Synthesis and characterization of cobalt nanocrystals

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,

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Chemistry.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2001

Author dc:creator

• Dinega, Dmitry P. (Dmitry Petrovich), 1969-

Advisor dc:contributor.advisor

• Moungi G. Bawendi.

• Chemistry.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/8250

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/8250