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phase of the CdS particles produced could be controlled between cubic and
hexagonal, by tuning reagent concentrations, and also temperature. The
anisotropy observed appeared to be driven by a small amount of hexagonal
structure in the cubic particles. Wurtzite-structured CdSe rods have also been
grown by redox-mediated Ostwald ripening.
165
In this case, spherical CdSe
particles with a water-soluble surface amine ligand, when annealed in water
at low temperature, grew asymmetrically at the expense of the smaller
particles. The surface selenium oxidised and was then reduced in solution by
the amine, thus generating Se
0
. When this occurred in the presence of extra
cadmium precursor, growth along the
c
-axis occurred. A similar phenom-
enon has been observed by the inclusion of oxygen in the growth of CdSe due
to oxygen passivation of speci
d
n
1
y
4
n
g
|
1
c crystal facets.
166
Cadmium telluride rods with a wurtzite crystal core were prepared by the
thermolysis of Cd(CO
2
(CH
2
)
16
CH
3
)
2
and TOPTe in TOPO, although the
concentration used was not particularly high (tens to hundreds of milligrams
of precursor in 4 g TOPO).
167
Zinc selenide rods and polypods could also be
prepared by varying the concentration of precursor solution (Et
2
Zn and
TOPSe), precursor delivery rate and temperature of a method described
earlier.
108
This route is unusual when compared to the analogous CdSe
system; in the case of ZnSe, a smaller amount of precursor was added to
a larger amount of solvent, resulting in a dilute solution. A slower injection
rate also promoted the growth of rods with a high aspect ratio of up to 8.
Injection of precursor at a high injection rate and a lower temperature
resulted in polypods, as did using the same conditions for rod growth except
for a larger volume of precursor. Anisotropic ZnSe particles displayed
di
d
n
4
.
raction patterns consistent with wurtzite crystals (as opposed to the zinc
blende spherical particles), with growth along the
c
-axis, although HRTEM
studies con
rmed the existence of both zinc blende and wurtzite phases in
all anisotropic ZnSe structures. Zinc selenide rods could also be grown using
zinc oxide or carboxylates as previously described.
109
1.6.1 Di
usion Model for Anisotropic Growth
Further work revealed a total of four growth stages, which could not be
explained entirely by the directed binding of phosphonic acids described
above, and a di
rst growth phase
was described as the 1D growth stage, where the monomer concentration was
1.4
usion mechanism was suggested.
168
The
2% and resulted in rods growing along the long axis. The second phase,
previously unobserved in the initial studies, occurred when the monomer
concentration had dropped to 0.5
-
1.4% and was termed the 3D growth stage.
In this region, the particles grew in all three dimensions, maintaining the
aspect ratio from the
-
rst phase but increasing the overall volume. The third
stage (1D/2D ripening stage) occurred when the monomer concentration
stabilised at 0.5%, resulting in the nanoparticles increasing along the short
axis and decreasing along the long axis. The volume and number of nano-
particles appeared constant, suggesting that the monomer actually shi
ed
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