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as long as the angle is not too extreme. The wires can then di
use across
grain boundaries to form a single zinc blende junction that connects the
arms. Where no zinc blende core is present, two arms join through a high
angle grain boundary, resulting in Y-shaped nanowires. It was also noted that
all higher structures retained their nanoparticle seed. The number of
observed blunt and sharp-tipped nanowires, where the blunt end of a rod is
the end associated with the catalyst particle and the sharp end is the terminal
point of growth, supports this hypothesis. Similar results, have been ob-
tained using bismuth nanoparticles, where the bandgap size dependence of
the rods was monitored.
208
An interesting extension is the preparation of
homo- and hetero-branched structures, where bismuth seeds were attached
to a poly(ethylenimine) (PEI) passivated ZnSe nanowire backbone and then
used as seeds for further growth from the particle.
209
The density of the
branching was controlled by the density of seeds along the wire, which was in
turn controlled by the amount of particles added during the colloidal work-
up. By expanding this method, ZnSe arms could be grown on CdSe wires,
with alloy junctions of Zn
1
x
Cd
x
Se. The optical properties of these materials
showed broad absorption spectra with two features assigned to CdSe and
ZnSe. Weak emission was also observed. Similarly, ZnE (E
d
n
1
y
4
n
g
|
1
d
n
4
.
S, Se, Te)
nanowires have also been grown using a SLS-type route using bismuth
nanoparticles as seeds, TOPO or trioctylamine (TOA) as solvents and capping
agents, and a range of Zn precursors.
210
Methods for the reproducible synthesis of CdTe have also been reported
using the SLS method.
211
In this work, Au/Bi particles were used as seeds,
mixed with Me
2
Cd and injected into a solution of TOP, TOPO, a long-chain
phosphonic acid and a large excess of tributylphosphine telluride (TBPTe)
relative to the cadmium, at an elevated temperature, producing either
branched wires or straight wires depending on injection temperature and
Cd : Te ratio, with lower injection temperatures resulting in wires that dis-
played no signi
¼
cant branching. The branched wires exhibited Y, V and
merged Y structures, the zinc blende core of the wires being attributed to the
central branching points. The wires had diameters of 8
-
10 nm, with lengths
m
o
m, and composed of a mixture of zinc blende and
wurtzite phases. As the diameters of the wires are below the excitonic
diameter of CdTe, a shi
en exceeding 10
in the optical band edge was observed, although
emission quantum yields were low. An optimisation has been reported,
where straight, unbranched, exclusively wurtzite-structured CdTe wires,
several micrometres long with diameter 5
11 nm were grown using cadmium
octadecylphosphonate and TOPTe as precursors in TOPO.
212,213
Similarly, multipods of CdSe have been grown using noble metal particles
as seeds, although at much lower temperatures than is usual for the SLS-type
growth.
214
In these cases, the rods generated had unusually high quantum
yields of up to
ca.
10%. The use of BiCl
3
instead of bismuth-based nano-
particles has also been reported as a simpler method of preparing aniso-
tropic II
-
VI nanomaterials, where an acetone solution of BiCl
3
was included
with the TOPSe injection into a standard solution of CdO, oleic acid and
-
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