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allowed the formation of an alloy layer rather than a distinct core/shell
structure.
114
Talapin
et al.
have carried out a detailed study into the preparation of
CdSe/CdS/ZnS and CdSe/ZnSe/ZnS nanorods and spherical nanoparticles.
115
In the case of spherical CdSe/CdS/ZnS particles, the CdSe/CdS constituent of
the material was prepared using the green chemical route described earlier.
The core/shell particles were then isolated and a ZnS shell deposited
by addition of Et
2
Zn and S(SiMe
3
)
2
to a TOPO solution of the preformed
core/shell particles. CdSe/CdS/ZnS rods were made in a similar fashion,
using CdSe/CdS rods prepared as described above, with a spherical CdSe
core and elongated CdS shell.
116
CdSe/ZnSe/ZnS particles were prepared by
preparing CdSe particles capped with TOPO, TOP and HDA, with shells
deposited using metal alkyls, TOPSe and S(SiMe
3
)
2
. The materials showed
strong emission (quantum yields of 70
d
n
1
y
4
n
g
|
3
-
85%) at
ca.
550 nm, with the ex-
pected red shi
upon deposition of a ZnSe shell on the CdSe core, consis-
tent with exciton leakage due to the relatively small potential barrier. Again,
ZnS shell thicknesses beyond 2 monolayers resulted in a decrease in
quantum yield. Prior to this work, CdSe/ZnS was considered the most
photochemically stable core/shell system, surpassing CdSe/CdS and CdSe/
ZnSe for resistance to environmental in
uences. Examination of compa-
rable CdSe/ZnS and CdSe/ZnSe/ZnS (shell thickness of 1.5/3 monolayers for
ZnSe/ZnS) particles under intense laser illumination showed that
the
double shell particles were the most stable.
The preparation of core/shell/shell materials and the gradual easing of the
lattice strain was advanced further using SILAR deposition to prepare a three-
shell system, CdSe/CdS/Zn
0.5
Cd
0.5
S/ZnS.
117,118
The nature of the deposition
allowed careful, exact shell growth. The CdSe core particles were grown from
CdO in TOPO and ODA as described by Peng (see Chapter 1), followed by
redispersion in an ODE solution of ODA. The shells were grown (using the
metal oxides and sulfur in ODE) by sequential addition of either ion in
solution to build the structure half a monolayer at a time. Over a 3 hour
period, a structure was grown that consisted of CdSe cores with 2 monolayers
of CdS, 3.5 monolayers of Zn
0.5
Cd
0.5
S and 2 monolayers of ZnS, capped with
the amine. The particles were found to be highly spherical in shape, in
contrast to CdSe/ZnS particles made by metal alkyl precursors. The gradual
addition of the shells could be tracked through the emission and absorption
spectra, as the exciton leaked into the extended shell structure, with the
.
nal
material exhibiting a quantum yield of 80
-
90%. The gradual overall red shi
was accompanied by a
er the addition of 1.5 monolayers of
the ZnS shell, attributed in this case to the formation of a Zn
x
Cd
1
x
S alloy
shell. Phase transfer to water resulted in the quantum yield dropping to
almost half its original value. Photo-oxidation studies of the three-shell
material showed enhanced stability, although the material gradually oxi-
dised over 120 hours of illumination, with the quantum yield dropping to
approximately half its original value, although this was found to be slightly
better than CdSe/ZnS material made using metal alkyls as described above.
nal blue shi
a
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