NANO LETTERS
Fabrication of hcp-Co Nanocrystals via Rapid Pyrolysis in Inverse PS-b-PVP Micelles and Thermal Annealing
2003 Vol. 3, No. 7 891-895
Fre´de´ric S. Diana,* Seung-Heon Lee, Pierre M. Petroff, and Edward J. Kramer Materials Department & Mitsubishi ChemicalsCenter for AdVanced Materials, UniVersity of California, Santa Barbara, California 93106 Received April 25, 2003; Revised Manuscript Received June 10, 2003
ABSTRACT Magnetic colloidal nanocrystals tend to aggregate in solution because of magnetic dipolar interactions. A diblock copolymer was used as a stabilizer to limit these effects, especially those leading to aggregation in solution. Cobalt nanoparticles have been synthesized within inverse micelles of polystyrene-block-poly(2-vinylpyridine) copolymer in toluene by the pyrolysis of dicobalt octacarbonyl at 115 °C. The nanoparticle structure at different reaction times was investigated using transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). At early reaction stages, the nanoparticles were found to be noncrystalline from TEM, and FT-IR showed that the precursor was only partially decomposed. After 15 min of reaction, the nanoparticles became crystalline, forming chains due to magnetic interactions. The noncrystalline nanoparticles could be crystallized upon heating to 420 °C on grids in the transmission electron microscope. This produced nearly monodisperse single nanocrystals inside each micelle, with limited aggregation, but such annealing led to the degradation of the polymer.
Magnetic colloidal or micellar chemically synthesized monodisperse nanoparticles (NPs) or nanocrystals (NCs) have recently been the subject of numerous studies because of their possible implementation into future ultrahigh density (>100 Gbit/in2) patterned magnetic media (PMM)1-5 or magnetoresistive devices,6-8 among other applications. It is now well established that such NCs can act as single bits for information storage.3,9 However, there are two major difficulties that should be overcome if PMM with such NCs is ever to be achieved. First, monodispersity (reduced standard deviation of the NCs population size distribution σ below 10%) has to be reached. This can be achieved by temporally separating the nucleation and growth stages involved in the synthesis of the NCs10 or by focusing the size distribution by adjusting the precursor concentration during the synthesis.11 Because monodisperse particles are difficult to synthesize in practical situations, postsynthesis operations such as filtering and/or size-selective precipitation12 are used, limiting the yield of the synthesis. Furthermore, chemically synthesized magnetic colloidal NCs, each consisting of a crystalline core surrounded by small organic molecule surfactants, have, in addition to van der Waals interactions, an attractive magnetic dipole13 that tends to cause these to aggregate in solution. Such a lack of stability in solution causes poor postsynthesis processing ability. It * To whom correspondence should be addressed. E-mail: frederic@ engineering.ucsb.edu. 10.1021/nl034262l CCC: $25.00 Published on Web 06/24/2003
© 2003 American Chemical Society
is possible to obtain soft magnetic or paramagnetic NCs and recrystallize them to harder magnetic phases. This method was introduced by Sun et al. with the transition from disordered fcc-FePt to fct-FePt at 560 °C.3 A similar idea was employed with cobalt (lower anisotropy than FePt but a higher magnetic moment per NC): -Co f hcp-Co at 300 °C5 or fcc-Co at 500 °C.14 Yet, there is still no real control over the interparticle distance, and this can give rise to magnetic coupling between adjacent particles after deposition so that long-range order is difficult to obtain. Three avenues can be used to improve the control over interparticle distances while maintaining a sharp size distribution: (i) changing the surfactant and solvent, (ii) crystallizing after deposition, and (iii) using patterned substrates. In this letter, we show a method combining the first two approaches, which are mainly related to the reduction of aggregation in solution, and a report on the latter method is in preparation.15 Our method successfully applies the idea of crystallization after deposition using a short-time (
