Chemistry Reference
In-Depth Information
The InP/ZnS materials could also be transferred to water using thiols, as
described in Chapter 6, without losing any brightness.
This improvement in the synthesis of InP is signi
cant as the materials are
highly monodispersed, crystalline and importantly cover the entire visible
region; an even wider range than CdSe particles, without size-selective
precipitation or harsh synthesis conditions. The use of long-chain amines to
avoid oxide formation is, however, in direct contrast to a report that o
d
n
1
y
4
n
g
|
3
ers
adi
ering explanation of the role of the amine; while the presence of an
oxide layer has been reported as essential for luminescent InP QD, this is
normally the result of exposure of the particle to ambient conditions.
However, the amine-induced growth of an oxide layer on InP has been
reported, which increased the emission quantum yield to 6
-
8%.
84
The
introduction of OAm into the reaction
ask with the indium precursor before
the addition of the phosphorus reagent reportedly resulted in the growth of
an In
2
O
3
shell. In this example, the use of indium myristate as the indium
precursor resulted in the generation of myristic acid anhydride, which
underwent a condensation reaction with the amine to generate an amide and
water, which hydrolysed excess indium precursor generating the wide-
bandgap oxide shell, which has a lattice mismatch of 14%. The use of the
amine altered the size distribution of the particles, again leading to the
suggestion that the amine activated the phosphorus precursor.
This activation claim is again in contrast to the report by Allen
et al.
,
85
who
reported a detailed study into the mechanism of InP formation and showed
that the phosphorus precursors are completely depleted a
er injection,
suggesting the growth step was due to ripening from non-InP species.
Investigations (using NMR) into InP formation using long-chain amines at
relatively low temperatures have shown that in the case of OA-capped parti-
cles, the indium precursor coordinates with the capping agent, before one
P(SiMe
3
)
3
reversibly coordinates to the solvation sphere, giving complex 1
(Figure 2.5). The complex then lost one ligand from the indium precursor,
forming a stable In
.
P bond, while the P(SiMe
3
)
3
lost a silylmethyl group,
forming intermediate 2 in Figure 2.5, which then formed InP clusters and
QDs. This convincingly suggested, contrary to prior results which indicated
that protic reagents such as primary amines induced the decomposition of
P(SiMe
3
)
3
and hence acted as activators, that amines are actually inhibitors
for InP QD formation, restricting precursor decomposition due to steric
factors where the bulky silylated precursor is unlikely to approach the indium
to allow the formation of reaction intermediate 1 (Figure 2.5).
Another study showed that when using indium carboxylate precursors,
further surfactants were not actually necessary to produce capped nano-
particles of InP.
86
In this case, the carboxylates were made prior to the
reaction, from In(C
5
H
5
)
3
and the carboxylic acid directly, followed by puri-
-
cation. The carboxylate was then added to ODE, which was then degassed
and stabilised at an elevated temperature before the injection of an ODE
solution of P(SiMe
3
)
3
.A
er several minutes, the nucleation stage was
terminated by the injection of room-temperature ODE, allowing a discrete
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