Chemistry Reference
In-Depth Information
peak in the absorption spectra, at
ca.
500 nm, with the onset of absorption
being approximately 600
700 nm. Broad photoluminescence was also
reported, between 600 nm and 900 nm, and was attributed to sub-gap surface
states.
This initial report was extended to describe the properties of InP in more
depth (notably describing the inability to prepare crystalline particles using
InCl
3
), while also reporting the preparation of GaP and GaInP
2
QDs.
30,31
In
the case of GaP particles, the amorphous particles were initially prepared by
mixing GaCl
3
and P(SiMe
3
)
3
with TOPO and TOP, giving particles with a 3 nm
diameter. These particles could be heated further with a large amount of
TOPO at 360
C for 3 days giving a crystalline nanomaterial. The synthesis of
2.5 nm diameter GaInP
2
required a mixture of both precursors and a slight
molar excess of P(SiMe
3
)
3
, followed by heating in the presence of a small
amount of TOPO and tris(2-diphenylphospinoethyl)phosphine at 400
C for 3
days. All particles could be isolated using methanol as a non-solvent,
followed by redispersion in toluene.
The size of the zinc blende InP particles could be controlled by varying the
amount of precursor used, from 2.6 nm with a band edge of
ca.
600 nm and
a clear excitonic peak, to 4.6 nm with a band edge at
ca.
800 nm. Emission
from the InP particles showed a band in the region of 400
-
d
n
1
y
4
n
g
|
3
-
600 nm, and
a second beyond 800 nm. GaP particles 2
3 nm diameter displayed band
edges between
ca.
400 and 450 nm, complicated by the fact that below 3 nm
the bandgap is predicted to decrease with decreasing size rather than
increase. GaInP
2
particles 2.5
-
6.5 nm in diameter were reported, with the 2.5
nm particles having a band edge at
ca.
2.7 eV (
ca.
460 nm). Importantly, the
photoluminescent properties of the particles could not be fully ascertained
because of interference from a luminescent decomposition product from
heated TOPO, and this remains a signi
-
.
cant factor when exploring the
optical characteristics of nanoparticles. An in-depth exploration of the
optical properties of capping agent decomposition products has highlighted
that TOA and hexadecylamine (HDA) also exhibit emission that may well be
mistakenly assigned as III
-
V QD luminescence.
32
The broad emission was
found to be signi
cantly reduced in 99% pure TOPO (as opposed to tech-
nical-grade, 90% pure TOPO), indicating the luminescent materials may well
be the thermal decomposition products of the impurities outlined in
Chapter 6.
Guzelian
et al.
extended the synthesis using essentially the same chem-
istry, but adding InCl
3
directly to TOPO before heating, forming an In
-
TOPO
complex.
33
A
er prolonged stirring and gentle heating, an equimolar amount
of P(SiMe
3
)
3
was injected into the In
TOPO complex and the temperature
increased to 265
C, where it was maintained for several days, with nucle-
ation and growth occurring simultaneously in direct contrast to the discrete
separation of nucleation and growth easily achieved in the preparation of
II
-
VI nanomaterials. The temperature was lowered, and another capping
agent added if required, followed by several days further heating at a lower
temperature yielding crystalline (zinc blende) InP QDs. The long period of
-
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