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particles were found to photo-brighten upon annealing, puri
cation and
photo-oxidation. The quantum yield was found to dip a
er the injection of
each shell component, a
er which the emission then increased. This was
attributed to the rapid growth of the shell causing structural disorder. This
same method has been used to prepare structures with a maximum quantum
yield of 92% when 5 monolayers of CdS had been deposited.
47
The SILAR
technique has been used to prepare CdSe/CdS in an in-depth study exam-
ining reaction conditions and optical properties, allowing a calibration plot
to be realised, allowing the accurate determination of shell thickness without
the need for high-resolution electron microscopy.
48
The study also high-
lighted that charge carriers can be found at the surface, despite shell thick-
nesses of up to 2.2 nm.
The use of the SILAR method has been extended to the preparation of CdSe
particles with a giant CdS shell, designed to isolate the core wave function from
the surface.
49
Core CdSe particles with a diameter of 3
d
n
1
y
4
n
g
|
3
19
monolayers of CdS deposited sequentially. The shell materials dominated the
absorption spectra, while the emission pro
-
4 nm had 18
-
ed from
the original band edge, although they were not attributed to the shell mate-
rials. As expected, the optical properties of the materials were insensitive to
ligand exchange, with quantum yields of up to 40%. The synthesis also allowed
for the preparation of CdSe/11CdS/6Cd
x
Zn
y
S/2ZnS shelled materials, with
a graded alloy Cd
x
Zn
y
S shell, which had quantum yields of
ca.
10%. Interest-
ingly, the materials exhibited signi
les were signi
cantly shi
cantly reduced optical blinking, and long
multiexciton lifetimes, consistent with suppressed Auger recombination.
50
The growth of CdSe/CdS core/shell particles can also be achieved by a two-
phase method in an autoclave as a reaction vessel, using similar precursors.
In this case, Cd(CO
2
(CH
2
)
12
CH
3
)
2
and oleic acid were dissolved in toluene
and heated until dissolved, followed by the addition of an aqueous solution
of SeC(NH
2
)
2
.A
.
er the reaction at 180
C for
ca.
20 minutes the reaction was
cooled and the nanocrystals precipitated with ethanol. The particles were
then dissolved in toluene, and fresh batches of cadmium precursor and oleic
acid were added. Further autoclaving was a
ected until the cadmium
precursor had dissolved, followed by addition of an aqueous solution of
thiourea. The reagents were then heated at 140
C for 4 hours, and then
allowed to cool and isolated. The material showed band edge emission with
quantum yields of 60
80%, with no evidence of deep trap emission. The
resulting emission was between
ca.
450 and 550 nm. The reaction was
notable for the small size of the core particles used; between 1.2 and 1.5 nm
core with up to 5 monolayers of shell deposited.
51
-
5.3.3 Other Type I Core/Shell Materials Based on Group II
Chalcogenides
The preparation of materials that emit e
ectively in the blue region of the
spectrum is desirable for nanoparticle chemists. The most studied system,
CdSe, would require particles below 2 nm in diameter to emit at such
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