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Structure of Cobalt Nanosphere Superlattice Films by Small Angle X-ray Scattering

Published online by Cambridge University Press:  21 March 2011

Michael Beerman ,
Masato Ohnuma ,
Yuping Bao  and
Kannan M. Krishnan
Michael Beerman
Affiliation:
Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195
Masato Ohnuma
Affiliation:
National Institute for Materials Science, Tsukuba, Ibaraki-ken, Japan305-0044
Yuping Bao
Affiliation:
Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195
Kannan M. Krishnan
Affiliation:
Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195
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Abstract

Cobalt nanocrystals, recently synthesized with narrow size distributions and controlled shapes, organize in a wide range of arrays as a function of shape, size and interparticle interactions. The nanocrystals (NCs) consist of a cobalt metal core with a nominal diameter of 11 nm, and an organic surfactant surface layer with a chain length of ∼1.7 nm. For the simplest case (ε-Co nanospheres, super-paramagnetic at room temperature) a hexagonal arrangement of NCs is observed in transmission electron microscope (TEM) images when precipitated from solution onto carbon films. For practical applications and for further understanding of the self-assembly process, long range order of the super lattice must be probed over regions that are greater in extent than may be examined by TEM. Hence, small angle x-ray scattering (SAXS) measurements were performed on cobalt nanospheres randomly dispersed in solution and assembled on glass substrates. Least squares fit to the intensity distribution as a function of the scattering vector q gave an average particle diameter of 11.0 ± 0.8 nm. Structure factor contribution to the intensity profile agrees well with a quasi-random model for scattering from a face centered cubic (FCC) superlattice composed of uniform radius cobalt spheres. The measured nearest neighbor interparticle spacing, 14.1 nm, agrees to within 2% of the predicted value of 14.4 nm based on a free energy model that governs the self-assembly of the nanoparticle system.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 818: Symposium M – Nanoparticles and Nanowire Building Blocks-Synthesis, Processing, Characterization and Theory , 2004 , M1.10.1
Copyright
Copyright © Materials Research Society 2004

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