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a narrowing of, the re
ections, and electron microscopy con
rmed their
spherical shape.
A larger study by the same group explored the use of GaAs, ZnS and ZnSe as
shell materials on InAs cores and discussed their suitability with regards to
lattice mismatch, band o
set (Figure 5.9) and chemical compatibility.
133
Attempts to grow a GaAs shell on a InAs core were limited in their success,
with a maximum of no more than two monolayers of shell. This was attrib-
uted to the strong gallium
d
n
1
y
4
n
g
|
3
TOP interactions, which is also a limiting factor in
the preparation of gallium-containing QDs, such as GaAs. The report also
highlighted the unsuitability of TOPO as a capping agent for CdSe shell
growth, as this induced CdSe particle nucleation, unlike the use of TOP.
Increasing the Se : Cd ratio was also found to reduce nucleation, although
this a
-
nal material. The use of TOP for ZnSe and
ZnS shell growth was found to be unsuitable, as the resulting particles were
poorly soluble a
ected the solubility of the
er processing with CH
3
OH. A mixture of TOP and TOPO was
found to reduce nucleation while making the particles soluble.
The optical properties of InAs capped with ZnS and ZnSe di
ered from
those capped with InP or CdSe, as no shi
in the band edge was observed,
due to the larger band o
nement energy of
the charge carriers. The emission quantum yields were up to 20% for ZnSe-
capped InAs, with a maximum of 8% for InAs/ZnS with a shell of 1.2
set which was larger than the con
-
1.8
monolayers, a
er which further shell growth reduced the quantum yields
due to charge carriers being trapped at the interface. One should also
consider that ZnS prefers the wurtzite structure, which might cause
.
Figure 5.9
A summary of lattice mismatches and band osets for InAs cores and
a range of potential shell materials. Reprinted with permission from
Y.-W. Cao and U. Banin,
J. Am. Chem. Soc.
, 2000, 122, 9692. Copyright
2000 American Chemical Society.
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