DOI: 10.1002/adma.200602130 Rod-Shaped Assemblies of FePt-PtTe 2 through Dynamic Templating** By Qingyu Yan, Makala S. Raghuveer, Huafang Li, Binay Singh, Taegyun Kim, Mutsuhiro Shima, Arijit Bose , and Ganapathiraman Ramanath* The synthesis of inorganic nanocrystals with control over shape and size is attractive for both fundamental studies of electrical transport, [1] optical phenomena [2] and magnetic properties, [3] as well as many emerging applications. Several key strategies have been developed to synthesize and assem- ble one-dimensional nanocrystals, e.g., vapor-liquid-solid, [4] physical [5] chemical [6,7] and biological [8] routes, which allow crystal shape control by selective adsorption of surfactants onto specific crystallographic facets. Furthermore, hierarchi- cal one-, [9] two- [10,11] and three-dimensional [12,13] architectures can be achieved by assembling these building blocks in orga- nized formations by controlling interparticle interactions using a suitable capping agent or solvent medium. However, to the best of our knowledge, there have been no reports on the formation nanoscopic ensembles of defined shapes com- prised of multiple inorganic phases assembled via the dynamic interaction between the phases. Here, we show that when shape control of one inorganic phase is difficult, e.g., due to limited or lack of interaction with molecular surfactants, another inorganic phase can serve as an intermediary to control the shape and the configuration in which the first phase is assembled. Such a strategy may of- fer a completely new alternative for controlling the shape of multi-component assemblies of nanocrystals. In particular, we demonstrate the synthesis of rod-shaped assemblies of FePt- PtTe 2 composites through co-precipitation of the two inorgan- ic phases by reducing the metal precursors by polyols, in the presence of sucrose and trioctylphosphine oxide (TOPO). Separate synthesis of FePt or PtTe 2 under identical conditions results in randomly shaped aggregates of nanoparticles, indi- cating that evolution of the assembly shape involves the inter- action of the two phases. Inside each biphasic nanorod assem- bly, the PtTe 2 resides as platelets embedded in a matrix of FePt nanoparticles. We chose FePt due to its promise for nanomagnetic device applications while we chose PtTe 2 be- cause of its hexagonal crystal structure (P3 ¯ m1), which can promote the growth of one-dimensional structures. [14] The FePt-PtTe 2 assemblies exhibit high coercivity H c > 500 mT upon annealing, and can be rendered thermally stable and amenable to biofunctionalization by coating a thin silica shell, making the assemblies attractive for magnetically-stimulated cell manipulation [15,16] or accessing novel magneto-optical phenomena. [17] Moreover, such a synthesis strategy can be possibly adapted to encourage new applications in other mul- ti-component/phase systems where shape control can be otherwise difficult. Figure 1a shows representative SEM images of rod-shaped nanostructures prepared from precursors with a molar ratio Te(OH) 6 :Pt(acac) 2 :Fe(acac) 3 = 44:37:19. Analysis of bright field TEM images (e.g., see Fig. 1b) reveals that these nano- rods have uniform lengths spanning ∼ 200–380 nm. Although the sidewalls of the rods are jagged, the nanorod width is nar- rowly distributed around ∼ 40 nm (see Fig. 1c). We note that the nanorods stick closely with each other due to surface hy- drogen bonding commonly observed in nanocrystals synthe- sized using polyols. [18] Energy dispersive X-ray (EDX) analy- sis of as-prepared nanorods reveals a Te:Pt:Fe ratio of 46:37:17, close to the precursor ratio. Phase contrast TEM im- ages (e.g., see Fig. 1d and e, and Supporting Information Fig. S1) show that each nanorod is an assembly of many plate- let-shaped crystals (e.g., length ∼ 10 nm and width ∼ 5 nm for this sample) embedded in a second-phase matrix. The (001) planes of PtTe 2 are stacked parallel to the nanorod axis (see Fig. 1e). Regions enclosed in dashed lines in Figure 1e depict instances where the platelets protrude perpendicular to the axis, giving rise to the jagged morphology, while in some cases the plates appear to have coalesced into larger grains. X-ray diffractograms (see Supporting Information Fig. S2) from as-prepared samples synthesized with different Fe/Pt molar ratios X FePt show Bragg reflections corresponding to cubic FePt and trigonal PtTe 2 phases. Selected area electron diffraction patterns exhibit rings corresponding to fcc FePt COMMUNICATION 4358 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19, 4358–4363 – [*] Prof.G. Ramanath, Dr. Q. Yan, M. S. Raghuveer, Dr. H. Li, B. Singh, T. Kim, Prof. M. Shima Rensselaer Polytechnic Institute, Materials Science and Engineering Department 110 Eighth Street, Troy, NY 12180 (USA) E-mail: Ramanath@rpi.edu Prof. G. Ramanath Indian Institute of Science, Materials Engineering Department Bangalore 560012 (India) Dr. Q. Yan Materials Science and Engineering Department Nanjing University Nanjing 210024 (P. R. China) Prof. A. Bose Chemical Engineering Department, University of Rhode Island Kingston, RI 02881 (USA) [**] The authors gratefully acknowledge funding from NSF through DMR 0519081 and ECS 0501488 grants, NY State through the Focus Center, a Honda Research Initiation Grant, and NSFC grant 10407035. Supporting Information is available online from Wiley In- terScience or from the authors.