Magnetophotoluminescence properties of Co-doped ZnO nanorods

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Magnetophotoluminescence properties of Co-doped ZnO nanorods

C. Y. Lin, W. H. Wang, C.-S. Lee, K. W. Sun, and Y. W. Suen

Citation: Applied Physics Letters 94, 151909 (2009); doi: 10.1063/1.3117203 View online: http://dx.doi.org/10.1063/1.3117203

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/15?ver=pdfcov Published by the AIP Publishing

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Magnetophotoluminescence properties of Co-doped ZnO nanorods

C. Y. Lin,1W. H. Wang,1C.-S. Lee,1K. W. Sun,1,a兲 and Y. W. Suen2

1_{Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan} 2_{Institute of NanoScience, National Chung Hsing University, Taichung 402, Taiwan}

共Received 26 February 2009; accepted 12 March 2009; published online 15 April 2009兲

We present the detailed experimental results of the magnetic and optical properties of cobalt doped ZnO nanorods, especially the temperature and magnetic field dependence of photoluminescence up to 14 T. The Raman measurements indicate that our Co-doped ZnO nanorods have the same lattice constant as crystalline bulk ZnO. Sharp luminescence peaks centered at around 670 nm were observed at low temperature and their intensity decreased with increasing magnetic field. The luminescence peaks were attributed to d-d transitions in the Ligand field from the doped Co ions. We also observed a diamagnetic shift at a temperature of 1.5 K when the magnetic field was scanned from 0 to 14 T. The exciton radius of the Co-doped ZnO nanorods was deduced from the magnetophotoluminescence results. © 2009 American Institute of Physics. 关DOI:10.1063/1.3117203兴

Diluted magnetic semiconductors 共DMSs兲, in which a fraction of nonmagnetic elements are substituted by mag-netic transition metal ions, are ideal candidates for spin-transport electronics. Wide band gap zinc oxide DMSs have been extensively studied due to their possible applications in spintronics and UV devices.1,2ZnO has a high solubility for transition metal ions3,4 when compared to its other III-V compound semiconductor counterparts. This makes the ox-ides ideal candidates for fabricating DMSs. The existence of room temperature ferromagnetism in transition metal doped GaN and ZnO thin films was predicted by Dietl et al.5 and Sato and Katayama-Yoshida6and was demonstrated experi-mentally in Co-doped ZnO thin films by Lin et al.7 The ferromagnetism measured at various temperatures was re-ported in Co-, Ni-, Mn-, and Fe-doped ZnO thin films,8–12as well as nanostructures,13–16 in the past few years. Recently, anisotropic ferromagnetism dependent on nanowire geom-etry and density at room temperature was reported by Cui

et al.17 in Co- and Ni-doped ZnO nanowires. More recently,

photoluminescence 共PL兲, electroluminescence 共EL兲, cathod-oluminescence 共CL兲, and magnetic properties were investi-gated in undoped and Mn-doped ZnO nanowires and nanorods.18–20 PL, EL, and CL emitted in the visible region were observed and were attributed to the ionized oxygen vacancies.

In this paper, we apply optical spectroscopy techniques in a low temperature and high magnetic field dilution refrig-erator to investigate the optical properties of hydrothermally prepared Co-doped ZnO nanorods. We report Raman mea-surements, as well as temperature and magnetic field depen-dence of PL peaks observed at the visible regions. The lumi-nescence peaks exhibited a weak diamagnetic shift due to the increase in wave function separation between the electron and hole pairs as the magnetic field intensity was increased. The Co-doped ZnO nanorod samples studied in this work were prepared by the hydrothermal method. Zinc ni-trate hydrate关Zn共NO₃兲₂, 0.06 g兴 and hexamethyleneteramine 共0.028 g兲 were first added to 40 mL of de-ionized 共DI兲 water

to form a 5 mM clear solution. Cobalt nitrate with different weight percentages from 0% to 200 %, with respect to zinc nitrate hydrate, was dissolved into the above solution. The mixture was heated in a Teflon-coated stainless steel auto-clave at 95 ° C for 2 h. After cooling to room temperature, the solid product was put into a centrifuge tube and was repeatedly cleaned with DI water to wash away the remain-ing Co ions for five to ten times, dependremain-ing on the amount of cobalt nitrate added. The solid was dried at 70 ° C for 5 h to obtain the ZnO nanorod powder. Doping concentration var-ied by adjusting the weight percentage of the cobalt nitrate. The color of the ZnO nanorod powder turned green as the doping became heavier. The composition of the solid was found by powder x-ray diffraction 共XRD兲 to be ZnO. The doping concentration of Co was analyzed using the induc-tively coupled plasma mass spectrometry.

The XRD patterns of the ZnO samples with different doping concentrations and the scanning electron microscopy 共SEM兲 image are given in Ref. 21. The strong peaks in the data indicate that well aligned ZnO nanorods with wurtzite structures were obtained, and the crystal structure was not altered by the doping. The nanorods were uniform in size, with a diameter of about 200 nm. However, the length of the rod varied from 6 to 1 ␮m as the doping concentration was increased from 0 % to 90 %. Figure 1shows the hysteresis behavior of 90% Co-doped ZnO 共Zn0.998Co0.002O兲 nanorods at room temperature. The sample exhibited weak ferromag-netic behavior with remanence permanent magnet of the or-der of 10−3 _emu_/g.

For the optical measurements under low temperature and high magnetic field, we first put the 90% Co-doped 共Zn0.998Co0.002O兲 ZnO nanorod powder on a 1⫻1 cm2 sili-con wafer. A 0.2 mm thick cover glass was placed on top and was sealed onto the silicon wafer using putty and silicone. The sample was stuck to a sample holder with N-grease at the bottom of an insert equipped with a fiber probe. The insert was put into a dilution refrigerator and cooled down to 1.5 K. We performed the Raman and PL spectroscopy mea-surements through the fiber for above samples at excitation wavelengths of UV and 488 nm. In Fig.2it is shown that we compared the Raman signal from the Co-doped nanorods a兲_{Author to whom correspondence should be addressed. Electronic mail:}

[email protected].

APPLIED PHYSICS LETTERS 94, 151909共2009兲

0003-6951/2009/94_{共15兲/151909/3/$25.00} 94, 151909-1 © 2009 American Institute of Physics 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:

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with pure ZnO nanorods. The spectra agree well with earlier reports.22–25 It also indicates that our samples have the same lattice constant as crystalline bulk ZnO.

The Co-doped nanorods showed a PL peak near 380 nm due to the near-band-edge emission.26 We also found two weak bands, centered around 500 and 670 nm, at an excita-tion wavelength of 266 nm. The green luminescence is gen-erally attributed to the single ionized oxygen vacancy in the ZnO.27 However, the emission band at around 670 nm de-serves greater attention.

Figures3shows the temperature dependence of PL spec-tra excited at 405 nm from T = 10 to T = 300 K. Sharp peaks were revealed at 659, 665, 671, 679, 685.5, and 692 nm at low temperature, respectively. We believe that the light ab-sorption is due to the isolated Co2+ _{ions in the ZnO lattice} and the mechanisms are the competing superpositions of 4_A

2共F兲 to2E共G兲, and4A2共F兲 to4T1共P兲 and4A2共F兲 to2T1共G兲 energy levels.28,29The observed emissions resulted from the transitions of the4T1共P兲,2T1共G兲,2E共G兲 levels to the4A2共F兲

ground state of the isolated Co2+ _{ions on zinc sites.}30

Sche-matics of the energy transitions are given in Fig. 3.

The magneto-PL spectra of Co-doped ZnO nanorods, as shown in Fig. 4, were recorded at 1.5 K by scanning the magnetic field up to 14 T with steps of 0.2 T at a time. The luminescence intensity decreased with increasing magnetic field, and dropped by about 25% at 14 T. These results sug-gest that the wave functions of the electron hole pairs in the excitons were separated under high magnetic field. However, the mechanism is not clear at this moment and needs further investigation.

Other than the intensity variations, a weak diamagnetic shift in luminescence peaks was also detected. The energy shift in the peak at 664 nm as a function of the applied magnetic field is plotted in the inset of Fig.4as an example. The diamagnetic shift was also clearly observed when the magnetic field was ramped down. The amount of energy shift is related to the magnetic field by the following equation:31 ⌬E=␤共B2_兲=e2_具␳2_典/8mⴱ_共B2_{兲, where}_␤_{is the diamagnetic} co-efficient,␳is the radius of exciton, mⴱis the exciton effective mass, and B is the applied magnetic field. By fitting the curve FIG. 1. 共Color online兲 Room temperature magnetic hysteresis loops 共M-H

curve兲 of Co-doped ZnO nanorods.

FIG. 2. 共Color online兲 Raman spectra of the Co-doped ZnO nanorods and pure ZnO nanorods.

FIG. 3. 共Color online兲 Temperature dependence of the PL spectra centered at 680 nm from T = 10 K to T = 300 K. The inset shows the schematics of energy transitions.

FIG. 4. 共Color online兲 Magneto-PL spectra of the Co-doped ZnO nanorods recorded at 1.5 K. Diamagnetic shift in the PL peak of 664 nm is shown in the inset.

151909-2 Lin et al. Appl. Phys. Lett. 94, 151909共2009兲

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in the inset of Fig. 4, the diamagnetic coefficient ␤ was found to be 3.37 ␮eV/T2. From the above equation, we obtained an exciton radius of 4.899 nm. It is in good agree-ment with the size of the exciton 共3.38 nm兲 obtained with the equation ␳=共␧r兲m₀/m_ⴱ共aB兲, where ␧r= 10 in ZnO nanostructures.32The weak diamagnetic shift observed in the experiments can be due to the smaller exciton radius in this particular material.

To conclude, we have measured the Raman, PL, and magneto-PL of hydrothermally prepared Co-doped ZnO na-norods in a dilution refrigerator with high magnetic field. The PL peaks at around 670 nm were attributed to the tran-sitions between energy levels of doped Co ions. PL intensity was also found to decrease with an increasing magnetic field. The diamagnetic shift in the luminescence peaks was found at a low temperature and allowed us to extract the exciton radius in Co-doped nanorods.

This work was supported by the National Science Coun-cil of Republic of China under Contract No. NSC 96-2112-M-009-024-MY3 and NSC 96-2120-M-009-004, the MOE ATU program, and National Nano Device Laboratories.

1_{Y. W. Heo, D. P. Norton, L. C. Chen, Y. Kwon, B. S. Kang, F. Ren, S. J.} Pearton, and J. R. LaRoche,Mater. Sci. Eng. R. 47, 1共2004兲.

2_{Z. L. Wang,}_{J. Phys.: Condens. Matter} _{16, R829}_共2004兲.

3_{Z. W. Jin, T. Fukumura, M. Kawasaki, K. Ando, H. Saito, and T.} Sekigu-chi,Appl. Phys. Lett. 78, 3824共2001兲.

4_{C. H. Bates, W. B. White, and R. Roy,}_{J. Inorg. Nucl. Chem.} _{28, 397} 共1966兲.

5_{T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand,}_Science _287, 1019共2000兲.

6_{K. Sato and H. Katayama-Yoshida,}_{Jpn. J. Appl. Phys., Part 2} _{39, L555} 共2000兲; 40, L334 共2001兲.

7_{H. T. Lin, T. S. Chin, J. C. Shih, S. H. Lin, T. M. Hong, R. T. Huang, F. R.} Chen, and J. J. Kai,Appl. Phys. Lett. 85, 621共2004兲.

8_{L. Yan, C. K. Ong, and X. S. Rao,}_{J. Appl. Phys.} _{96, 508}_共2004兲. 9_{K. Rode, A. Anane, R. Mattana, J.-P. Contour, O. Durand, and R.}

LeBourgeois,J. Appl. Phys. 93, 7676共2003兲.

10_{P. Sharma, A. Gupta, K. V. Rao, F. J. Owens, R. Sharma, R. Ahuja, J. M.} Osorio Guillen, B. Johansson, and G. A. Gehring,Nature Mater. 2, 673

共2003兲.

11_{M. Venkatesan, C. B. Fitzgerald, J. G. Lunney, and J. M. D. Coey,}_Phys.

Rev. Lett. 93, 177206共2004兲.

12_{S. J. Han, T. H. Jang, Y. B. Kim, B.-G. Park, J.-H. Park, and Y. H. Jeong,}

Appl. Phys. Lett. 83, 920共2003兲.

13_{D. A. Schwartz, K. R. Kittilstved, and D. R. Gamelin,}_{Appl. Phys. Lett.}

85, 1395共2004兲.

14_{Y. Q. Chang, D. B. Wang, X. H. Luo, X. Y. Xu, X. H. Chen, L. Li, C. P.} Chen, R. M. Wang, J. Xu, and D. P. Yu,Appl. Phys. Lett.83, 4020共2003兲.

15_{J. J. Wu, S. C. Liu, and M. H. Yang,}_{Appl. Phys. Lett.} _{85, 1027}_共2004兲. 16_{K. Ip, R. M. Frazier, Y. W. Heo, D. P. Norton, C. R. Abernathy, S. J.} Pearton, J. Kelly, R. Rairigh, A. F. Hebard, J. M. Zavada, and R. G. Wilson,J. Vac. Sci. Technol. B 21, 1476共2003兲.

17_{J. B. Cui and U. J. Gibson,}_{Appl. Phys. Lett.} _{87, 133108}_共2005兲. 18_{J. Cui, Q. Zeng, and U. Gibson,}_{J. Appl. Phys.} _{99, 08M113}_共2006兲. 19_{C.-Y. Wong, L.-M. Lai, S.-L. Leung, V. A. L. Roy, and E. Y.-B. Pun,}_Appl.

Phys. Lett. 93, 023502共2008兲.

20_{H. L. Yan, X. L. Zhong, J. B. Wang, G. J. Huang, S. L. Ding, G. C. Zhou,} and Y. C. Zhou,Appl. Phys. Lett. 90, 082503共2007兲.

21_{See EPAPS Document No. E-APPLAB-94-024915 for the XRD pattern} and the SEM image of ZnO nanorods. For more information of EPAPS, see http://www.aip.org/pubservs/epaps.html.

22_{R. P. Wang, G. Xu, and P. Jin,}_{Phys. Rev. B} _{69, 113303}_共2004兲. 23_{Z. W. Dong, C. F. Zhang, H. Deng, G. J. You, and S. X. Qian,} _Mater.

Chem. Phys. 99, 160共2006兲.

24_{R. Cusco, J. Jimenez, B. Wang, and M. J. Callahan,}_{Phys. Rev. B} _75, 165202共2007兲.

25_{C.-T. Chien, H.-H. Yang, and Y.-F. Chen,}_{Appl. Phys. Lett.} _{92, 223102} 共2008兲.

26_{P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J.} Pham, R. He, and H.-J. Choi,Adv. Funct. Mater. 12, 323共2002兲.

27_{K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt,}

Appl. Phys. Lett. 68, 403共1996兲.

28_{H.-J. Schulz and M. Thiede,}_{Phys. Rev. B} _{35, 18}_共1987兲. 29_{P. Koidl,}_{Phys. Rev. B} _{15, 2493}_共1977兲.

30_{B. D. Yuhas, D. O. Zitoun, P. J. Pauzauskie, R. He, and P. Yang,}_Angew.

Chem., Int. Ed. 45, 420共2006兲.

31_{S. N. Walck and T. L. Reinecke,}_{Phys. Rev. B} _{57, 9088}_共1998兲. 32_{I. Mora-Sero, F. Fabregat-Santiago, B. Denier, J. Bisquert, R. Tena-Zaera,}

J. Elias, and C. Levy-Clement,Appl. Phys. Lett. 89, 203117共2006兲.

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