Structure of Self-Assembled Fe and FePt Nanoparticle Arrays
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Structure of Self-Assembled Fe and FePt Nanoparticle Arrays S. Yamamuro, D. Farrell, K. D. Humfeld, and S. A. Majetich Dept. of Physics, Carnegie Mellon University Pittsburgh, PA 15213, U.S.A
ABSTRACT Arrays were self-assembled by evaporating suspensions of 4 nm FePt or 8 nm Fe nanoparticles. The monolayers had a hexagonal close packed (hcp) structure, but the multilayer structure varied. To identify the multilayer structures, transmission electron microscopy (TEM) images were compared with phase contrast image simulations. The results showed that Fe could be grown as both hcp and face-centered cubic (fcc), or fcc-like, structures. The results of image analysis of the FePt arrays were consistent with fcc structures.
INTRODUCTION Monodisperse nano particles of Fe or FePt can self-assemble into mono- and multilayer arrays. Monodomain magnetic nanoparticles behave like tiny dipoles. At low concentrations this causes them to form chain-like structures, and at higher concentrations simulations predict the formation of hexagonal sheets in an applied field [1]. However our arrays were self-assembled without an external field. The particles in this study are superparamagnetic, so that the magnetic moment direction varies due to thermal fluctuations. Arrays can be formed when this occurs much more rapidly than the time necessary for the self-assembly. Self-assembled arrays made from spherical nanoparticles have been reported with several different structures: the close-packed lattices hcp and fcc [2-6], and also body-centered cubic (bcc) [7]. Interference between electrons passing through successive layers of the superlattice makes interpretation of TEM images difficult, but image simulations of known lattice structures can be compared to the experimental images to determine the experimental structure. Here we investigate the structures of arrays made from 8.6 nm Fe or 4.2 nm FePt nanoparticles using a combination of TEM and image simulations.
EXPERIMENTAL The preparation of monodisperse nanoparticles requires rapid and nearly simultaneous nucleation of clusters, followed by relatively slow growth through the addition of individual atoms, rather than by the coalescence of clusters [8, 9]. Several other groups have developed methods to prepare iron nanoparticles [10-12], all under inert atmosphere to prevent oxidation. To synthesize 8.6 nm Fe nanoparticles, we used a procedure similar to that of Sun et al. for synthesizing FePt [2]. 4 mg of platinum acetylacetonate and 150 mg of the mild reducing agent 1,2-hexadecanediol were dissolved in 15 mL of dioctyl ether. The solution was heated and 0.2 mL of iron pentacarbonyl was added at 100 °C. The heating continued until the solution reached 280 °C. During this process the diol reduces the platinum salt to form platinum clusters, and the D10.8.1
Fe(CO)5 thermally decomposes, releasing iron atoms which preferentially deposit onto the platinum clusters. After cooling, 15 mL of octyl ether and 4.2 mL of Fe(CO)5 were added at 100 °C before reheating to 260 °C. The final solution
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