Chemistry Reference
In-Depth Information
Core/shell systems are not restricted to semiconducting materials of the
same family. For example, CdTe cores, prepared by the standard green route,
have been layered with a shell of InP, which was grown using standard
chemistry used to prepare III
gu-
ration suggested an inverse structure, with InP possessing a narrower
bandgap than CdTe. This resulted in the red shi
-
V nanomaterials.
131
The energy gap con
of the emission wavelength
while the excitonic feature in the absorption spectra disappeared, not unlike
the optical characteristics in type II core/shell materials. Quantum yields of
the core/shell structures improved from 20% to 70% upon shell deposition,
which dropped to 40% with increasing shell thickness. At this point,
attempts to phase-transfer the material to water resulted in signi
d
n
1
y
4
n
g
|
3
cant
quenching of the emission, and attempts to grow a ZnSe shell on the CdTe/
InP failed. A ZnS shell was successfully deposited on the core/shell system
using Et
2
Zn and S as precursors in ODE at 160
C. Notably, the use of
S(SiMe
3
)
2
as a precursor resulted in the complete quenching of the emis-
sion. Addition of the
ect the emission quantum yield
or wavelength, and yielded material stable enough to be successfully
phase-transferred into water and utilised in imaging experiments.
nal shell did not a
5.5 Core/Shell Structures Based on III
V Materials
So far, we have discussed core/shell particles with the core based on the II
-
VI
family of materials. However, other materials have been explored as cores,
notably the III
-
-
V family of QDs. Simple unshelled III
-
V particles such as InP
.
have notably low quantum yields, o
en well below 1%, which limits their
applications. The
rst attempts to deposit wide-bandgap semiconductor
shells on III
V materials were reported by the Banin group who described the
synthesis of InAs/CdSe and InAs/InP
132
which might be considered unusual,
as InAs is a less popular material than, for example, InP. Shells were
deposited by dispersing 1.7 nm diameter InAs cores (prepared, puri
-
ed and
the size distribution narrowed as described in Chapter 2, with a clear exci-
tonic shoulder at 990 nm) in TOPO, followed by stabilisation at 260
C. This
high temperature o
en leads to Ostwald ripening when depositing shells on
-
II
VI materials, resulting in a wide size distribution, although this was not
observed when using InAs cores. Standard precursor solutions in TOP (either
InCl
3
and P(SiMe
3
)
3
,orMe
2
Cd and TOPSe) were added dropwise at
this temperature, and the growth monitored by absorption spectroscopy.
Isolation of the particles was achieved by standard solvent/non-solvent
interactions. The addition of either the InP or CdSe shell had a distinct e
ect
on the absorption spectra, red-shi
ing the excitonic peak by almost 100 nm.
The resulting emission from InAs/CdSe particles was found to be band edge,
with quantum yields of 18% (1.2 monolayers), up from 1% for the naked
cores. Emission from InAs particles was quenched upon InP shell deposition,
due to the electrons residing in the shell, as con
rmed by the increase in
emission when oxidised, as opposed to InAs/CdSe, which quenched upon
surface damage. XRD con
rmed epitaxial shell growth with a shi
in, and
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