Chemistry Reference
In-Depth Information
The synthesis of CdTe/CdSe has been improved by using CdO as
a precursor rather than the metal alkyls, and using TOP rather than TOPO,
building on the earlier results described in Chapter 2 which highlighted the
preferred surfactant system for CdTe.
85
Sequential addition of the selenide
precursor to the CdTe solution allowed the core/shell system to be prepared,
with particles growing from
ca.
3 nm up to 7 nm upon shell addition. Using
this system, the initial core CdTe particles were prepared with quantum
yields of up to 31%. Upon addition of the CdSe shell, the emission shi
d
n
1
y
4
n
g
|
3
ed
from
ca.
660 nm to
ca.
715 nm over a growth period of 4 hours at 200
C.
During this period, the quantum yield dropped immediately from 31% to
below 5%, but recovered over the growth period to almost 40%, then dropped
slightly to below 30% upon further shell growth. Also noteworthy were the
change in the emission FWHM which narrowed then widened upon pro-
longed growth, and the gradual overall increase in the Stokes shi
. The
highest quantum yield obtained (38%) was attributed to CdTe with one
monolayer of CdSe on the surface. The use of a SILAR-type technique in the
preparation of CdTe/CdSe in more traditional solvents such as long-chain
amines and TOPO resulted in particles with quantum yields of up to 82%.
Initially, CdSe
were observed on the CdTe particles, growing into
pyramids and ultimately multipods. The remarkably high quantum yields
were attributed to a low defect density due to the slow epitaxial growth and
the low degree of strain at the interface due to formation of the rod-like
particles.
86
The shape tuning of CdTe/CdSe heterostructures has also been
achieved by preparing CdTe cores by standard chemistry, followed by the
addition of a cadmium precursor to the particles with a mixture of phos-
phonic acids and the dropwise addition of TOPSe at a set temperature.
87
The
de
'
tips
'
.
ning parameter was determined to be the size of the CdTe seed, where
small (<4 nm) seeds incurred unidirectional growth of the CdSe component,
while larger (
ca.
4.3 nm) seeds resulted in Y-shaped particles, and seeds
larger than 5 nm resulted in the formation of tetrapod heterostructures.
Surprisingly, growth of CdSe on 3.5 nm seeds resulted in a signi
cant red
shi
le of up to
ca.
200 nm, consistent with type II
behaviour, while maintaining impressive emission quantum yields of up
to
ca.
39%.
Type II CdSe/CdTe have also been prepared using the SILAR method, using
standard precursors in a non-coordinating solvent.
88
The use of the standard
SILAR technique resulted in peanut-shaped particles, whereas the use of
thermal cycling
in the emission pro
—
the repeated low injection temperature of
the shell
precursor followed by a high growth temperature
—
resulted in spherical
particles. The resulting particles had signi
cantly higher quantum yields
(up to 60%) than other similar type II materials, suggested to be due to the
presence of CdTe as a shell layer where the heavy hole was better con
ned to
the interface junction by the electrostatic attractions from the CdSe core, and
the improved shell deposition a
orded by the SILAR shell deposition tech-
nique. Further work on the structure of typical CdTe/CdSe particles con
rmed
the material was indeed a core/shell structure, although CdSe/CdTe particles
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