Chemistry Reference
In-Depth Information
core/shell structure where the CdS electronically passivated the surface and
protects the emitting core, and similar observations have been made with
gold nanoparticles.
45
This, however, does not take into account the fact that
thiol-capped CdSe particles are not as luminescent as thiol-capped CdTe, and
in fact have relatively poor optically properties which one would not expect
from CdSe/CdS core/shell particles. Another explanation is based on the
redox energy levels of the thiol, which act as inhibitors for hole-trapping
processes for the CdTe particles.
46
This highlights the important role of the
capping agent, which should be viewed not as a simple inert surfactant, but
as an integral constituent of any nanostructure with its own discrete chem-
istry and physics.
d
n
1
y
4
n
g
|
1
1.3.1 Alternatives to Metal Alkyls
Although the use of organometallic precursors resulted in high-quality
materials, this route still utilised metal alkyls as precursors, which are
notoriously toxic and di
d
n
4
.
cult to handle. One method of circumventing the
problem is the use of simple Lewis base adducts of the metal alkyls, related to
the
method of purifying metal alkyls for chemical vapour deposi-
tion.
47
Adducts are generally more stable, (and in some cases, crystalline
solids whereas the parent metal alkyls are liquids), making the precursors
easier to handle. Replacing Me
2
Cd with a triethylamine adduct of Me
2
Cd
appeared to have no remedial impact on the quality of nanoparticles
prepared.
48
Likewise, a study into the use of the solid adduct (2,2
0
-bipyridine)
Me
2
Cd, the adduct bis(3-diethylaminopropyl)-cadmium or the more stable
metal alkyl dineopentylcadmium, ((CH
3
)
3
CH
2
)
2
Cd, as substitutes for Me
2
Cd
revealed no detrimental e
'
diphos
'
nal product.
49
Similarly,
cadmium alkoxide complexes have been used as replacements for metals
alkyls in CdE (E
ects on the quality of the
¼
S, Se, Te) nanoparticle synthesis.
50
Brien reported one of the simplest methods of preparing
CdS nanoparticles using CdCl
2
and sulfur, and although not organome-
tallic, this and related routes found their genesis in the original organo-
metallic synthetic pathways. In this method, sulfur was dissolved in TOP
giving trioctylphosphine sul
Lazell and O
'
de (TOPS), which was mixed with the
cadmium salt in excess TOP, then injected into TOPO at an elevated
temperature. This can be seen as the
TOPO-related synthesis of
nanoparticles, avoiding toxic metal alkyls or chalcogen species,
51
although
Stuczynski
et al.
rst
'
green
'
rst used tributylphosphine sul
de (TBPS) in the prepa-
ration of cobalt sul
de
via
cluster precursors.
52
(It is worth noting a report
that suggested TOPS was too unreactive towards zinc salts to be used in the
synthesis of nanomaterials.
53
)
This method has been developed, noticeably by Peng, who has reported
several new green routes to passivated particles.
54
Peng made a detailed study
of the decomposition of dimethylcadmium and observed metal precipitates
that coordinated to impurities in technical-grade TOPO, giving complexes
such as cadmium
-
hexadecylphosphonic acid.
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