Size and Shape Control of Monodisperse FePt Nanoparticles Vikas Nandwana, Kevin E. Elkins, Narayan Poudyal, Girija S. Chaubey, Kazuaki Yano, and J. Ping Liu* Department of Physics, UniVersity of Texas at Arlington, Arlington, Texas 76019 ReceiVed: December 4, 2006; In Final Form: January 17, 2007 Morphological control of FePt nanoparticles has been systematically studied. By varying synthetic parameters including precursors, solvents, amount of surfactants, and heating rate of the solution, the particle size from 2 to 9 nm can be tuned with 1 nm accuracy. While most particles are spherical in shape, cubic particles can be obtained when particles are greater than 7 nm. Rod-shape nanoparticles have also been obtained. The as-synthesized nanoparticles are found to be superparamagnetic at room temperature and their blocking temperature is size dependent that increases with particle size. After annealing in a reducing atmosphere, the nanoparticles form hard magnetic films with ordered fct structure and high coercivity up to 2.7 T. Introduction There has been renewed research interest in monodisperse magnetic nanoparticles in recent years, driven by new applica- tions such as ultrahigh-density magnetic storage media, 1 biologi- cal imaging and therapy, 2 and exchange-coupled nanocomposite magnets. 3 Recently, various magnetic nanoparticles including FePt, 4 Co, 5 CoPt, 6 and Fe 3 O 4 7 have been fabricated by solution- phase synthesis. Preparation of high quality nanoparticles with desired size and shape is a prerequisite to investigate and utilize their properties. Relatively simple and reproducible approaches for the synthesis of crystalline nanoparticles of controllable size are of great fundamental and technological interest. The control of the nanocrystal size and shape is also a key in the formation of two or three-dimensional self-assembled structures where individual nanocrystals are the building blocks of the next level of material hierarchy. 8 Although many factors affect particle size and shape of nanoparticles during chemical synthesis, only few parameters have been studied for FePt system. Recently, size control of monodisperse FePt nanoparticles from organometallic precursors and microemulsion was reported. 9,10 It was found that separation of nucleation and growth in time is required for the formation of particles with a near-monodisperse size distribution. 11 However, fine tuning of the particle size remains a challenging task. Here, we report our effort in FePt particle size control with 1 nm accuracy by changing several synthetic parameters. Experimental Methods FePt nanoparticles were prepared via chemical reduction of Pt(acac) 2 and thermal decomposition of Fe(CO) 5 in the presence of oleic acid and oleyl amine. The synthetic experiments were carried out using standard airless technique in argon atmosphere. In a typical procedure, 0.5 mmol of platinum acetylacetonate was added to 125 mL flask containing a magnetic stir bar and mixed with 20 mL of octyl/benzyl ether. After purging with argon for 30 min at room temperature, the flask was heated up to 120 °C for 10 min and a designated amount of oleic acid and oleyl amine was added. Iron pentacarbonyl or iron acety- lacetonate were used as an iron precursor. Iron acetylacetonate (0.5 mmol) was added at room temperature while iron pentac- arbonyl (1.0 mmol) was added at 120 °C when the platinum precursor dissolved completely. The dissolution of Pt(acac) 2 in solvent could be followed experimentally by the change of color of the solution from off yellow to transparent yellow. After the addition of Fe(CO) 5 , the color transition from golden to black suggested formation of nanoparticles in the solution. Then it was heated to 298 °C for 1 h before cooling to room temperature under the argon blanket. Argon gas was flowed throughout the experiment. The heating rate was varied from 1 to 15 °C per minute according to the experimental design. The black product was precipitated by adding ethanol and separated by centrifugation and redispersed in hexane. To achieve the highest purity, extra ethanol was added in this dispersion and the dispersion was centrifuged again. Because all the particles were quite homogeneous, size selection was not necessary. After washing the particles in ethanol three or more times, they were dispersed in hexane and stored in glass bottles under refrigeration. Samples for magnetic characteriza- tion were prepared by depositing a drop of the final hexane dispersion on a 3 × 3 mm silicon substrate, evaporating the solvent at room temperature and further drying in vacuum, which led to the formation of FePt nanoparticle-assembled thin films. The samples were then annealed at 650 °C for 1 h under the flow of forming gas (Ar + 7% H 2 ) in a tube furnace. The transmission electron microscopy (TEM) images were recorded on a JEOL 1200 EX electron microscope at an accelerating voltage of 120 kV. Powder X-ray diffraction (XRD) spectra were recorded on a Philips MPD diffractometer with a Cu KR X-ray source (λ ) 1.5405 Å). The magnetic hysteresis measurements have been carried out by using superconducting quantum interference device (SQUID) magnetometer with magnetic field up to 7 T. The composition analysis was done by energy dispersive X-ray spectroscopy (EDX) and inductively coupled plasma (ICP). Results and Discussions We examined several parameters that have an effect on the size and shape of monodisperse FePt nanoparticles. After examining each parameter thoroughly, we found that surfactants * Corresponding author. E-mail: pliu@uta.edu. Tel: +1-817-272-2815. Fax: +1-817-272-3637. 4185 J. Phys. Chem. C 2007, 111, 4185-4189 10.1021/jp068330e CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007