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precursor from M
2+
to M
0
, which then reacted with a trialkylphosphine
chalcogenide eliminating the phosphine, which reacted further to provide
the particles, phosphine oxide and metal counterion anhydride. The use of
trialkylphosphine chalcogenides is considered the standard group-VI
precursor system, but the use of other chalcogen sources has also been
described, such as NaHSe
64
and NaHTe.
65
d
n
1
y
4
n
g
|
1
1.3.2 Alternatives to TOPO and Novel Solvents
The search for novel solvents is o
en driven by the need for a phosphine-
free system, as TOP and TOPO are considered toxic,
66
and to a lesser degree,
by cost. Peng demonstrated that non-coordinating solvents could also be
used in nanoparticle synthesis, as opposed to the thermolysis in TOPO. In
a typical example, CdO and oleic acid were dissolved in octadecene (ODE),
a room-temperature liquid with a boiling point of
ca.
320
C. Elemental
sulfur dissolved in ODE was then injected into the reagents at elevated
temperatures, forming CdS nanoparticle capped with oleic acid. An inter-
esting examination revealed that upon isolation of CdS particles prepared in
ODE, a decrease in the emission intensity was observed. Redispersion of the
particles in CHCl
3
resulted in an increase of trap-related emission associ-
ated with the removal of surface ligands, while redispersion in ODE resulted
in the recovery of band edge emission, attributable to the solvent possibly
blocking surface trapping states.
67
The delicate balance between forming too many nuclei, which resulted in
the defocusing of the size distribution by Ostwald ripening, or too few nuclei,
which resulted in growth that was too fast to reach the required size and
distribution, was easily achieved by simply altering the concentration of the
capping agent and thus the amount of metal complex available for the
reaction (the monomer concentration). Another advantage is the extreme
control over particle size; the synthesis of CdSe particles less than 4 nm in
diameter is di
d
n
4
.
cult to achieve using methods discussed previously, but
using ODE as a solvent, particles in the size range 1.5
20 nm were routinely
prepared.
68
The role of ODE was envisaged as a non-coordinating solvent and
a simple chalcogen delivery agent. Further work has shown that in the case of
selenium in ODE, the viability of ODE/Se is dependent on the length of time
the reagents are heated and the temperature. Selenium reportedly formed
rings and chain species that formed bridged structures between alkene/
alkane chains consisting of multiple selenium atoms. This form of precursor
was found to be highly reactive and useful, unlike the monoselenide species
obtained upon prolonged heating.
69
Using this route, zinc blende-structured
CdSe could be prepared by adding a phosphonic acid in the TOPSe solu-
tion.
70
The generation of zinc blende-structured CdSe by the organometallic
route is unusual, and few reports describe CdSe with such a crystalline core.
71
The absence of phosphine or phosphine oxide capping agents, or the use of
a non-coordinating solvent, appears to be a constant factor with such
a synthesis; for example, the use of H
2
Se gas as a chalcogen precursor and the
-
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