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cores were prepared by green chemical methods as outlined by Peng,
followed by the growth of 1
-
5 monolayers of ZnS deposited by the SILAR
technique. The
nal shell, of either CdSe or CdS, was again grown by SILAR.
All precursors were metal oxides/oleic acid, and elemental chalcogens used
with ODE and ODA. Temperatures for growth and annealing varied between
190
C and 240
C. High-resolution TEM showed particles with a spherical
shape and narrow size distribution, highlighting the e
d
n
1
y
4
n
g
|
3
ectiveness of shell
deposition by SILAR. Optical spectra are shown in Figure 5.8, where the two
emission bands are clearly visible. The total emission quantum yield was
measured at 30%, and each peak could be tuned, by altering the core size
and/or the shell thickness. The intensity of emission could also be tuned by
varying the length of time the particles were annealed, with the 2D emission
increasing upon prolonged heating. Coupling between the two emission
centres was observed, with no emission being detected from the 2D layers
when only one monolayer of ZnS was used, and core emission shi
ed
towards the blue end of the spectrum with the increase in thickness of
the bu
er layer. Forster energy transfer was also observed in the system,
especially when the outer layer of CdSe was 5 monolayers thick.
Few multiple-shell systems have been reported with a CdTe core, although
CdTe/CdSe/ZnS particles have been prepared using typical green chemistry
as described in Chapter 1 for the CdTe cores, a modi
ed SILAR route for the
middle CdSe shell, and a single-source precursor (Chapter 7) for the
nal
ZnS shell.
130
The resulting structures exhibited emission which could be
tuned between 550 and 850 nm, with a maximum quantum yield of 94% for
CdTe with 1 monolayer of CdSe, which dropped signi
.
cantly upon further
layer depositions to
ca.
46%.
Figure 5.8
(Le
) Diagram showing band structure of CdSe/ZnS/CdSe. (Right)
Absorption, emission and excitation spectra of both emission peaks
for CdSe/ZnS/CdSe. Reprinted with permission from D. Battaglia,
B. Blackman and X. Peng,
J. Am. Chem. Soc.
, 2005, 127, 10889. Copyright
2005 American Chemical Society.
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