Chemistry Reference
In-Depth Information
prepared in a similar manner were actually a single-phase structure (solid
solution) with a uniform distribution of CdSe and CdTe, more akin to an
alloy than a heterostructure.
89
Related to this is the preparation of CdTe/CdSe bar bells, where CdTe tips
are grown on the end of short CdSe rods. These materials are of course not
strictly core/shell materials, and although type II charge carrier separation
was observed, no emission was detected.
90
Similar results were reported by
Kumar
et al.
, who also reported the growth of CdTe particles on the tips of
CdSe rods when the tellurium precursor was added slowly with an excess of
tellurium relative to selenium.
91
The same group also reported that rapid
addition of the tellurium precursor to the CdSe rods resulted in the depo-
sition of a CdTe layer along both the lateral and
c
-axis, forming a type II
quantum rod heterostructure. The heterostructures exhibited a clear extra
excitonic feature in the near infrared region which could be further shi
d
n
1
y
4
n
g
|
3
ed
towards the red with further growth, consistent with formation of the het-
erostructure. Interestingly, two distinct emission features were observed:
weak band edge emission from the CdTe exciton, and a stronger red-shi
ed
charge transfer emission originating from recombination of the CdSe elec-
tron and CdTe hole, with quantum yields of 5
10%, with trap states playing
a key role in the population of radiative states.
92
Similar chemistry was
employed to prepare curved CdSe/CdTe structures, using CdSe rods as seeds.
Addition of standard precursors for CdTe resulted in the rods growing
slightly longer and
-
at the ends, while the centre of the rods
increased by only a few monolayers. The rods also exhibited a signi
'
fattening
'
cant
.
curvature, attributed to a de
ection in the lattice planes due to deposition of
CdTe on the tips and the sides of the rod.
93
It is worth noting that in related work, Shieh
et al.
reported CdSe/CdTe/
CdSe heterostructured rods, again with almost totally quenched emission,
yet reported CdTe/CdS/CdTe rods with enhanced emission with quantum
yields up to a maximum of 20% depending upon reaction conditions.
94
Xi
et al.
have also reported the synthesis of CdTe/CdSe multiblock heter-
ostructures in a one-pot reaction.
95
In this case, CdTe rods were prepared
by the green route at 330
C,
96
using TOPTe as a precursor. A
er the rods
were prepared by multiple injections, TOPSe was introduced by two
further separate injections,
nally followed by more TOPTe. Electron
microscopy and powder di
rmed the growth of the CdSe
block epitaxially on CdTe, followed by a further CdTe block. The structures
varied in morphology, with some materials exhibiting branching. The
addition of the secondary precursors resulted in the quenching of the rod
emission.
Using similar chemistry, core/shell particles of CdSe/ZnTe 4
raction con
6nmin
diameter have also been made, although quantum yields were found to be in
the region of 10
3
%,
97
while particles of CdTe/ZnSe have also been prepared
in ODE, which had quantum yields of up to 24%. These particles were of
especial interest as the material was reported to exhibit a novel carrier
distribution along the material junction, and as such exhibited a blue shi
-
in
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