©2008 American Institute of Physics. The electronic version of this article is the complete one and can be found online at: http://link.aip.org/link/?APPLAB/92/162102/1DOI:10.1063/1.2905274Mn doped ZnO nanowires (NWs) were fabricated by a one-step vapor-solid process at 500°C. The doped Mn exists in the wurtzite lattice as substitutional atom without forming secondary phases. X-ray absorption near-edge structure reveals that the doped Mn atoms occupy the Zn sites, and they lead to an expansion in lattice constants. The I-V characteristic of a single Zn₁₋ₓMnₓO NW shows a typical Ohmic contact with gold electrodes. The as-received NWs could be suitable for studying spintronics in one-dimensional diluted magnetic semiconductors

Chu, Wangsheng

Gu, Yudong

Mai, Wenjie

Wang, Z. L. (Zhong Lin)

Wu, Ziyu

Zhang, Xiaomei

Zhang, Yue

Applied Physics Letters

English

Scholarly Materials And Research @ Georgia Tech

Synthesis and characterization of Zn1−xMnxO nanowiresXiaomei Zhang,1,2 Yue Zhang,1,a Zhong Lin Wang,2,a Wenjie Mai,2 Yudong Gu,2Wangsheng Chu,3 and Ziyu Wu31Department of Materials Physics and Chemistry, State Key Laboratory for Advanced Metals and Materials,University of Science and Technology Beijing, Beijing 100083, People’s Republic of China2School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta,Georgia 30332-0245, USA3Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences,Beijing 100049, People’s Republic of ChinaReceived 7 February 2008; accepted 13 March 2008; published online 21 April 2008Mn doped ZnO nanowires NWs were fabricated by a one-step vapor-solid process at 500 °C. Thedoped Mn exists in the wurtzite lattice as substitutional atom without forming secondary phases.X-ray absorption near-edge structure reveals that the doped Mn atoms occupy the Zn sites, and theylead to an expansion in lattice constants. The I-V characteristic of a single Zn1−xMnxO NW showsa typical Ohmic contact with gold electrodes. The as-received NWs could be suitable for studyingspintronics in one-dimensional diluted magnetic semiconductors. © 2008 American Institute ofPhysics. DOI: 10.1063/1.2905274Due to a high Curie temperature above 300 K as pre-dicted by theoretical calculation, diluted magnetic semicon-ductors DMSs are regarded as important materials forspintronics.1 Combining ferromagnetism and semiconductiv-ity gives an additional degree of freedom and functionalityfor fabricating unique devices with applications ranging fromnonvolatile memory to quantum computing.2,3 Synthesismethods such as chemical vapor deposition and pulse laserdeposition have been developed to prepare DMS thinfilms.4–6 It is known that different synthetic methods result invariance in environment of doping element in the matrix orforming second phase.Exploring one-dimensional 1D DMS materials are ofgreat interest because nanowires NWs and nanobelts areideal materials for fabricating field effect transistors, sensors,optoelectronic devices, logic circuits, and lasers.7–11 Thereare two approaches for receiving DMS NWs. One method isdoping the as-synthesized NWs by ion implantation.12 This isa postgrowth doping via physical method, which usually re-quires postannealing to reduce the defects created by ionimplantation. Moreover, ion implantation usually results in adepth variation in doping concentration. The other approachis by incorporating doping elements in the precursors duringgrowth, which is a methodology for receiving uniformly dis-tributed doping element in the matrix.13,14 For the NWs syn-thesized by the second approach, a key question is if thedoping element is incorporated into the volume lattice or atthe surface. Establishing an effective approach for synthesiz-ing volume-doped DMS NWs is of fundamental importancefor investigating spin transport in 1D nanomaterials and fab-rication of NW based spintronic devices.In this letter, we used a vapor-solid process to synthesizeMn doped ZnO NWs. The electronic structure and nearneighbor distribution around the Mn atoms have been inves-tigated by x-ray absorption near-edge structure XANES.The results confirmed that the Mn atoms have been dopedinto the lattice of ZnO NWs and the doping is substitutional.The electric transport of a single Zn1−xMnxO NW has alsobeen measured and a conductivity of 0.025 −1 cm−1 hasbeen received.Our synthesis was based on a vapor-solid process viathermal evaporation. High purity Zn powders 99.99% andMnCl2 ·4H2O powders 99.99% were used as source mate-rials. The source materials were placed in an alumina boatthat was placed at the center of a horizontal tube furnace.A silicon substrate was placed above the boat. The tube fur-nace was heated to 500 °C with a constant argon flow of100 SCCM SCCM denotes cubic centimeter per minute atSTP. After 3 h, a yellowish layer was received on the sili-con wafer. The color of the as-received sample is clearlydifferent from the white color of conventional ZnO NWs.The morphology and chemical composition of the as-synthesized sample were characterized by scanning electronmicroscopy SEM. Figure 1a shows an SEM image ofZn1−xMnxO NWs, with uniform diameters and lengths up to100 m. From the typical TEM image of Mn doped ZnONWs in Fig. 1b, the average diameter of the NWs is60 nm. To precisely define the chemical composition of theNWs, we used energy dispersive x-ray spectroscopy EDSattached to a transmission electron microscope TEM Hita-chi HF-2000 to quantify the chemical composition of asingle NW without the interference from the substrate. TheMn peak is clearly shown in the EDS spectrum Fig. 1band the Mn content in the ZnO NW is determined to be4.8 at. %. This apparently indicates that the NW containsMn.The phase of the as-synthesized NWs is identified byx-ray diffraction XRD. The result indicate that the as-received Zn1−xMnxO x=0.048 NWs has the wurtzite struc-ture Fig. 2. No secondary phase was found from the XRDspectrum, thus, the doping element Mn must be incorporatedinto the lattice as substitutional atom. The doping causessignificant change in lattice constants, resulting in measur-able shift in the 10−10 and 0002 peaks see the insets inFig. 2, corresponding to an expansion in lattice constants bya /a=0.59%, c /c=0.5% after doping.aAuthors to whom correspondence should be addressed. Electronic ad-dresses: yuezhang@ustb.edu.cn and zlwang@gatech.edu.APPLIED PHYSICS LETTERS 92, 162102 20080003-6951/2008/9216/162102/3/$23.00 © 2008 American Institute of Physics92, 162102-1Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspThe occupation sites for the Mn dopants in the lattice aredetermined by XANES. The Mn K edge of the Zn1−xMnxONWs Fig. 3a was measured in fluorescence yield mode byusing synchrotron radiation with a silicon 111 double crys-tal monochromater at the x-ray absorption station Beamline 1W1B of the Beijing Synchrotron Radiation FacilityBSRF. The storage ring was operated at the typical energyof 2.5 GeV with the electron current decreasing from200 to 100 mA in the span time of 12 h. XANES spectrumis very sensitive to the near neighbor distribution of atomsaround the selected atom due to the multiple-scattering pro-cesses of the photoelectron emitted by the x-ray-absorbingatom.15–17 The shoulder, main peak, and features at10–90 eV beyond the Mn K edge of the Zn1−xMnxO areclearly different from those of standard samples such as Mn,MnO, Mn2O3, and MnO2 Fig. 3a. This excludes the pos-sibility of Mn forming Mn clusters and other manganeseoxides in the Zn1−xMnxO NWs. To find out the possibility ofMn substituting Zn in the lattice, the Mn K edge of theZn1−xMnxO NWs is compared to the Zn K edge feature ofZnO at an energy of 10–90 eV beyond the edges Fig. 3b.The striking similarity between the XANES’s from Mn Kand Zn K edges reveals that the occupancy sites of Mn atomsin the Zn1−xMnxO NWs are the Zn sites. Therefore, thedoped Mn atoms are in the volume of the NWs and theyoccupy the Zn sites.The doping element is likely to affect the transport prop-erty of the NWs. We have fabricated two-electrode bridgeddevices using individual NWs, and their transport propertieshave been measured. Figure 4 gives the I-V characteristic ofa NW based device, in which a Zn1−xMnxO NW was in con-tact with two gold electrodes. The NW was laid on prefabri-cated electrodes by dielectrophoresis. To reduce the contactresistance, Pt was deposited at the contacts using a focusedion beam microscope FEI Nova. An SEM image of thedevice is shown as the inset in Fig. 4. The I-V curve showsan Ohmic behavior of the NW, from the slope of which andthe diameter and length of the NW measured from the SEMimage, the conductivity of the Zn1−xMnxO NW is measuredto be 0.025 −1 cm−1.In conclusion, Mn doped ZnO NWs were fabricated by aone-step vapor-solid process at 500 °C. XANES reveals thatthe doping elements occupy the Zn sites. Electrical measure-ment on individual Mn-doped ZnO NW showed that Auelectrodes made good Ohmic contacts with the Zn1−xMnxOFIG. 1. a SEM image of Zn1−xMnxO NWs. b TypicalEDX spectrum taken from individual Zn1−xMnxO NWin TEM, which gives that the Mn content is 4.8 at. %.Inset in b is a TEM image of a 60 nm NWs withsingle crystal structure.FIG. 2. Color online a and b XRD patterns of Zn1−xMnxO NWs andstandard ZnO powders, respectively. The insets are the enlarged 100or 10-10 and 002 or 0002 diffraction peaks.162102-2 Zhang et al. Appl. Phys. Lett. 92, 162102 2008Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspNW. The as-received NWs could be suitable for studyingspintronics in 1D DMS.Thanks to Changshi Lao and Yong Ding at Georgia In-stitute of Technology Atlanta, USA for helping with theFIB and TEM experiments. This work was supported by theMajor Project of International Cooperation and ExchangesNos. 50620120439 and 2006DFB51000, the National BasicResearch Program of China No. 2007CB936201, the Na-tional Natural Science Foundation of China No. 50772011,USA DOE BES DE-FG02-07ER46394, and NSF DMS0706436.1T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287,1019 2000.2H. Ohno, Science 281, 951 1998.3S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. vonMolnár, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Science294, 1488 2001.4Q. Wan, Appl. Phys. Lett. 89, 082515 2006.5Q. Xu, H. Schmidt, L. Hartmann, H. Hochmuth, M. Lorenz, A. Setzer, P.Esquinazi, C. Meinecke, and M. Grundmann, Appl. Phys. Lett. 91,092503 2007.6W. Yan, Z. Sun, Q. Liu, Z. Li, T. Shi, F. Wang, Z. Qi, G. Zhang, and S.Wei, Appl. Phys. Lett. 90, 242509 2007.7Y. Nakayama, P. J. Pauzauskie, A. Radenovic, R. M. Onorato, R. J.Saykally, J. Liphardt, and P. D. Yang, Nature London 447, 1098 2007.8S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner, Prog. Mater.Sci. 50, 293 2005.9S. N. Cha, J. E. Jang, Y. Choi, G. A. J. Amaratunga, G. W. Ho, M. E.Welland, D. G. Hasko, D. J. Kang, and J. M. Kim, Appl. Phys. Lett. 89,263102 2006.10Z. L. Wang, Adv. Mater. 19, 889 2007.11Z. L. Wang, Mater. Today 10, 20 2007.12C. Ronning, P. X. Gao, Y. Ding, Z. L. Wang, and D. Schwen, Appl. Phys.Lett. 84, 783 2004.13J. J. Liu, M. H. Yu, and W. L. Zhou, Appl. Phys. Lett. 87, 172505 2005.14H. 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 2007.15Z. Wu, A. Marcelli, A. Mottana, G. Giuli, E. Paris, and F. Seifert, Phys.Rev. B 54, 2976 1996.16Z. Y. Wu, G. Ouvrard, P. Gressier, and C. R. Natoli, Phys. Rev. B 55,10382 1997.17Z. Wu, D. C. Xian, C. R. Natoli, A. Marcelli, E. Paris, and A. Mottana,Appl. Phys. Lett. 79, 1918 2001.FIG. 3. Color online a Mn K-edge XANES spectrafor Zn1−xMnxO NWs, Mn, MnO, Mn2O3, and MnO2standards, respectively. b Mn K-edge XANES spec-trum of Zn1−xMnxO NWs and the Zn K-edge XANESspectrum of ZnO.FIG. 4. I-V curve of a single Zn1−xMnxO NW. The inset shows an SEMimage of a single NW contacted with two gold electrodes.162102-3 Zhang et al. Appl. Phys. Lett. 92, 162102 2008Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Synthesis and characterization of Zn₁₋ₓMnₓO nanowires

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Synthesis and characterization of Zn₁₋ₓMnₓO nanowires

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