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enhanced strain when grown on a cubic core. Electron microscopy, XRD and
XPS all con
rmed the core/shell structure, particularly important with InAs/
CdSe which have identical lattice constants and might therefore have formed
alloys. These materials (InAs/ZnSe) have been used to probe the electron
structure of QDs, notably in imaging the arti
cial atom-like states.
134,135
V particles to be shelled, other groups
have explored this system. InAs/CdSe core/shells have also been grown in
non-coordinating solvents. The shell was grown using the SILAR technique at
190
C with cadmium oxide and TOPSe, the resulting particles again exhib-
iting a red shi
Although InAs QDs were the
rst III
-
d
n
1
y
4
n
g
|
3
in the optical properties.
136
Impressively, the emission
quantum yields were reportedly as high as 90%, and emission could be tuned
from
ca.
800 nm to
ca.
1400 nm. The use of non-coordinating solvents has
also been explored in the synthesis of InAs/ZnSe, the shell being grown
in situ
without isolation of the core particles, using Me
2
Zn and TOPSe as precur-
sors
137
(the use of S(SiMe
3
)
2
in place of TOPSe resulted in InAs/ZnS parti-
cles
138
). This could be amended to allow the growth of InAs/ZnCdSe by the
addition of a small amount of Me
2
Cd during shell synthesis, which extended
the emission wavelength of the particles further into the red. The resulting
particles were notable by possessing a clear excitonic peak in the absorption
spectra past 800 nm, which became more pronounced on shell deposition,
with quantum yields of up 10% in hexane which dropped only slightly upon
phase transfer into water. The QDs also possessed an extremely small
hydrodynamic diameter of below 10 nm when phase-transferred using vari-
ants of DHLA, and hence were extremely useful in sentinel lymph node
imaging. The lattice constant for InAs (6.058 A) is almost identical to that of
CdSe (6.05 A) which inspired the same research group to explore other alloy
shell systems based on the successful CdSe/shell materials described earlier,
with the aim of preparing material exhibiting near infrared emission
between 700 and 900 nm, ideal for biological imaging.
139
The growth of a low
lattice mismatched CdS shell pushed the emission beyond this window, in
a similar manner to CdSe shells described above, but materials with the high
lattice mismatched ZnS shell were found to be di
.
cult to phase-transfer,
hence the alloy system was deemed ideal. The core particles were grown
again in ODE, using TOP as a capping agent, giving extremely small InAs
cores (1.4 nm diameter), with an emission wavelength of
ca.
700 nm. The
growth of the shell was achieved with the addition of OAm to provide surface
passivation prior to precursor injection, followed by dropwise addition of
Me
2
Cd, Me
2
Zn and S(SiMe
3
)
2
over several hours at 170
C, which was stopped
once the ideal emission wavelength (800 nm) had been reached. The
resulting shell was 2.5 monolayers of Zn
0.7
Cd
0.3
S, producing particles with an
overall diameter of 2.9 nm and an emission quantum yield of 35
50%. Once
the surface ligands were exchanged for a water-soluble polymer, the hydro-
dynamic diameter was less than 10 nm, and the water-soluble core/shell QDs
were found to be useful in cell imaging.
A ZnS shell has also been deposited on InAs particles that were grown
using In(OCOCH
3
)
3
and AsH
3
in ODE, giving InAs/ZnS particles that had
-
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