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be used, giving wurtzite-structured material; however, no noticeable emis-
sion was detected. A related non-injection route has also been described
using similar precursors, reporting ZnCdS particles with a cadmium-rich
core and an increasingly zinc-rich outer shell. The particles, with quantum
yields of up to 23%, exhibited a cubic crystalline phase.
226
Homogenous Zn
x
Cd
1
x
Se particles have been prepared by the reaction
between preformed, cooled CdSe nanoparticles and Me
2
Zn/TOPSe.
227
Conventional synthetic methodologies suggest a core/shell system would
result; however, the resulting blue shi
d
n
1
y
4
n
g
|
1
in the band edge absorption and
emission of the preformed CdSe particles suggested either the particles were
getting smaller, which was unlikely, or an alloy system formed. When
x
¼
0
(pure CdSe), the band edge emission was found at
ca.
625 nm. Gradual
increase in the zinc content shi
ed the absorption and emission by
ca.
125 nm to a band edge of
ca.
500 nm, tuning the emission from red to blue.
The formation of a core/shell system is ruled out by these observations, as
a slight red shi
d
n
4
.
is expected in the absorption spectrum of a compatible core/
shell material. XRD con
rmed the wurtzite structure and the gradual shi
to
larger angle with the increased zinc content, again con
rming the formation
of an alloy.
To investigate why the alloy formed rather than the expected core/shell
material, pre-prepared core/shell QD structures were subjected to high-
temperature annealing. Nanoparticles of CdSe/ZnS and CdSe/CdS were
heated to 300
C for 10 minutes, and the absorption spectra recorded,
revealing a slight red shi
due to Ostwald ripening. The annealing of CdSe/
ZnSe at 300
C resulted in a blue shi
in the absorption spectrum consistent
with alloying. CdSe/ZnSe di
use
easily through a semiconductor structure, the formation of an alloy structure
is easier than that of the CdSe/ZnS structure, where anion di
er only by the cations, and as cations di
usion is more
cult. Three regions of dynamics were observed: up to 270
C there was
a ripening step, with a red shi
di
in the absorption spectrum signifying slight
growth; between 270
C (the alloying point) and 290
C the alloying process
began, signi
in the absorption and emission spectra; and
heating above 290
C resulted in the rapid complete formation of stable
alloys. The particles prepared had quantum yields of 70
ed by the blue shi
-
cantly
higher than the parent CdSe particles. This was attributed to the larger
particle size, the high degree of crystallinity, the hardened lattice structure
and the spatial composition
85%, signi
uctuations. A notable property of alloyed
nanoparticles prepared by organometallic chemistry is their stability when
phase-transferred to water. Usually, the exchange of surfactants damages the
particle surface, quenching the luminescence to some degree; hence core/
shell particles are used instead. The alloy particles reported here retain their
strong emissive properties, making them attractive alternatives to core/shell
particles, which are di
cult to prepare.
In related work, embryonic nuclei-induced routes have been used to
prepare CdZnSe alloy particles, where either CdSe seeds are grown, followed
by the immediate addition of Et
2
Zn, or by the growth of ZnSe seeds followed
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