Chemistry Reference
In-Depth Information
CdSe core particles were grown using Cd(CH
3
CO
2
)
2
, TOPO, HDA, a phos-
phonic acid and TOPSe. Unlike previous reactions, the cadmium precursor
was injected to the hot surfactants/selenium solution, resulting in particle
formation and the conversion of only 20% of the Cd(CH
3
CO
2
)
2
, compared to
almost 100% conversion of Me
2
Cd under similar conditions. The excess
cadmium precursor was then used to form an epitaxial CdS shell by the
addition of H
2
S gas into a closed reaction
d
n
1
y
4
n
g
|
3
ask (above, not into the reaction
C. The particles prepared exhibited narrow emission
mixture) at 140
(FWHM 27
35 nm) in the green to red regions of the spectrum, with quantum
yields up to 85%, while CdSe core with
ca.
3.5 monolayers of CdS exhibited
similar stability to the analogous CdSe/ZnS system.
The control over the deposition of the shell material is a key factor as
described above. The growth of too thick a shell resulted in the quenching of
the emission due to strain-induced defects, therefore a method of epitaxially
growing individual monolayers controllably was of immense importance. An
interesting development is the use of successive ion layer adsorption and
reaction (SILAR),
44,45
a solution analogue of atomic layer epitaxy (ALE), where
cationic and anionic species were deposited sequentially, essentially half
a monolayer at a time. Since both species are not present at the same time,
separate nucleation is avoided.
This method was initially used to prepare CdSe/CdS particles.
46
In this
case, the core particles of CdSe,
ca.
3.5 nm in diameter were prepared using
the green method described by Peng (see Chapter 1), utilising CdO/stearate
and TBPSe as precursors in a non-coordinating solvent. The particles were
puri
-
.
ed and isolated into hexane, removing excess capping agents, and the
concentration of nanoparticles calculated using Beer
s law. The isolation of
core nanoparticles as a powder was avoided as this resulted in substandard
core/shell materials. The amount required to add a monolayer was calculated
and the cadmium precursor then added, followed by the sulfur precursor;
this process was repeated to add a speci
'
c number of monolayers. When the
required number of monolayers had been grown, the reaction was quenched
by removal from the heat source. Contrary to other methods of core/shell
preparation, a high temperature (240
C) was required for discrete shell
growth, avoiding the formation of CdS particles. The absorption spectra were
consistent with shell growth, and the emission intensity increased as ex-
pected. Con
rmation of a core/shell structure was obtained using XPS
measurements. Emission quantum yields were found to increase with the
increasing number of shells to a maximum of
ca.
40%, with no drop-o
er
5 monolayers had been deposited, in contrast to CdSe/ZnS. What is notable is
that the core particles capped with amines exhibited quantum yields of up to
80%, which actually dropped upon shell addition. Core particles capped with
just TOPO had low quantum yields, which increased to a similar level to that
of CdSe/CdS particles prepared from amine-capped cores upon addition of
the shells. The emission width remained narrow throughout the experi-
ments. The emission spectra of particles with thick shells (
a
ve monolayers)
displayed a blue-shi
ed shoulder, the origins of which were not clear. The
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