DOI: 10.1002/adma.200602612 Ferromagnetic Semiconducting EuO Nanorods** By Matthew J. Bierman, Katherine M. Van Heuvelen, Dieter Schmeißer, Thomas C. Brunold, and Song Jin* Europium chalcogenides (EuX, X=O, S, Se, and Te) are magnetic semiconductor materials [1,2] that are of interest for magneto-optical applications and spintronics. [3] The science of spintronics, which seeks to couple spin with charge degrees of freedom in electronic and photonic devices, promises applica- tions involving nonvolatile circuits, faster data processing, and markedly decreased power consumption. [4,5] Of particular in- terest is the ferromagnetic semiconductor europium oxide (EuO) with a bandgap of 1.1 eV (1130 nm) and a bulk Curie temperature (T c ) of 69 K. [1,2] In EuO, the Eu 2+ has an exactly half-filled and high spin 4f shell where these localized 4f elec- trons give EuO an 8 S 7/2 spectroscopic ground state. It is well understood that the seven unpaired electrons of the half-filled 4f shell of the Eu 2+ ions at every lattice site of this rocksalt crystal structure are coupled to each other below the T c to be- come ferromagnetically aligned while preserving the materi- al’s semiconducting character. [2] This alignment gives EuO a large localized magnetic moment of 7.9 l B . In the ferromag- netic semiconducting state the conduction band is Zeeman- split by up to 0.3 eV [4,6] leading to spin-polarized electronic states and large spin-polarized currents that are central to spintronic applications. While dilute magnetic semiconductors have received the majority of attention due to potential room temperature (RT) application, [4] the concentrated magnetic semiconductor EuO offers the advantages of both theoretical clarity and a large spin polarization. [4,7] Because of the diffi- culties in preparing bulk EuO samples via traditional solid state synthesis techniques, [1,2] nanoscale building blocks of this important material such as one-dimensional wires or rods [8] provide an alternate approach to high quality materials by following the bottom-up paradigm of nanoscience and nano- technology—using the building blocks directly as the compo- nents for device fabrication and fundamental study, [9] allowing for fabrication of basic spintronic devices such as spin FETs and LEDs and magneto-optical modulators. To our knowledge no synthetic route to concentrated ferro- magnetic semiconducting nanorods, including EuO, pre- viously existed, even though there have been successful syn- theses of dilute magnetic semiconductor nanowires including Mn doped GaN, [10] CdS, [11] and others. [12] Besides the signifi- cant body of work on the bulk, [1,2] there has been a revival of interest in thin films of EuO [13,14] and EuS [15,16] because of their potential spintronic applications. Nanomaterials of rare earth oxides and sulfides are emerging as an active research area as well. Nanocrystals of EuO have been synthesized through photochemical reduction of Eu(III) [17] and nanocrys- tals of EuS have been synthesized by decomposition of a europium dithiocarbamate precursor in a surfactant. [18,19] The synthesis of nanorods and nanoparticles of the higher oxides and hydroxides of europium and other rare earths have also been reported, [20–23] though nanorods of the technologically important EuO have not been investigated nor was the mag- netic semiconducting nature examined for any EuO or EuS nanocrystals. Herein we report the first successful preparation of EuO nanorods through a facile solution synthesis of Eu(OH) 3 nanorods and their subsequent solid state conver- sion. The ferromagnetic semiconducting properties of the rods are confirmed with magnetic and magneto-optical measure- ments. EuO nanorods were synthesized in high yield via a three step reaction process starting with aqueous synthesis of Eu(OH) 3 nanorods by the hydrolysis of Eu(NO 3 ) 3 together with hexamethylenetetramine (HMT, see Fig. 1a). Eu(OH) 3 nanorods (Fig. 1b) were converted first to Eu 2 O 3 (Fig. 1d) by heating in air and then to EuO (Fig. 1f) by a reduction in Eu vapor. [24] Powder X-ray diffraction (PXRD) of the resulting powders (Fig. 1c, e, and g) clearly match the standard diffrac- tion patterns for Eu(OH) 3 , Eu 2 O 3 , EuO (JCPDS PDF# 00-017-0781, 00-034-0392, 00-018-0507, respectively). The morphology was preserved along the reaction pathway as shown in the SEM images (Fig. 1b, d, and f). Nanorods had lengths that were between 1–2 lm and diameters that could be as small as 100 nm, depending on reaction conditions. EuO is not the thermodynamically stable phase in ambient air conditions, [25] therefore reduction to EuO is necessary when Eu(III) is used as the starting materials. Among the various possible schemes of reducing Eu 2 O 3 to EuO, [26] reduction with europium vapor does not introduce impurities. Furthermore, since the necessary diffusion length is nanoscale, europium COMMUNICATION Adv. Mater. 2007, 19, 2677–2681 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2677 – [*] Prof. S. Jin, M. J. Bierman,K. M. Van Heuvelen, Prof. T. C. Brunold Department of Chemistry, University of Wisconsin-Madison Madison, WI 53706 (USA) E-mail: jin@chem.wisc.edu Prof. D. Schmeißer Lehrstuhl Angewandte Physik—Sensorik, BTU Cottbus 03046 Cottbus (Germany) [**] This work is supported by an NSF CAREER award (DMR-0548232). S.J. also thanks UW-Madison NSEC (NSF DMR-0425880) and the 3M Nontenured Faculty Award for financial support. K.M.V.H. thanks the NSF Graduate Research Fellowship Program. We thank Prof. Robert J. Hamers for the access to SEM.