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the thermolysis of Et
2
Zn and phosphine chalcogenides in TOP.
17
It was
observed that the use of TOPO as a capping agent was unsuccessful, with the
dialkylzinc forming stable TOPO adducts. Hines and Guyot-Sionnest
reported zinc selenide (ZnSe) particles with a cubic crystalline core, prepared
by the thermolysis of Et
2
Zn and TOPSe in HDA.
105
The use of TOPO was again
found to be unsuccessful, attributed to TOPO binding too strongly to zinc
and TOP binding too weakly. The use of long-chain amines was found to be
ideal; they are slightly weaker Lewis bases than phosphines, so the coordi-
nation to the particle surface through the electron-rich nitrogen atom
provided ideal binding characteristics that allowed growth while passivating
the particles, and the long alkyl chain imparted solubility in organic solvents
while allowing solvent
d
n
1
y
4
n
g
|
1
-
non-solvent interactions. The emission from ZnSe
capped with HDA was found to be band edge with quantum yields of 20
-
50%,
attributable to the high growth temperatures and e
cient capping of the
amine and the phosphine. The use of amines as capping agents has been
investigated by Woo,
106
who suggested that the linear steric features of
primary amines (TOP has a cone angle of
ca.
130
)
107
allowed for more e
d
n
4
.
-
cient packing on the particle surface resulting in a more e
cient surface
passivation, and that amines etched the particle surface removing defects
and trapping sites.
The Hines method of preparing ZnSe particles was advanced by Cozzoli
et al.
, who amended the method to include the use of amines with longer
chain length/higher boiling point.
108
The amended method also reported the
additional slow introduction of further precursor to refocus the size distri-
bution. The route is also interesting due to the growth conditions; a
er the
rapid injection of precursor at
ca.
300
C, the reaction cooled to
ca.
265
C
and was allowed to proceed at this temperature. Increasing the concentration
of the precursor stock solution resulted in larger particles (up to 8 nm). The
method also covered altering growth conditions to prepare particles of
various morphologies, which will be described later.
Li
et al.
prepared cubic ZnS and ZnSe particles in ODE and tetracosane
(a non-coordinating solvent with a boiling point of 391
C) using zinc
carboxylates and tributylphosphine selenide (TBPSe) or an ODE solution of
sulfur (ODE/S) as precursors, and a variety of capping agents such as amines
and fatty acids.
109
The growth of zinc chalcogenides demands a more strin-
gent set of conditions; again using capping ligands with a long chain length
slowed the growth by increasing steric e
ects of the zinc monomer. The
growth of particles using just a fatty acid as a capping agent was possible, but
gave substandard material. The use of a high concentration of fatty acids
resulted in no particle nucleation, attributed to the activity coe
cient being
too low to form the required amount of nuclei. Activation of the zinc
precursor by inclusion of an amine during the reaction increased the reac-
tion rate and gave balanced growth and nucleation, observable by the sharp
features in the optical spectra. Interestingly, ZnO nanoparticles were
prepared if no selenium was added, suggesting that ZnSe particles may be
prepared by a ZnO intermediate or small cluster. The amine was found to be
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