DOI: 10.1002/adma.200702387 Ferromagnetism in ZnO Nanowires Derived from Electro-deposition on AAO Template and Subsequent Oxidation** By J. B. Yi, * H. Pan, * J. Y. Lin, J. Ding, * Y. P. Feng, * S. Thongmee, T. Liu, H. Gong, and L. Wang The wide-gap semiconductor ZnO is a potential candidate for many applications, including gas sensing, varistors, light- emitting devices, and solar cells. [1–5] The nanostructure of ZnO has been extensively studied. [6–8] ZnO nanowires have been found to be promising for nanometer-scale optoelectronics, electronic devices, and biotechnology. [9–12] Many methods have been used to fabricate ZnO nanowires, such as high-pressure pulse laser deposition (PLD), vapor– liquid–solid methods, chemical vapor deposition, and template- assisted methods. [13–15] It is known anodic aluminum oxide (AAO) templates are widely used for the preparation of na- nowires. [16–18] However, there have been only a few reports on the fabrication of ZnO nanowires via electro-deposition and oxidation of Zn nanowires into AAO templates, [19,20] probably because of the limitation imposed by the formation of ZnAlO 3 . Besides the potential applications because of optical and electronic properties, ZnO recently received extensive interest owing to a predication as candidate for room-temperature magnetic semiconductors. [21] Magnetic semiconductors may play important roles in future spintronic devices for the manip- ulation of both charges and spins. [22,23] Apart from a few magnetic semiconductors such as EuO, EuS, CdCr 2 S 4 , and CdCr 2 Se 4 , another type of well-established magnetic semicon- ductor, known as dilute magnetic semiconductor (DMS), is formed by doping III–V and II–VI semiconductors with transi- tion-metal impurities,. As predicted by Dietl et al., [21] ZnO and GaN can be the host candidate for the room-temperature ferromagnetic DMS. Coey et al. found room-temperature fer- romagnetism in non-transition-metal-doped ZnO. [24] Room- temperature ferromagnetism was also achieved in non-metal- doped ZnO. [25] Nevertheless, in spite of accelerated interest in these materials, the origin of the ferromagnetism is still under dispute. Several studies reported the nonintrinsic ferromagne- tism is due to phase separation and the formation of ferromag- netic clusters, [26] the formation of bound magnetic polaron, [27] or the metal deficient centers. [28] At the same time, some other experimental and theoretical works indicated intrinsic ferro- magnetism was due to the interaction between defects and 3d ions. [24] Moreover, ZnO, In 2 O 3 , and TiO 2 without doping have been found ferromagnetic. [29,30] It was explained that the oxygen vacancies play an important role, similar to HfO 2 without doping. [31] The controversy of the results suggests that the preparation method may greatly affect the magnetic prop- erties. In this work, we report room-temperature ferromagne- tism in ZnO nanowires (and thin films) through a carefully controlled synthesis process. ZnO nanowires with the presence of Zn clusters were found to be ferromagnetic at room temper- ature. When the Zn clusters disappeared (after the completion of oxidation), no room-temperature ferromagnetism was detected. ZnO nanowires were prepared by electro-deposition into an AAO template, and subsequent oxidation through annealing under air atmosphere. The preparation of the AAO template has been reported elsewhere. [10] Zn nanowires were firstly formed during electro-deposition into the AAO template with nanometer-sized pores. XRD measurements showed that the Zn nanowires had a wurtzite hcp-Zn phase. The as-deposited Zn nanowires were non-ferromagnetic. ZnO nanowires were formed by oxidation of the Zn nanowires. To form ZnO nanowires, at the first step, Zn nanowires were annealed at different temperatures (25–500  C) with a constant duration (10 h). It was found that an annealing temperature higher than 400  C resulted in the formation of a ZnAlO 3 phase. Hence, in this work, we fixed the annealing temperature at 350  C and studied the effect of annealing duration on the microstructure and properties. For the as-deposited Zn nanowires, XRD measurements showed a wurtzite hcp-Zn phase (see Supporting Information Figure S1). The intensity of the peak of Zn (220) is much stronger than other peaks, indicating a texture in the (220) orientation. When the deposited wires were annealed at 350  C for 10 h in air atmosphere, polycrystalline hcp-ZnO could be [*] J. B. Yi, Prof. J. Ding, S. Thongmee, Prof. H. Gong Department of Materials Science and Engineering National University of Singapore, 119260 (Singapore) E-mail: mseyj@nus.edu.sg; msedingj@nus.edu.sg H. Pan, Prof. Y. P. Feng, Prof. J. Y. Lin, T. Liu Department of Physics National University of Singapore, 119260 (Singapore) E-mail: phyph@nus.edu.sg; phyfyp@nus.edu.sg Prof. L. Wang Division of Physics and Applied Physics School of Physics and Mathematics Sciences Nanyang Technological University, 637371 (Singapore) [**] D.J. thanks support from the Singapore National Research Foundation (NRF). Supporting Information is available online from Wiley InterScience or from the author. 1170 ª 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim Adv. Mater. 2008, 20, 1170–1174 COMMUNICATION