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nanomaterials.
175
In this synthesis, preformed PbSe particles were reacted
with cadmium oleate at 100
C, resulting in cation exchange on the surface,
yielding PbSe/CdSe particles with a CdSe shell of up to 1.5 nm thick irre-
spective of reagent concentration and reaction conditions. XRD con
rmed
the growth of a core/shell species rather than an alloyed material. The
emission was found to shi
to shorter wavelength as the emitting core
d
n
1
y
4
n
g
|
3
e
ectively became smaller, while the quantum yield of the smaller particles
remained high (
ca.
70%), and the smaller core/shell particles had signi
-
cantly improved emission of up to 17%. The deposition of the shell improved
the particle stability, from a time scale measured in days, to materials that
still exhibited excellent properties months later. Once a CdSe shell had been
deposited, the epitaxial growth of a further ZnS shell was attempted onto the
pre-existing CdSe layer, using the typical precursors such as S(SiMe
3
)
2
and
Me
2
Zn in the presence of TOP, giving PbSe/CdSe/ZnS particles. Although
successful,
ciency.
A similar method was used to grow PbSe/CdSe/CdS particles, using cadmium
oleate and sulfur dissolved in ODE at a growth temperature of 240
C.
176
It
was found that the reduction in emission quantum yield while depositing the
the additional
layer actually reduced emission e
nal shell was actually due to annealing at the core/shell interface due to
heating, and the observed blue shi
was due to alloy formation, which was
reversed upon CdS deposition as the electron wavefunction delocalised into
the new shell. The emission quantum yield was found to stabilise at 10%.
These particles are notable for their ultra-long recombination life time of
80
S due to the reduced electron/hole overlap, almost two orders of
magnitude longer than the core particles. The use of a lower deposition
temperature (170
C) for the CdS shell resulted in an unusual tetrapod
morphology, with CdS arms on spherical PbSe/CdSe particles.
Core/shell particles of PbS/CdS could also be grown using ion exchange,
and these particles could be phase-transferred to aqueous solution using an
amphiphilic polymer while maintaining an impressive emission quantum
yield of
ca.
30% in the infrared region.
177
The cation exchange method was
extended to the preparation of PbTe/CdTe particles
178
as both semi-
conducting materials exist in the rock salt phase, with almost no lattice
mismatch. Extensive high-resolution electron microscopy revealed the
seamless match between core and shell, when viewed along a speci
m
.
c crys-
talline axis which allowed extremely high-quality images of core/shell parti-
cles to be produced (Figure 5.10). The core particles were found to have
almost entirely (111) crystalline plane edges, suggesting an anisotropic
exchange mechanism.
Other notable systems include the use of Cu
2
S, which was used as both
a core and a shell with CdS.
179
As the bandgaps were staggered, either
combination gave a type II alignment and both structures (Cu
2
S/CdS and
CdS/Cu
2
S) were prepared simply using existing precursor routes. By tuning
precursor ratios and controlling reaction times (giving di
ering core sizes
and shell thicknesses), the entire visible range was accessible and growth was
allowed to continue until the desired emission wavelength was reached, with
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