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in a hot coordinating solvent (long-chain phosphine oxides, such as
tri-
n
-octylphosphine oxide, TOPO) rather than a room-temperature reaction
in an inverse micelle. The precursors employed, Me
2
Cd and either a silylated
chalcogenide or a chalcogen dissolved in trioctylphosphine (TOP), were
chosen with reference to the work of Steigerwald, and the route elegantly
incorporated both the precursor (phosphine chalcogenide) and capping
agent (phosphine oxide) chemistries described above. It should be assumed
that all reactions and preparations described herea
d
n
1
y
4
n
g
|
1
er are carried out under
inert atmosphere conditions unless stated otherwise.
Cadmium selenide (CdSe) is generally considered the prototypical QD
material as size quantisation e
ects result in tuneable emission across the
entire visible range of the electromagnetic spectrum, making the material
attractive for a wide range of optoelectronic applications. A notable report by
Donega
et al.
highlighted the importance of reaction temperature, reagents
and the ratio of reagents. By carefully selecting optimum condition, CdSe
particles with quantum yields as high as 85% were prepared.
18
In contrast,
CdS, one of the earliest materials to be studied because of its ease of prep-
aration, demands less attention as it displays only small changes in the
optical properties when prepared on the nano scale. This can be attributed to
the di
d
n
4
.
32 A; CdS
a
B
¼
19 A)
erence in Bohr radius of the exciton (CdSe,
a
B
¼
which means CdS particles have to be signi
cantly smaller than CdSe to
exhibit size quantisation e
in the
band edge. (In comparison, TiO
2
, a wide-bandgap semiconductor, has an
excitonic radius of only 8 A and would require crystals to be little more than
clusters to display any optical e
ects and therefore display a smaller shi
ects from the con
nement of charge
carriers. TiO
2
displays little, if any, shi
in the optical band edge. In this case,
size quantisation e
ects are manifest as variations in the oscillator
strength.
19
)
In a typical reaction, Me
2
Cd and a trioctylphosphine chalcogenide (such as
trioctylphosphine selenide, TOPSe) were dissolved in TOP. The Lewis base
phosphine, a liquid at room temperature, served as both a solvent for
precursor delivery and a capping agent once the nanoparticles had formed.
The precursor solution was then injected into TOPO (which had been
rigorously dried and degassed), under an inert atmosphere at temperatures
of between 100 and 350
C, with lower temperatures producing smaller
particles. The sudden introduction of reagents into a hot solvent and the
subsequent immediate supersaturation resulted in the formation of nuclei.
The drop in temperature a
er the injection of room-temperature reagents
prevented further nucleation, and further heating resulted in growth of
particles by Ostwald ripening. The sudden nucleation and slow growth steps,
originally described by La Mer,
20
resulted in a monodispersed product. The
surfactants, TOPO and TOP coordinated to the surface of the nanoparticles,
providing physical and electronic passivation. The labile nature of the
surfactants is a key requirement, desorbing from the particle surface to allow
growth, yet coordinating strongly enough to allow particle isolation and
provide the required protection for the nanoparticle.
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