3366 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011
Structural Dependence of Photoluminescence and Room-TemperatureFerromagnetism in Lightly Cu-Doped ZnO Nanorods
C. C. Lin , S. L. Young , C. Y. Kung , H. Z. Chen , M. C. Kao , Lance Horng , and Y. T. Shih
Department of Electrical Engineering, National Chung Hsing University, Taichung, Taiwan, ROCDepartment of Electronic Engineering, Hsiuping Institute of Technology, Taichung, Taiwan, ROCDepartment of Physics, National Changhua University of Education, Changhua, Taiwan, ROC
Well-defined 1-D ZnO and ZnO:Cu semiconductor nanostructures have been fabricated by a low temperature solution process. Cu(0.073 nm) is chosen as a dopant due to the similar ionic radius with Zn (0.074 nm). The radius of ZnO:Cu nanorods observed by FE-SEMis lager than that of pure ZnO which indicates the growth rate of the nanorods can be obviously enhanced by the light doping of Cu. TheXRD patterns of both compositions with single diffraction peak (002) show the same wurtzite hexagonal structure. Photoluminescencespectra show a red-shift of the UV emission peak position and a decrease of the luminescence intensity in green-yellow region. From theroom temperature hysteresis loop, ferromagnetism is observed and the saturation magnetization decreases with the increase of the Cuconcentration for ZnO:Cu nanorods.
Index Terms—Nanostructure, photoluminescence, saturation magnetization.
I. INTRODUCTION
W IDE band gap semiconductors have been hot researchsubjects in recent years because of the increasing de-
mand for short wavelength lasers and blue light emitting diodes.Zinc oxide (ZnO) with wide and direct band gap (3.37 eV) isa potential UV emission material. Not only does it have sim-ilar crystal lattice and electrical properties as GaN, but it alsohas higher melting point and larger exciton binding energy (60meV) than GaN (24 meV). In the recent development of nan-otechnology, ZnO nanostructure is offering the brand new pos-sible application in optoelectronics devices [1]–[4]. Nowadays,progressive researches in the synthesis and characterization ofthe ZnO nanotubes, nanowires, and nanorods have been drivenby the need to investigate the novel physical properties. ZnOdoped with selective elements offers an effective method to ad-just the electrical, optical, and magnetic properties which is cru-cial for practical industrial application [5]–[7]. Many theoreticaland experimental studies showed that ZnO doped with transi-tion metal (TM) is a potential diluted magnetic semiconductor(DMS) with a high Curie temperature above room temperature.However, the origin of the ferromagnetism in the ZnO-basedDMS may be affected by the precipitates, clustering or sec-ondary phase of doped magnetic elements and needs further dis-cussion [7]. Furthermore, ZnO-based DMS have attracted broadinterests for their possible use as spintronics materials [8]–[11].ZnO nanostructures have been fabricated by variousmethods,
such as thermal evaporation, metal-organic vapor phase epitaxy,and laser ablation within high vacuum or high temperature envi-ronment. In this paper, we report a simple hydrothermal methodto prepared ZnO and ZnO:Cu nanorods at the growth tempera-ture less than 100 and concentrate on an attempt to study thesubstitution effect of Cu for Zn on the microstructural, optical,and magnetic properties. It is found that the crystallization andthe corresponding optical and magnetic properties are affectedby the doping of Cu in the ZnO nanorods.
Manuscript received February 20, 2011; May 27, 2011 accepted May28, 2011. Date of current version September 23, 2011. Corresponding au-thor: S.-L. Young (e-mail: [email protected]) and C. Y. Kung (e-mail:[email protected]).Digital Object Identifier 10.1109/TMAG.2011.2159299
II. EXPERIMENTAL
The ZnO and ZnO:Cu nanorods were fabricated by hy-drothermal method on the seeded silicon substrate. Detailsof the hydrothermal growth mechanism of the samples hadbeen reported previously [12], [13]. The source solutions forZnO and ZnO:Cu nanorods growth were prepared using theprecursors, pure zinc acetate dehydrate(100%), and zinc acetate dehydrate(99%) + Cupric acetate monohydrate(1%), respectively, within a blending solvent of D.I. water andHMTA . The seeded substrate was placed upsidedown into the solution in a closed vial at 90 for 3 h to growthe nanorods. Finally, the samples were rinsed by deionizedwater and dried in air for characterization.The crystal structure of ZnO and ZnO:Cu nanorods was
determined using a Rigaku Dmax 2200 x-ray diffractometerwith radiation. Morphological characterization wasobserved using a field emission scanning electron microscopy(FE-SEM JEOL JSM-6700F). The chemical analysis of el-emental composition of the nanorods was recorded by thesecondary ion mass spectrometry (SIMS). The photolumines-cence spectroscopy was used to measure optical emissionsfrom 350 to 645 nm using the He-Cd laser with wavelength 325nm. Finally, the magnetization measurements were performedusing a MicroMag2900 Alternative Gradient Magnetometerwith a magnetic moment measurement sensitivity of 10 nemustandard deviation and a magnetic field control accuracy of 2%at room temperature to investigate the magnetic properties ofZnO and ZnO:Cu nanorods.
III. RESULTS AND DISCUSSION
The XRD spectra of ZnO and ZnO:Cu are illustrated inFig. 1 which shows the same wurtzite hexagonal structure forboth compositions and no structural disturbance caused by thesubstitution of Cu for Zn in the nanorods. Besides, no tracesof Cu related secondary phases, such as Cu, , and CuO,were detected within the XRD analysis. However, the peak ofthe ZnO:Cu nanorods slightly shifts toward lower scatteringangle compared with that of pure ZnO due to the differencein ionic radii of ions (0.073 nm) and ions (0.074nm). The higher intensity of (002) oriented peak for Cu-dopedZnO nanorods indicate the enhancement of growth rate and
LIN et al.: STRUCTURAL DEPENDENCE OF PHOTOLUMINESCENCE AND ROOM-TEMPERATURE FERROMAGNETISM 3367
Fig. 1. X-ray diffraction patterns of the pure ZnO and ZnO:Cu nanorods.
Fig. 2. Plane view SEM images of the (a) pure ZnO and (b) ZnO:Cu nanorods.
crystallization due to the doping of Cu. Fig. 2 shows the surfacemorphology of FE-SEM images of both the pure ZnO andZnO:Cu nanorods. Obviously, the radius of ZnO:Cu nanorodsis lager than that of pure ZnO which also indicates the growthrate of these nanorods can be enhanced by the introduction ofCu.Previous report explored that the as-prepared ZnO semi-
conductor exhibits n-type characteristics due to the naturalexistence of oxygen vacancies [14]. Secondary ion mass spec-trometry spectrum (SIMS) can be used to observe the chemicalcomposition and presence of oxygen vacancies by the countsof Zn and O elements. Fig. 3 shows SIMS profiles of Zn, O,and Cu in both ZnO and ZnO:Cu nanorods. The O/Zn ratiodecreases from 41% (ZnO) to 38% (ZnO:Cu) obtained fromthe SIMS spectrum, as shown in Fig. 3. The result reveals theincrease of oxygen vacancies when Zn is slightly substituted byCu. It is found that Cu concentration is approximately 0.5 at.%.The photoluminescence (PL) spectra of the pure ZnO and
ZnO:Cu nanorods are recorded at room temperature using anexcitation source at 325 nm using the He-Cd laser. The PL emis-sion spectrum shown in Fig. 4 corresponds to the typical emis-sion spectrum of ZnO bulks and films, which consists of broadgreen and yellow emissions and a strong UV emission. The UVemission peak is originated from excitonic recombination corre-sponding to the near-band-edge emission and the green-yellow
Fig. 3. SIMS spectrum of the (a) pure ZnO and (b) Zn:Cu nanorods.
Fig. 4. Photoluminescence spectra of the pure ZnO and ZnO:Cu nanorods.
visible emission, a deep-level emission, is attributed to the re-combination of electrons trapped in single ionized oxygen va-cancies with holes. As shown in Fig. 4, there are an emissionpeak (372.8 nm for ZnO and 373.6 nm for ZnO:Cu) in UV bandand two weak broad emission peaks at 520 nm (green) and 577nm (yellow) in visible band, with an intensity ratio of UV toyellow emission ca. 24.98. In ZnO:Cu nanorods, thetwo peaks are at 373.6 nm for UV and 577 nm for yellow emis-sion, and is ca. 9.27. In particular, the ZnO:Cu nanorodsshowed the suppression of deep-level emission and decreased
intensity ratio to 9.27, indicating the improved visibleemission efficiency caused by the oxygen vacancies. The inten-sity of UV emission peak for Cu-doped ZnO nanorods shows ared-shifted reduction than that of pure ZnO, indicating the re-strain of UV emission due to the slight distortion caused by Cudoping. The results can be explained by the change of defecttypes as well as the increase of defect level related to Cu dopant.Fig. 5 shows the field dependence magnetization curves of
the pure ZnO and ZnO:Cu nanorods with the magnetic field ap-plied along the axial direction. Obvious ferromagnetic behavioris observed for both compositions. The saturation magnetiza-tion are 0.197 emu/g for pure ZnO and 0.163 emu/g forZnO:Cu nanorods, respectively. Inset in Fig. 5 shows the lowfield region of the magnetization curves by the fine measure-ment with a field step of 10 Oe. The coercivity is deduced
3368 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011
Fig. 5. Field dependence magnetization curves of the pure ZnO and ZnO:Cunanorods. The inset shows the low field magnetization of the pure ZnO andZnO:Cu nanorods.
by interpolation method where the nanorods are demagnetized.The decreases from 58 Oe for pure ZnO nanorods to 47 Oefor ZnO:Cu nanorods. The ferromagnetism including and
is reduced by the doping of Cu which results from the anti-ferromagnetic coupling between Cu-Cu ions and had also beenobserved in Cu-doped ZnO nanoparticles [15].
IV. CONCLUSION
The vertically aligned ZnO and ZnO:Cu nanorods havebeen prepared on seeded Si substrate by a low temperature hy-drothermal method. It is found that the microstructure and thecorresponding optical and magnetic properties of the ZnO andZnO:Cu nanorods are affected by the introduction of Cu in theZnO nanorods. The growth rate of the nanorods is obviouslyenhanced by the light substitution of Cu for Zn. The XRDpatterns of Cu-doped ZnO nanorods show the wurtzite struc-ture as that of ZnO nanorods. The decay of UV emission andimprovement of visible emissiom of Cu-doped ZnO nanorodshas been observed from the PL spectroscopy. Magnetizationmeasurement shows the room temperature ferromagnetism ofboth nanorods and the saturation magnetization decreases withthe doping of the Cu in the nanorods.
ACKNOWLEDGMENT
This work was sponsored by the National ScienceCouncil of the Republic of China under the grants No.NSC 99-2112-M-164-001 and NSC 99-2221-E-005-103. The
authors express their thanks to the Precious Instrument Centerof National Chiao Tung University for providing the SEM andCL instruments and technical assistance from Dr. L. Lee.
REFERENCES
[1] W. Guo, Y. Yang, J. J. Qi, J. Zhao, and Y. Zhang, “Localized ultravioletphotoresponse in single bent ZnO micro/nanowires,” Appl. Phys. Lett.,vol. 97, pp. 133112–133114, 2010.
[2] V. Chivukula, D. Ciplys, M. Shur, and P. Dutta, “ZnO nanoparticlesurface acoustic wave UV sensor,” Appl. Phys. Lett., vol. 96, pp.233512–233514, 2010.
[3] C. H. Chen, S. J. Chang, S. P. Chang, M. J. Li, I. C. Chen, T. J. Hsueh,and C. L. Hsu, “Electroluminescence from n-ZnO nanowires/p-GaNheterostructure light-emitting diodes,” Appl. Phys. Lett., vol. 95, pp.223101–223101, 2009.
[4] M. A. Zimmler, T. Voss, C. Ronning, and F. Capasso, “Exciton-relatedelectroluminescence from ZnO nanowire light-emitting diodes,,” Appl.Phys. Lett., vol. 94, pp. 241120–241122, 2009.
[5] N. Ito, Y. Sato, P. K. Song, A. Kaijio, K. Inoue, and Y. Shigesato,“Electrical and optical properties of amorphous indium zinc oxidefilms,” Thin Solid Films, vol. 496, pp. 99–103, 2006.
[6] T. F. Chung, L. B. Luo, Z. B. He, Y. H. Leung, I. Shafiq, Z. Q. Yao,and S. T. Lee, “Selective growth of catalyst-free ZnO nanowire arrayson Al : ZnO for device application,” Appl. Phys. Lett., vol. 91, pp.233112–233114, 2007.
[7] T. Li, H. Fan, J. Yi, T. S. Herng, Y. Ma, X. Huang, J. Xue, and J. Ding,“Structural and magnetic studies of Cu-doped ZnO films synthesizedvia a hydrothermal route,” J. Mater. Chem., vol. 20, pp. 5756–5762,2010.
[8] S. Gupta, W. E. Fenwick, A. Melton, T. Zaidi, H. Yu, V. Rengarajan, J.Nause, A. Ougazzaden, and I. T. Ferguson, “MOVPE growth of transi-tion-metal-doped GaN and ZnO for spintronic applications,” J. Cryst.Growth, vol. 310, pp. 5032–5038, 2008.
[9] A. Fouchet, W. Prellier, and L. Méchin, “Growth and characterizationsof ZnO and Co-doped ZnO films for their use in spintronic,” Superlat-tices Microstruct., vol. 42, pp. 185–190, 2007.
[10] D. P. Joseph, G. S. Kumar, and C. Venkateswaran, “Structural, mag-netic and optical studies of Zn0.95Mn0.05O DMS,” Mater. Lett., vol.59, pp. 2720–2724, 2005.
[11] N. Kouklin, M. Omari, A. Gupta, and S. Sen, “Pre-avalanche ultra-violet photoconduction properties of transitional-metal-doped ZnOnanowires,” J. Elec. Mater., vol. 38, pp. 596–600, 2009.
[12] W. J. Li, E.-W. Shi, W.-Z. Zhong, and Z.-W. Yin, “Growth mechanismand growth habit of oxide crystals,” J. Crystal Growth, vol. 203, pp.186–196, 1999.
[13] S. Yamabi and H. Imai, “Growth conditions for wurtzite zinc oxidefilms in aqueous solutions,” J. Mater. Chem., vol. 12, pp. 3773–3778,2002.
[14] L. Liao, Z. Zhang, Y. Yang, B. Yan, H. T. Cao, L. L. Chen, G. P. Li,T. Wu, Z. X. Shen, B. K. Tay, T. Yu, and X. W. Sun, “Tunable trans-port properties of n-type ZnO nanowires by Ti plasma immersion ionimplantation,” J. Appl. Phys., vol. 104, pp. 076104–076106, 2008.
[15] H. L. Liu, J. H. Yang, Y. J. Zhang, Y. X.Wang, M. B. Wei, D. D.Wang,L. Y. Zhao, J. H. Lang, and M. Gao, “Ferromagnetism in Cu-dopedZnO nanoparticles at room temperature,” J. Mater. Sci.: Mater. Elec.,vol. 20, pp. 628–631, 2009.
