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in diameter, were grown in TOPO at 250
C using [Cd(SeCNMe(
n
-hex))
2
]
2
.
A shell of CdS 0.4 nm thick was deposited using [Cd(SCNMe(
n
-hex))
2
]
2
which
was simply added to the reaction vessel a
er 30 minutes of core particle
growth. Although quantum yield measurements were not reported, the
exciton leakage into the CdS shell was clearly visible in the optical spectrum.
This work was advanced to cover the preparation of CdSe/ZnS and CdSe/
ZnSe core/shell particles using asymmetrical single-source precursors,
[M(ECNMe(
n
-hex))
2
]
2
(M
d
n
1
y
4
n
g
|
4
S, Se).
49
Again, the main evidence of
the formation of core/shell materials was the optical spectrum, which
showed leakage of the excitons into the shell; the shi
¼
Cd, Zn; E
¼
was only slight for
CdSe/ZnS, yet signi
cantly more pronounced for CdSe/ZnSe. All emission
was near band edge and reportedly strong, with no evidence of deep trap
emission. High-resolution transmission electron microscopy (HRTEM)
studies showed crystalline particles with clear grain boundaries, highlighting
that the growth of CdSe/ZnS particles is not always a simple clean epitaxial
process. Interestingly, in this report, the authors attempted to grow CdSe/
CdS composites by thermolysing two precursors simultaneously. The mate-
rial obtained exhibited the expected optical and physical properties, but the
X-ray di
values, indicative of
alloy particles, as described in Chapter 2. The use of the symmetrical zinc
diethyldithiocarbamate has also been reported as a method of depositing
a ZnS shell on CdSe core particles prepared by the green route,
50
and has
become more popular as a reproducible method of shell deposition,
51
even
for other semiconducting phases such as CuInS
2
.
52
The use of zinc dieth-
yldithiocarbamate as a shelling precursor has also been applied to the
thermal cycling technique of depositing ZnS shells.
53
In this case, CdS core
particles were prepared and puri
raction (XRD) pattern was shi
ed to lower 2
q
.
ed by standard green techniques, then the
single-source precursor added (in a OAm solution, used to reduce the
decomposition temperature) at 120
C, then the temperature raised to 160
-
200
C for shell growth. This was then repeated to grow several layers of the
shell. The quantum yield of the resulting CdS/ZnS particles rose gradually
upon layer addition, peaking at four monolayers of ZnS coverage (50%
quantum yield), with no evidence of trap emission, unlike CdS/ZnS grown by
the successive ion layer adsorption and reaction (SILAR) route.
Single-source precursors have also been used to grow QD-quantum well
structures, where a wide-bandgap core is grown, followed by a relatively
narrow-bandgap shell, which is capped by another wide-bandgap material,
con
ning the charge carriers to the middle shell.
54
7.5 Other Single-Source Precursors
Although compounds with the general formula M(ER)
2
were used in the
formation of nanomaterials in seminal work described earlier, the materials
obtained were substandard relative to the high-quality TOPO-capped parti-
cles described in Chapter 1. This can in part be attributed to the low
temperature of growth. The thermolysis of related compounds in TOPO has
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