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evidence of the core/shell structure, displaying band edge emission with no
evidence of surface trapping states. The quantum yields of the core particles
were reported as 40%, which rose to 85% with the deposition of one
monolayer. This value was maintained until 3 monolayers were deposited,
dropping to 25% with the deposition of 6 monolayers. This drop was
attributed to weaker con
ects as the particle increased in size. In
this case, the charge carriers were mainly delocalised in the CdSe core. This is
an excellent example of how the emission wavelength can be varied in
core/shell structures, with photoluminescence being tuned through the
entire visible region of the spectrum by growth conditions. This range of
colours cannot be achieved by either the CdSe or ZnSe materials on their
own. Also, the emission obtained appeared to show no evidence of trapping
states. In comparison, emission from CdSe/ZnS nanoparticles cannot be
tuned, with the emission of the CdSe remaining constant.
nement e
d
n
1
y
4
n
g
|
3
5.4.2 Multiple-Shell Structures
The core/shell CdSe/ZnS system with a
of CdS was described
earlier and found to produce better-quality nanorods, as a result of the
decreased lattice strain. This strain resulted in uneven particle formation
and defects at the core/shell boundary resulting in the reduction of quantum
yields. The incorporation of a bu
'
bu
er layer
'
er layer has been developed further,
leading to the preparation of double shell particles that overcame the strain
between the interface of core/shell systems, notably CdSe/ZnS. The CdSe/ZnS
system is ideal in many respects, when compared to CdSe/ZnSe, for example.
The barriers for charge carrier con
.
nement are better in the ZnS shell, where
the hole has an energetic o
set of 0.6 eV as compared to only 0.1 eV for the
analogous ZnSe shell. The lattice mismatch is, however, better for the ZnSe
shell with a mismatch of only 6.3%. CdSe/ZnSe/ZnS nanoparticles have
been reported, building on the synthesis of CdSe/ZnSe core/shell particles
described earlier including the addition of the ZnS shell using
Zn(CO
2
(CH
2
)
16
CH
3
)
2
and either S(SiMe
3
)
2
or CH
3
C(S)NH
2
. The ZnS shell was
deposited at 200
C with no evidence of separate ZnS nucleation observed.
110
Thick shells, 5 monolayers of both the ZnSe and ZnS, were deposited, and
shell growth was evidenced by the bathochromic shi
of the emission spectra
as the exciton leaked into the shell. The quantum yields were found to be
14.8% for the CdSe/ZnS system, 17.6% for the analogous CdSe/ZnSe system,
and 25.3% for CdSe/ZnSe/ZnS.
111
CdSe/ZnSe/ZnS nanoparticles have also
been prepared for optical data storage applications; in this case, the core
particles of CdSe were synthesised using CdO, oleic acid and TOPO in ODE as
described by Peng (see Chapter 1). The shell of ZnSe, 1 nm thick, was
deposited using Et
2
Zn and TOPSe and the ZnS shell, 1.5 nm thick, was
deposited using Et
2
Zn and S(SiMe
3
)
2
.
112
Few other details were given in this
case. CdSe/CdS/ZnS particles have also been grown using octanethiol as the
sulfur source.
113
As octanethiol is signi
cantly more stable than silylated
sulfur sources, it can be used at higher deposition temperatures which
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