Chemistry Reference
In-Depth Information
CdTe particles prepared by organometallic chemistry were phase-transferred
to water using long-chain thiols.
56
Nanoparticles of CdTe made by this
method are expected to have all
uorescence quenched in a matter of hours,
and phase transfer is usually accompanied by further loss of emission. In this
case, where the thiol was used to phase-transfer, the emission was enhanced
and no evidence of trapping was observed. To take advantage of the suit-
ability of the CdTe/thiol system in water, a hybrid approach to core/shell
particles has been developed in which core CdTe (and CdHgTe) particles
were prepared by the aqueous methods.
57
The particles were then phase-
transferred from the aqueous phase to the organic phase using dodeca-
nethiol. Once in the organic phase (toluene) the particles were capped with
a ZnS shell using S(SiMe
3
)
2
and dialkylzinc precursors using TOPO and TBP
as surfactants. Once phase-transferred to the organic phase, the quantum
yields dropped, although this increased to the initial value (and above) upon
shell deposition. A
d
n
1
y
4
n
g
|
3
er shell growth, the absorption and emission spectra
were found to have signi
was too large to be
explained by particle growth or exciton leakage into the shell. Although the
exact cause of the red shi
cantly red-shi
ed. The shi
is not clear, it increased upon increasing shell
thickness, so was possibly related to lattice strain.
Other core materials have also been investigated. As highlighted in
Chapter 1, zinc chalcogenides are potentially ideal blue emitters, but their air
sensitivity restricts potential applications. The deposition of a suitable cap
should therefore make the particles more stable and useful in devices. ZnSe
nanoparticles capped with HDA, synthesised as described earlier, have been
capped with a ZnS shell using dialkylzinc and S(SiMe
3
)
2
, giving strong
emission at
ca.
3eV(
ca.
415 nm) with quantum yields of around 15%, even
months a
.
er synthesis.
58
Chen
et al.
have reported a more in-depth study,
where ZnSe particles were capped with either ZnS or ZnSeS.
59
The core
particles, made from ZnO and lauric acid in HDA, were 2
-
6 nm in diameter,
emitting at 400
440 nmwith quantum yields of up to 10%. Interestingly, ZnO
was found only to fully dissolve with over 3 molar equivalents of lauric acid,
attributed to the decomposition of the carboxylic acid at high temperatures
or the slow formation of the zinc
-
-
acid complex. The ZnS shell was deposited
using ZnO/lauric acid and trioctylphospine sul
er deposition
of 1.8 monolayers, the emission quantum yield was found to increase to 32%.
To prepare the ZnSeS shell, TOPS was injected into the reaction
de (TOPS). A
ask while
the core was still growing. A
er deposition of 1.6 monolayers of the alloy
shell, the emission was found to increase to 26%. In related reports, the
optical properties of ZnSe/ZnS have been examined although few experi-
mental details were provided.
60,61
The desire for cadmium-free materials (notably in bioimaging) has also
led to other materials being examined, notably doped materials. Since
transition metals have been successfully used as dopants in numerous
nanomaterials, a natural progression is the use of doping chemistry in core/
shell synthesis, resulting in unique structures that can be considered hybrid
doped or core/shell. Doped structures can be grown using techniques
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