Synthesis and characterization of cobalt nanocrystals
Name
50323868-MIT.pdf
Description
Full printable version
Size
10.38 MB
Format
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Checksum (MD5)
7fa9fc3928c77d8cdc1b7548453edfd8
Author(s)
Dinega, Dmitry P. (Dmitry Petrovich), 1969-
Advisor(s)
Moungi G. Bawendi.
Date Issued
2001
Publisher
Massachusetts Institute of Technology
Abstract
Although several theoretical models for the behavior of magnetic crystals smaller than a single domain size were developed in the 1950's and 60's, they have hardly been verified experimentally because of the lack of appropriate material systems. This thesis is an attempt to develop such a system using metallic cobalt as a magnetic material and to verify its magnetic behavior in the context of a Stoner-Wohlfarth model of coherent rotation. The problem of preparing crystals of a desired shape and the effect of the crystal shape on its magnetic properties is also addressed. Cobalt nanocrystals are prepared by thermal decomposition of dicobalt octacarbonyl in solution and in the presence of suitable surfactants and coordinating ligands, which influence the shape of the resulting crystals as well as their internal structure. The presence of trialkylphosphines in the growth solution leads to the formation of spherical nanocrystals with mixed fcc-hcp structure, where as trioctylphosphine oxide leads to a newly discovered structure of [epsilon]-cobalt. The final size of the crystals is controlled by the precursor-to-ligand ratio, and low polydispersity is achieved by the separation of nucleation and growth stages. Size-selective precipitation is used to further reduce the size variation of the samples. As a result, cobalt nanocrystals in the size range of 4-12 nm in diameter can be routinely produced with size distributions as small as 6%. The study of magnetic properties reveals the superparamagnetic nature of cobalt nanocrystals of this size range at room temperature. At low temperatures, a good qualitative agreement with the theoretical (Stoner-Wohlfarth) model is found,
(cont.) although quantitative results are strongly influenced by the presence of an oxide shell around each nanocrystal. The presence of two surfactants (trialkylphosphines and sodium carboxylates) during the growth leads to the formation of a significant number of triangular and rod-shaped nanocrystals. Unlike disordered spherical particles, these nanocrystals have pure fcc structure without visible defects. The length of the rods is roughly controlled by the concentration of carboxylates in the growth solution and can be changed within a 40-400 nm range. Unlike spherical crystals of comparable volume, the rods are ferromagnetic even at room temperature due to an added effect of shape anisotropy. A growth mechanism for the formation of nanorods with cubic structure is also proposed.
Description
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2001.
Includes bibliographical references.
Subjects
Chemistry.
MIT Department
Massachusetts Institute of Technology. Department of Chemistry
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