Chemistry Reference
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have been used for the deposition of the ZnSe shell.
54
ZnSe is an ideal shell
material for CdSe dots; the bandgap (2.72 eV) is wider than that of CdSe
(1.74 eV) giving a type I structure, the lattice mismatch is low (6.3%) and the
same anion is maintained. The core particles were prepared as described in
Chapter 1, by the thermolysis of a cadmium carboxylate and TOPSe in TOPO,
TOP and a long-chain amine at 250
C. The particles were isolated and
redispersed in a hexane solution of TOPO and long-chain amine, then heated
to 190
C. A syringe pump then injected a toluene solution consisting of an
equimolar amount of Zn(CO
2
(CH
2
)
16
CH
3
)
2
and TOPSe within 1 hour, at
a temperature of 190
d
n
1
y
4
n
g
|
3
200
C, which was followed by annealing for up to
1.5 hours. The quantum yield increased up to a maximum of 85%, although
the number of monolayers this related to was not reported. The absorption
spectra red-shi
-
ed slightly as the exciton leaked into the shell, although the
width (FWHM) of the emission pro
le remained unchanged at
ca.
28 nm.
The CdSe/ZnSe particles were stable when phase-transferred to water and
exhibited no drop-o
in emission intensity, an essential property of particles
to be used in biological imaging.
An unusual route to core/shell particles has been reported by Schreder
et al.
who described the synthesis of CdTe/CdS structures starting from CdS
particles,
i.e.
, with the shell material
rst.
55
In this route, the initial CdS
particles were prepared by complexing CdCl
2
to TBP in CHCl
3
, followed by
the addition of S(SiMe
3
)
2
and eventual re
cation
resulted in a yellow powder of CdS nanoparticles, estimated to be
ca.
3nmin
diameter as determined from XRD measurements. The particles were then
dissolved in CH
2
Cl
2
, to which was added CdCl
2
and Te(SiMe
3
)
2
, with a Cd : Te
ratio of 2 : 1. It is worth noting that no excess stabilising ligand was used. The
reaction started immediately and took up to 30 hours at room temperature to
complete. The cause of this seemingly reverse reaction was the rapid anion
exchange, with the tellurium ions expelling and replacing the sulfur ions,
which then in turn reacted with the excess cadmium chloride in solution to
yield the shell. The
uxing. Rigorous puri
.
nal material was estimated to be
ca.
3 nm in diameter as
determined by the position of the excitonic peak in the absorption spectra,
which supported the ion exchange hypothesis. The material structure was
con
ections from the as-prepared particles were
weak as a result of the low temperature of synthesis. Annealing of spin-coated
rmed by XRD, although re
rmed the zinc blende structure. Emission quantum yields increase
from virtually zero to 10% a
lms con
er the reaction and formation of the core/shell
structure, with a shi
in the absorption edge observed during the conversion
from the CdS core to the CdTe core. Addition of excess sulfur precursor to the
in the emission spectrum from
550 nm to
ca.
650 nm. This highlights the complex growth mechanism, and
the shi
nal product resulted in the further red shi
was attributed to the increased shell straining the core.
One interesting point regarding the growth of tellurium-containing
nanoparticles is that aqueous-based routes utilising thiols as capping agents
appear to be more e
ective than organometallic routes, attributed to the
thiol blocking trapping states. Wuister has nicely highlighted this, where
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