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structures, and that chemical potential, di
usion of monomers and strongly
binding monomer ligands (to maintain the high monomer concentration) all
need to be taken into account when assessing the complicated mode of
growth. It is also noteworthy that the classical Gibbs
-
Thompson model of
crystal growth does not
t with the observed growth phases of CdSe
nanoparticles.
Interestingly, the growth of anisotropic CdSe particles has also been ach-
ieved simply by changing the chain length on the chalcogen delivery system,
without the need for high concentrations, temperatures or phosphonic
acids.
184
The use of TBP instead of TOP (along with 2-octenoic acid) increased
the di
d
n
1
y
4
n
g
|
1
usion coe
cient and hence resulted in CdSe nanorod formation.
1.6.2 Tetrapods
Tetrapods of CdSe, present in most experiments that formed rods, were
explained as wurtzite arms growing from the four equal (111) faces out of
a zinc blende core crystal, with HPA increasing the growth of the (111) facets
in the same manner as a (001
d
n
4
.
) facet in a wurtzite-structured particle
(Figure 1.4). Notably, the arms were terminated with the (0001
) facet, atomi-
cally identical to the zinc blende (111) facet. If the arms were pure wurtzite,
they continued to grow upon further addition of precursor; if they possessed
stacking faults or had zinc blende layers, up to three additional arms could be
grown at the end of the initial arm, forming dendritic structures. The pres-
ence of magic clusters has also been used to explain the growth of branched
particles, as the small clusters exist in a zinc blende-type geometry.
153
Figure 1.4
a: high-resolution TEM image of CdSe tetrapod looking down the (100)
arm. b: two-dimensional cartoon of tetrapod structure. Reprinted with
permission from L. Manna, E. C. Scher and A. P. Alivisatos,
J. Am.
Chem. Soc.
, 2000, 122, 12700. Copyright 2000 American Chemical
Society.
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