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sulfosuccinate/water/heptane microemulsion was prepared and an aqueous
solution of cadmium perchlorate added, followed by a heptane solution of
Se(SiMe
3
)
2
. The room-temperature reaction yielded particles of CdSe, which
had to be reduced to dryness to remove water and avoid
occulation. The
powders were then redissolvable in hydrocarbons. Addition of Cd
2+
stock
solution followed by phenyl(trimethylsilyl)selenide resulted in a phenyl-
passivated surface, following which, the particles could be isolated by
centrifugation. Importantly, this is the
d
n
1
y
4
n
g
|
1
rst example of monomer passivation
of a nanoparticle surface and the phenyl-passivated clusters were soluble in
pyridine, but insoluble in petroleum ether. Absorption spectroscopy showed
materials with a shi
in bandgap with decreasing particle size and a slight
excitonic shoulder. The paper also reported growth of the particles with
further addition of precursor, demonstrating Ostwald ripening-type growth.
Following isolation, annealing of these particles in 4-ethylpyridine improved
the crystallinity giving crystalline zinc blende (cubic) type clusters.
15
Annealing of small (20 A) particles in a mixture of tributylphosphine (TBP)
and tributylphosphine oxide (TBPO) resulted in further particle growth to
ca.
40 A, giving particles with a wurtzite (hexagonal) crystalline core. This
highlighted the suitability of phosphine oxides as capping agents and was
used as the basis for further work.
Clusters of CdSe prepared by the inverse micelle route were then used in
core/shell studies, where a ZnS shell was deposited on the surface of the
particles, inorganically passivating the surface.
16
The use of an inorganic
layer rather than a surfactant to protect the surface is an area of immense
interest, as the inorganic layer is not restricted by factors such as the
surfactant cone angle, is generally more stable and is a more complete pro-
tecting layer. By choosing the correct materials, speci
d
n
4
.
c heterojunctions with
engineered band mismatches can be prepared, which will be discussed later.
Once the layer of ZnS had been deposited on the CdSe core, the emission
spectrum, previously broad with luminescence attributed to surface defect
states, became more band edge, suggesting blocking of the defects. In this
case, the sul
de shell was deposited using inorganic rather than organo-
metallic reagents: however, once the surface trapping states had been
removed, the band edge emission observed originated from a nanoparticle
core prepared by organometallic-based precursors.
1.3 Organometallic Routes to CdE (E
S, Se, Te)
The seminal paper describing solution-based organometallic routes to
nanomaterials was published in 1993,
17
and reported the
¼
rst
totally
organ-
ometallic/organic-based synthesis of CdE (E
S, Se, Te) nanoparticles which
resembled a solution analogue of the MOCVD process, using an inert
atmosphere, appropriate precursors with origins in vapour deposition and
coordinating solvents suitable for high-temperature reactions. Although
organometallic precursors had previously been investigated as discussed,
this route relied entirely on organometallic reagents thermolysed directly
¼
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