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Interestingly, rods of CdSe have also been used to make alloyed rods of
ZnCdSe by adding ZnSe precursors to CdSe rods, followed by a prolonged
annealing step.
38
The emission quantum yield of these rods was up to 10%,
signi
cantly higher than that of simple CdSe rods.
Spherical CdSe particles have also been capped with a ternary shell of
CdZnS using an amendment on the usual precursors.
39
In this case, puri
ed
TOPO, TOP, HDA and HPA were degassed, and injected with a mixture of
Cd(CH
3
COCHCOCH
3
)
2
, TOPSe and hexadecanediol in TOP at 360
C. The
particles were le
d
n
1
y
4
n
g
|
3
to grow at 280
C, annealed overnight at 80
C and then
isolated. The size distribution was narrowed by size-selective precipitation,
followed by redispersion in TOPO with HPA. Dropwise addition of a TOP
solution of Et
2
Zn, Me
2
Cd (4 : 1 ratio) and a threefold excess of S(SiMe
3
)
2
at
145
C followed by overnight annealing resulted in core/shell particles with
a quantum yield maximum of 46%.
5.3.2 CdSe/CdS
Nanoparticles of CdSe can also be prepared with a shell of just CdS.
40
The
energy gap is aligned to con
ne charge carriers suggesting a type I structure,
and the lattice mismatch (only 3.9%) is su
cient to prevent alloying while
allowing epitaxial shell growth. In the seminal report of CdSe/CdS, the core
particles were grown by the injection of Me
2
Cd and TBPSe (with a slight
excess of cadmium precursor) into TOPO at elevated temperatures, followed
by growth to the required size. Isolation by non-solvent interactions was
followed by dissolution in pyridine, exchanging the surface cap for the more
labile ligand. A
.
uxing under nitrogen, the shell stock
solution of Me
2
Cd and S(SiMe
3
)
2
(1 : 0.4 Cd : S ratio) was added dropwise at
100
er prolonged re
C. A
er growth, DDA was added until the particles precipitated,
exchanging the capping ligand. The particles were then soluble in chlori-
nated solvent, but not pyridine. Growth of discrete CdS particles was also
observed, and avoided by changing the Cd : S ratio to 1 : 2.1, whereupon only
shell growth was observed. This unusual method of growth was developed
a
er the failure to grow CdS shells on CdSe cores in TOPO at 200
C: a more
labile surface ligand was required. The low Cd : S ratio used in shell growth
was explained due to the cadmium-rich precursor solution for the core
particles, which may have given particles with a metal-rich surface. The
particles were analysed by the usual methods, notably XPS which was used to
probe shell thickness using the mean free path of the emitted photoelectron.
High-resolution electron microscopy also revealed particles with a reduced
image contrast on the edges, consistent with a material with fewer electrons
per unit cell than the core.
The absorption spectra were found to be shi
ed, which was explained by
the perturbation of the core energy levels by the shell, as previously described
for CdSe/ZnS core/shell particles. This is neatly explained in Figure 5.3a and
b, where the new energy levels emerged when the core/shell particle formed.
The molecular-like energy levels, a consequence of quantum con
nement,
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