Chemistry Reference
In-Depth Information
During attempts to prepare Mn- and Fe-doped CdSe, CdSe tetrapods have
serendipitously been produced in large amounts due to the generation of
acidic protons by addition of the metal salts and acids to the cadmium
precursor prior to TOPSe injection.
185
The
in situ
generated H
+
ions directed
the growth of a zinc blende crystalline core, which resulted in wurtzite arms
growing on the (111) facets. CdSe tetrapods have also been prepared using
quaternary ammonium salts, which were introduced into the reaction with
TOPSe in a typical reaction. Of the particles generated by this route, 90%were
tetrapods with numerous rods, bipods and tripods being observed
alongside.
186
A key factor identi
d
n
1
y
4
n
g
|
1
ed in the preparation of tetrapods is the energy
di
erence between the two polytypes,
187
which dictates the reaction
temperature, as ideally one polytype should form during nucleation and the
other during growth. In cases where the energy is more than 10 meV per
atom, it is di
ering structures. In the case of
materials such as CdS, CdSe and ZnS the energy gap is signi
cult to alternate between the di
d
n
4
.
cantly smaller,
making it di
c phase. CdTe is an intermediate
material where the preparation of zinc blende particles is possible at
temperatures normally associated with the wurtzite structure. Phosphonic
acids used during CdTe growth were found to bind speci
cult to grow one speci
cally to non-polar
faces in wurtzite structures (Figure 1.5),
187
(which have no equivalent in the
cubic material) reducing the growth of these facets, facilitating the growth of
a cubic core and wurtzite arms resulting in tetrapods. By speci
cally
controlling the kinetics, the ratio of Cd : Te monomers and amount of
n
-
octadecylphosphonic acid, high yields (
ca.
70%) of CdTe tetrapods with arms
of almost equal length were obtained, although bipods and tripods were also
found. Once the monomer concentration had dropped, the facet with the
highest interfacial energy started to dissolve, and formmore stable facets. As
this facet is the one that grows fastest in solution, this resulted in the round
ends on the arms observed in some samples. Tripods were found to be
extremely soluble relative to simple rods of an equal length, reinforcing the
comparison to dendrimers and polymers. The optics of CdTe tetrapods have
been investigated and the properties found to be highly dependant on the
particle size and structure.
188
Later work also suggested that an impurity in
n
-octadecylphosphonic acid (the shorter-chained methylphosphonic acid)
was responsible for forming the multiple wurtzite twin boundaries which is
responsible for driving the branching.
189
Tetrapods have also been prepared
at a relatively low temperature of only 180
C using a non-coordinating
solvent and oleic acid as a capping agent, a weaker binding ligand than the
phosphonic acids. In this case, the rapid release of monomers from the
cadmium oleate complex was suggested to be su
cient to allow di
usion-
limited growth.
190
A notable calculation from Thomas and O
Brien has sug-
gested that the formation of branched particles may be in response to
minimising the percentage of surface atoms and therefore lowering the
internal energy or maximising the lattice energy. This also explains why
systems without templates can grow into multipod structures.
191
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