Synthesis of hcp-Co nanodisks Page: 3 of 28
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elongation is along the 100 direction, for which there are three symmetry equivalent
faces. Thus, there may be many avenues for symmetry breaking and shape control at
work in both gas phase and solution phase methods. One possibility suggested by Peng
for solution phase shape control is that a momentary fluctuation may lower the surfactant
coverage on a particular face, allowing that face to grow rapidly, depleting the local
monomer concentration. It has been suggested that this could set up a self-sustaining
gradient in monomer that continuously acts to feed more monomer to the growing facet
. Banfield and coworkers  and more recently Weller et al.  have discussed the
concept of oriented attachment, in which anisotropic shapes are obtained by selective
fusion of faceted nanocrystals along particular directions. Similar results have been found
in Pt particles collapsing to form wires  and iron nanorods . Other disk-like
nanoparticles have been recently reported on the literature .
Of current interest in the literature of nanocrystal shape control is the case of
Co. Co possesses multiple crystal structures (hcp, fcc, and epsilon [11,12]) that
are close in energy, suggesting that surfactant adhesion or modest variation in
temperature may be used to control the crystal phase, size and shape, and thus,
ultimately determine the physical and chemical properties. In this paper we extend
our earlier work on the synthesis of Co nanocrystals via the thermal decomposition of
Co2(CO)s in a mixture of hot organic solvent and surfactants . We report conditions
of temperature and surfactant composition for which there is a high yield of anisotropic
nanocrystals. Second, we demonstrate that the anisotropically shaped nanocrystals we
reported earlier and that we confirm now, are in fact hcp disks and not rods as previously
thought. The disks appear in TEM to be rods due to their propensity to form stacks that
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Puntes, Victor F.; Zanchet, Daniela; Erdonmez, Can K. & Alivisatos, A.P. Synthesis of hcp-Co nanodisks, article, June 11, 2002; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc737319/m1/3/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.