Chemistry Reference
In-Depth Information
The brightness of the emission was dependent on emission wavelength, with
quantum yields reaching up to 70% for particles that emitted at
ca.
540 nm.
The zinc and sulfur precursors were also found to a
ect growth rate, with
lower concentration of precursor resulting in larger particles, but reducing
the emission stability. Further photoelectron spectroscopy studies have since
shown that the structures grown from such a one-pot reaction are actually an
InPZnS alloy with a thin ZnS shell,
156
and further work on the role of zinc
carboxylates suggests that inclusion of such materials during QD growth
leads to the formation of a luminescent InZnP alloy rather than InP particles
passivated with the zinc species.
157
A related route by Ryu
et al.
explored the
role of the zinc precursor in the preparation of InP/ZnS, and also used a long-
chain thiol as a sulfur precursor.
158
The group reported a notable increase in
emission and red shi
d
n
1
y
4
n
g
|
3
in wavelength when Zn(CO
2
CH
3
)
2
was added to
a solution of InP dots and heated at 230
C for 5 hours, reportedly forming
a zinc carboxylate on the surface. Later addition of the thiol increased the
emission quantum yield up to 38%. It is still unclear whether the addition of
the Zn(CO
2
CH
3
)
2
actually formed a surface carboxylate species, or, referring
to results reported by Reiss, formed an alloy. The increase in emission
intensity of InP dots by the addition of a zinc carboxylate was also reported by
Li
et al.
159
The emission was then increased to a
nal quantum yield of 22%
by further addition of zinc ethylxanthate, a single-source precursor for ZnS
deposition. Whether the addition of the zinc stearate also resulted in alloy
formation was not reported.
Other similar families of core/shell QDs have been reported, such as
InGaP/ZnS materials, which displayed band edge emission at
ca.
675 nm,
although this was a commercially available product (at the time) and no
experimental details were provided.
160
Emission at this wavelength is of
interest to biologists, and these alloyed core materials have been utilised in
deep tissue labelling experiments where the emission can be detected even
through a mouse skull.
161
GaInP
2
shells of 0.5
.
2 nm have been deposited on
InP cores, using GaCl
3
, InCl
3
and P(SiMe
3
)
3
(in the ratio of 2 : 2 : 1), added
slowly to the core particles immediately a
-
er growth, at 260
C followed by
heating for 16 hours using TOPO and hexylamine as capping agents.
162
Few
optical properties were reported. InP shells were also grown on MnP cores
using InCl
3
and P(SiMe
3
)
3
, dissolved in TOP, which were added to the core
solution of MnP.
163
The resulting MnP/InP core/shell structures maintained
the magnetic properties of the MnP core, but did not show any optical
characteristics consistent with the InP shell.
5.6 Core/Shell Structures Based on IV
VI Materials
Lead chalcogenide QDs are predominantly infrared-emitting materials, with
excellent optical and electronic properties which have been exploited in
numerous applications. Although the particles have superior attributes and
have few organic dye equivalents, they are extremely sensitive to oxidation.
The optical properties have reportedly exhibited blue shi
-
s, emission
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