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Tips could also be grown on the arms of tetrapods and on CdSe/ZnS rods.
Coupling between gold tips and CdSe/ZnS rods was evidenced in the
absorption spectra where the
ne structure was lost and increased absorp-
tion in the visible region was observed. Quenching of the emission was also
observed, attributable to the electron transfer to the metal.
192
Conductive
atomic force microscopy (AFM) experiments con
rmed the metal tips were
conducting, while the rods remained semiconducting. Linking the rods was
achieved by adding a solution of dithiols, which selectively bound the ends of
the rods.
Similarly, asymmetric tetrapods have been prepared by depositing a layer
of CdTe tetrapods on a substrate in a tetrahedral con
d
n
1
y
4
n
g
|
1
guration, followed by
partial coating with a polymer. The exposed arms were passivated with
dithiols, and then exposed to gold nanoparticles, which coordinated to the
thiol linker. As a result, multiple nanoparticles coordinated to the arm. The
polymer was dissolved, leaving the structures exposed on the substrate.
Removing the tetrapods was problematic, as the deposition process results in
the
d
n
4
.
attening of the three tetrahedral legs. By mechanical interactions, the
gold-capped arms could be removed, leaving CdTe rods capped with gold
particles.
196
Hyperbranched CdTe particles have also been capped with gold
on the tips, using preformed CdTe nanostructures, DDAB and HAuCl
4
.
197
Other semiconducting phases have also been grown on the tips of nano-
rods; lead selenide (PbSe) tips have been grown on both ends of CdSe and
CdS rods by addition of the relevant precursors (to be discussed later) to
a diphenylether/oleic acid solution of rods at 130
C.
198,199
Higher tempera-
tures of synthesis resulted in the unselective growth of PbSe on the lateral
faces of the crystal. A lower concentration of precursor delivered by a fast
injection resulted in the formation of tips on just one end of the rods (65% of
the sample
—
the remaining structures were asymmetric dumbbells, with one
tips signi
cantly larger than the other). Growth of PbSe tips on CdS rods
required a higher growth temperature and faster precursor injection,
although growth was always uniquely on the ends of the rods. Due to the
higher temperature of growth, separate PbSe crystals always formed. Unlike
the gold tips on CdSe rods described above which had no preferred growth
with respect to the wurtzite structure of the rod, the (002) planes of the PbSe
rock salt structure were aligned with the (100) planes of either CdSe or CdS.
The preparation of tipped rods was extended to the preparation of highly
organised, designed heterostructures with speci
c topologies. By controlling
the chemistry, distinct structures were prepared, o
en in a single-pot reac-
tion.
200
In a typical reaction, CdS rods were prepared, and tipped with CdSe
(extensions) on both ends. This was achieved by adding the anion precursor
to the reaction mixture at the required point, controlling the precise length
and composition of the rods. Addition of a tellurium precursor resulted in
the formation of a branching point
via
a CdTe tripod on the terminal CdSe
extension. Attempts to make the same structures in reverse order resulted in
CdTe tripods with a shell of CdSe. In a similar manner, CdSe tripods could
have CdTe extensions added to the wurtzite arms, which could also form
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