Chemistry Reference
In-Depth Information
by the addition of Me
2
Cd.
228
The absorption and emission of alloy particles
starting from CdSe seeds gradually blue-shi
ed over time due to the incor-
poration of the wide-bandgap ZnSe into the narrow-bandgap CdSe. A
er 7
hours growth,
little change was detected, suggesting the particles had
nished growing and formed a homogenous, stable alloy. When CdSe is
incorporated into a ZnSe lattice, a red shi
in the optical properties was
d
n
1
y
4
n
g
|
1
observed. The red shi
, although evident, was not as pronounced as when
the alloys are prepared from the CdSe seeds. A
er
ca.
10 hours growth, the
emission started to broaden, attributable to Ostwald ripening and the
gradual oxidation and desorption of the surfactants. Emission quantum
yields were 45
-
ciencies being obtained when
growth continued to the stable phase. Prolonged growth past this stage
resulted in the gradual decrease in the quantum yield. Phase-transfer to
water resulted in particles with quantum yields of 25
70%, with the highest e
35%.
Likewise, CdSeS nanoparticles were prepared in a one-pot reaction
between CdO/oleic acid and a premixed stock solution of TOPSe/TOPS in
TOA.
229
The emission could be tuned between
ca.
460 and 580 nm, with
a maximum quantum e
-
d
n
4
.
ciency of 85% being reported. In this case, sele-
nium was reported to react faster than sulfur with cadmium, resulting, in
some cases, in particles with a CdSe-rich core, which was the dominant
species when considering the bandgap of the resulting particles. Investiga-
tions into the structure using X-ray photoelectron spectroscopy (XPS)
con
rmed a gradual graded structure that avoided problems with lattice
mismatch and dislocations at the interface.
230
Further studies into the role
of surfactants in the synthesis of CdSeS alloyed particles highlighted
that choosing di
erent surfactants could control the crystalline phase of
the particles.
231
These particles have been capped with SiO
2
and used in
cell imaging.
232
Other attempts to prepare CdSeS particles using non-
coordinating solvents and precursors reported earlier
68
resulted in homog-
enous alloyed particles with excitonic features in the absorption spectra
tuneable from
ca.
400 nm to
ca.
575 nm.
233,234
The emission from these
particles was notable due to the presence of trap emission. Inclusion of TBP
in the reaction mixture resulted in the coordination of sulfur, allowing
selenium to react
rst, followed by the sulfur once the TOPS had been
exhausted, resulting in a graded structure. Large amounts of TBP drastically
a
ected the morphology of the particles, yielding nanorods.
Attempts to prepare quaternary alloyed Cd
1
x
Zn
x
S
y
Se
1
y
particles resulted
in a CdSe-rich core, with a hybrid CdS and ZnS shell
235
(in contrast to alloys of
HgSe
1
x
S
x
,
135
where such particles were found to be sulfur rich, attributed to
sulfur
s increased reactivity towards mercury). ZnS shells have purposely
been deposited on Cd
1
x
Zn
x
Se using zinc ethylxanthate and a zinc carbox-
ylate as shell precursors, giving a core/shell system that emitted in the blue
region of the spectrum with a quantum e
'
ciency of 67%.
236
Other quaternary materials include ZnCdSSe, prepared by a phosphine-
free synthesis.
237
The particles remained at
ca.
6 nm diameter, but could be
tuned to emit across the entire visible spectrum by altering the composition.
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