction. HRTEM ws pplied to imge microstructures of single rnch s shown in the left inset of Fig. 2. In the HRTEM imge, perfect nd continuous lttice fringes disclose tht Ni 2? ions were vergely dulterted into the crystl lttice of ZnO nd formed single crystl structure with hexgonl phse, which is comptile with results of XRD nd SAED. Bsed on clcultions of HRTEM, SAED, nd XRD, the lttice spcing (0001) of Ni-doped NCs (0.58 nm) is little lrger thn tht of undoped ZnO smple (0.52 nm) [1], which is induced y doped Ni 2? ion into Zn 2? site. ICP nlyses were crried out to otin the contents of Zn, O, nd Ni in the ulk smples. Dt (not shown) confirm 3 M rtios of three kinds of smples re 80.4:19.6:98.6 (Zn 0.8 O), 90.1:9.9:98.8 (Zn 0.9 Ni 0.1 O), nd 50.1:49.9 (ZnO), respectively, which re lmost equl to their nominl compositions. In ddition, HRXPS spectr of the ulk NCs (Zn 0.8 O) were shown in Fig. 3,. Bsed on peks of Zn ( ev, 2p3/2) nd Ni 2p3/2 (853.7 ev, 2p3/2), nd their integrl res, the products must contin smll mount (up to *20%) Ni element. For single Zn 0.8 O NCs, EDS ws used to confirm its microcomposition s shown in Fig. 3c. It cn e clerly seen tht every NC hs lmost the sme composition nd contins smll mount of Ni. Further quntittive nlysis finds tht the tomic rtio etween Zn nd Ni is out 80.4:19.6, which is comptile with the dt of IPC nd HRXPS. c O Ni,Cu,Zn Zn Ni Cu Zn ZnNiO or ZnO could e simply formed y chemicl rection; however, the formtion of NCs would involve very complicted process. Becuse of the fct tht the NCs cn only e formed t lower temperture region, we my speculte the NC formtion procedure s follows: (1) the nucletion nd growth long [2 1 10] forms the com-stems; (2) slower growth long [0001] direction y fceted epitxil growth process cretes the rnched chin. Possily, it is relted to the supersturted environments, nd gret del of Zn vpor exists in the rection chmer, which grows ZnNiO or ZnO nucletion with O 2. Then, nucletion occurs s sites of ZnNiO or ZnO nnoprticles. Continuous feeding of n evported [(Zn? little Ni 2+ )or Ni Energy(KeV) Fig. 3 HRXPS spectr [Ni 2p (); Zn 2p ()] nd EDS pttern (c) of Zn 0.8 O NCs Cu

4 Nnoscle Res Lett (2010) 5: Zn] source nd elementl oxygen would led to form wire-/ rod-like rry (hierrchicl) nnostructure (minly from eutectoid of ZnNiO or ZnO). The more detiled growth mechnism ws reported in our erlier ppers [2 4, 6]. RTPL mesurements were crried out y using the Xe lmp with n excittion wvelength of 330 nm. After nneling t different tempertures [873 K (lue, nmed for smple 1), 973 K (red, smple 2), 1,073 K (lck, smple 3)] in Ar gs, RTPL spectr from ulk quntity of Zn 0.8 O NCs re shown in Fig. 4. There re two ovious PL regions for the three different nneling tempertures. It is found tht the strongest ultrviolet (UV) emission pek centered t out 3.27 ev (*379 nm) nd wekest rod green one nd centered t *2.46 ev (*504 nm) re from smple 1. Further nneled, however, the green emission peks hppen into mgnifiction intensity wheres counterprts of UV emissions re decresing. For exmple, the green pek of smple 3 is the highest while the UV emission chnges into the weker. The UV pek of ZnO is generlly ttriuted to recomintion of free excitons, nmely ner-nd-edge trnsition of wide nd gp of ZnO, nd the green emission is thought tht origintes from the recomintion of the holes with the electrons occupying the singly ionized oxygen vcncy [3 6, 11 21]. After smples re nneled t high tempertures, the excess crriers supplied y the impurities to the conduction nd contriute to decrese the electricl conductivity of ZnO. The intensity of the green emission increses strongly, which mens tht the oxygen vcncy concentrtion in Ni-doped ZnO NCs increses fter nneling t high tempertures [12, 15, 16, 18]. It is resonle ecuse oxygen toms re esily evported during the higher temperture, especilly in Ar mient [21]. M-T properties were chrcterized y using quntum design SQUID mgnetometer equipped with 5 T mgnet in the temperture rnge K. Since the NCs re the hierrchicl ligned structures, it is nturl to expect tht the smples should show the perpendiculr nisotropy, i.e., the mgnetiztion prefers to e perpendiculr to the NCs. In order to chrcterize the mcroscopic nisotropy of the smple which is n verge nisotropy of the individul NCs, the mgnetic field, H hs een pplied perpendiculr to nd prllel to the NCs (Si wfer for the sustrte). For our experiment, however, no significnt difference hs een oserved in the two directions. The sturtion mgnetiztion nd coercive field ws found to e the sme for the two directions within the experimentl resolution, which is comptile with our previous report [6]. Figure 4 shows mgnetiztion of smple 3 s function of temperture otined t the zero-field-cooled (ZFC) nd field cooled (FC) processes with smll pplied mgnetic field of 500 Oe. It is evident tht the Zn 0.8 O NCs hve ferromgneticlly with Curie temperture higher thn M (x10 2, emu/g) c M(x10, emu/g) Zn 0.8 O ZFC Zn 0.8 O FC H=500 Oe lck: 1073 K red: 973 K lue: 873 K yellow: undop NCs green: ZnO CPs Energy(eV) T (K) H(Oe) lck:1073 K red: 973 K lue: 873 K Fig. 4 Three RTPL spectr () of smple 1 (lue), 2 (red), nd 3 (lck); The mgnetiztion s function of temperture () for FC nd ZFC [tken from smple 3]; RT M-H (c) of smple 1, 2, nd 3, s well s undoped NCs nd CPs nneled only t 1,073 K 400 K due to the cler seprtion etween the FC nd ZFC. The ZFC FC mgnetiztion curves clerly indicte tht the smple is quite thermlly stle without locking (or superprmgnetic ehvior), which is similr to erlier reports on Ni-doped nnostructures [11 20]. In ddition, Fig. 4c is presented for RT M-H properties of smple (1, 2, nd 3), s well s oth of undoped NCs nd conventionl

5 1288 Nnoscle Res Lett (2010) 5: powders (CPs) (oth of them were nneled only t 1,073 K). It cn e clerly seen tht the highest temperture nneling Zn 0.8 O NCs hve sturtion mgnetiztion (M s, * 0.62 emu/g) nd coercivity vlue (H c ) mounts to out 88 Oe, which re more thn Zn 0.95 Ni 0.05 O nnorods (M s : 0.4 emu/g, H c : 72 Oe) [17]. As for smple 2, however, the studied Zn 0.8 O NCs exhiit RT superprmgnetic ehvior, in which no coercivity or remnence is oserved. However, oth of the undoped ZnO NCs nd smple 3 re prmgnetic properties wheres CPs hve dimgnetic ehvior, which is prtly comptile with previous report [22]. As to the origin of ferromgnetic ehvior oserved in Ni-doped ZnO nnostructures, there re few of controversil explntions, one of which is the formtion of some nnoscle Ni-relted secondry phse, such s metllic Ni precipittion nd NiO. In fct, however, NiO phse cn e esily ruled out, since ulk NiO is ntiferromgnetism with Neel temperture of 520 K [17]. Secondly, sed on our experimentl conditions, metllic Ni is lso n unlikely source of this ferromgnetism ecuse the synthesis of Ni-doped ZnO NCs is performed in ir which cn prevent the formtion of metllic Ni nnoclusters to some extent. In ddition, XRD nd HRTEM results clerly show no metllic Ni clusters in the NCs. According to the opticl nd mgnetic dt, one cn see tht it is pprent tht the ferromgnetic properties of Zn 0.8 O NCs hve strong correltion with oxygen vcncies, which is comptile with erlier litertures [6, 11 21]. Of course, convincing explntion on the reltion of ARTF is our further work. Conclusion To summrize, ARTF nd RT superprmgnetic ehviors of Ni-doped ZnO NCs hve firstly een oserved fter het treting. The present study not only demonstrtes Ni-doped ZnO NCs emit green light ut lso suggests tht introducing oxygen vcncies is n effective wy to oost the ARTF. In ddition, Ni-doped ZnO NCs hve een fricted y using simple chemicl vpor-deposition method, in which the process is friendly environment. Such doping hierrchicl nnostructures with crefully tilored composition my find potentil pplictions in high-density dt storge nd other mgnetic/semiconducting-device pplictions. Acknowledgments This work ws prtilly supported y the Progrm for Science & Technology Innovtion Tlents in Universities of Henn Province (No HASTIT002), Innovtion Scientists nd Technicins Troop Construction Projects of Henn Province (No ), nd y the Nturl Science Foundtion of Chin under Grnt No Open Access This rticle is distriuted under the terms of the Cretive Commons Attriution Noncommercil License which permits ny noncommercil use, distriution, nd reproduction in ny medium, provided the originl uthor(s) nd source re credited. References 1. Z. Wng, X. Kong, J. Zuo, Phys. Rev. Lett. 91, (2003) 2. S. Zhou, X. Meng, X. Zhng, X. Fn, K. Zou, S. Wu, S. Lee, Micron 36, 55 (2005) 3. B. Zhng, X. Zhng, H. Gong, Z. Wu, S. Zhou, Z. Du, Phys. Lett. A 372, 2300 (2008) 4. B. Zhng, S. Zhou, H. Wng, Z. Du, Chinese Sci. Bull. 53, 1639 (2008) 5. Q. Ahsnulhq, J. Kim, J.H. Kim, Y. Hhn, Nnosc. Res. Lett. 5, 669 (2010) 6. S. Zhou, P. Wng, S. Li, B. Zhng, H. Gong, L. Du, Chinese Phys. Lett. 25, 4446 (2008) 7. P. Shrm, A. Gupt, K. Ro, F. Owens, R. Shrm, R. Ahuj, J. Guillen, B. Johnsson, G. Gehring, Nt. Mter. 2, 673 (2003) 8. J. Coey, M. Venktesn, C. Fitzgerld, Nt. Mter. 4, 173 (2005) 9. F. Owens, Nnosc. Res. Lett. 2, 447 (2007) 10. R. Snz, J. Jensen, G. Gonzlez, O. Mrtinez, M. Vzquez, M. Hernndez-Velez, Nnosc. Res. Lett. 4, 878 (2009) 11. C. Cong, J. Hong, Q. Liu, L. Lio, K. Zhng, Solid Stte Commun. 138, 511 (2006) 12. H. Wng, Y. Chen, H. Wng, C. Zhng, F. Yng, J. Dun, C. Yng, Y. Xu, M. Zhou, Q. Li, Appl. Phys. Lett. 90, (2007) 13. G. Hung, J. Wng, X. Zhong, G. Zhou, H. Yn, J. Mter. Sci. 42, 6464 (2007) 14. M. El-Hilo, A. Dkhel, A. Ali-Mohmed, J. Mgn. Mgn. Mter. 321, 2279 (2009) 15. J. Cui, U. Gison, Appl. Phys. Lett. 87, (2005) 16. J. Iql, B. Wng, X. Liu, D. Yu, B. He, R. Yu, New J. Phys. 11, (2009) 17. C. Cheng, G. Xu, H. Zhng, Y. Luo, Mter. Lett. 62, 1617 (2008) 18. X. Hung, G. Li, B. Co, M. Wng, C. Ho, J. Phys. Chem. C 113, 4381 (2009) 19. H. Shi, Y. Dun, Nnosc. Res. Lett. 4, 480 (2009) 20. G. Xing, D. Wng, J. Yi, L. Yng, M. Go, M. He, J. Yng, J. Ding, T. Sum, T. Wu, Appl. Phys. Lett. 96, (2010) 21. H. Hsu, J. Hung, Y. Hung, Y. Lio, M. Lin, C. Lee, J. Lee, S. Chen, L. Li, C. Liu, Appl. Phys. Lett. 88, (2006) 22. Q. Xu, S. Zhou, H. Schmidt, J. Alloys Compd. 487, 665 (2009)

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