Chemistry Reference
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yields of almost 100%, although no speci
c details were provided. Typically,
shells up to 3 monolayers thick were grown on core between 2.3 and 3.9 nm
in diameter. A
er 2 monolayers of shell growth, the quantum yield was found
to drop, consistent with other core/shell particles. The ability to grow thicker
shells of CdS on CdSe while still increasing the emission when compared to
the CdSe/ZnS system can be attributed directly to the smaller lattice
mismatch, possibly providing a heterojunction with increased stability
towards environmental factors such as oxidation.
Altering the ratio of Cd : S in the precursor solution for shell growth
resulted in the formation of core/shell particles with unusual morphol-
ogies.
41
Core particles of CdSe were prepared using the precursors described
immediately above, with the inclusion of HDA. The shells were grown by the
slow addition of a TOP solution of Me
2
Cd and S(SiMe
3
)
2
to the crude CdSe
solution at 130
C followed by growth at 90
C. When the ratio of Cd : S was
between 1 : 1 and 1 : 1.6, spherical core/shell particles were formed. When an
excess of sulfur precursor was used (ratio 1 : 3
d
n
1
y
4
n
g
|
3
1 : 5), an anisotropic shell
grew along the
c
-axis giving rods with an aspect ratio of up to 5. The width of
initial rods was the same as the core particle diameter (3
-
-
6 nm); however,
180
C was used, the width of the rod also
increased. Two main points were highlighted for the anisotropic shell
growth; the lattice mismatch along the (001)
c
-axis was larger than along the
perpendicular (100) plane (
ca.
4.2% compared to 3.8%) and CdS growth on
the (100) CdSe face proceeded with more strain than on the (001) or (001
when a growth temperature of 140
-
)
face. Also, cadmium atoms on the (100) and (001) had only one dangling
bond, whereas cadmium atoms on the (001
.
) facet had three, making it more
active. The increased amount of sulfur precursor resulted in preferential
growth along the (001
) facet on the cadmium sites. The growth of isotropic
shells at higher temperature was explained by the temperature-dependent
transition from the low-temperature reaction kinetic-limited growth, to the
higher-temperature di
usion-limited growth. The particles prepared
exhibited large extinction coe
cients of 10
7
cm
1
M
1
, large Stokes shi
s
and quantum yields greater than 70% for rods with aspect ratios of 1.6
2, the
largest recorded for anisotropic core/shell particles at that time. An increase
in the quantum yields was also observed for rods with the same diameter as
the core particle, suggesting there was no CdS shell growth on the (100) facet
of the CdSe core. This implied a surface reconstruction or annealing e
-
ect
during the shell growth process, and that the passivation of the dangling
bonds on the (001) and (001
) facets were essential for high emission e
-
ciencies. The CdSe-seeded CdS-rod core/shell systems could also be tuned to
exhibit type II behaviour, as discussed later, by controlling the morphology of
the structures by using a signi
cantly smaller (<2.8 nm diameter) CdSe
core.
42
Despite the high quantum yields obtained for spherical CdSe/CdS parti-
cles, the reaction required the isolation of the cores, and a one-pot synthesis
is preferred utilising simple precursors. A
one-pot route to CdSe/CdS
has been described based on the organometallic synthesis.
43
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green
'
In this case,
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